Merge branch 'develop' into correct-cc

# Conflicts:
#	lib/spack/env/cc

84 files changed, 2893 insertions, 1183 deletions

diff --git a/lib/spack/docs/basic_usage.rst b/lib/spack/docs/basic_usage.rst
index 0578f0c8db..accf09cc2a 100644
--- a/lib/spack/docs/basic_usage.rst
+++ b/lib/spack/docs/basic_usage.rst
@@ -357,7 +357,7 @@ Spack, you can simply run ``spack compiler add`` with the path to
 where the compiler is installed.  For example::
 
     $ spack compiler add /usr/local/tools/ic-13.0.079
-    ==> Added 1 new compiler to /Users/gamblin2/.spackconfig
+    ==> Added 1 new compiler to /Users/gamblin2/.spack/compilers.yaml
         intel@13.0.079
 
 Or you can run ``spack compiler add`` with no arguments to force
@@ -367,7 +367,7 @@ installed, but you know that new compilers have been added to your
 
     $ module load gcc-4.9.0
     $ spack compiler add
-    ==> Added 1 new compiler to /Users/gamblin2/.spackconfig
+    ==> Added 1 new compiler to /Users/gamblin2/.spack/compilers.yaml
         gcc@4.9.0
 
 This loads the environment module for gcc-4.9.0 to get it into the
@@ -398,27 +398,34 @@ Manual compiler configuration
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 If auto-detection fails, you can manually configure a compiler by
-editing your ``~/.spackconfig`` file.  You can do this by running
-``spack config edit``, which will open the file in your ``$EDITOR``.
+editing your ``~/.spack/compilers.yaml`` file.  You can do this by running
+``spack config edit compilers``, which will open the file in your ``$EDITOR``.
 
 Each compiler configuration in the file looks like this::
 
     ...
-    [compiler "intel@15.0.0"]
-        cc = /usr/local/bin/icc-15.0.024-beta
-        cxx = /usr/local/bin/icpc-15.0.024-beta
-        f77 = /usr/local/bin/ifort-15.0.024-beta
-        fc = /usr/local/bin/ifort-15.0.024-beta
-    ...
+    chaos_5_x86_64_ib:
+      ...
+      intel@15.0.0:
+          cc: /usr/local/bin/icc-15.0.024-beta
+          cxx: /usr/local/bin/icpc-15.0.024-beta
+          f77: /usr/local/bin/ifort-15.0.024-beta
+          fc: /usr/local/bin/ifort-15.0.024-beta
+      ...
+
+The chaos_5_x86_64_ib string is an architecture string, and multiple
+compilers can be listed underneath an architecture.  The architecture
+string may be replaced with the string 'all' to signify compilers that
+work on all architectures.
 
 For compilers, like ``clang``, that do not support Fortran, put
 ``None`` for ``f77`` and ``fc``::
 
-    [compiler "clang@3.3svn"]
-        cc = /usr/bin/clang
-        cxx = /usr/bin/clang++
-        f77 = None
-        fc = None
+    clang@3.3svn:
+        cc: /usr/bin/clang
+        cxx: /usr/bin/clang++
+        f77: None
+        fc: None
 
 Once you save the file, the configured compilers will show up in the
 list displayed by ``spack compilers``.
@@ -896,7 +903,7 @@ Or, similarly with modules, you could type:
    $ spack load mpich %gcc@4.4.7
 
 These commands will add appropriate directories to your ``PATH``,
-``MANPATH``, and ``LD_LIBRARY_PATH``.  When you no longer want to use
+``MANPATH``, ``CPATH``, and ``LD_LIBRARY_PATH``.  When you no longer want to use
 a package, you can type unload or unuse similarly:
 
 .. code-block:: sh
diff --git a/lib/spack/docs/developer_guide.rst b/lib/spack/docs/developer_guide.rst
index db47de80f5..0b618aa683 100644
--- a/lib/spack/docs/developer_guide.rst
+++ b/lib/spack/docs/developer_guide.rst
@@ -73,19 +73,32 @@ with a high level view of Spack's directory structure::
   spack/                  <- installation root
      bin/
         spack             <- main spack executable
+
+     etc/
+        spack/            <- Spack config files.
+                             Can be overridden by files in ~/.spack.
+
      var/
         spack/            <- build & stage directories
+        repos/            <- contains package repositories
+           builtin/       <- pkg repository that comes with Spack
+              repo.yaml   <- descriptor for the builtin repository
+              packages/   <- directories under here contain packages
+
      opt/
         spack/            <- packages are installed here
+
      lib/
         spack/
            docs/          <- source for this documentation
            env/           <- compiler wrappers for build environment
 
+           external/      <- external libs included in Spack distro
+           llnl/          <- some general-use libraries
+
            spack/         <- spack module; contains Python code
               cmd/        <- each file in here is a spack subcommand
               compilers/  <- compiler description files
-              packages/   <- each file in here is a spack package
               test/       <- unit test modules
               util/       <- common code
 
diff --git a/lib/spack/docs/features.rst b/lib/spack/docs/features.rst
index fcb810086d..0998ba8da4 100644
--- a/lib/spack/docs/features.rst
+++ b/lib/spack/docs/features.rst
@@ -103,7 +103,7 @@ creates a simple python file:
 It doesn't take much python coding to get from there to a working
 package:
 
-.. literalinclude:: ../../../var/spack/packages/libelf/package.py
+.. literalinclude:: ../../../var/spack/repos/builtin/packages/libelf/package.py
    :lines: 25-
 
 Spack also provides wrapper functions around common commands like
diff --git a/lib/spack/docs/getting_started.rst b/lib/spack/docs/getting_started.rst
index 67ca18e71a..2c5b68ea65 100644
--- a/lib/spack/docs/getting_started.rst
+++ b/lib/spack/docs/getting_started.rst
@@ -22,7 +22,7 @@ go:
    $ spack install libelf
 
 For a richer experience, use Spack's `shell support
-<http://llnl.github.io/spack/basic_usage.html#environment-modules>`_:
+<http://software.llnl.gov/spack/basic_usage.html#environment-modules>`_:
 
 .. code-block:: sh
 
diff --git a/lib/spack/docs/index.rst b/lib/spack/docs/index.rst
index 79757208c9..d6ce52b747 100644
--- a/lib/spack/docs/index.rst
+++ b/lib/spack/docs/index.rst
@@ -18,7 +18,7 @@ configurations can coexist on the same system.
 Most importantly, Spack is *simple*.  It offers a simple *spec* syntax
 so that users can specify versions and configuration options
 concisely.  Spack is also simple for package authors: package files
-are writtin in pure Python, and specs allow package authors to
+are written in pure Python, and specs allow package authors to
 maintain a single file for many different builds of the same package.
 
 See the :doc:`features` for examples and highlights.
diff --git a/lib/spack/docs/mirrors.rst b/lib/spack/docs/mirrors.rst
index d732a3dd54..dad04d053b 100644
--- a/lib/spack/docs/mirrors.rst
+++ b/lib/spack/docs/mirrors.rst
@@ -38,7 +38,7 @@ contains tarballs for each package, named after each package.
 
 .. note::
 
-   Archives are **not** named exactly they were in the package's fetch
+   Archives are **not** named exactly the way they were in the package's fetch
    URL.  They have the form ``<name>-<version>.<extension>``, where
    ``<name>`` is Spack's name for the package, ``<version>`` is the
    version of the tarball, and ``<extension>`` is whatever format the
@@ -186,7 +186,7 @@ Each mirror has a name so that you can refer to it again later.
 ``spack mirror list``
 ----------------------------
 
-If you want to see all the mirrors Spack knows about you can run ``spack mirror list``::
+To see all the mirrors Spack knows about, run ``spack mirror list``::
 
    $ spack mirror list
    local_filesystem    file:///Users/gamblin2/spack-mirror-2014-06-24
@@ -196,7 +196,7 @@ If you want to see all the mirrors Spack knows about you can run ``spack mirror
 ``spack mirror remove``
 ----------------------------
 
-And, if you want to remove a mirror, just remove it by name::
+To remove a mirror by name::
 
    $ spack mirror remove local_filesystem
    $ spack mirror list
@@ -205,12 +205,11 @@ And, if you want to remove a mirror, just remove it by name::
 Mirror precedence
 ----------------------------
 
-Adding a mirror really just adds a section in ``~/.spackconfig``::
+Adding a mirror really adds a line in ``~/.spack/mirrors.yaml``::
 
-   [mirror "local_filesystem"]
-       url = file:///Users/gamblin2/spack-mirror-2014-06-24
-   [mirror "remote_server"]
-       url = https://example.com/some/web-hosted/directory/spack-mirror-2014-06-24
+   mirrors:
+     local_filesystem: file:///Users/gamblin2/spack-mirror-2014-06-24
+     remote_server: https://example.com/some/web-hosted/directory/spack-mirror-2014-06-24
 
 If you want to change the order in which mirrors are searched for
 packages, you can edit this file and reorder the sections.  Spack will
diff --git a/lib/spack/docs/packaging_guide.rst b/lib/spack/docs/packaging_guide.rst
index 59ba63fa35..169899212d 100644
--- a/lib/spack/docs/packaging_guide.rst
+++ b/lib/spack/docs/packaging_guide.rst
@@ -84,7 +84,7 @@ always choose to download just one tarball initially, and run
 
    If it fails entirely, you can get minimal boilerplate by using
    :ref:`spack-edit-f`, or you can manually create a directory and
-   ``package.py`` file for the package in ``var/spack/packages``.
+   ``package.py`` file for the package in ``var/spack/repos/builtin/packages``.
 
 .. note::
 
@@ -203,7 +203,7 @@ edit`` command:
 So, if you used ``spack create`` to create a package, then saved and
 closed the resulting file, you can get back to it with ``spack edit``.
 The ``cmake`` package actually lives in
-``$SPACK_ROOT/var/spack/packages/cmake/package.py``, but this provides
+``$SPACK_ROOT/var/spack/repos/builtin/packages/cmake/package.py``, but this provides
 a much simpler shortcut and saves you the trouble of typing the full
 path.
 
@@ -269,18 +269,18 @@ live in Spack's directory structure.  In general, `spack-create`_ and
 `spack-edit`_ handle creating package files for you, so you can skip
 most of the details here.
 
-``var/spack/packages``
+``var/spack/repos/builtin/packages``
 ~~~~~~~~~~~~~~~~~~~~~~~
 
 A Spack installation directory is structured like a standard UNIX
 install prefix (``bin``, ``lib``, ``include``, ``var``, ``opt``,
 etc.).  Most of the code for Spack lives in ``$SPACK_ROOT/lib/spack``.
-Packages themselves live in ``$SPACK_ROOT/var/spack/packages``.
+Packages themselves live in ``$SPACK_ROOT/var/spack/repos/builtin/packages``.
 
 If you ``cd`` to that directory, you will see directories for each
 package:
 
-.. command-output::  cd $SPACK_ROOT/var/spack/packages;  ls -CF
+.. command-output::  cd $SPACK_ROOT/var/spack/repos/builtin/packages;  ls -CF
    :shell:
    :ellipsis: 10
 
@@ -288,7 +288,7 @@ Each directory contains a file called ``package.py``, which is where
 all the python code for the package goes.  For example, the ``libelf``
 package lives in::
 
-   $SPACK_ROOT/var/spack/packages/libelf/package.py
+   $SPACK_ROOT/var/spack/repos/builtin/packages/libelf/package.py
 
 Alongside the ``package.py`` file, a package may contain extra
 directories or files (like patches) that it needs to build.
@@ -301,7 +301,7 @@ Packages are named after the directory containing ``package.py``.  So,
 ``libelf``'s ``package.py`` lives in a directory called ``libelf``.
 The ``package.py`` file defines a class called ``Libelf``, which
 extends Spack's ``Package`` class.  for example, here is
-``$SPACK_ROOT/var/spack/packages/libelf/package.py``:
+``$SPACK_ROOT/var/spack/repos/builtin/packages/libelf/package.py``:
 
 .. code-block:: python
    :linenos:
@@ -328,7 +328,7 @@ these:
    $ spack install libelf@0.8.13
 
 Spack sees the package name in the spec and looks for
-``libelf/package.py`` in ``var/spack/packages``.  Likewise, if you say
+``libelf/package.py`` in ``var/spack/repos/builtin/packages``.  Likewise, if you say
 ``spack install py-numpy``, then Spack looks for
 ``py-numpy/package.py``.
 
@@ -401,6 +401,35 @@ construct the new one for ``8.2.1``.
 When you supply a custom URL for a version, Spack uses that URL
 *verbatim* and does not perform extrapolation.
 
+Skipping the expand step
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Spack normally expands archives automatically after downloading
+them. If you want to skip this step (e.g., for self-extracting
+executables and other custom archive types), you can add
+``expand=False`` to a ``version`` directive.
+
+.. code-block:: python
+
+   version('8.2.1', '4136d7b4c04df68b686570afa26988ac',
+           url='http://example.com/foo-8.2.1-special-version.tar.gz', 'expand=False')
+
+When ``expand`` is set to ``False``, Spack sets the current working
+directory to the directory containing the downloaded archive before it
+calls your ``install`` method.  Within ``install``, the path to the
+downloaded archive is available as ``self.stage.archive_file``.
+
+Here is an example snippet for packages distributed as self-extracting
+archives.  The example sets permissions on the downloaded file to make
+it executable, then runs it with some arguments.
+
+.. code-block:: python
+
+   def install(self, spec, prefix):
+       set_executable(self.stage.archive_file)
+       installer = Executable(self.stage.archive_file)
+       installer('--prefix=%s' % prefix, 'arg1', 'arg2', 'etc.')
+
 Checksums
 ~~~~~~~~~~~~~~~~~
 
@@ -632,7 +661,7 @@ Default
   revision instead.
 
 Revisions
-  Add ``hg`` and ``revision``parameters:
+  Add ``hg`` and ``revision`` parameters:
 
   .. code-block:: python
 
@@ -703,7 +732,7 @@ supply is a filename, then the patch needs to live within the spack
 source tree.  For example, the patch above lives in a directory
 structure like this::
 
-   $SPACK_ROOT/var/spack/packages/
+   $SPACK_ROOT/var/spack/repos/builtin/packages/
        mvapich2/
            package.py
            ad_lustre_rwcontig_open_source.patch
@@ -1524,6 +1553,69 @@ This is useful when you want to know exactly what Spack will do when
 you ask for a particular spec.
 
 
+``Concretization Policies``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A user may have certain preferences for how packages should
+be concretized on their system.  For example, one user may prefer packages
+built with OpenMPI and the Intel compiler.  Another user may prefer
+packages be built with MVAPICH and GCC.  
+
+Spack can be configured to prefer certain compilers, package
+versions, depends_on, and variants during concretization.
+The preferred configuration can be controlled via the 
+``~/.spack/packages.yaml`` file for user configuations, or the
+``etc/spack/packages.yaml`` site configuration.
+
+
+Here's an example packages.yaml file that sets preferred packages:
+
+.. code-block:: sh
+
+    packages:
+      dyninst:
+        compiler: [gcc@4.9]
+        variants: +debug
+      gperftools:
+        version: [2.2, 2.4, 2.3]
+      all:
+        compiler: [gcc@4.4.7, gcc@4.6:, intel, clang, pgi]
+        providers:
+          mpi: [mvapich, mpich, openmpi]
+          
+
+At a high level, this example is specifying how packages should be
+concretized.  The dyninst package should prefer using gcc 4.9 and 
+be built with debug options.  The gperftools package should prefer version
+2.2 over 2.4.  Every package on the system should prefer mvapich for
+its MPI and gcc 4.4.7 (except for Dyninst, which overrides this by preferring gcc 4.9).  
+These options are used to fill in implicit defaults.  Any of them can be overwritten 
+on the command line if explicitly requested.
+
+Each packages.yaml file begins with the string ``packages:`` and 
+package names are specified on the next level. The special string ``all``
+applies settings to each package. Underneath each package name is 
+one or more components: ``compiler``, ``variants``, ``version``, 
+or ``providers``.  Each component has an ordered list of spec 
+``constraints``, with earlier entries in the list being preferred over
+later entries.
+
+Sometimes a package installation may have constraints that forbid 
+the first concretization rule, in which case Spack will use the first
+legal concretization rule.  Going back to the example, if a user
+requests gperftools 2.3 or later, then Spack will install version 2.4 
+as the 2.4 version of gperftools is preferred over 2.3.
+
+An explicit concretization rule in the preferred section will always 
+take preference over unlisted concretizations.  In the above example, 
+xlc isn't listed in the compiler list.  Every listed compiler from
+gcc to pgi will thus be preferred over the xlc compiler.
+
+The syntax for the ``provider`` section differs slightly from other
+concretization rules.  A provider lists a value that packages may
+``depend_on`` (e.g, mpi) and a list of rules for fulfilling that
+dependency.
+
 .. _install-method:
 
 Implementing the ``install`` method
@@ -1533,7 +1625,7 @@ The last element of a package is its ``install()`` method.  This is
 where the real work of installation happens, and it's the main part of
 the package you'll need to customize for each piece of software.
 
-.. literalinclude::  ../../../var/spack/packages/libelf/package.py
+.. literalinclude::  ../../../var/spack/repos/builtin/packages/libelf/package.py
    :start-after: 0.8.12
    :linenos:
 
@@ -1711,15 +1803,15 @@ Compile-time library search paths
   * ``-L$dep_prefix/lib``
   * ``-L$dep_prefix/lib64``
 Runtime library search paths (RPATHs)
-  * ``-Wl,-rpath=$dep_prefix/lib``
-  * ``-Wl,-rpath=$dep_prefix/lib64``
+  * ``-Wl,-rpath,$dep_prefix/lib``
+  * ``-Wl,-rpath,$dep_prefix/lib64``
 Include search paths
   * ``-I$dep_prefix/include``
 
 An example of this would be the ``libdwarf`` build, which has one
 dependency: ``libelf``.  Every call to ``cc`` in the ``libdwarf``
 build will have ``-I$LIBELF_PREFIX/include``,
-``-L$LIBELF_PREFIX/lib``, and ``-Wl,-rpath=$LIBELF_PREFIX/lib``
+``-L$LIBELF_PREFIX/lib``, and ``-Wl,-rpath,$LIBELF_PREFIX/lib``
 inserted on the command line.  This is done transparently to the
 project's build system, which will just think it's using a system
 where ``libelf`` is readily available.  Because of this, you **do
@@ -2108,6 +2200,15 @@ Filtering functions
 
   Examples:
 
+  #. Filtering a Makefile to force it to use Spack's compiler wrappers:
+
+     .. code-block:: python
+
+        filter_file(r'^CC\s*=.*',  spack_cc,  'Makefile')
+        filter_file(r'^CXX\s*=.*', spack_cxx, 'Makefile')
+        filter_file(r'^F77\s*=.*', spack_f77, 'Makefile')
+        filter_file(r'^FC\s*=.*',  spack_fc,  'Makefile')
+
   #. Replacing ``#!/usr/bin/perl`` with ``#!/usr/bin/env perl`` in ``bib2xhtml``:
 
      .. code-block:: python
diff --git a/lib/spack/docs/site_configuration.rst b/lib/spack/docs/site_configuration.rst
index b03df29573..3abfa21a9d 100644
--- a/lib/spack/docs/site_configuration.rst
+++ b/lib/spack/docs/site_configuration.rst
@@ -54,87 +54,73 @@ more elements to the list to indicate where your own site's temporary
 directory is.
 
 
-.. _concretization-policies:
-
-Concretization policies
-----------------------------
-
-When a user asks for a package like ``mpileaks`` to be installed,
-Spack has to make decisions like what version should be installed,
-what compiler to use, and how its dependencies should be configured.
-This process is called *concretization*, and it's covered in detail in
-:ref:`its own section <abstract-and-concrete>`.
-
-The default concretization policies are in the
-:py:mod:`spack.concretize` module, specifically in the
-:py:class:`spack.concretize.DefaultConcretizer` class.  These are the
-important methods used in the concretization process:
-
-* :py:meth:`concretize_version(self, spec) <spack.concretize.DefaultConcretizer.concretize_version>`
-* :py:meth:`concretize_architecture(self, spec) <spack.concretize.DefaultConcretizer.concretize_architecture>`
-* :py:meth:`concretize_compiler(self, spec) <spack.concretize.DefaultConcretizer.concretize_compiler>`
-* :py:meth:`choose_provider(self, spec, providers) <spack.concretize.DefaultConcretizer.choose_provider>`
-
-The first three take a :py:class:`Spec <spack.spec.Spec>` object and
-modify it by adding constraints for the version.  For example, if the
-input spec had a version range like `1.0:5.0.3`, then the
-``concretize_version`` method should set the spec's version to a
-*single* version in that range.  Likewise, ``concretize_architecture``
-selects an architecture when the input spec does not have one, and
-``concretize_compiler`` needs to set both a concrete compiler and a
-concrete compiler version.
-
-``choose_provider()`` affects how concrete implementations are chosen
-based on a virtual dependency spec.  The input spec is some virtual
-dependency and the ``providers`` index is a :py:class:`ProviderIndex
-<spack.packages.ProviderIndex>` object.  The ``ProviderIndex`` maps
-the virtual spec to specs for possible implementations, and
-``choose_provider()`` should simply choose one of these.  The
-``concretize_*`` methods will be called on the chosen implementation
-later, so there is no need to fully concretize the spec when returning
-it.
-
-The ``DefaultConcretizer`` is intended to provide sensible defaults
-for each policy, but there are certain choices that it can't know
-about.  For example, one site might prefer ``OpenMPI`` over ``MPICH``,
-or another might prefer an old version of some packages.  These types
-of special cases can be integrated with custom concretizers.
-
-Writing a custom concretizer
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-To write your own concretizer, you need only subclass
-``DefaultConcretizer`` and override the methods you want to change.
-For example, you might write a class like this to change *only* the
-``concretize_version()`` behavior:
-
-.. code-block:: python
-
-   from spack.concretize import DefaultConcretizer
-
-   class MyConcretizer(DefaultConcretizer):
-       def concretize_version(self, spec):
-           # implement custom logic here.
-
-Once you have written your custom concretizer, you can make Spack use
-it by editing ``globals.py``.  Find this part of the file:
-
-.. code-block:: python
-
-   #
-   # This controls how things are concretized in spack.
-   # Replace it with a subclass if you want different
-   # policies.
-   #
-   concretizer = DefaultConcretizer()
-
-Set concretizer to *your own* class instead of the default:
-
-.. code-block:: python
-
-   concretizer = MyConcretizer()
-
-The next time you run Spack, your changes should take effect.
+External Packages
+~~~~~~~~~~~~~~~~~~~~~
+Spack can be configured to use externally-installed
+packages rather than building its own packages. This may be desirable
+if machines ship with system packages, such as a customized MPI
+that should be used instead of Spack building its own MPI.
+
+External packages are configured through the ``packages.yaml`` file found
+in a Spack installation's ``etc/spack/`` or a user's ``~/.spack/``
+directory. Here's an example of an external configuration:
+
+.. code-block:: yaml
+
+   packages:
+      openmpi:
+         paths:
+            openmpi@1.4.3%gcc@4.4.7=chaos_5_x86_64_ib: /opt/openmpi-1.4.3
+            openmpi@1.4.3%gcc@4.4.7=chaos_5_x86_64_ib+debug: /opt/openmpi-1.4.3-debug
+            openmpi@1.6.5%intel@10.1=chaos_5_x86_64_ib: /opt/openmpi-1.6.5-intel
+
+This example lists three installations of OpenMPI, one built with gcc,
+one built with gcc and debug information, and another built with Intel.
+If Spack is asked to build a package that uses one of these MPIs as a
+dependency, it will use the the pre-installed OpenMPI in
+the given directory.  
+
+Each ``packages.yaml`` begins with a ``packages:`` token, followed
+by a list of package names.  To specify externals, add a ``paths``
+token under the package name, which lists externals in a
+``spec : /path`` format.  Each spec should be as
+well-defined as reasonably possible.  If a
+package lacks a spec component, such as missing a compiler or
+package version, then Spack will guess the missing component based
+on its most-favored packages, and it may guess incorrectly.
+
+Each package version and compilers listed in an external should
+have entries in Spack's packages and compiler configuration, even
+though the package and compiler may not every be built.
+
+The packages configuration can tell Spack to use an external location
+for certain package versions, but it does not restrict Spack to using
+external packages.  In the above example, if an OpenMPI 1.8.4 became
+available Spack may choose to start building and linking with that version
+rather than continue using the pre-installed OpenMPI versions.
+
+To prevent this, the ``packages.yaml`` configuration also allows packages
+to be flagged as non-buildable.  The previous example could be modified to
+be:
+
+.. code-block:: yaml
+
+  packages:
+    openmpi:
+      paths:
+        openmpi@1.4.3%gcc@4.4.7=chaos_5_x86_64_ib: /opt/openmpi-1.4.3
+        openmpi@1.4.3%gcc@4.4.7=chaos_5_x86_64_ib+debug: /opt/openmpi-1.4.3-debug
+        openmpi@1.6.5%intel@10.1=chaos_5_x86_64_ib: /opt/openmpi-1.6.5-intel                
+      buildable: False
+
+The addition of the ``buildable`` flag tells Spack that it should never build
+its own version of OpenMPI, and it will instead always rely on a pre-built
+OpenMPI.  Similar to ``paths``, ``buildable`` is specified as a property under
+a package name.
+
+The ``buildable`` does not need to be paired with external packages.  
+It could also be used alone to forbid packages that may be 
+buggy or otherwise undesirable.
 
 
 Profiling
diff --git a/lib/spack/env/cc b/lib/spack/env/cc
index ac8f717cc7..df6795c264 100755
--- a/lib/spack/env/cc
+++ b/lib/spack/env/cc
@@ -90,15 +90,15 @@ case "$command" in
         command="$SPACK_CC"
         language="C"
         ;;
-    c++|CC|g++|clang++|icpc|pgCC|xlc++)
+    c++|CC|g++|clang++|icpc|pgc++|xlc++)
         command="$SPACK_CXX"
         language="C++"
         ;;
-    f90|fc|f95|gfortran|ifort|pgf90|xlf90|nagfor)
+    f90|fc|f95|gfortran|ifort|pgfortran|xlf90|nagfor)
         command="$SPACK_FC"
         language="Fortran 90"
         ;;
-    f77|gfortran|ifort|pgf77|xlf|nagfor)
+    f77|gfortran|ifort|pgfortran|xlf|nagfor)
         command="$SPACK_F77"
         language="Fortran 77"
         ;;
@@ -113,14 +113,22 @@ case "$command" in
         ;;
 esac
 
-# Finish setting up the mode.
+# If any of the arguments below is present then the mode is vcheck. In vcheck mode nothing is added in terms of extra search paths or libraries
 if [ -z "$mode" ]; then
-    mode=ccld
     for arg in "$@"; do
         if [ "$arg" = -v -o "$arg" = -V -o "$arg" = --version -o "$arg" = -dumpversion ]; then
             mode=vcheck
             break
-        elif [ "$arg" = -E ]; then
+    fi
+    done
+fi
+
+# Finish setting up the mode.
+
+if [ -z "$mode" ]; then
+    mode=ccld
+    for arg in "$@"; do
+        if [ "$arg" = -E ]; then
             mode=cpp
             break
         elif [ "$arg" = -c ]; then
@@ -130,7 +138,7 @@ if [ -z "$mode" ]; then
     done
 fi
 
-# Dump the version and exist if we're in testing mode.
+# Dump the version and exit if we're in testing mode.
 if [ "$SPACK_TEST_COMMAND" = "dump-mode" ]; then
     echo "$mode"
     exit
@@ -146,6 +154,7 @@ fi
 input_command="$@"
 args=("$@")
 
+<<<<<<< HEAD
 # Dump parsed values for unit testing if asked for
 if [[ -n $SPACK_TEST_COMMAND ]]; then
 
@@ -213,6 +222,88 @@ if [[ -n $SPACK_TEST_COMMAND ]]; then
         esac
         shift
     done
+=======
+if [ "$mode" == vcheck ] ; then
+    exec ${command} "$@"
+fi
+
+#
+# Now do real parsing of the command line args, trying hard to keep
+# non-rpath linker arguments in the proper order w.r.t. other command
+# line arguments.  This is important for things like groups.
+#
+includes=()
+libraries=()
+libs=()
+rpaths=()
+other_args=()
+
+while [ -n "$1" ]; do
+    case "$1" in
+        -I*)
+            arg="${1#-I}"
+            if [ -z "$arg" ]; then shift; arg="$1"; fi
+            includes+=("$arg")
+            ;;
+        -L*)
+            arg="${1#-L}"
+            if [ -z "$arg" ]; then shift; arg="$1"; fi
+            libraries+=("$arg")
+            ;;
+        -l*)
+            arg="${1#-l}"
+            if [ -z "$arg" ]; then shift; arg="$1"; fi
+            libs+=("$arg")
+            ;;
+        -Wl,*)
+            arg="${1#-Wl,}"
+            # TODO: Handle multiple -Wl, continuations of -Wl,-rpath
+            if [[ $arg == -rpath=* ]]; then
+                arg="${arg#-rpath=}"
+                for rpath in ${arg//,/ }; do
+                    rpaths+=("$rpath")
+                done
+            elif [[ $arg == -rpath,* ]]; then
+                arg="${arg#-rpath,}"
+                for rpath in ${arg//,/ }; do
+                    rpaths+=("$rpath")
+                done
+            elif [[ $arg == -rpath ]]; then
+                shift; arg="$1"
+                if [[ $arg != '-Wl,'* ]]; then
+                    die "-Wl,-rpath was not followed by -Wl,*"
+                fi
+                arg="${arg#-Wl,}"
+                for rpath in ${arg//,/ }; do
+                    rpaths+=("$rpath")
+                done
+            else
+                other_args+=("-Wl,$arg")
+            fi
+            ;;
+        -Xlinker)
+            shift; arg="$1";
+            if [[ $arg = -rpath=* ]]; then
+                rpaths+=("${arg#-rpath=}")
+            elif [[ $arg = -rpath ]]; then
+                shift; arg="$1"
+                if [[ $arg != -Xlinker ]]; then
+                    die "-Xlinker -rpath was not followed by -Xlinker <arg>"
+                fi
+                shift; arg="$1"
+                rpaths+=("$arg")
+            else
+                other_args+=("-Xlinker")
+                other_args+=("$arg")
+            fi
+            ;;
+        *)
+            other_args+=("$1")
+            ;;
+    esac
+    shift
+done
+>>>>>>> develop
 
     IFS=$'\n'
     case "$SPACK_TEST_COMMAND" in
diff --git a/lib/spack/env/pgi/case-insensitive/pgCC b/lib/spack/env/pgi/case-insensitive/pgCC
deleted file mode 120000
index e2deb67f3b..0000000000
--- a/lib/spack/env/pgi/case-insensitive/pgCC
+++ /dev/null
@@ -1 +0,0 @@
-../../cc
\ No newline at end of file
diff --git a/lib/spack/env/pgi/pgf77 b/lib/spack/env/pgi/pgc++
index 82c2b8e90a..82c2b8e90a 120000
--- a/lib/spack/env/pgi/pgf77
+++ b/lib/spack/env/pgi/pgc++
diff --git a/lib/spack/env/pgi/pgf90 b/lib/spack/env/pgi/pgfortran
index 82c2b8e90a..82c2b8e90a 120000
--- a/lib/spack/env/pgi/pgf90
+++ b/lib/spack/env/pgi/pgfortran
diff --git a/lib/spack/llnl/util/filesystem.py b/lib/spack/llnl/util/filesystem.py
index 24cfbfde71..c4665c284c 100644
--- a/lib/spack/llnl/util/filesystem.py
+++ b/lib/spack/llnl/util/filesystem.py
@@ -25,7 +25,9 @@
 __all__ = ['set_install_permissions', 'install', 'install_tree', 'traverse_tree',
            'expand_user', 'working_dir', 'touch', 'touchp', 'mkdirp',
            'force_remove', 'join_path', 'ancestor', 'can_access', 'filter_file',
-           'FileFilter', 'change_sed_delimiter', 'is_exe', 'force_symlink']
+           'FileFilter', 'change_sed_delimiter', 'is_exe', 'force_symlink',
+           'set_executable', 'copy_mode', 'unset_executable_mode',
+           'remove_dead_links', 'remove_linked_tree']
 
 import os
 import sys
@@ -152,15 +154,28 @@ def set_install_permissions(path):
 def copy_mode(src, dest):
     src_mode = os.stat(src).st_mode
     dest_mode = os.stat(dest).st_mode
-    if src_mode | stat.S_IXUSR: dest_mode |= stat.S_IXUSR
-    if src_mode | stat.S_IXGRP: dest_mode |= stat.S_IXGRP
-    if src_mode | stat.S_IXOTH: dest_mode |= stat.S_IXOTH
+    if src_mode & stat.S_IXUSR: dest_mode |= stat.S_IXUSR
+    if src_mode & stat.S_IXGRP: dest_mode |= stat.S_IXGRP
+    if src_mode & stat.S_IXOTH: dest_mode |= stat.S_IXOTH
     os.chmod(dest, dest_mode)
 
 
+def unset_executable_mode(path):
+    mode = os.stat(path).st_mode
+    mode &= ~stat.S_IXUSR
+    mode &= ~stat.S_IXGRP
+    mode &= ~stat.S_IXOTH
+    os.chmod(path, mode)
+
+
 def install(src, dest):
     """Manually install a file to a particular location."""
     tty.debug("Installing %s to %s" % (src, dest))
+
+    # Expand dsst to its eventual full path if it is a directory.
+    if os.path.isdir(dest):
+        dest = join_path(dest, os.path.basename(src))
+
     shutil.copy(src, dest)
     set_install_permissions(dest)
     copy_mode(src, dest)
@@ -235,7 +250,7 @@ def touchp(path):
 def force_symlink(src, dest):
     try:
         os.symlink(src, dest)
-    except OSError, e:
+    except OSError as e:
         os.remove(dest)
         os.symlink(src, dest)
 
@@ -339,3 +354,41 @@ def traverse_tree(source_root, dest_root, rel_path='', **kwargs):
 
     if order == 'post':
         yield (source_path, dest_path)
+
+
+def set_executable(path):
+    st = os.stat(path)
+    os.chmod(path, st.st_mode | stat.S_IEXEC)
+
+
+def remove_dead_links(root):
+    """
+    Removes any dead link that is present in root
+
+    Args:
+        root: path where to search for dead links
+
+    """
+    for file in os.listdir(root):
+        path = join_path(root, file)
+        if os.path.islink(path):
+            real_path = os.path.realpath(path)
+            if not os.path.exists(real_path):
+                os.unlink(path)
+
+def remove_linked_tree(path):
+    """
+    Removes a directory and its contents.  If the directory is a
+    symlink, follows the link and removes the real directory before
+    removing the link.
+
+    Args:
+        path: directory to be removed
+
+    """
+    if os.path.exists(path):
+        if os.path.islink(path):
+            shutil.rmtree(os.path.realpath(path), True)
+            os.unlink(path)
+        else:
+            shutil.rmtree(path, True)
diff --git a/lib/spack/llnl/util/lang.py b/lib/spack/llnl/util/lang.py
index 1c4d1ed623..13d301f84e 100644
--- a/lib/spack/llnl/util/lang.py
+++ b/lib/spack/llnl/util/lang.py
@@ -235,11 +235,11 @@ def key_ordering(cls):
     if not has_method(cls, '_cmp_key'):
         raise TypeError("'%s' doesn't define _cmp_key()." % cls.__name__)
 
-    setter('__eq__', lambda s,o: o is not None and s._cmp_key() == o._cmp_key())
+    setter('__eq__', lambda s,o: (s is o) or (o is not None and s._cmp_key() == o._cmp_key()))
     setter('__lt__', lambda s,o: o is not None and s._cmp_key() <  o._cmp_key())
     setter('__le__', lambda s,o: o is not None and s._cmp_key() <= o._cmp_key())
 
-    setter('__ne__', lambda s,o: o is None or s._cmp_key() != o._cmp_key())
+    setter('__ne__', lambda s,o: (s is not o) and (o is None or s._cmp_key() != o._cmp_key()))
     setter('__gt__', lambda s,o: o is None or s._cmp_key() >  o._cmp_key())
     setter('__ge__', lambda s,o: o is None or s._cmp_key() >= o._cmp_key())
 
diff --git a/lib/spack/spack/__init__.py b/lib/spack/spack/__init__.py
index ab78ecef30..3051d3f742 100644
--- a/lib/spack/spack/__init__.py
+++ b/lib/spack/spack/__init__.py
@@ -82,6 +82,20 @@ from spack.directory_layout import YamlDirectoryLayout
 install_layout = YamlDirectoryLayout(install_path)
 
 #
+# This controls how packages are sorted when trying to choose
+# the most preferred package.  More preferred packages are sorted
+# first.
+#
+from spack.preferred_packages import PreferredPackages
+pkgsort = PreferredPackages()
+
+#
+# This tests ABI compatibility between packages
+#
+from spack.abi import ABI
+abi = ABI()
+
+#
 # This controls how things are concretized in spack.
 # Replace it with a subclass if you want different
 # policies.
diff --git a/lib/spack/spack/abi.py b/lib/spack/spack/abi.py
new file mode 100644
index 0000000000..7e565bcbf9
--- /dev/null
+++ b/lib/spack/spack/abi.py
@@ -0,0 +1,128 @@
+##############################################################################
+# Copyright (c) 2015, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://scalability-llnl.github.io/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+
+import os
+import spack
+import spack.spec
+from spack.spec import CompilerSpec
+from spack.util.executable import Executable, ProcessError
+from llnl.util.lang import memoized
+
+class ABI(object):
+    """This class provides methods to test ABI compatibility between specs.
+       The current implementation is rather rough and could be improved."""
+
+    def architecture_compatible(self, parent, child):
+        """Returns true iff the parent and child specs have ABI compatible architectures."""
+        return not parent.architecture or not child.architecture or parent.architecture == child.architecture
+
+
+    @memoized
+    def _gcc_get_libstdcxx_version(self, version):
+        """Returns gcc ABI compatibility info by getting the library version of
+           a compiler's libstdc++.so or libgcc_s.so"""
+        spec = CompilerSpec("gcc", version)
+        compilers = spack.compilers.compilers_for_spec(spec)
+        if not compilers:
+            return None
+        compiler = compilers[0]
+        rungcc = None
+        libname = None
+        output = None
+        if compiler.cxx:
+            rungcc = Executable(compiler.cxx)
+            libname = "libstdc++.so"
+        elif compiler.cc:
+            rungcc = Executable(compiler.cc)
+            libname = "libgcc_s.so"
+        else:
+            return None
+        try:
+            output = rungcc("--print-file-name=%s" % libname, return_output=True)
+        except ProcessError, e:
+            return None
+        if not output:
+            return None
+        libpath = os.readlink(output.strip())
+        if not libpath:
+            return None
+        return os.path.basename(libpath)
+
+
+    @memoized
+    def _gcc_compiler_compare(self, pversion, cversion):
+        """Returns true iff the gcc version pversion and cversion
+          are ABI compatible."""
+        plib = self._gcc_get_libstdcxx_version(pversion)
+        clib = self._gcc_get_libstdcxx_version(cversion)
+        if not plib or not clib:
+            return False
+        return plib == clib
+
+
+    def _intel_compiler_compare(self, pversion, cversion):
+        """Returns true iff the intel version pversion and cversion
+           are ABI compatible"""
+
+        # Test major and minor versions.  Ignore build version.
+        if (len(pversion.version) < 2 or len(cversion.version) < 2):
+            return False
+        return pversion.version[:2] == cversion.version[:2]
+
+
+    def compiler_compatible(self, parent, child, **kwargs):
+        """Returns true iff the compilers for parent and child specs are ABI compatible"""
+        if not parent.compiler or not child.compiler:
+            return True
+
+        if parent.compiler.name != child.compiler.name:
+            # Different compiler families are assumed ABI incompatible
+            return False
+
+        if kwargs.get('loose', False):
+            return True
+
+        # TODO: Can we move the specialized ABI matching stuff
+        # TODO: into compiler classes?
+        for pversion in parent.compiler.versions:
+            for cversion in child.compiler.versions:
+                # For a few compilers use specialized comparisons.  Otherwise
+                # match on version match.
+                if pversion.satisfies(cversion):
+                    return True
+                elif (parent.compiler.name == "gcc" and
+                      self._gcc_compiler_compare(pversion, cversion)):
+                    return True
+                elif (parent.compiler.name == "intel" and
+                      self._intel_compiler_compare(pversion, cversion)):
+                    return True
+        return False
+
+
+    def compatible(self, parent, child, **kwargs):
+        """Returns true iff a parent and child spec are ABI compatible"""
+        loosematch = kwargs.get('loose', False)
+        return self.architecture_compatible(parent, child) and \
+               self.compiler_compatible(parent, child, loose=loosematch)
diff --git a/lib/spack/spack/build_environment.py b/lib/spack/spack/build_environment.py
index 1b87778080..87fc310b5a 100644
--- a/lib/spack/spack/build_environment.py
+++ b/lib/spack/spack/build_environment.py
@@ -177,8 +177,6 @@ def set_module_variables_for_package(pkg, m):
     """Populate the module scope of install() with some useful functions.
        This makes things easier for package writers.
     """
-    m = pkg.module
-
     # number of jobs spack will to build with.
     jobs = multiprocessing.cpu_count()
     if not pkg.parallel:
@@ -214,6 +212,13 @@ def set_module_variables_for_package(pkg, m):
     m.std_cmake_args.append('-DCMAKE_INSTALL_RPATH_USE_LINK_PATH=FALSE')
     m.std_cmake_args.append('-DCMAKE_INSTALL_RPATH=%s' % ":".join(get_rpaths(pkg)))
 
+    # Put spack compiler paths in module scope.
+    link_dir = spack.build_env_path
+    m.spack_cc  = join_path(link_dir, pkg.compiler.link_paths['cc'])
+    m.spack_cxx = join_path(link_dir, pkg.compiler.link_paths['cxx'])
+    m.spack_f77 = join_path(link_dir, pkg.compiler.link_paths['f77'])
+    m.spack_f90 = join_path(link_dir, pkg.compiler.link_paths['fc'])
+
     # Emulate some shell commands for convenience
     m.pwd          = os.getcwd
     m.cd           = os.chdir
diff --git a/lib/spack/spack/cmd/checksum.py b/lib/spack/spack/cmd/checksum.py
index b1ad89dbb8..518d2703dc 100644
--- a/lib/spack/spack/cmd/checksum.py
+++ b/lib/spack/spack/cmd/checksum.py
@@ -22,23 +22,18 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import os
-import re
 import argparse
 import hashlib
-from pprint import pprint
-from subprocess import CalledProcessError
 
 import llnl.util.tty as tty
-from llnl.util.tty.colify import colify
-
 import spack
 import spack.cmd
 import spack.util.crypto
 from spack.stage import Stage, FailedDownloadError
 from spack.version import *
 
-description ="Checksum available versions of a package."
+description = "Checksum available versions of a package."
+
 
 def setup_parser(subparser):
     subparser.add_argument(
@@ -58,25 +53,23 @@ def get_checksums(versions, urls, **kwargs):
 
     tty.msg("Downloading...")
     hashes = []
-    for i, (url, version) in enumerate(zip(urls, versions)):
-        stage = Stage(url)
+    i = 0
+    for url, version in zip(urls, versions):
         try:
-            stage.fetch()
-            if i == 0 and first_stage_function:
-                first_stage_function(stage)
-
-            hashes.append(
-                spack.util.crypto.checksum(hashlib.md5, stage.archive_file))
-        except FailedDownloadError, e:
+            with Stage(url, keep=keep_stage) as stage:
+                stage.fetch()
+                if i == 0 and first_stage_function:
+                    first_stage_function(stage)
+
+                hashes.append((version,
+                               spack.util.crypto.checksum(hashlib.md5, stage.archive_file)))
+                i += 1
+        except FailedDownloadError as e:
             tty.msg("Failed to fetch %s" % url)
-            continue
-
-        finally:
-            if not keep_stage:
-                stage.destroy()
-
-    return zip(versions, hashes)
+        except Exception as e:
+            tty.msg('Something failed on %s, skipping.\n    (%s)' % (url, e))
 
+    return hashes
 
 
 def checksum(parser, args):
@@ -95,13 +88,13 @@ def checksum(parser, args):
     else:
         versions = pkg.fetch_remote_versions()
         if not versions:
-            tty.die("Could not fetch any versions for %s." % pkg.name)
+            tty.die("Could not fetch any versions for %s" % pkg.name)
 
     sorted_versions = sorted(versions, reverse=True)
 
-    tty.msg("Found %s versions of %s." % (len(versions), pkg.name),
+    tty.msg("Found %s versions of %s" % (len(versions), pkg.name),
             *spack.cmd.elide_list(
-            ["%-10s%s" % (v, versions[v]) for v in sorted_versions]))
+                ["%-10s%s" % (v, versions[v]) for v in sorted_versions]))
     print
     archives_to_fetch = tty.get_number(
         "How many would you like to checksum?", default=5, abort='q')
@@ -116,7 +109,7 @@ def checksum(parser, args):
         keep_stage=args.keep_stage)
 
     if not version_hashes:
-        tty.die("Could not fetch any versions for %s." % pkg.name)
+        tty.die("Could not fetch any versions for %s" % pkg.name)
 
     version_lines = ["    version('%s', '%s')" % (v, h) for v, h in version_hashes]
     tty.msg("Checksummed new versions of %s:" % pkg.name, *version_lines)
diff --git a/lib/spack/spack/cmd/compiler.py b/lib/spack/spack/cmd/compiler.py
index 75b51f6b49..3e58e82184 100644
--- a/lib/spack/spack/cmd/compiler.py
+++ b/lib/spack/spack/cmd/compiler.py
@@ -96,7 +96,7 @@ def compiler_remove(args):
     compilers = spack.compilers.compilers_for_spec(cspec, scope=args.scope)
 
     if not compilers:
-        tty.die("No compilers match spec %s." % cspec)
+        tty.die("No compilers match spec %s" % cspec)
     elif not args.all and len(compilers) > 1:
         tty.error("Multiple compilers match spec %s. Choose one:" % cspec)
         colify(reversed(sorted([c.spec for c in compilers])), indent=4)
@@ -105,7 +105,7 @@ def compiler_remove(args):
 
     for compiler in compilers:
         spack.compilers.remove_compiler_from_config(compiler.spec, scope=args.scope)
-        tty.msg("Removed compiler %s." % compiler.spec)
+        tty.msg("Removed compiler %s" % compiler.spec)
 
 
 def compiler_info(args):
@@ -114,7 +114,7 @@ def compiler_info(args):
     compilers = spack.compilers.compilers_for_spec(cspec, scope=args.scope)
 
     if not compilers:
-        tty.error("No compilers match spec %s." % cspec)
+        tty.error("No compilers match spec %s" % cspec)
     else:
         for c in compilers:
             print str(c.spec) + ":"
diff --git a/lib/spack/spack/cmd/create.py b/lib/spack/spack/cmd/create.py
index edcea0718c..f0cd50b8df 100644
--- a/lib/spack/spack/cmd/create.py
+++ b/lib/spack/spack/cmd/create.py
@@ -156,7 +156,7 @@ def guess_name_and_version(url, args):
     # Try to deduce name and version of the new package from the URL
     version = spack.url.parse_version(url)
     if not version:
-        tty.die("Couldn't guess a version string from %s." % url)
+        tty.die("Couldn't guess a version string from %s" % url)
 
     # Try to guess a name.  If it doesn't work, allow the user to override.
     if args.alternate_name:
@@ -189,7 +189,7 @@ def find_repository(spec, args):
         try:
             repo = Repo(repo_path)
             if spec.namespace and spec.namespace != repo.namespace:
-                tty.die("Can't create package with namespace %s in repo with namespace %s."
+                tty.die("Can't create package with namespace %s in repo with namespace %s"
                         % (spec.namespace, repo.namespace))
         except RepoError as e:
             tty.die(str(e))
@@ -208,7 +208,7 @@ def find_repository(spec, args):
     return repo
 
 
-def fetch_tarballs(url, name, args):
+def fetch_tarballs(url, name, version):
     """Try to find versions of the supplied archive by scraping the web.
 
     Prompts the user to select how many to download if many are found.
@@ -252,11 +252,11 @@ def create(parser, args):
     name = spec.name  # factors out namespace, if any
     repo = find_repository(spec, args)
 
-    tty.msg("This looks like a URL for %s version %s." % (name, version))
+    tty.msg("This looks like a URL for %s version %s" % (name, version))
     tty.msg("Creating template for package %s" % name)
 
     # Fetch tarballs (prompting user if necessary)
-    versions, urls = fetch_tarballs(url, name, args)
+    versions, urls = fetch_tarballs(url, name, version)
 
     # Try to guess what configure system is used.
     guesser = ConfigureGuesser()
@@ -266,7 +266,7 @@ def create(parser, args):
         keep_stage=args.keep_stage)
 
     if not ver_hash_tuples:
-        tty.die("Could not fetch any tarballs for %s." % name)
+        tty.die("Could not fetch any tarballs for %s" % name)
 
     # Prepend 'py-' to python package names, by convention.
     if guesser.build_system == 'python':
@@ -291,4 +291,4 @@ def create(parser, args):
 
     # If everything checks out, go ahead and edit.
     spack.editor(pkg_path)
-    tty.msg("Created package %s." % pkg_path)
+    tty.msg("Created package %s" % pkg_path)
diff --git a/lib/spack/spack/cmd/diy.py b/lib/spack/spack/cmd/diy.py
index 9df53312f8..45f13e4463 100644
--- a/lib/spack/spack/cmd/diy.py
+++ b/lib/spack/spack/cmd/diy.py
@@ -46,6 +46,9 @@ def setup_parser(subparser):
         '--skip-patch', action='store_true',
         help="Skip patching for the DIY build.")
     subparser.add_argument(
+        '-q', '--quiet', action='store_true', dest='quiet',
+        help="Do not display verbose build output while installing.")
+    subparser.add_argument(
         'spec', nargs=argparse.REMAINDER,
         help="specs to use for install.  Must contain package AND verison.")
 
@@ -72,8 +75,8 @@ def diy(self, args):
                 edit_package(spec.name, spack.repo.first_repo(), None, True)
                 return
 
-        if not spec.version.concrete:
-            tty.die("spack diy spec must have a single, concrete version.")
+        if not spec.versions.concrete:
+            tty.die("spack diy spec must have a single, concrete version.  Did you forget a package version number?")
 
         spec.concretize()
         package = spack.repo.get(spec)
@@ -92,4 +95,5 @@ def diy(self, args):
         package.do_install(
             keep_prefix=args.keep_prefix,
             ignore_deps=args.ignore_deps,
+            verbose=not args.quiet,
             keep_stage=True)   # don't remove source dir for DIY.
diff --git a/lib/spack/spack/cmd/md5.py b/lib/spack/spack/cmd/md5.py
index 879ef9f7b7..f99fc0f8c2 100644
--- a/lib/spack/spack/cmd/md5.py
+++ b/lib/spack/spack/cmd/md5.py
@@ -22,51 +22,51 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import os
 import argparse
 import hashlib
-
-from contextlib import contextmanager
+import os
 
 import llnl.util.tty as tty
-from llnl.util.filesystem import *
-
 import spack.util.crypto
 from spack.stage import Stage, FailedDownloadError
 
 description = "Calculate md5 checksums for files/urls."
 
-@contextmanager
-def stager(url):
-    _cwd = os.getcwd()
-    _stager = Stage(url)
-    try:
-        _stager.fetch()
-        yield _stager
-    except FailedDownloadError:
-        tty.msg("Failed to fetch %s" % url)
-    finally:
-        _stager.destroy()
-        os.chdir(_cwd) # the Stage class changes the current working dir so it has to be restored
 
 def setup_parser(subparser):
     setup_parser.parser = subparser
     subparser.add_argument('files', nargs=argparse.REMAINDER,
                            help="Files to checksum.")
 
+
+def compute_md5_checksum(url):
+    if not os.path.isfile(url):
+        with Stage(url) as stage:
+            stage.fetch()
+            value = spack.util.crypto.checksum(hashlib.md5, stage.archive_file)
+    else:
+        value = spack.util.crypto.checksum(hashlib.md5, url)
+    return value
+
+
 def md5(parser, args):
     if not args.files:
         setup_parser.parser.print_help()
         return 1
 
-    for f in args.files:
-        if not os.path.isfile(f):
-            with stager(f) as stage:
-                checksum = spack.util.crypto.checksum(hashlib.md5, stage.archive_file)
-                print "%s  %s" % (checksum, f)
-        else:
-            if not can_access(f):
-                tty.die("Cannot read file: %s" % f)
+    results = []
+    for url in args.files:
+        try:
+            checksum = compute_md5_checksum(url)
+            results.append((checksum, url))
+        except FailedDownloadError as e:
+            tty.warn("Failed to fetch %s" % url)
+            tty.warn("%s" % e)
+        except IOError as e:
+            tty.warn("Error when reading %s" % url)
+            tty.warn("%s" % e)
 
-            checksum = spack.util.crypto.checksum(hashlib.md5, f)
-            print "%s  %s" % (checksum, f)
+    # Dump the MD5s at last without interleaving them with downloads
+    tty.msg("%d MD5 checksums:" % len(results))
+    for checksum, url in results:
+        print "%s  %s" % (checksum, url)
diff --git a/lib/spack/spack/cmd/mirror.py b/lib/spack/spack/cmd/mirror.py
index 8e9438c1a3..fcd15a6a90 100644
--- a/lib/spack/spack/cmd/mirror.py
+++ b/lib/spack/spack/cmd/mirror.py
@@ -126,7 +126,7 @@ def mirror_remove(args):
 
     old_value = mirrors.pop(name)
     spack.config.update_config('mirrors', mirrors, scope=args.scope)
-    tty.msg("Removed mirror %s with url %s." % (name, old_value))
+    tty.msg("Removed mirror %s with url %s" % (name, old_value))
 
 
 def mirror_list(args):
@@ -203,7 +203,7 @@ def mirror_create(args):
 
     verb = "updated" if existed else "created"
     tty.msg(
-        "Successfully %s mirror in %s." % (verb, directory),
+        "Successfully %s mirror in %s" % (verb, directory),
         "Archive stats:",
         "  %-4d already present"  % p,
         "  %-4d added"            % m,
diff --git a/lib/spack/spack/cmd/module.py b/lib/spack/spack/cmd/module.py
index a5a9570eb5..1d6867c1d9 100644
--- a/lib/spack/spack/cmd/module.py
+++ b/lib/spack/spack/cmd/module.py
@@ -58,7 +58,7 @@ def module_find(mtype, spec_array):
        should type to use that package's module.
     """
     if mtype not in module_types:
-        tty.die("Invalid module type: '%s'.  Options are %s." % (mtype, comma_or(module_types)))
+        tty.die("Invalid module type: '%s'.  Options are %s" % (mtype, comma_or(module_types)))
 
     specs = spack.cmd.parse_specs(spec_array)
     if len(specs) > 1:
@@ -78,7 +78,7 @@ def module_find(mtype, spec_array):
     mt = module_types[mtype]
     mod = mt(specs[0])
     if not os.path.isfile(mod.file_name):
-        tty.die("No %s module is installed for %s." % (mtype, spec))
+        tty.die("No %s module is installed for %s" % (mtype, spec))
 
     print mod.use_name
 
@@ -94,7 +94,7 @@ def module_refresh():
             shutil.rmtree(cls.path, ignore_errors=False)
         mkdirp(cls.path)
         for spec in specs:
-            tty.debug("   Writing file for %s." % spec)
+            tty.debug("   Writing file for %s" % spec)
             cls(spec).write()
 
 
diff --git a/lib/spack/spack/cmd/patch.py b/lib/spack/spack/cmd/patch.py
index 44fc8696db..b04b402738 100644
--- a/lib/spack/spack/cmd/patch.py
+++ b/lib/spack/spack/cmd/patch.py
@@ -24,6 +24,7 @@
 ##############################################################################
 import argparse
 
+import llnl.util.tty as tty
 import spack.cmd
 import spack
 
diff --git a/lib/spack/spack/cmd/repo.py b/lib/spack/spack/cmd/repo.py
index 34c755fb67..87c782000f 100644
--- a/lib/spack/spack/cmd/repo.py
+++ b/lib/spack/spack/cmd/repo.py
@@ -6,7 +6,7 @@
 # Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
 # LLNL-CODE-647188
 #
-# For details, see https://llnl.github.io/spack
+# For details, see https://software.llnl.gov/spack
 # Please also see the LICENSE file for our notice and the LGPL.
 #
 # This program is free software; you can redistribute it and/or modify
@@ -74,51 +74,7 @@ def setup_parser(subparser):
 
 def repo_create(args):
     """Create a new package repository."""
-    root = canonicalize_path(args.directory)
-    namespace = args.namespace
-
-    if not args.namespace:
-        namespace = os.path.basename(root)
-
-    if not re.match(r'\w[\.\w-]*', namespace):
-        tty.die("'%s' is not a valid namespace." % namespace)
-
-    existed = False
-    if os.path.exists(root):
-        if os.path.isfile(root):
-            tty.die('File %s already exists and is not a directory' % root)
-        elif os.path.isdir(root):
-            if not os.access(root, os.R_OK | os.W_OK):
-                tty.die('Cannot create new repo in %s: cannot access directory.' % root)
-            if os.listdir(root):
-                tty.die('Cannot create new repo in %s: directory is not empty.' % root)
-        existed = True
-
-    full_path = os.path.realpath(root)
-    parent = os.path.dirname(full_path)
-    if not os.access(parent, os.R_OK | os.W_OK):
-        tty.die("Cannot create repository in %s: can't access parent!" % root)
-
-    try:
-        config_path = os.path.join(root, repo_config_name)
-        packages_path = os.path.join(root, packages_dir_name)
-
-        mkdirp(packages_path)
-        with open(config_path, 'w') as config:
-            config.write("repo:\n")
-            config.write("  namespace: '%s'\n" % namespace)
-
-    except (IOError, OSError) as e:
-        tty.die('Failed to create new repository in %s.' % root,
-                "Caused by %s: %s" % (type(e), e))
-
-        # try to clean up.
-        if existed:
-            shutil.rmtree(config_path, ignore_errors=True)
-            shutil.rmtree(packages_path, ignore_errors=True)
-        else:
-            shutil.rmtree(root, ignore_errors=True)
-
+    full_path, namespace = create_repo(args.directory, args.namespace)
     tty.msg("Created repo with namespace '%s'." % namespace)
     tty.msg("To register it with spack, run this command:",
             'spack repo add %s' % full_path)
@@ -133,11 +89,11 @@ def repo_add(args):
 
     # check if the path exists
     if not os.path.exists(canon_path):
-        tty.die("No such file or directory: '%s'." % path)
+        tty.die("No such file or directory: %s" % path)
 
     # Make sure the path is a directory.
     if not os.path.isdir(canon_path):
-        tty.die("Not a Spack repository: '%s'." % path)
+        tty.die("Not a Spack repository: %s" % path)
 
     # Make sure it's actually a spack repository by constructing it.
     repo = Repo(canon_path)
@@ -147,7 +103,7 @@ def repo_add(args):
     if not repos: repos = []
 
     if repo.root in repos or path in repos:
-        tty.die("Repository is already registered with Spack: '%s'" % path)
+        tty.die("Repository is already registered with Spack: %s" % path)
 
     repos.insert(0, canon_path)
     spack.config.update_config('repos', repos, args.scope)
@@ -166,7 +122,7 @@ def repo_remove(args):
         if canon_path == repo_canon_path:
             repos.remove(repo_path)
             spack.config.update_config('repos', repos, args.scope)
-            tty.msg("Removed repository '%s'." % repo_path)
+            tty.msg("Removed repository %s" % repo_path)
             return
 
     # If it is a namespace, remove corresponding repo
@@ -176,13 +132,13 @@ def repo_remove(args):
             if repo.namespace == path_or_namespace:
                 repos.remove(path)
                 spack.config.update_config('repos', repos, args.scope)
-                tty.msg("Removed repository '%s' with namespace %s."
+                tty.msg("Removed repository %s with namespace '%s'."
                         % (repo.root, repo.namespace))
                 return
         except RepoError as e:
             continue
 
-    tty.die("No repository with path or namespace: '%s'"
+    tty.die("No repository with path or namespace: %s"
             % path_or_namespace)
 
 
diff --git a/lib/spack/spack/compiler.py b/lib/spack/spack/compiler.py
index 12c02e0ea2..d38c0b00b1 100644
--- a/lib/spack/spack/compiler.py
+++ b/lib/spack/spack/compiler.py
@@ -256,12 +256,12 @@ class Compiler(object):
 
 
     def __repr__(self):
-        """Return a string represntation of the compiler toolchain."""
+        """Return a string representation of the compiler toolchain."""
         return self.__str__()
 
 
     def __str__(self):
-        """Return a string represntation of the compiler toolchain."""
+        """Return a string representation of the compiler toolchain."""
         return "%s(%s)" % (
             self.name, '\n     '.join((str(s) for s in (self.cc, self.cxx, self.f77, self.fc))))
 
diff --git a/lib/spack/spack/compilers/__init__.py b/lib/spack/spack/compilers/__init__.py
index 6159ef576c..3a04bc2ebc 100644
--- a/lib/spack/spack/compilers/__init__.py
+++ b/lib/spack/spack/compilers/__init__.py
@@ -74,28 +74,36 @@ def _to_dict(compiler):
 def get_compiler_config(arch=None, scope=None):
     """Return the compiler configuration for the specified architecture.
     """
-    # If any configuration file has compilers, just stick with the
-    # ones already configured.
-    config = spack.config.get_config('compilers', scope=scope)
-
+    # Check whether we're on a front-end (native) architecture.
     my_arch = spack.architecture.sys_type()
     if arch is None:
         arch = my_arch
 
-    if arch in config:
-        return config[arch]
-
-    # Only for the current arch in *highest* scope: automatically try to
-    # find compilers if none are configured yet.
-    if arch == my_arch and scope == 'user':
+    def init_compiler_config():
+        """Compiler search used when Spack has no compilers."""
         config[arch] = {}
         compilers = find_compilers(*get_path('PATH'))
         for compiler in compilers:
             config[arch].update(_to_dict(compiler))
         spack.config.update_config('compilers', config, scope=scope)
-        return config[arch]
 
-    return {}
+    config = spack.config.get_config('compilers', scope=scope)
+
+    # Update the configuration if there are currently no compilers
+    # configured.  Avoid updating automatically if there ARE site
+    # compilers configured but no user ones.
+    if arch == my_arch and arch not in config:
+        if scope is None:
+            # We know no compilers were configured in any scope.
+            init_compiler_config()
+        elif scope == 'user':
+            # Check the site config and update the user config if
+            # nothing is configured at the site level.
+            site_config = spack.config.get_config('compilers', scope='site')
+            if not site_config:
+                init_compiler_config()
+
+    return config[arch] if arch in config else {}
 
 
 def add_compilers_to_config(compilers, arch=None, scope=None):
diff --git a/lib/spack/spack/compilers/pgi.py b/lib/spack/spack/compilers/pgi.py
index 9ac74cfbdb..c6a1078bd9 100644
--- a/lib/spack/spack/compilers/pgi.py
+++ b/lib/spack/spack/compilers/pgi.py
@@ -29,28 +29,28 @@ class Pgi(Compiler):
     cc_names = ['pgcc']
 
     # Subclasses use possible names of C++ compiler
-    cxx_names = ['pgCC']
+    cxx_names = ['pgc++', 'pgCC']
 
     # Subclasses use possible names of Fortran 77 compiler
-    f77_names = ['pgf77']
+    f77_names = ['pgfortran', 'pgf77']
 
     # Subclasses use possible names of Fortran 90 compiler
-    fc_names = ['pgf95', 'pgf90']
+    fc_names = ['pgfortran', 'pgf95', 'pgf90']
 
     # Named wrapper links within spack.build_env_path
     link_paths = { 'cc'  : 'pgi/pgcc',
-                   'cxx' : 'pgi/case-insensitive/pgCC',
-                   'f77' : 'pgi/pgf77',
-                   'fc'  : 'pgi/pgf90' }
+                   'cxx' : 'pgi/pgc++',
+                   'f77' : 'pgi/pgfortran',
+                   'fc'  : 'pgi/pgfortran' }
 
     @classmethod
     def default_version(cls, comp):
         """The '-V' option works for all the PGI compilers.
            Output looks like this::
 
-               pgf95 10.2-0 64-bit target on x86-64 Linux -tp nehalem-64
-               Copyright 1989-2000, The Portland Group, Inc.  All Rights Reserved.
-               Copyright 2000-2010, STMicroelectronics, Inc.  All Rights Reserved.
+               pgcc 15.10-0 64-bit target on x86-64 Linux -tp sandybridge
+               The Portland Group - PGI Compilers and Tools
+               Copyright (c) 2015, NVIDIA CORPORATION.  All rights reserved.
         """
         return get_compiler_version(
             comp, '-V', r'pg[^ ]* ([^ ]+) \d\d\d?-bit target')
diff --git a/lib/spack/spack/concretize.py b/lib/spack/spack/concretize.py
index 85cdb202d5..8083f91982 100644
--- a/lib/spack/spack/concretize.py
+++ b/lib/spack/spack/concretize.py
@@ -33,12 +33,16 @@ or user preferences.
 TODO: make this customizable and allow users to configure
       concretization  policies.
 """
+import spack
 import spack.spec
 import spack.compilers
 import spack.architecture
 import spack.error
 from spack.version import *
-
+from functools import partial
+from spec import DependencyMap
+from itertools import chain
+from spack.config import *
 
 class DefaultConcretizer(object):
     """This class doesn't have any state, it just provides some methods for
@@ -46,10 +50,92 @@ class DefaultConcretizer(object):
        default concretization strategies, or you can override all of them.
     """
 
+    def _valid_virtuals_and_externals(self, spec):
+        """Returns a list of candidate virtual dep providers and external
+           packages that coiuld be used to concretize a spec."""
+        # First construct a list of concrete candidates to replace spec with.
+        candidates = [spec]
+        if spec.virtual:
+            providers = spack.repo.providers_for(spec)
+            if not providers:
+                raise UnsatisfiableProviderSpecError(providers[0], spec)
+            spec_w_preferred_providers = find_spec(
+                spec, lambda(x): spack.pkgsort.spec_has_preferred_provider(x.name, spec.name))
+            if not spec_w_preferred_providers:
+                spec_w_preferred_providers = spec
+            provider_cmp = partial(spack.pkgsort.provider_compare, spec_w_preferred_providers.name, spec.name)
+            candidates = sorted(providers, cmp=provider_cmp)
+
+        # For each candidate package, if it has externals, add those to the usable list.
+        # if it's not buildable, then *only* add the externals.
+        usable = []
+        for cspec in candidates:
+            if is_spec_buildable(cspec):
+                usable.append(cspec)
+            externals = spec_externals(cspec)
+            for ext in externals:
+                if ext.satisfies(spec):
+                    usable.append(ext)
+
+        # If nothing is in the usable list now, it's because we aren't
+        # allowed to build anything.
+        if not usable:
+            raise NoBuildError(spec)
+
+        def cmp_externals(a, b):
+            if a.name != b.name:
+                # We're choosing between different providers, so
+                # maintain order from provider sort
+                return candidates.index(a) - candidates.index(b)
+
+            result = cmp_specs(a, b)
+            if result != 0:
+                return result
+
+            # prefer external packages to internal packages.
+            if a.external is None or b.external is None:
+                return -cmp(a.external, b.external)
+            else:
+                return cmp(a.external, b.external)
+
+        usable.sort(cmp=cmp_externals)
+        return usable
+
+
+    def choose_virtual_or_external(self, spec):
+        """Given a list of candidate virtual and external packages, try to
+           find one that is most ABI compatible.
+        """
+        candidates = self._valid_virtuals_and_externals(spec)
+        if not candidates:
+            return candidates
+
+        # Find the nearest spec in the dag that has a compiler.  We'll
+        # use that spec to calibrate compiler compatibility.
+        abi_exemplar = find_spec(spec, lambda(x): x.compiler)
+        if not abi_exemplar:
+            abi_exemplar = spec.root
+
+        # Make a list including ABI compatibility of specs with the exemplar.
+        strict = [spack.abi.compatible(c, abi_exemplar) for c in candidates]
+        loose  = [spack.abi.compatible(c, abi_exemplar, loose=True) for c in candidates]
+        keys = zip(strict, loose, candidates)
+
+        # Sort candidates from most to least compatibility.
+        # Note:
+        #   1. We reverse because True > False.
+        #   2. Sort is stable, so c's keep their order.
+        keys.sort(key=lambda k:k[:2], reverse=True)
+
+        # Pull the candidates back out and return them in order
+        candidates = [c for s,l,c in keys]
+        return candidates
+
+
     def concretize_version(self, spec):
         """If the spec is already concrete, return.  Otherwise take
-           the most recent available version, and default to the package's
-           version if there are no avaialble versions.
+           the preferred version from spackconfig, and default to the package's
+           version if there are no available versions.
 
            TODO: In many cases we probably want to look for installed
                  versions of each package and use an installed version
@@ -67,20 +153,14 @@ class DefaultConcretizer(object):
         # If there are known available versions, return the most recent
         # version that satisfies the spec
         pkg = spec.package
-
-        # Key function to sort versions first by whether they were
-        # marked `preferred=True`, then by most recent.
-        def preferred_key(v):
-            prefer = pkg.versions[v].get('preferred', False)
-            return (prefer, v)
-
+        cmp_versions = partial(spack.pkgsort.version_compare, spec.name)
         valid_versions = sorted(
             [v for v in pkg.versions
              if any(v.satisfies(sv) for sv in spec.versions)],
-            key=preferred_key)
+            cmp=cmp_versions)
 
         if valid_versions:
-            spec.versions = ver([valid_versions[-1]])
+            spec.versions = ver([valid_versions[0]])
         else:
             # We don't know of any SAFE versions that match the given
             # spec.  Grab the spec's versions and grab the highest
@@ -134,7 +214,7 @@ class DefaultConcretizer(object):
            the default variants from the package specification.
         """
         changed = False
-        for name, variant in spec.package.variants.items():
+        for name, variant in spec.package_class.variants.items():
             if name not in spec.variants:
                 spec.variants[name] = spack.spec.VariantSpec(name, variant.default)
                 changed = True
@@ -145,10 +225,10 @@ class DefaultConcretizer(object):
         """If the spec already has a compiler, we're done.  If not, then take
            the compiler used for the nearest ancestor with a compiler
            spec and use that.  If the ancestor's compiler is not
-           concrete, then give it a valid version.  If there is no
-           ancestor with a compiler, use the system default compiler.
+           concrete, then used the preferred compiler as specified in
+           spackconfig.
 
-           Intuition: Use the system default if no package that depends on
+           Intuition: Use the spackconfig default if no package that depends on
            this one has a strict compiler requirement.  Otherwise, try to
            build with the compiler that will be used by libraries that
            link to this one, to maximize compatibility.
@@ -160,40 +240,91 @@ class DefaultConcretizer(object):
             spec.compiler in all_compilers):
             return False
 
-        try:
-            nearest = next(p for p in spec.traverse(direction='parents')
-                           if p.compiler is not None).compiler
+        #Find the another spec that has a compiler, or the root if none do
+        other_spec = find_spec(spec, lambda(x) : x.compiler)
+        if not other_spec:
+            other_spec = spec.root
+        other_compiler = other_spec.compiler
+        assert(other_spec)
+
+        # Check if the compiler is already fully specified
+        if other_compiler in all_compilers:
+            spec.compiler = other_compiler.copy()
+            return True
+
+        # Filter the compilers into a sorted list based on the compiler_order from spackconfig
+        compiler_list = all_compilers if not other_compiler else spack.compilers.find(other_compiler)
+        cmp_compilers = partial(spack.pkgsort.compiler_compare, other_spec.name)
+        matches = sorted(compiler_list, cmp=cmp_compilers)
+        if not matches:
+            raise UnavailableCompilerVersionError(other_compiler)
+
+        # copy concrete version into other_compiler
+        spec.compiler = matches[0].copy()
+        assert(spec.compiler.concrete)
+        return True  # things changed.
 
-            if not nearest in all_compilers:
-                # Take the newest compiler that saisfies the spec
-                matches = sorted(spack.compilers.find(nearest))
-                if not matches:
-                    raise UnavailableCompilerVersionError(nearest)
 
-                # copy concrete version into nearest spec
-                nearest.versions = matches[-1].versions.copy()
-                assert(nearest.concrete)
+def find_spec(spec, condition):
+    """Searches the dag from spec in an intelligent order and looks
+       for a spec that matches a condition"""
+    # First search parents, then search children
+    dagiter = chain(spec.traverse(direction='parents',  root=False),
+                    spec.traverse(direction='children', root=False))
+    visited = set()
+    for relative in dagiter:
+        if condition(relative):
+            return relative
+        visited.add(id(relative))
 
-            spec.compiler = nearest.copy()
+    # Then search all other relatives in the DAG *except* spec
+    for relative in spec.root.traverse():
+        if relative is spec: continue
+        if id(relative) in visited: continue
+        if condition(relative):
+            return relative
 
-        except StopIteration:
-            spec.compiler = spack.compilers.default_compiler().copy()
+    # Finally search spec itself.
+    if condition(spec):
+        return spec
 
-        return True  # things changed.
+    return None   # Nohting matched the condition.
 
 
-    def choose_provider(self, spec, providers):
-        """This is invoked for virtual specs.  Given a spec with a virtual name,
-           say "mpi", and a list of specs of possible providers of that spec,
-           select a provider and return it.
-        """
-        assert(spec.virtual)
-        assert(providers)
+def cmp_specs(lhs, rhs):
+    # Package name sort order is not configurable, always goes alphabetical
+    if lhs.name != rhs.name:
+        return cmp(lhs.name, rhs.name)
+
+    # Package version is second in compare order
+    pkgname = lhs.name
+    if lhs.versions != rhs.versions:
+        return spack.pkgsort.version_compare(
+            pkgname, lhs.versions, rhs.versions)
+
+    # Compiler is third
+    if lhs.compiler != rhs.compiler:
+        return spack.pkgsort.compiler_compare(
+            pkgname, lhs.compiler, rhs.compiler)
+
+    # Variants
+    if lhs.variants != rhs.variants:
+        return spack.pkgsort.variant_compare(
+            pkgname, lhs.variants, rhs.variants)
+
+    # Architecture
+    if lhs.architecture != rhs.architecture:
+        return spack.pkgsort.architecture_compare(
+            pkgname, lhs.architecture, rhs.architecture)
+
+    # Dependency is not configurable
+    lhash, rhash = hash(lhs), hash(rhs)
+    if lhash != rhash:
+        return -1 if lhash < rhash else 1
+
+    # Equal specs
+    return 0
 
-        index = spack.spec.index_specs(providers)
-        first_key = sorted(index.keys())[0]
-        latest_version = sorted(index[first_key])[-1]
-        return latest_version
 
 
 class UnavailableCompilerVersionError(spack.error.SpackError):
@@ -211,3 +342,11 @@ class NoValidVersionError(spack.error.SpackError):
     def __init__(self, spec):
         super(NoValidVersionError, self).__init__(
             "There are no valid versions for %s that match '%s'" % (spec.name, spec.versions))
+
+
+class NoBuildError(spack.error.SpackError):
+    """Raised when a package is configured with the buildable option False, but
+       no satisfactory external versions can be found"""
+    def __init__(self, spec):
+        super(NoBuildError, self).__init__(
+            "The spec '%s' is configured as not buildable, and no matching external installs were found" % spec.name)
diff --git a/lib/spack/spack/config.py b/lib/spack/spack/config.py
index 6fecde9980..6afd69b3ac 100644
--- a/lib/spack/spack/config.py
+++ b/lib/spack/spack/config.py
@@ -129,6 +129,7 @@ from ordereddict_backport import OrderedDict
 
 import llnl.util.tty as tty
 from llnl.util.filesystem import mkdirp
+import copy
 
 import spack
 from spack.error import SpackError
@@ -194,6 +195,49 @@ section_schemas = {
                 'default': [],
                 'items': {
                     'type': 'string'},},},},
+    'packages': {
+        '$schema': 'http://json-schema.org/schema#',
+        'title': 'Spack package configuration file schema',
+        'type': 'object',
+        'additionalProperties': False,
+        'patternProperties': {
+            r'packages:?': {
+                'type': 'object',
+                'default': {},
+                'additionalProperties': False,
+                'patternProperties': {
+                    r'\w[\w-]*': { # package name
+                        'type': 'object',
+                        'default': {},
+                        'additionalProperties': False,
+                        'properties': {
+                            'version': {
+                                'type' : 'array',
+                                'default' : [],
+                                'items' : { 'anyOf' : [ { 'type' : 'string' },
+                                                        { 'type' : 'number'}]}}, #version strings
+                            'compiler': {
+                                'type' : 'array',
+                                'default' : [],
+                                'items' : { 'type' : 'string' } }, #compiler specs
+                            'buildable': {
+                                'type':  'boolean',
+                                'default': True,
+                             },
+                            'providers': {
+                                'type':  'object',
+                                'default': {},
+                                'additionalProperties': False,
+                                'patternProperties': {
+                                    r'\w[\w-]*': {
+                                        'type' : 'array',
+                                        'default' : [],
+                                        'items' : { 'type' : 'string' },},},},
+                            'paths': {
+                                'type' : 'object',
+                                'default' : {},
+                            }
+                        },},},},},}
 }
 
 """OrderedDict of config scopes keyed by name.
@@ -205,7 +249,7 @@ config_scopes = OrderedDict()
 def validate_section_name(section):
     """Raise a ValueError if the section is not a valid section."""
     if section not in section_schemas:
-        raise ValueError("Invalid config section: '%s'.  Options are %s."
+        raise ValueError("Invalid config section: '%s'.  Options are %s"
                          % (section, section_schemas))
 
 
@@ -335,7 +379,7 @@ def validate_scope(scope):
         return config_scopes[scope]
 
     else:
-        raise ValueError("Invalid config scope: '%s'.  Must be one of %s."
+        raise ValueError("Invalid config scope: '%s'.  Must be one of %s"
                          % (scope, config_scopes.keys()))
 
 
@@ -350,7 +394,7 @@ def _read_config_file(filename, schema):
             "Invlaid configuration. %s exists but is not a file." % filename)
 
     elif not os.access(filename, os.R_OK):
-        raise ConfigFileError("Config file is not readable: %s." % filename)
+        raise ConfigFileError("Config file is not readable: %s" % filename)
 
     try:
         tty.debug("Reading config file %s" % filename)
@@ -494,6 +538,39 @@ def print_section(section):
         raise ConfigError("Error reading configuration: %s" % section)
 
 
+def spec_externals(spec):
+    """Return a list of external specs (with external directory path filled in),
+       one for each known external installation."""
+    allpkgs = get_config('packages')
+    name = spec.name
+
+    external_specs = []
+    pkg_paths = allpkgs.get(name, {}).get('paths', None)
+    if not pkg_paths:
+        return []
+
+    for external_spec, path in pkg_paths.iteritems():
+        if not path:
+            # skip entries without paths (avoid creating extra Specs)
+            continue
+
+        external_spec = spack.spec.Spec(external_spec, external=path)
+        if external_spec.satisfies(spec):
+            external_specs.append(external_spec)
+    return external_specs
+
+
+def is_spec_buildable(spec):
+    """Return true if the spec pkgspec is configured as buildable"""
+    allpkgs = get_config('packages')
+    name = spec.name
+    if not spec.name in allpkgs:
+        return True
+    if not 'buildable' in allpkgs[spec.name]:
+        return True
+    return allpkgs[spec.name]['buildable']
+
+
 class ConfigError(SpackError): pass
 class ConfigFileError(ConfigError): pass
 
@@ -509,7 +586,7 @@ class ConfigFormatError(ConfigError):
         # Try to get line number from erroneous instance and its parent
         instance_mark = getattr(validation_error.instance, '_start_mark', None)
         parent_mark = getattr(validation_error.parent, '_start_mark', None)
-        path = getattr(validation_error, 'path', None)
+        path = [str(s) for s in getattr(validation_error, 'path', None)]
 
         # Try really hard to get the parent (which sometimes is not
         # set) This digs it out of the validated structure if it's not
diff --git a/lib/spack/spack/database.py b/lib/spack/spack/database.py
index 9cbe7de44a..089d29325e 100644
--- a/lib/spack/spack/database.py
+++ b/lib/spack/spack/database.py
@@ -330,7 +330,7 @@ class Database(object):
             found = rec.ref_count
             if not expected == found:
                 raise AssertionError(
-                    "Invalid ref_count: %s: %d (expected %d), in DB %s."
+                    "Invalid ref_count: %s: %d (expected %d), in DB %s"
                     % (key, found, expected, self._index_path))
 
 
diff --git a/lib/spack/spack/directives.py b/lib/spack/spack/directives.py
index 0b98211cb9..61cd303012 100644
--- a/lib/spack/spack/directives.py
+++ b/lib/spack/spack/directives.py
@@ -125,7 +125,7 @@ class directive(object):
             dicts = (dicts,)
         elif type(dicts) not in (list, tuple):
             raise TypeError(
-                "dicts arg must be list, tuple, or string. Found %s."
+                "dicts arg must be list, tuple, or string. Found %s"
                 % type(dicts))
 
         self.dicts = dicts
@@ -174,7 +174,11 @@ def version(pkg, ver, checksum=None, **kwargs):
 
 
 def _depends_on(pkg, spec, when=None):
-    if when is None:
+    # If when is False do nothing
+    if when is False:
+        return
+    # If when is None or True make sure the condition is always satisfied
+    if when is None or when is True:
         when = pkg.name
     when_spec = parse_anonymous_spec(when, pkg.name)
 
@@ -296,8 +300,8 @@ def resource(pkg, **kwargs):
         raise RuntimeError(message)
     when_spec = parse_anonymous_spec(when, pkg.name)
     resources = pkg.resources.setdefault(when_spec, [])
-    fetcher = from_kwargs(**kwargs)
     name = kwargs.get('name')
+    fetcher = from_kwargs(**kwargs)
     resources.append(Resource(name, fetcher, destination, placement))
 
 
@@ -313,5 +317,5 @@ class CircularReferenceError(DirectiveError):
     def __init__(self, directive, package):
         super(CircularReferenceError, self).__init__(
             directive,
-            "Package '%s' cannot pass itself to %s." % (package, directive))
+            "Package '%s' cannot pass itself to %s" % (package, directive))
         self.package = package
diff --git a/lib/spack/spack/directory_layout.py b/lib/spack/spack/directory_layout.py
index 3e416a6a1f..39ee4e203d 100644
--- a/lib/spack/spack/directory_layout.py
+++ b/lib/spack/spack/directory_layout.py
@@ -85,6 +85,16 @@ class DirectoryLayout(object):
         raise NotImplementedError()
 
 
+    def check_installed(self, spec):
+        """Checks whether a spec is installed.
+
+        Return the spec's prefix, if it is installed, None otherwise.
+
+        Raise an exception if the install is inconsistent or corrupt.
+        """
+        raise NotImplementedError()
+
+
     def extension_map(self, spec):
         """Get a dict of currently installed extension packages for a spec.
 
@@ -173,7 +183,9 @@ class YamlDirectoryLayout(DirectoryLayout):
 
         self.spec_file_name      = 'spec.yaml'
         self.extension_file_name = 'extensions.yaml'
-        self.build_log_name      = 'build.out' # TODO: use config file.
+        self.build_log_name      = 'build.out'  # build log.
+        self.build_env_name      = 'build.env'  # build environment
+        self.packages_dir        = 'repos'      # archive of package.py files
 
         # Cache of already written/read extension maps.
         self._extension_maps = {}
@@ -186,6 +198,10 @@ class YamlDirectoryLayout(DirectoryLayout):
 
     def relative_path_for_spec(self, spec):
         _check_concrete(spec)
+
+        if spec.external:
+            return spec.external
+
         dir_name = "%s-%s-%s" % (
             spec.name,
             spec.version,
@@ -231,29 +247,49 @@ class YamlDirectoryLayout(DirectoryLayout):
                          self.build_log_name)
 
 
+    def build_env_path(self, spec):
+        return join_path(self.path_for_spec(spec), self.metadata_dir,
+                         self.build_env_name)
+
+
+    def build_packages_path(self, spec):
+        return join_path(self.path_for_spec(spec), self.metadata_dir,
+                         self.packages_dir)
+
+
     def create_install_directory(self, spec):
         _check_concrete(spec)
 
+        prefix = self.check_installed(spec)
+        if prefix:
+            raise InstallDirectoryAlreadyExistsError(prefix)
+
+        mkdirp(self.metadata_path(spec))
+        self.write_spec(spec, self.spec_file_path(spec))
+
+
+    def check_installed(self, spec):
+        _check_concrete(spec)
         path = self.path_for_spec(spec)
         spec_file_path = self.spec_file_path(spec)
 
-        if os.path.isdir(path):
-            if not os.path.isfile(spec_file_path):
-                raise InconsistentInstallDirectoryError(
-                    'No spec file found at path %s' % spec_file_path)
+        if not os.path.isdir(path):
+            return None
 
-            installed_spec = self.read_spec(spec_file_path)
-            if installed_spec == self.spec:
-                raise InstallDirectoryAlreadyExistsError(path)
+        if not os.path.isfile(spec_file_path):
+            raise InconsistentInstallDirectoryError(
+                'Inconsistent state: install prefix exists but contains no spec.yaml:',
+                "  " + path)
 
-            if spec.dag_hash() == installed_spec.dag_hash():
-                raise SpecHashCollisionError(installed_hash, spec_hash)
-            else:
-                raise InconsistentInstallDirectoryError(
-                    'Spec file in %s does not match hash!' % spec_file_path)
+        installed_spec = self.read_spec(spec_file_path)
+        if installed_spec == spec:
+            return path
 
-        mkdirp(self.metadata_path(spec))
-        self.write_spec(spec, spec_file_path)
+        if spec.dag_hash() == installed_spec.dag_hash():
+            raise SpecHashCollisionError(installed_hash, spec_hash)
+        else:
+            raise InconsistentInstallDirectoryError(
+                'Spec file in %s does not match hash!' % spec_file_path)
 
 
     def all_specs(self):
@@ -323,7 +359,7 @@ class YamlDirectoryLayout(DirectoryLayout):
 
                         if not dag_hash in by_hash:
                             raise InvalidExtensionSpecError(
-                                "Spec %s not found in %s." % (dag_hash, prefix))
+                                "Spec %s not found in %s" % (dag_hash, prefix))
 
                         ext_spec = by_hash[dag_hash]
                         if not prefix == ext_spec.prefix:
@@ -387,8 +423,8 @@ class YamlDirectoryLayout(DirectoryLayout):
 
 class DirectoryLayoutError(SpackError):
     """Superclass for directory layout errors."""
-    def __init__(self, message):
-        super(DirectoryLayoutError, self).__init__(message)
+    def __init__(self, message, long_msg=None):
+        super(DirectoryLayoutError, self).__init__(message, long_msg)
 
 
 class SpecHashCollisionError(DirectoryLayoutError):
@@ -410,8 +446,8 @@ class RemoveFailedError(DirectoryLayoutError):
 
 class InconsistentInstallDirectoryError(DirectoryLayoutError):
     """Raised when a package seems to be installed to the wrong place."""
-    def __init__(self, message):
-        super(InconsistentInstallDirectoryError, self).__init__(message)
+    def __init__(self, message, long_msg=None):
+        super(InconsistentInstallDirectoryError, self).__init__(message, long_msg)
 
 
 class InstallDirectoryAlreadyExistsError(DirectoryLayoutError):
@@ -438,7 +474,7 @@ class ExtensionConflictError(DirectoryLayoutError):
     """Raised when an extension is added to a package that already has it."""
     def __init__(self, spec, ext_spec, conflict):
         super(ExtensionConflictError, self).__init__(
-            "%s cannot be installed in %s because it conflicts with %s."% (
+            "%s cannot be installed in %s because it conflicts with %s"% (
                 ext_spec.short_spec, spec.short_spec, conflict.short_spec))
 
 
diff --git a/lib/spack/spack/fetch_strategy.py b/lib/spack/spack/fetch_strategy.py
index 337dd1e198..0d0a7db8a9 100644
--- a/lib/spack/spack/fetch_strategy.py
+++ b/lib/spack/spack/fetch_strategy.py
@@ -44,6 +44,7 @@ import os
 import sys
 import re
 import shutil
+import copy
 from functools import wraps
 import llnl.util.tty as tty
 from llnl.util.filesystem import *
@@ -55,23 +56,28 @@ from spack.util.string import *
 from spack.version import Version, ver
 from spack.util.compression import decompressor_for, extension
 
+import spack.util.pattern as pattern
+
 """List of all fetch strategies, created by FetchStrategy metaclass."""
 all_strategies = []
 
+
 def _needs_stage(fun):
     """Many methods on fetch strategies require a stage to be set
        using set_stage().  This decorator adds a check for self.stage."""
+
     @wraps(fun)
     def wrapper(self, *args, **kwargs):
         if not self.stage:
             raise NoStageError(fun)
         return fun(self, *args, **kwargs)
+
     return wrapper
 
 
 class FetchStrategy(object):
     """Superclass of all fetch strategies."""
-    enabled = False             # Non-abstract subclasses should be enabled.
+    enabled = False  # Non-abstract subclasses should be enabled.
     required_attributes = None  # Attributes required in version() args.
 
     class __metaclass__(type):
@@ -80,28 +86,28 @@ class FetchStrategy(object):
             type.__init__(cls, name, bases, dict)
             if cls.enabled: all_strategies.append(cls)
 
-
     def __init__(self):
         # The stage is initialized late, so that fetch strategies can be constructed
         # at package construction time.  This is where things will be fetched.
         self.stage = None
 
-
     def set_stage(self, stage):
         """This is called by Stage before any of the fetching
            methods are called on the stage."""
         self.stage = stage
 
-
     # Subclasses need to implement these methods
     def fetch(self):   pass  # Return True on success, False on fail.
+
     def check(self):   pass  # Do checksum.
+
     def expand(self):  pass  # Expand archive.
+
     def reset(self):   pass  # Revert to freshly downloaded state.
 
     def archive(self, destination): pass  # Used to create tarball for mirror.
 
-    def __str__(self):       # Should be human readable URL.
+    def __str__(self):  # Should be human readable URL.
         return "FetchStrategy.__str___"
 
     # This method is used to match fetch strategies to version()
@@ -111,6 +117,15 @@ class FetchStrategy(object):
         return any(k in args for k in cls.required_attributes)
 
 
+@pattern.composite(interface=FetchStrategy)
+class FetchStrategyComposite(object):
+    """
+    Composite for a FetchStrategy object. Implements the GoF composite pattern.
+    """
+    matches = FetchStrategy.matches
+    set_stage = FetchStrategy.set_stage
+
+
 class URLFetchStrategy(FetchStrategy):
     """FetchStrategy that pulls source code from a URL for an archive,
        checks the archive against a checksum,and decompresses the archive.
@@ -129,6 +144,8 @@ class URLFetchStrategy(FetchStrategy):
         self.digest = kwargs.get('md5', None)
         if not self.digest: self.digest = digest
 
+        self.expand_archive = kwargs.get('expand', True)
+
         if not self.url:
             raise ValueError("URLFetchStrategy requires a url for fetching.")
 
@@ -137,20 +154,20 @@ class URLFetchStrategy(FetchStrategy):
         self.stage.chdir()
 
         if self.archive_file:
-            tty.msg("Already downloaded %s." % self.archive_file)
+            tty.msg("Already downloaded %s" % self.archive_file)
             return
 
         tty.msg("Trying to fetch from %s" % self.url)
 
-        curl_args = ['-O',        # save file to disk
-                     '-f',        # fail on >400 errors
-                     '-D', '-',   # print out HTML headers
-                     '-L', self.url,]
+        curl_args = ['-O',  # save file to disk
+                     '-f',  # fail on >400 errors
+                     '-D', '-',  # print out HTML headers
+                     '-L', self.url, ]
 
         if sys.stdout.isatty():
             curl_args.append('-#')  # status bar when using a tty
         else:
-            curl_args.append('-sS') # just errors when not.
+            curl_args.append('-sS')  # just errors when not.
 
         # Run curl but grab the mime type from the http headers
         headers = spack.curl(
@@ -164,24 +181,23 @@ class URLFetchStrategy(FetchStrategy):
             if spack.curl.returncode == 22:
                 # This is a 404.  Curl will print the error.
                 raise FailedDownloadError(
-                    self.url, "URL %s was not found!" % self.url)
+                        self.url, "URL %s was not found!" % self.url)
 
             elif spack.curl.returncode == 60:
                 # This is a certificate error.  Suggest spack -k
                 raise FailedDownloadError(
-                    self.url,
-                    "Curl was unable to fetch due to invalid certificate. "
-                    "This is either an attack, or your cluster's SSL configuration "
-                    "is bad.  If you believe your SSL configuration is bad, you "
-                    "can try running spack -k, which will not check SSL certificates."
-                    "Use this at your own risk.")
+                        self.url,
+                        "Curl was unable to fetch due to invalid certificate. "
+                        "This is either an attack, or your cluster's SSL configuration "
+                        "is bad.  If you believe your SSL configuration is bad, you "
+                        "can try running spack -k, which will not check SSL certificates."
+                        "Use this at your own risk.")
 
             else:
                 # This is some other curl error.  Curl will print the
                 # error, but print a spack message too
                 raise FailedDownloadError(
-                    self.url, "Curl failed with error %d" % spack.curl.returncode)
-
+                        self.url, "Curl failed with error %d" % spack.curl.returncode)
 
         # Check if we somehow got an HTML file rather than the archive we
         # asked for.  We only look at the last content type, to handle
@@ -196,7 +212,6 @@ class URLFetchStrategy(FetchStrategy):
         if not self.archive_file:
             raise FailedDownloadError(self.url)
 
-
     @property
     def archive_file(self):
         """Path to the source archive within this stage directory."""
@@ -204,12 +219,16 @@ class URLFetchStrategy(FetchStrategy):
 
     @_needs_stage
     def expand(self):
+        if not self.expand_archive:
+            tty.msg("Skipping expand step for %s" % self.archive_file)
+            return
+
         tty.msg("Staging archive: %s" % self.archive_file)
 
         self.stage.chdir()
         if not self.archive_file:
             raise NoArchiveFileError("URLFetchStrategy couldn't find archive file",
-                                      "Failed on expand() for URL %s" % self.url)
+                                     "Failed on expand() for URL %s" % self.url)
 
         decompress = decompressor_for(self.archive_file)
 
@@ -241,7 +260,6 @@ class URLFetchStrategy(FetchStrategy):
         # Set the wd back to the stage when done.
         self.stage.chdir()
 
-
     def archive(self, destination):
         """Just moves this archive to the destination."""
         if not self.archive_file:
@@ -252,7 +270,6 @@ class URLFetchStrategy(FetchStrategy):
 
         shutil.move(self.archive_file, destination)
 
-
     @_needs_stage
     def check(self):
         """Check the downloaded archive against a checksum digest.
@@ -263,9 +280,8 @@ class URLFetchStrategy(FetchStrategy):
         checker = crypto.Checker(self.digest)
         if not checker.check(self.archive_file):
             raise ChecksumError(
-                "%s checksum failed for %s." % (checker.hash_name, self.archive_file),
-                "Expected %s but got %s." % (self.digest, checker.sum))
-
+                    "%s checksum failed for %s" % (checker.hash_name, self.archive_file),
+                    "Expected %s but got %s" % (self.digest, checker.sum))
 
     @_needs_stage
     def reset(self):
@@ -277,12 +293,10 @@ class URLFetchStrategy(FetchStrategy):
             shutil.rmtree(self.stage.source_path, ignore_errors=True)
         self.expand()
 
-
     def __repr__(self):
         url = self.url if self.url else "no url"
         return "URLFetchStrategy<%s>" % url
 
-
     def __str__(self):
         if self.url:
             return self.url
@@ -298,33 +312,30 @@ class VCSFetchStrategy(FetchStrategy):
         # Set a URL based on the type of fetch strategy.
         self.url = kwargs.get(name, None)
         if not self.url: raise ValueError(
-            "%s requires %s argument." % (self.__class__, name))
+                "%s requires %s argument." % (self.__class__, name))
 
         # Ensure that there's only one of the rev_types
         if sum(k in kwargs for k in rev_types) > 1:
             raise FetchStrategyError(
-                "Supply only one of %s to fetch with %s." % (
-                    comma_or(rev_types), name))
+                    "Supply only one of %s to fetch with %s" % (
+                        comma_or(rev_types), name))
 
         # Set attributes for each rev type.
         for rt in rev_types:
             setattr(self, rt, kwargs.get(rt, None))
 
-
     @_needs_stage
     def check(self):
-        tty.msg("No checksum needed when fetching with %s." % self.name)
-
+        tty.msg("No checksum needed when fetching with %s" % self.name)
 
     @_needs_stage
     def expand(self):
         tty.debug("Source fetched with %s is already expanded." % self.name)
 
-
     @_needs_stage
     def archive(self, destination, **kwargs):
-        assert(extension(destination) == 'tar.gz')
-        assert(self.stage.source_path.startswith(self.stage.path))
+        assert (extension(destination) == 'tar.gz')
+        assert (self.stage.source_path.startswith(self.stage.path))
 
         tar = which('tar', required=True)
 
@@ -338,16 +349,13 @@ class VCSFetchStrategy(FetchStrategy):
         self.stage.chdir()
         tar('-czf', destination, os.path.basename(self.stage.source_path))
 
-
     def __str__(self):
         return "VCS: %s" % self.url
 
-
     def __repr__(self):
         return "%s<%s>" % (self.__class__, self.url)
 
 
-
 class GitFetchStrategy(VCSFetchStrategy):
     """Fetch strategy that gets source code from a git repository.
        Use like this in a package:
@@ -368,30 +376,31 @@ class GitFetchStrategy(VCSFetchStrategy):
     required_attributes = ('git',)
 
     def __init__(self, **kwargs):
+        # Discards the keywords in kwargs that may conflict with the next call to __init__
+        forwarded_args = copy.copy(kwargs)
+        forwarded_args.pop('name', None)
+
         super(GitFetchStrategy, self).__init__(
-            'git', 'tag', 'branch', 'commit', **kwargs)
+                'git', 'tag', 'branch', 'commit', **forwarded_args)
         self._git = None
 
-
     @property
     def git_version(self):
         vstring = self.git('--version', output=str).lstrip('git version ')
         return Version(vstring)
 
-
     @property
     def git(self):
         if not self._git:
             self._git = which('git', required=True)
         return self._git
 
-
     @_needs_stage
     def fetch(self):
         self.stage.chdir()
 
         if self.stage.source_path:
-            tty.msg("Already fetched %s." % self.stage.source_path)
+            tty.msg("Already fetched %s" % self.stage.source_path)
             return
 
         args = []
@@ -418,7 +427,7 @@ class GitFetchStrategy(VCSFetchStrategy):
             if self.branch:
                 args.extend(['--branch', self.branch])
             elif self.tag and self.git_version >= ver('1.8.5.2'):
-                    args.extend(['--branch', self.tag])
+                args.extend(['--branch', self.tag])
 
             # Try to be efficient if we're using a new enough git.
             # This checks out only one branch's history
@@ -429,7 +438,7 @@ class GitFetchStrategy(VCSFetchStrategy):
             # Yet more efficiency, only download a 1-commit deep tree
             if self.git_version >= ver('1.7.1'):
                 try:
-                    self.git(*(args + ['--depth','1', self.url]))
+                    self.git(*(args + ['--depth', '1', self.url]))
                     cloned = True
                 except spack.error.SpackError:
                     # This will fail with the dumb HTTP transport
@@ -452,18 +461,15 @@ class GitFetchStrategy(VCSFetchStrategy):
                 self.git('pull', '--tags', ignore_errors=1)
                 self.git('checkout', self.tag)
 
-
     def archive(self, destination):
         super(GitFetchStrategy, self).archive(destination, exclude='.git')
 
-
     @_needs_stage
     def reset(self):
         self.stage.chdir_to_source()
         self.git('checkout', '.')
         self.git('clean', '-f')
 
-
     def __str__(self):
         return "[git] %s" % self.url
 
@@ -483,26 +489,28 @@ class SvnFetchStrategy(VCSFetchStrategy):
     required_attributes = ['svn']
 
     def __init__(self, **kwargs):
+        # Discards the keywords in kwargs that may conflict with the next call to __init__
+        forwarded_args = copy.copy(kwargs)
+        forwarded_args.pop('name', None)
+
         super(SvnFetchStrategy, self).__init__(
-            'svn', 'revision', **kwargs)
+                'svn', 'revision', **forwarded_args)
         self._svn = None
         if self.revision is not None:
             self.revision = str(self.revision)
 
-
     @property
     def svn(self):
         if not self._svn:
             self._svn = which('svn', required=True)
         return self._svn
 
-
     @_needs_stage
     def fetch(self):
         self.stage.chdir()
 
         if self.stage.source_path:
-            tty.msg("Already fetched %s." % self.stage.source_path)
+            tty.msg("Already fetched %s" % self.stage.source_path)
             return
 
         tty.msg("Trying to check out svn repository: %s" % self.url)
@@ -515,7 +523,6 @@ class SvnFetchStrategy(VCSFetchStrategy):
         self.svn(*args)
         self.stage.chdir_to_source()
 
-
     def _remove_untracked_files(self):
         """Removes untracked files in an svn repository."""
         status = self.svn('status', '--no-ignore', output=str)
@@ -529,23 +536,19 @@ class SvnFetchStrategy(VCSFetchStrategy):
             elif os.path.isdir(path):
                 shutil.rmtree(path, ignore_errors=True)
 
-
     def archive(self, destination):
         super(SvnFetchStrategy, self).archive(destination, exclude='.svn')
 
-
     @_needs_stage
     def reset(self):
         self.stage.chdir_to_source()
         self._remove_untracked_files()
         self.svn('revert', '.', '-R')
 
-
     def __str__(self):
         return "[svn] %s" % self.url
 
 
-
 class HgFetchStrategy(VCSFetchStrategy):
     """Fetch strategy that gets source code from a Mercurial repository.
        Use like this in a package:
@@ -567,11 +570,14 @@ class HgFetchStrategy(VCSFetchStrategy):
     required_attributes = ['hg']
 
     def __init__(self, **kwargs):
+        # Discards the keywords in kwargs that may conflict with the next call to __init__
+        forwarded_args = copy.copy(kwargs)
+        forwarded_args.pop('name', None)
+
         super(HgFetchStrategy, self).__init__(
-            'hg', 'revision', **kwargs)
+                'hg', 'revision', **forwarded_args)
         self._hg = None
 
-
     @property
     def hg(self):
         if not self._hg:
@@ -583,7 +589,7 @@ class HgFetchStrategy(VCSFetchStrategy):
         self.stage.chdir()
 
         if self.stage.source_path:
-            tty.msg("Already fetched %s." % self.stage.source_path)
+            tty.msg("Already fetched %s" % self.stage.source_path)
             return
 
         args = []
@@ -597,11 +603,9 @@ class HgFetchStrategy(VCSFetchStrategy):
 
         self.hg(*args)
 
-
     def archive(self, destination):
         super(HgFetchStrategy, self).archive(destination, exclude='.hg')
 
-
     @_needs_stage
     def reset(self):
         self.stage.chdir()
@@ -619,7 +623,6 @@ class HgFetchStrategy(VCSFetchStrategy):
         shutil.move(scrubbed, source_path)
         self.stage.chdir_to_source()
 
-
     def __str__(self):
         return "[hg] %s" % self.url
 
@@ -693,9 +696,10 @@ class FetchError(spack.error.SpackError):
 
 class FailedDownloadError(FetchError):
     """Raised wen a download fails."""
+
     def __init__(self, url, msg=""):
         super(FailedDownloadError, self).__init__(
-            "Failed to fetch file from URL: %s" % url, msg)
+                "Failed to fetch file from URL: %s" % url, msg)
         self.url = url
 
 
@@ -718,12 +722,14 @@ class InvalidArgsError(FetchError):
 
 class ChecksumError(FetchError):
     """Raised when archive fails to checksum."""
+
     def __init__(self, message, long_msg=None):
         super(ChecksumError, self).__init__(message, long_msg)
 
 
 class NoStageError(FetchError):
     """Raised when fetch operations are called before set_stage()."""
+
     def __init__(self, method):
         super(NoStageError, self).__init__(
-            "Must call FetchStrategy.set_stage() before calling %s" % method.__name__)
+                "Must call FetchStrategy.set_stage() before calling %s" % method.__name__)
diff --git a/lib/spack/spack/hooks/sbang.py b/lib/spack/spack/hooks/sbang.py
new file mode 100644
index 0000000000..d78adb576e
--- /dev/null
+++ b/lib/spack/spack/hooks/sbang.py
@@ -0,0 +1,98 @@
+##############################################################################
+# Copyright (c) 2013-2015, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/llnl/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+import os
+
+from llnl.util.filesystem import *
+import llnl.util.tty as tty
+
+import spack
+import spack.modules
+
+# Character limit for shebang line.  Using Linux's 127 characters
+# here, as it is the shortest I could find on a modern OS.
+shebang_limit = 127
+
+def shebang_too_long(path):
+    """Detects whether a file has a shebang line that is too long."""
+    with open(path, 'r') as script:
+        bytes = script.read(2)
+        if bytes != '#!':
+            return False
+
+        line = bytes + script.readline()
+        return len(line) > shebang_limit
+
+
+def filter_shebang(path):
+    """Adds a second shebang line, using sbang, at the beginning of a file."""
+    with open(path, 'r') as original_file:
+        original = original_file.read()
+
+    # This line will be prepended to file
+    new_sbang_line = '#!/bin/bash %s/bin/sbang\n' % spack.spack_root
+
+    # Skip files that are already using sbang.
+    if original.startswith(new_sbang_line):
+        return
+
+    backup = path + ".shebang.bak"
+    os.rename(path, backup)
+
+    with open(path, 'w') as new_file:
+        new_file.write(new_sbang_line)
+        new_file.write(original)
+
+    copy_mode(backup, path)
+    unset_executable_mode(backup)
+
+    tty.warn("Patched overly long shebang in %s" % path)
+
+
+def filter_shebangs_in_directory(directory):
+    for file in os.listdir(directory):
+        path = os.path.join(directory, file)
+
+        # only handle files
+        if not os.path.isfile(path):
+            continue
+
+        # only handle links that resolve within THIS package's prefix.
+        if os.path.islink(path):
+            real_path = os.path.realpath(path)
+            if not real_path.startswith(directory + os.sep):
+                continue
+
+        # test the file for a long shebang, and filter
+        if shebang_too_long(path):
+            filter_shebang(path)
+
+
+def post_install(pkg):
+    """This hook edits scripts so that they call /bin/bash
+       $spack_prefix/bin/sbang instead of something longer than the
+       shebang limit."""
+    if not os.path.isdir(pkg.prefix.bin):
+        return
+    filter_shebangs_in_directory(pkg.prefix.bin)
diff --git a/lib/spack/spack/mirror.py b/lib/spack/spack/mirror.py
index 341cc4cb88..6981f69ac0 100644
--- a/lib/spack/spack/mirror.py
+++ b/lib/spack/spack/mirror.py
@@ -45,25 +45,31 @@ from spack.version import *
 from spack.util.compression import extension, allowed_archive
 
 
-def mirror_archive_filename(spec):
+def mirror_archive_filename(spec, fetcher):
     """Get the name of the spec's archive in the mirror."""
     if not spec.version.concrete:
         raise ValueError("mirror.path requires spec with concrete version.")
 
-    fetcher = spec.package.fetcher
     if isinstance(fetcher, fs.URLFetchStrategy):
-        # If we fetch this version with a URLFetchStrategy, use URL's archive type
-        ext = url.downloaded_file_extension(fetcher.url)
+        if fetcher.expand_archive:
+            # If we fetch this version with a URLFetchStrategy, use URL's archive type
+            ext = url.downloaded_file_extension(fetcher.url)
+        else:
+            # If the archive shouldn't be expanded, don't check for its extension.
+            ext = None
     else:
         # Otherwise we'll make a .tar.gz ourselves
         ext = 'tar.gz'
 
-    return "%s-%s.%s" % (spec.package.name, spec.version, ext)
+    filename = "%s-%s" % (spec.package.name, spec.version)
+    if ext:
+        filename += ".%s" % ext
+    return filename
 
 
-def mirror_archive_path(spec):
+def mirror_archive_path(spec, fetcher):
     """Get the relative path to the spec's archive within a mirror."""
-    return join_path(spec.name, mirror_archive_filename(spec))
+    return join_path(spec.name, mirror_archive_filename(spec, fetcher))
 
 
 def get_matching_versions(specs, **kwargs):
@@ -74,7 +80,7 @@ def get_matching_versions(specs, **kwargs):
 
         # Skip any package that has no known versions.
         if not pkg.versions:
-            tty.msg("No safe (checksummed) versions for package %s." % pkg.name)
+            tty.msg("No safe (checksummed) versions for package %s" % pkg.name)
             continue
 
         num_versions = kwargs.get('num_versions', 0)
@@ -111,7 +117,6 @@ def suggest_archive_basename(resource):
     return basename
 
 
-
 def create(path, specs, **kwargs):
     """Create a directory to be used as a spack mirror, and fill it with
        package archives.
@@ -159,82 +164,65 @@ def create(path, specs, **kwargs):
                 "Cannot create directory '%s':" % mirror_root, str(e))
 
     # Things to keep track of while parsing specs.
-    present  = []
-    mirrored = []
-    error    = []
+    categories = {
+        'present': [],
+        'mirrored': [],
+        'error': []
+    }
 
     # Iterate through packages and download all the safe tarballs for each of them
-    everything_already_exists = True
     for spec in version_specs:
-        pkg = spec.package
-
-        stage = None
-        try:
-            # create a subdirectory for the current package@version
-            archive_path = os.path.abspath(join_path(mirror_root, mirror_archive_path(spec)))
-            subdir = os.path.dirname(archive_path)
-            try:
+        add_single_spec(spec, mirror_root, categories, **kwargs)
+
+    return categories['present'], categories['mirrored'], categories['error']
+
+
+def add_single_spec(spec, mirror_root, categories, **kwargs):
+    tty.msg("Adding package {pkg} to mirror".format(pkg=spec.format("$_$@")))
+    spec_exists_in_mirror = True
+    try:
+        with spec.package.stage:
+            # fetcher = stage.fetcher
+            # fetcher.fetch()
+            # ...
+            # fetcher.archive(archive_path)
+            for ii, stage in enumerate(spec.package.stage):
+                fetcher = stage.fetcher
+                if ii == 0:
+                    # create a subdirectory for the current package@version
+                    archive_path = os.path.abspath(join_path(mirror_root, mirror_archive_path(spec, fetcher)))
+                    name = spec.format("$_$@")
+                else:
+                    resource = stage.resource
+                    archive_path = join_path(subdir, suggest_archive_basename(resource))
+                    name = "{resource} ({pkg}).".format(resource=resource.name, pkg=spec.format("$_$@"))
+                subdir = os.path.dirname(archive_path)
                 mkdirp(subdir)
-            except OSError as e:
-                raise MirrorError(
-                    "Cannot create directory '%s':" % subdir, str(e))
-
-            if os.path.exists(archive_path):
-                tty.msg("Already added %s" % spec.format("$_$@"))
-            else:
-                everything_already_exists = False
-                # Set up a stage and a fetcher for the download
-                unique_fetch_name = spec.format("$_$@")
-                fetcher = fs.for_package_version(pkg, pkg.version)
-                stage = Stage(fetcher, name=unique_fetch_name)
-                fetcher.set_stage(stage)
-
-                # Do the fetch and checksum if necessary
-                fetcher.fetch()
-                if not kwargs.get('no_checksum', False):
-                    fetcher.check()
-                    tty.msg("Checksum passed for %s@%s" % (pkg.name, pkg.version))
-
-                # Fetchers have to know how to archive their files.  Use
-                # that to move/copy/create an archive in the mirror.
-                fetcher.archive(archive_path)
-                tty.msg("Added %s." % spec.format("$_$@"))
-
-            # Fetch resources if they are associated with the spec
-            resources = pkg._get_resources()
-            for resource in resources:
-                resource_archive_path = join_path(subdir, suggest_archive_basename(resource))
-                if os.path.exists(resource_archive_path):
-                    tty.msg("Already added resource %s (%s@%s)." % (resource.name, pkg.name, pkg.version))
-                    continue
-                everything_already_exists = False
-                resource_stage_folder = pkg._resource_stage(resource)
-                resource_stage = Stage(resource.fetcher, name=resource_stage_folder)
-                resource.fetcher.set_stage(resource_stage)
-                resource.fetcher.fetch()
-                if not kwargs.get('no_checksum', False):
-                    resource.fetcher.check()
-                    tty.msg("Checksum passed for the resource %s (%s@%s)" % (resource.name, pkg.name, pkg.version))
-                resource.fetcher.archive(resource_archive_path)
-                tty.msg("Added resource %s (%s@%s)." % (resource.name, pkg.name, pkg.version))
-
-            if everything_already_exists:
-                present.append(spec)
-            else:
-                mirrored.append(spec)
-
-        except Exception, e:
-            if spack.debug:
-                sys.excepthook(*sys.exc_info())
-            else:
-                tty.warn("Error while fetching %s." % spec.format('$_$@'), e.message)
-            error.append(spec)
-
-        finally:
-            if stage:
-                stage.destroy()
-
-    return (present, mirrored, error)
+
+                if os.path.exists(archive_path):
+                    tty.msg("{name} : already added".format(name=name))
+                else:
+                    spec_exists_in_mirror = False
+                    fetcher.fetch()
+                    if not kwargs.get('no_checksum', False):
+                        fetcher.check()
+                        tty.msg("{name} : checksum passed".format(name=name))
+
+                    # Fetchers have to know how to archive their files.  Use
+                    # that to move/copy/create an archive in the mirror.
+                    fetcher.archive(archive_path)
+                    tty.msg("{name} : added".format(name=name))
+
+        if spec_exists_in_mirror:
+            categories['present'].append(spec)
+        else:
+            categories['mirrored'].append(spec)
+    except Exception as e:
+        if spack.debug:
+            sys.excepthook(*sys.exc_info())
+        else:
+            tty.warn("Error while fetching %s" % spec.format('$_$@'), e.message)
+        categories['error'].append(spec)
 
 
 class MirrorError(spack.error.SpackError):
diff --git a/lib/spack/spack/modules.py b/lib/spack/spack/modules.py
index 7036626e29..c27043db8c 100644
--- a/lib/spack/spack/modules.py
+++ b/lib/spack/spack/modules.py
@@ -33,6 +33,7 @@ number of directories to be appended to paths in the user's environment:
 
   * /bin directories to be appended to PATH
   * /lib* directories for LD_LIBRARY_PATH
+  * /include directories for CPATH
   * /man* and /share/man* directories for MANPATH
   * the package prefix for CMAKE_PREFIX_PATH
 
@@ -121,6 +122,7 @@ class EnvModule(object):
                     ('LIBRARY_PATH', self.spec.prefix.lib64),
                     ('LD_LIBRARY_PATH', self.spec.prefix.lib),
                     ('LD_LIBRARY_PATH', self.spec.prefix.lib64),
+                    ('CPATH', self.spec.prefix.include),
                     ('PKG_CONFIG_PATH', join_path(self.spec.prefix.lib, 'pkgconfig')),
                     ('PKG_CONFIG_PATH', join_path(self.spec.prefix.lib64, 'pkgconfig'))]:
 
@@ -194,12 +196,14 @@ class Dotkit(EnvModule):
     @property
     def file_name(self):
         return join_path(Dotkit.path, self.spec.architecture,
-                         self.spec.format('$_$@$%@$+$#.dk'))
+                         '%s.dk' % self.use_name)
 
     @property
     def use_name(self):
-        return self.spec.format('$_$@$%@$+$#')
-
+      return "%s-%s-%s-%s-%s" % (self.spec.name, self.spec.version,
+                                 self.spec.compiler.name,
+                                 self.spec.compiler.version, 
+                                 self.spec.dag_hash())
 
     def _write(self, dk_file):
         # Category
@@ -235,7 +239,10 @@ class TclModule(EnvModule):
 
     @property
     def use_name(self):
-        return self.spec.format('$_$@$%@$+$#')
+      return "%s-%s-%s-%s-%s" % (self.spec.name, self.spec.version,
+                                 self.spec.compiler.name,
+                                 self.spec.compiler.version, 
+                                 self.spec.dag_hash())
 
 
     def _write(self, m_file):
diff --git a/lib/spack/spack/multimethod.py b/lib/spack/spack/multimethod.py
index df9b9b2ab1..3cd17e796a 100644
--- a/lib/spack/spack/multimethod.py
+++ b/lib/spack/spack/multimethod.py
@@ -138,7 +138,7 @@ class when(object):
        methods like install() that depend on the package's spec.
        For example:
 
-       .. code-block::
+       .. code-block:: python
 
           class SomePackage(Package):
               ...
@@ -163,26 +163,28 @@ class when(object):
        if you only have part of the install that is platform specific, you
        could do this:
 
-       class SomePackage(Package):
-           ...
-           # virtual dependence on MPI.
-           # could resolve to mpich, mpich2, OpenMPI
-           depends_on('mpi')
+       .. code-block:: python
 
-           def setup(self):
-               # do nothing in the default case
-               pass
+          class SomePackage(Package):
+              ...
+              # virtual dependence on MPI.
+              # could resolve to mpich, mpich2, OpenMPI
+              depends_on('mpi')
+
+              def setup(self):
+                  # do nothing in the default case
+                  pass
 
-           @when('^openmpi')
-           def setup(self):
-               # do something special when this is built with OpenMPI for
-               # its MPI implementations.
+              @when('^openmpi')
+              def setup(self):
+                  # do something special when this is built with OpenMPI for
+                  # its MPI implementations.
 
 
-           def install(self, prefix):
-               # Do common install stuff
-               self.setup()
-               # Do more common install stuff
+              def install(self, prefix):
+                  # Do common install stuff
+                  self.setup()
+                  # Do more common install stuff
 
        There must be one (and only one) @when clause that matches the
        package's spec.  If there is more than one, or if none match,
@@ -193,10 +195,11 @@ class when(object):
        platform-specific versions.  There's not much we can do to get
        around this because of the way decorators work.
     """
-class when(object):
     def __init__(self, spec):
         pkg = get_calling_module_name()
-        self.spec = parse_anonymous_spec(spec, pkg)
+        if spec is True:
+            spec = pkg
+        self.spec = parse_anonymous_spec(spec, pkg) if spec is not False else None
 
     def __call__(self, method):
         # Get the first definition of the method in the calling scope
@@ -207,7 +210,9 @@ class when(object):
         if not type(original_method) == SpecMultiMethod:
             original_method = SpecMultiMethod(original_method)
 
-        original_method.register(self.spec, method)
+        if self.spec is not None:
+            original_method.register(self.spec, method)
+
         return original_method
 
 
diff --git a/lib/spack/spack/package.py b/lib/spack/spack/package.py
index 8cb947c276..02fb3e5834 100644
--- a/lib/spack/spack/package.py
+++ b/lib/spack/spack/package.py
@@ -58,14 +58,16 @@ import spack.compilers
 import spack.mirror
 import spack.hooks
 import spack.directives
+import spack.repository
 import spack.build_environment
 import spack.url
 import spack.util.web
 import spack.fetch_strategy as fs
 from spack.version import *
-from spack.stage import Stage
+from spack.stage import Stage, ResourceStage, StageComposite
 from spack.util.compression import allowed_archive, extension
 from spack.util.executable import ProcessError
+from spack.util.environment import dump_environment
 
 """Allowed URL schemes for spack packages."""
 _ALLOWED_URL_SCHEMES = ["http", "https", "ftp", "file", "git"]
@@ -316,6 +318,17 @@ class Package(object):
     """Most packages are NOT extendable.  Set to True if you want extensions."""
     extendable = False
 
+    """List of prefix-relative file paths. If these do not exist after
+       install, or if they exist but are not files, sanity checks fail.
+    """
+    sanity_check_files = []
+
+    """List of prefix-relative directory paths. If these do not exist
+       after install, or if they exist but are not directories, sanity
+       checks will fail.
+    """
+    sanity_check_dirs = []
+
 
     def __init__(self, spec):
         # this determines how the package should be built.
@@ -433,23 +446,51 @@ class Package(object):
         return spack.url.substitute_version(self.nearest_url(version),
                                             self.url_version(version))
 
+    def _make_resource_stage(self, root_stage, fetcher, resource):
+        resource_stage_folder = self._resource_stage(resource)
+        resource_mirror = join_path(self.name, os.path.basename(fetcher.url))
+        stage = ResourceStage(resource.fetcher, root=root_stage, resource=resource,
+                              name=resource_stage_folder, mirror_path=resource_mirror)
+        return stage
+
+    def _make_root_stage(self, fetcher):
+        # Construct a mirror path (TODO: get this out of package.py)
+        mp = spack.mirror.mirror_archive_path(self.spec, fetcher)
+        # Construct a path where the stage should build..
+        s = self.spec
+        stage_name = "%s-%s-%s" % (s.name, s.version, s.dag_hash())
+        # Build the composite stage
+        stage = Stage(fetcher, mirror_path=mp, name=stage_name)
+        return stage
+
+    def _make_stage(self):
+        # Construct a composite stage on top of the composite FetchStrategy
+        composite_fetcher = self.fetcher
+        composite_stage = StageComposite()
+        resources = self._get_needed_resources()
+        for ii, fetcher in enumerate(composite_fetcher):
+            if ii == 0:
+                # Construct root stage first
+                stage = self._make_root_stage(fetcher)
+            else:
+                # Construct resource stage
+                resource = resources[ii - 1]  # ii == 0 is root!
+                stage = self._make_resource_stage(composite_stage[0], fetcher, resource)
+            # Append the item to the composite
+            composite_stage.append(stage)
+
+        # Create stage on first access.  Needed because fetch, stage,
+        # patch, and install can be called independently of each
+        # other, so `with self.stage:` in do_install isn't sufficient.
+        composite_stage.create()
+        return composite_stage
 
     @property
     def stage(self):
         if not self.spec.concrete:
             raise ValueError("Can only get a stage for a concrete package.")
-
         if self._stage is None:
-            # Construct a mirror path (TODO: get this out of package.py)
-            mp = spack.mirror.mirror_archive_path(self.spec)
-
-            # Construct a path where the stage should build..
-            s = self.spec
-            stage_name = "%s-%s-%s" % (s.name, s.version, s.dag_hash())
-
-            # Build the stage
-            self._stage = Stage(self.fetcher, mirror_path=mp, name=stage_name)
-
+            self._stage = self._make_stage()
         return self._stage
 
 
@@ -459,17 +500,27 @@ class Package(object):
         self._stage = stage
 
 
+    def _make_fetcher(self):
+        # Construct a composite fetcher that always contains at least
+        # one element (the root package). In case there are resources
+        # associated with the package, append their fetcher to the
+        # composite.
+        root_fetcher = fs.for_package_version(self, self.version)
+        fetcher = fs.FetchStrategyComposite()  # Composite fetcher
+        fetcher.append(root_fetcher)  # Root fetcher is always present
+        resources = self._get_needed_resources()
+        for resource in resources:
+            fetcher.append(resource.fetcher)
+        return fetcher
+
     @property
     def fetcher(self):
         if not self.spec.versions.concrete:
-            raise ValueError(
-                "Can only get a fetcher for a package with concrete versions.")
-
+            raise ValueError("Can only get a fetcher for a package with concrete versions.")
         if not self._fetcher:
-            self._fetcher = fs.for_package_version(self, self.version)
+            self._fetcher = self._make_fetcher()
         return self._fetcher
 
-
     @fetcher.setter
     def fetcher(self, f):
         self._fetcher = f
@@ -632,7 +683,7 @@ class Package(object):
 
 
     def do_fetch(self, mirror_only=False):
-        """Creates a stage directory and downloads the taball for this package.
+        """Creates a stage directory and downloads the tarball for this package.
            Working directory will be set to the stage directory.
         """
         if not self.spec.concrete:
@@ -654,24 +705,10 @@ class Package(object):
 
             if not ignore_checksum:
                 raise FetchError(
-                    "Will not fetch %s." % self.spec.format('$_$@'), checksum_msg)
+                    "Will not fetch %s" % self.spec.format('$_$@'), checksum_msg)
 
         self.stage.fetch(mirror_only)
 
-        ##########
-        # Fetch resources
-        resources = self._get_resources()
-        for resource in resources:
-            resource_stage_folder = self._resource_stage(resource)
-            # FIXME : works only for URLFetchStrategy
-            resource_mirror = join_path(self.name, os.path.basename(resource.fetcher.url))
-            resource_stage = Stage(resource.fetcher, name=resource_stage_folder, mirror_path=resource_mirror)
-            resource.fetcher.set_stage(resource_stage)
-            # Delegate to stage object to trigger mirror logic
-            resource_stage.fetch()
-            resource_stage.check()
-        ##########
-
         self._fetch_time = time.time() - start_time
 
         if spack.do_checksum and self.version in self.versions:
@@ -684,49 +721,8 @@ class Package(object):
         if not self.spec.concrete:
             raise ValueError("Can only stage concrete packages.")
 
-        def _expand_archive(stage, name=self.name):
-            archive_dir = stage.source_path
-            if not archive_dir:
-                stage.expand_archive()
-                tty.msg("Created stage in %s." % stage.path)
-            else:
-                tty.msg("Already staged %s in %s." % (name, stage.path))
-
         self.do_fetch(mirror_only)
-        _expand_archive(self.stage)
-
-        ##########
-        # Stage resources in appropriate path
-        resources = self._get_resources()
-        # TODO: this is to allow nested resources, a better solution would be
-        # good
-        for resource in sorted(resources, key=lambda res: len(res.destination)):
-            stage = resource.fetcher.stage
-            _expand_archive(stage, resource.name)
-            # Turn placement into a dict with relative paths
-            placement = os.path.basename(stage.source_path) if resource.placement is None else resource.placement
-            if not isinstance(placement, dict):
-                placement = {'': placement}
-            # Make the paths in the dictionary absolute and link
-            for key, value in placement.iteritems():
-                target_path = join_path(self.stage.source_path, resource.destination)
-                link_path = join_path(target_path, value)
-                source_path = join_path(stage.source_path, key)
-
-                try:
-                    os.makedirs(target_path)
-                except OSError as err:
-                    if err.errno == errno.EEXIST and os.path.isdir(target_path):
-                        pass
-                    else: raise
-
-                # NOTE: a reasonable fix for the TODO above might be to have
-                # these expand in place, but expand_archive does not offer
-                # this
-
-                if not os.path.exists(link_path):
-                    shutil.move(source_path, link_path)
-        ##########
+        self.stage.expand_archive()
         self.stage.chdir_to_source()
 
 
@@ -744,7 +740,7 @@ class Package(object):
 
         # If there are no patches, note it.
         if not self.patches and not has_patch_fun:
-            tty.msg("No patches needed for %s." % self.name)
+            tty.msg("No patches needed for %s" % self.name)
             return
 
         # Construct paths to special files in the archive dir used to
@@ -757,7 +753,7 @@ class Package(object):
         # If we encounter an archive that failed to patch, restage it
         # so that we can apply all the patches again.
         if os.path.isfile(bad_file):
-            tty.msg("Patching failed last time.  Restaging.")
+            tty.msg("Patching failed last time. Restaging.")
             self.stage.restage()
 
         self.stage.chdir_to_source()
@@ -767,7 +763,7 @@ class Package(object):
             tty.msg("Already patched %s" % self.name)
             return
         elif os.path.isfile(no_patches_file):
-            tty.msg("No patches needed for %s." % self.name)
+            tty.msg("No patches needed for %s" % self.name)
             return
 
         # Apply all the patches for specs that match this one
@@ -788,10 +784,10 @@ class Package(object):
         if has_patch_fun:
             try:
                 self.patch()
-                tty.msg("Ran patch() for %s." % self.name)
+                tty.msg("Ran patch() for %s" % self.name)
                 patched = True
             except:
-                tty.msg("patch() function failed for %s." % self.name)
+                tty.msg("patch() function failed for %s" % self.name)
                 touch(bad_file)
                 raise
 
@@ -822,12 +818,15 @@ class Package(object):
         mkdirp(self.prefix.man1)
 
 
-    def _get_resources(self):
+    def _get_needed_resources(self):
         resources = []
         # Select the resources that are needed for this build
         for when_spec, resource_list in self.resources.items():
             if when_spec in self.spec:
                 resources.extend(resource_list)
+        # Sorts the resources by the length of the string representing their destination. Since any nested resource
+        # must contain another resource's name in its path, it seems that should work
+        resources = sorted(resources, key=lambda res: len(res.destination))
         return resources
 
     def _resource_stage(self, resource):
@@ -846,7 +845,9 @@ class Package(object):
 
         Args:
         keep_prefix -- Keep install prefix on failure. By default, destroys it.
-        keep_stage  -- Keep stage on successful build. By default, destroys it.
+        keep_stage  -- By default, stage is destroyed only if there are no
+                       exceptions during build. Set to True to keep the stage
+                       even with exceptions.
         ignore_deps -- Do not install dependencies before installing this package.
         fake        -- Don't really build -- install fake stub files instead.
         skip_patch  -- Skip patch stage of build if True.
@@ -856,99 +857,103 @@ class Package(object):
         if not self.spec.concrete:
             raise ValueError("Can only install concrete packages.")
 
-        if os.path.exists(self.prefix):
-            tty.msg("%s is already installed in %s." % (self.name, self.prefix))
+        # No installation needed if package is external
+        if self.spec.external:
+            tty.msg("%s is externally installed in %s" % (self.name, self.spec.external))
+            return
+
+        # Ensure package is not already installed
+        if spack.install_layout.check_installed(self.spec):
+            tty.msg("%s is already installed in %s" % (self.name, self.prefix))
             return
 
         tty.msg("Installing %s" % self.name)
 
+        # First, install dependencies recursively.
         if not ignore_deps:
             self.do_install_dependencies(
                 keep_prefix=keep_prefix, keep_stage=keep_stage, ignore_deps=ignore_deps,
-                fake=fake, skip_patch=skip_patch, verbose=verbose,
-                make_jobs=make_jobs)
-
-        start_time = time.time()
-        if not fake:
-            if not skip_patch:
-                self.do_patch()
-            else:
-                self.do_stage()
-
-        # create the install directory.  The install layout
-        # handles this in case so that it can use whatever
-        # package naming scheme it likes.
-        spack.install_layout.create_install_directory(self.spec)
+                fake=fake, skip_patch=skip_patch, verbose=verbose, make_jobs=make_jobs)
 
-        def cleanup():
-            if not keep_prefix:
-                # If anything goes wrong, remove the install prefix
-                self.remove_prefix()
-            else:
-                tty.warn("Keeping install prefix in place despite error.",
-                         "Spack will think this package is installed." +
-                         "Manually remove this directory to fix:",
-                         self.prefix, wrap=True)
+        # Set parallelism before starting build.
+        self.make_jobs = make_jobs
 
+        # Then install the package itself.
+        def build_process():
+            """Forked for each build. Has its own process and python
+               module space set up by build_environment.fork()."""
+            start_time = time.time()
+            if not fake:
+                if not skip_patch:
+                    self.do_patch()
+                else:
+                    self.do_stage()
 
-        def real_work():
-            try:
-                tty.msg("Building %s." % self.name)
+            tty.msg("Building %s" % self.name)
 
+            self.stage.keep = keep_stage
+            with self.stage:
                 # Run the pre-install hook in the child process after
                 # the directory is created.
                 spack.hooks.pre_install(self)
 
-                # Set up process's build environment before running install.
                 if fake:
                     self.do_fake_install()
                 else:
                     # Do the real install in the source directory.
-                    self.stage.chdir_to_source()
-
-                    # This redirects I/O to a build log (and optionally to the terminal)
-                    log_path = join_path(os.getcwd(), 'spack-build.out')
-                    log_file = open(log_path, 'w')
-                    with log_output(log_file, verbose, sys.stdout.isatty(), True):
-                        self.install(self.spec, self.prefix)
+                     self.stage.chdir_to_source()
 
-                # Ensure that something was actually installed.
-                self._sanity_check_install()
+                     # Save the build environment in a file before building.
+                     env_path = join_path(os.getcwd(), 'spack-build.env')
 
-                # Move build log into install directory on success
-                if not fake:
-                    log_install_path = spack.install_layout.build_log_path(self.spec)
-                    install(log_path, log_install_path)
+                     try:
+                        # Redirect I/O to a build log (and optionally to the terminal)
+                        log_path = join_path(os.getcwd(), 'spack-build.out')
+                        log_file = open(log_path, 'w')
+                        with log_output(log_file, verbose, sys.stdout.isatty(), True):
+                            dump_environment(env_path)
+                            self.install(self.spec, self.prefix)
 
-                # On successful install, remove the stage.
-                if not keep_stage:
-                    self.stage.destroy()
+                     except ProcessError as e:
+                         # Annotate ProcessErrors with the location of the build log.
+                         e.build_log = log_path
+                         raise e
 
-                # Stop timer.
-                self._total_time = time.time() - start_time
-                build_time = self._total_time - self._fetch_time
+                     # Ensure that something was actually installed.
+                     self.sanity_check_prefix()
 
-                tty.msg("Successfully installed %s." % self.name,
-                        "Fetch: %s.  Build: %s.  Total: %s."
-                        % (_hms(self._fetch_time), _hms(build_time), _hms(self._total_time)))
-                print_pkg(self.prefix)
+                     # Copy provenance into the install directory on success
+                     log_install_path = spack.install_layout.build_log_path(self.spec)
+                     env_install_path = spack.install_layout.build_env_path(self.spec)
+                     packages_dir = spack.install_layout.build_packages_path(self.spec)
 
-            except ProcessError, e:
-                # Annotate with location of build log.
-                e.build_log = log_path
-                cleanup()
-                raise e
+                     install(log_path, log_install_path)
+                     install(env_path, env_install_path)
+                     dump_packages(self.spec, packages_dir)
 
-            except:
-                # other exceptions just clean up and raise.
-                cleanup()
-                raise
+            # Stop timer.
+            self._total_time = time.time() - start_time
+            build_time = self._total_time - self._fetch_time
 
-        # Set parallelism before starting build.
-        self.make_jobs = make_jobs
+            tty.msg("Successfully installed %s" % self.name,
+                    "Fetch: %s.  Build: %s.  Total: %s."
+                    % (_hms(self._fetch_time), _hms(build_time), _hms(self._total_time)))
+            print_pkg(self.prefix)
 
-        # Do the build.
-        spack.build_environment.fork(self, real_work)
+        try:
+            # Create the install prefix and fork the build process.
+            spack.install_layout.create_install_directory(self.spec)
+            spack.build_environment.fork(self, build_process)
+        except:
+            # remove the install prefix if anything went wrong during install.
+            if not keep_prefix:
+                self.remove_prefix()
+            else:
+                tty.warn("Keeping install prefix in place despite error.",
+                         "Spack will think this package is installed. " +
+                         "Manually remove this directory to fix:",
+                         self.prefix, wrap=True)
+            raise
 
         # note: PARENT of the build process adds the new package to
         # the database, so that we don't need to re-read from file.
@@ -958,7 +963,18 @@ class Package(object):
         spack.hooks.post_install(self)
 
 
-    def _sanity_check_install(self):
+    def sanity_check_prefix(self):
+        """This function checks whether install succeeded."""
+        def check_paths(path_list, filetype, predicate):
+            for path in path_list:
+                abs_path = os.path.join(self.prefix, path)
+                if not predicate(abs_path):
+                    raise InstallError("Install failed for %s. No such %s in prefix: %s"
+                                       % (self.name, filetype, path))
+
+        check_paths(self.sanity_check_files, 'file', os.path.isfile)
+        check_paths(self.sanity_check_dirs, 'directory', os.path.isdir)
+
         installed = set(os.listdir(self.prefix))
         installed.difference_update(spack.install_layout.hidden_file_paths)
         if not installed:
@@ -1035,7 +1051,7 @@ class Package(object):
         # Uninstalling in Spack only requires removing the prefix.
         self.remove_prefix()
         spack.installed_db.remove(self.spec)
-        tty.msg("Successfully uninstalled %s." % self.spec.short_spec)
+        tty.msg("Successfully uninstalled %s" % self.spec.short_spec)
 
         # Once everything else is done, run post install hooks
         spack.hooks.post_uninstall(self)
@@ -1082,7 +1098,7 @@ class Package(object):
         self.extendee_spec.package.activate(self, **self.extendee_args)
 
         spack.install_layout.add_extension(self.extendee_spec, self.spec)
-        tty.msg("Activated extension %s for %s."
+        tty.msg("Activated extension %s for %s"
                 % (self.spec.short_spec, self.extendee_spec.format("$_$@$+$%@")))
 
 
@@ -1134,7 +1150,7 @@ class Package(object):
         if self.activated:
             spack.install_layout.remove_extension(self.extendee_spec, self.spec)
 
-        tty.msg("Deactivated extension %s for %s."
+        tty.msg("Deactivated extension %s for %s"
                 % (self.spec.short_spec, self.extendee_spec.format("$_$@$+$%@")))
 
 
@@ -1162,8 +1178,7 @@ class Package(object):
 
     def do_clean(self):
         """Removes the package's build stage and source tarball."""
-        if os.path.exists(self.stage.path):
-            self.stage.destroy()
+        self.stage.destroy()
 
 
     def format_doc(self, **kwargs):
@@ -1202,7 +1217,7 @@ class Package(object):
         try:
             return spack.util.web.find_versions_of_archive(
                 *self.all_urls, list_url=self.list_url, list_depth=self.list_depth)
-        except spack.error.NoNetworkConnectionError, e:
+        except spack.error.NoNetworkConnectionError as e:
             tty.die("Package.fetch_versions couldn't connect to:",
                     e.url, e.message)
 
@@ -1220,8 +1235,8 @@ class Package(object):
 
     @property
     def rpath_args(self):
-        """Get the rpath args as a string, with -Wl,-rpath= for each element."""
-        return " ".join("-Wl,-rpath=%s" % p for p in self.rpath)
+        """Get the rpath args as a string, with -Wl,-rpath, for each element."""
+        return " ".join("-Wl,-rpath,%s" % p for p in self.rpath)
 
 
 def validate_package_url(url_string):
@@ -1234,6 +1249,52 @@ def validate_package_url(url_string):
         tty.die("Invalid file type in URL: '%s'" % url_string)
 
 
+def dump_packages(spec, path):
+    """Dump all package information for a spec and its dependencies.
+
+       This creates a package repository within path for every
+       namespace in the spec DAG, and fills the repos wtih package
+       files and patch files for every node in the DAG.
+    """
+    mkdirp(path)
+
+    # Copy in package.py files from any dependencies.
+    # Note that we copy them in as they are in the *install* directory
+    # NOT as they are in the repository, because we want a snapshot of
+    # how *this* particular build was done.
+    for node in spec.traverse():
+        if node is not spec:
+            # Locate the dependency package in the install tree and find
+            # its provenance information.
+            source = spack.install_layout.build_packages_path(node)
+            source_repo_root = join_path(source, node.namespace)
+
+            # There's no provenance installed for the source package.  Skip it.
+            # User can always get something current from the builtin repo.
+            if not os.path.isdir(source_repo_root):
+                continue
+
+            # Create a source repo and get the pkg directory out of it.
+            try:
+                source_repo = spack.repository.Repo(source_repo_root)
+                source_pkg_dir = source_repo.dirname_for_package_name(node.name)
+            except RepoError as e:
+                tty.warn("Warning: Couldn't copy in provenance for %s" % node.name)
+
+        # Create a destination repository
+        dest_repo_root = join_path(path, node.namespace)
+        if not os.path.exists(dest_repo_root):
+            spack.repository.create_repo(dest_repo_root)
+        repo = spack.repository.Repo(dest_repo_root)
+
+        # Get the location of the package in the dest repo.
+        dest_pkg_dir = repo.dirname_for_package_name(node.name)
+        if node is not spec:
+            install_tree(source_pkg_dir, dest_pkg_dir)
+        else:
+            spack.repo.dump_provenance(node, dest_pkg_dir)
+
+
 def print_pkg(message):
     """Outputs a message with a package icon."""
     from llnl.util.tty.color import cwrite
@@ -1284,7 +1345,7 @@ class PackageVersionError(PackageError):
     """Raised when a version URL cannot automatically be determined."""
     def __init__(self, version):
         super(PackageVersionError, self).__init__(
-            "Cannot determine a URL automatically for version %s." % version,
+            "Cannot determine a URL automatically for version %s" % version,
             "Please provide a url for this version in the package.py file.")
 
 
diff --git a/lib/spack/spack/preferred_packages.py b/lib/spack/spack/preferred_packages.py
new file mode 100644
index 0000000000..4d8526c75f
--- /dev/null
+++ b/lib/spack/spack/preferred_packages.py
@@ -0,0 +1,175 @@
+##############################################################################
+# Copyright (c) 2013, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://scalability-llnl.github.io/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+
+import spack
+from spack.version import *
+
+class PreferredPackages(object):
+    _default_order = {'compiler' : [ 'gcc', 'intel', 'clang', 'pgi', 'xlc' ] } # Arbitrary, but consistent
+
+    def __init__(self):
+        self.preferred = spack.config.get_config('packages')
+        self._spec_for_pkgname_cache = {}
+
+    # Given a package name, sort component (e.g, version, compiler, ...), and
+    # a second_key (used by providers), return the list
+    def _order_for_package(self, pkgname, component, second_key, test_all=True):
+        pkglist = [pkgname]
+        if test_all:
+            pkglist.append('all')
+        for pkg in pkglist:
+            order = self.preferred.get(pkg, {}).get(component, {})
+            if type(order) is dict:
+                order = order.get(second_key, {})
+            if not order:
+                continue
+            return [str(s).strip() for s in order]
+        return []
+
+
+    # A generic sorting function. Given a package name and sort
+    # component, return less-than-0, 0, or greater-than-0 if
+    # a is respectively less-than, equal to, or greater than b.
+    def _component_compare(self, pkgname, component, a, b, reverse_natural_compare, second_key):
+        if a is None:
+            return -1
+        if b is None:
+            return 1
+        orderlist = self._order_for_package(pkgname, component, second_key)
+        a_in_list = str(a) in orderlist
+        b_in_list = str(b) in orderlist
+        if a_in_list and not b_in_list:
+            return -1
+        elif b_in_list and not a_in_list:
+            return 1
+
+        cmp_a = None
+        cmp_b = None
+        reverse = None
+        if not a_in_list and not b_in_list:
+            cmp_a = a
+            cmp_b = b
+            reverse = -1 if reverse_natural_compare else 1
+        else:
+            cmp_a = orderlist.index(str(a))
+            cmp_b = orderlist.index(str(b))
+            reverse = 1
+
+        if cmp_a < cmp_b:
+            return -1 * reverse
+        elif cmp_a > cmp_b:
+            return 1 * reverse
+        else:
+            return 0
+
+
+    # A sorting function for specs.  Similar to component_compare, but
+    # a and b are considered to match entries in the sorting list if they
+    # satisfy the list component.
+    def _spec_compare(self, pkgname, component, a, b, reverse_natural_compare, second_key):
+        if not a or not a.concrete:
+            return -1
+        if not b or not b.concrete:
+            return 1
+        specs = self._spec_for_pkgname(pkgname, component, second_key)
+        a_index = None
+        b_index = None
+        reverse = -1 if reverse_natural_compare else 1
+        for i, cspec in enumerate(specs):
+            if a_index == None and (cspec.satisfies(a) or a.satisfies(cspec)):
+                a_index = i
+                if b_index:
+                    break
+            if b_index == None and (cspec.satisfies(b) or b.satisfies(cspec)):
+                b_index = i
+                if a_index:
+                    break
+
+        if   a_index != None and b_index == None: return -1
+        elif a_index == None and b_index != None: return 1
+        elif a_index != None and b_index == a_index: return -1 * cmp(a, b)
+        elif a_index != None and b_index != None and a_index != b_index: return cmp(a_index, b_index)
+        else: return cmp(a, b) * reverse
+
+
+
+    # Given a sort order specified by the pkgname/component/second_key, return
+    # a list of CompilerSpecs, VersionLists, or Specs for that sorting list.
+    def _spec_for_pkgname(self, pkgname, component, second_key):
+        key = (pkgname, component, second_key)
+        if not key in self._spec_for_pkgname_cache:
+            pkglist = self._order_for_package(pkgname, component, second_key)
+            if not pkglist:
+                if component in self._default_order:
+                    pkglist = self._default_order[component]
+            if component == 'compiler':
+                self._spec_for_pkgname_cache[key] = [spack.spec.CompilerSpec(s) for s in pkglist]
+            elif component == 'version':
+                self._spec_for_pkgname_cache[key] = [VersionList(s) for s in pkglist]
+            else:
+                self._spec_for_pkgname_cache[key] = [spack.spec.Spec(s) for s in pkglist]
+        return self._spec_for_pkgname_cache[key]
+
+
+    def provider_compare(self, pkgname, provider_str, a, b):
+        """Return less-than-0, 0, or greater than 0 if a is respecively less-than, equal-to, or
+           greater-than b. A and b are possible implementations of provider_str.
+           One provider is less-than another if it is preferred over the other.
+           For example, provider_compare('scorep', 'mpi', 'mvapich', 'openmpi') would return -1 if
+           mvapich should be preferred over openmpi for scorep."""
+        return self._spec_compare(pkgname, 'providers', a, b, False, provider_str)
+
+
+    def spec_has_preferred_provider(self, pkgname, provider_str):
+        """Return True iff the named package has a list of preferred provider"""
+        return bool(self._order_for_package(pkgname, 'providers', provider_str, False))
+
+
+    def version_compare(self, pkgname, a, b):
+        """Return less-than-0, 0, or greater than 0 if version a of pkgname is
+           respecively less-than, equal-to, or greater-than version b of pkgname.
+           One version is less-than another if it is preferred over the other."""
+        return self._spec_compare(pkgname, 'version', a, b, True, None)
+
+
+    def variant_compare(self, pkgname, a, b):
+        """Return less-than-0, 0, or greater than 0 if variant a of pkgname is
+           respecively less-than, equal-to, or greater-than variant b of pkgname.
+           One variant is less-than another if it is preferred over the other."""
+        return self._component_compare(pkgname, 'variant', a, b, False, None)
+
+
+    def architecture_compare(self, pkgname, a, b):
+        """Return less-than-0, 0, or greater than 0 if architecture a of pkgname is
+           respecively less-than, equal-to, or greater-than architecture b of pkgname.
+           One architecture is less-than another if it is preferred over the other."""
+        return self._component_compare(pkgname, 'architecture', a, b, False, None)
+
+
+    def compiler_compare(self, pkgname, a, b):
+        """Return less-than-0, 0, or greater than 0 if compiler a of pkgname is
+           respecively less-than, equal-to, or greater-than compiler b of pkgname.
+           One compiler is less-than another if it is preferred over the other."""
+        return self._spec_compare(pkgname, 'compiler', a, b, False, None)
diff --git a/lib/spack/spack/repository.py b/lib/spack/spack/repository.py
index f58cd52125..d2fdc937f7 100644
--- a/lib/spack/spack/repository.py
+++ b/lib/spack/spack/repository.py
@@ -6,7 +6,7 @@
 # Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
 # LLNL-CODE-647188
 #
-# For details, see https://llnl.github.io/spack
+# For details, see https://software.llnl.gov/spack
 # Please also see the LICENSE file for our notice and the LGPL.
 #
 # This program is free software; you can redistribute it and/or modify
@@ -33,7 +33,7 @@ from bisect import bisect_left
 from external import yaml
 
 import llnl.util.tty as tty
-from llnl.util.filesystem import join_path
+from llnl.util.filesystem import *
 
 import spack.error
 import spack.config
@@ -156,7 +156,7 @@ class RepoPath(object):
 
         if repo.namespace in self.by_namespace:
             raise DuplicateRepoError(
-                "Package repos '%s' and '%s' both provide namespace %s."
+                "Package repos '%s' and '%s' both provide namespace %s"
                 % (repo.root, self.by_namespace[repo.namespace].root, repo.namespace))
 
         # Add repo to the pkg indexes
@@ -316,6 +316,21 @@ class RepoPath(object):
         return self.repo_for_pkg(spec).get(spec)
 
 
+    def get_pkg_class(self, pkg_name):
+        """Find a class for the spec's package and return the class object."""
+        return self.repo_for_pkg(pkg_name).get_pkg_class(pkg_name)
+
+
+    @_autospec
+    def dump_provenance(self, spec, path):
+        """Dump provenance information for a spec to a particular path.
+
+           This dumps the package file and any associated patch files.
+           Raises UnknownPackageError if not found.
+        """
+        return self.repo_for_pkg(spec).dump_provenance(spec, path)
+
+
     def dirname_for_package_name(self, pkg_name):
         return self.repo_for_pkg(pkg_name).dirname_for_package_name(pkg_name)
 
@@ -535,12 +550,12 @@ class Repo(object):
             raise UnknownPackageError(spec.name)
 
         if spec.namespace and spec.namespace != self.namespace:
-            raise UnknownPackageError("Repository %s does not contain package %s."
+            raise UnknownPackageError("Repository %s does not contain package %s"
                                       % (self.namespace, spec.fullname))
 
         key = hash(spec)
         if new or key not in self._instances:
-            package_class = self._get_pkg_class(spec.name)
+            package_class = self.get_pkg_class(spec.name)
             try:
                 copy = spec.copy() # defensive copy.  Package owns its spec.
                 self._instances[key] = package_class(copy)
@@ -552,6 +567,35 @@ class Repo(object):
         return self._instances[key]
 
 
+    @_autospec
+    def dump_provenance(self, spec, path):
+        """Dump provenance information for a spec to a particular path.
+
+           This dumps the package file and any associated patch files.
+           Raises UnknownPackageError if not found.
+        """
+        # Some preliminary checks.
+        if spec.virtual:
+            raise UnknownPackageError(spec.name)
+
+        if spec.namespace and spec.namespace != self.namespace:
+            raise UnknownPackageError("Repository %s does not contain package %s."
+                                      % (self.namespace, spec.fullname))
+
+        # Install any patch files needed by packages.
+        mkdirp(path)
+        for spec, patches in spec.package.patches.items():
+            for patch in patches:
+                if patch.path:
+                    if os.path.exists(patch.path):
+                        install(patch.path, path)
+                    else:
+                        tty.warn("Patch file did not exist: %s" % patch.path)
+
+        # Install the package.py file itself.
+        install(self.filename_for_package_name(spec), path)
+
+
     def purge(self):
         """Clear entire package instance cache."""
         self._instances.clear()
@@ -676,7 +720,7 @@ class Repo(object):
         return self._modules[pkg_name]
 
 
-    def _get_pkg_class(self, pkg_name):
+    def get_pkg_class(self, pkg_name):
         """Get the class for the package out of its module.
 
         First loads (or fetches from cache) a module for the
@@ -705,6 +749,58 @@ class Repo(object):
         return self.exists(pkg_name)
 
 
+def create_repo(root, namespace=None):
+    """Create a new repository in root with the specified namespace.
+
+       If the namespace is not provided, use basename of root.
+       Return the canonicalized path and the namespace of the created repository.
+    """
+    root = canonicalize_path(root)
+    if not namespace:
+        namespace = os.path.basename(root)
+
+    if not re.match(r'\w[\.\w-]*', namespace):
+        raise InvalidNamespaceError("'%s' is not a valid namespace." % namespace)
+
+    existed = False
+    if os.path.exists(root):
+        if os.path.isfile(root):
+            raise BadRepoError('File %s already exists and is not a directory' % root)
+        elif os.path.isdir(root):
+            if not os.access(root, os.R_OK | os.W_OK):
+                raise BadRepoError('Cannot create new repo in %s: cannot access directory.' % root)
+            if os.listdir(root):
+                raise BadRepoError('Cannot create new repo in %s: directory is not empty.' % root)
+        existed = True
+
+    full_path = os.path.realpath(root)
+    parent = os.path.dirname(full_path)
+    if not os.access(parent, os.R_OK | os.W_OK):
+        raise BadRepoError("Cannot create repository in %s: can't access parent!" % root)
+
+    try:
+        config_path = os.path.join(root, repo_config_name)
+        packages_path = os.path.join(root, packages_dir_name)
+
+        mkdirp(packages_path)
+        with open(config_path, 'w') as config:
+            config.write("repo:\n")
+            config.write("  namespace: '%s'\n" % namespace)
+
+    except (IOError, OSError) as e:
+        raise BadRepoError('Failed to create new repository in %s.' % root,
+                           "Caused by %s: %s" % (type(e), e))
+
+        # try to clean up.
+        if existed:
+            shutil.rmtree(config_path, ignore_errors=True)
+            shutil.rmtree(packages_path, ignore_errors=True)
+        else:
+            shutil.rmtree(root, ignore_errors=True)
+
+    return full_path, namespace
+
+
 class RepoError(spack.error.SpackError):
     """Superclass for repository-related errors."""
 
@@ -713,6 +809,10 @@ class NoRepoConfiguredError(RepoError):
     """Raised when there are no repositories configured."""
 
 
+class InvalidNamespaceError(RepoError):
+    """Raised when an invalid namespace is encountered."""
+
+
 class BadRepoError(RepoError):
     """Raised when repo layout is invalid."""
 
@@ -730,7 +830,7 @@ class UnknownPackageError(PackageLoadError):
     def __init__(self, name, repo=None):
         msg = None
         if repo:
-            msg = "Package %s not found in repository %s." % (name, repo)
+            msg = "Package %s not found in repository %s" % (name, repo)
         else:
             msg = "Package %s not found." % name
         super(UnknownPackageError, self).__init__(msg)
diff --git a/lib/spack/spack/resource.py b/lib/spack/spack/resource.py
index 2bf92947fd..ddfaaf4cb0 100644
--- a/lib/spack/spack/resource.py
+++ b/lib/spack/spack/resource.py
@@ -6,7 +6,7 @@
 # Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
 # LLNL-CODE-647188
 #
-# For details, see https://llnl.github.io/spack
+# For details, see https://software.llnl.gov/spack
 # Please also see the LICENSE file for our notice and the LGPL.
 #
 # This program is free software; you can redistribute it and/or modify
diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 10e246bf2e..d04135860e 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -353,7 +353,7 @@ class VariantMap(HashableMap):
     @property
     def concrete(self):
         return self.spec._concrete or all(
-            v in self for v in self.spec.package.variants)
+            v in self for v in self.spec.package_class.variants)
 
 
     def copy(self):
@@ -418,9 +418,12 @@ class Spec(object):
         # cases we've read them from a file want to assume normal.
         # This allows us to manipulate specs that Spack doesn't have
         # package.py files for.
-        self._normal = kwargs.get('normal', False)
+        self._normal   = kwargs.get('normal', False)
         self._concrete = kwargs.get('concrete', False)
 
+        # Allow a spec to be constructed with an external path.
+        self.external  = kwargs.get('external', None)
+
         # This allows users to construct a spec DAG with literals.
         # Note that given two specs a and b, Spec(a) copies a, but
         # Spec(a, b) will copy a but just add b as a dep.
@@ -498,6 +501,14 @@ class Spec(object):
 
 
     @property
+    def package_class(self):
+        """Internal package call gets only the class object for a package.
+           Use this to just get package metadata.
+        """
+        return spack.repo.get_pkg_class(self.name)
+
+
+    @property
     def virtual(self):
         """Right now, a spec is virtual if no package exists with its name.
 
@@ -770,12 +781,11 @@ class Spec(object):
             # Concretize virtual dependencies last.  Because they're added
             # to presets below, their constraints will all be merged, but we'll
             # still need to select a concrete package later.
-            if not self.virtual:
-                changed |= any(
-                    (spack.concretizer.concretize_architecture(self),
-                     spack.concretizer.concretize_compiler(self),
-                     spack.concretizer.concretize_version(self),
-                     spack.concretizer.concretize_variants(self)))
+            changed |= any(
+                (spack.concretizer.concretize_architecture(self),
+                 spack.concretizer.concretize_compiler(self),
+                 spack.concretizer.concretize_version(self),
+                 spack.concretizer.concretize_variants(self)))
             presets[self.name] = self
 
         visited.add(self.name)
@@ -786,8 +796,30 @@ class Spec(object):
         """Replace this virtual spec with a concrete spec."""
         assert(self.virtual)
         for name, dependent in self.dependents.items():
+            # remove self from all dependents.
+            del dependent.dependencies[self.name]
+
+            # add the replacement, unless it is already a dep of dependent.
+            if concrete.name not in dependent.dependencies:
+                dependent._add_dependency(concrete)
+
+
+    def _replace_node(self, replacement):
+        """Replace this spec with another.
+
+        Connects all dependents of this spec to its replacement, and
+        disconnects this spec from any dependencies it has. New spec
+        will have any dependencies the replacement had, and may need
+        to be normalized.
+
+        """
+        for name, dependent in self.dependents.items():
             del dependent.dependencies[self.name]
-            dependent._add_dependency(concrete)
+            dependent._add_dependency(replacement)
+
+        for name, dep in self.dependencies.items():
+            del dep.dependents[self.name]
+            del self.dependencies[dep.name]
 
 
     def _expand_virtual_packages(self):
@@ -807,22 +839,81 @@ class Spec(object):
               this are infrequent, but should implement this before it is
               a problem.
         """
+        # Make an index of stuff this spec already provides
+        self_index = ProviderIndex(self.traverse(), restrict=True)
+
         changed = False
-        while True:
-            virtuals =[v for v in self.traverse() if v.virtual]
-            if not virtuals:
-                return changed
+        done = False
+        while not done:
+            done = True
+            for spec in list(self.traverse()):
+                replacement = None
+                if spec.virtual:
+                    replacement = self._find_provider(spec, self_index)
+                    if replacement:
+                        # TODO: may break if in-place on self but
+                        # shouldn't happen if root is traversed first.
+                        spec._replace_with(replacement)
+                        done=False
+                        break
+
+                if not replacement:
+                    # Get a list of possible replacements in order of preference.
+                    candidates = spack.concretizer.choose_virtual_or_external(spec)
+
+                    # Try the replacements in order, skipping any that cause
+                    # satisfiability problems.
+                    for replacement in candidates:
+                        if replacement is spec:
+                            break
+
+                        # Replace spec with the candidate and normalize
+                        copy = self.copy()
+                        copy[spec.name]._dup(replacement.copy(deps=False))
+
+                        try:
+                            # If there are duplicate providers or duplicate provider
+                            # deps, consolidate them and merge constraints.
+                            copy.normalize(force=True)
+                            break
+                        except SpecError as e:
+                            # On error, we'll try the next replacement.
+                            continue
+
+                # If replacement is external then trim the dependencies
+                if replacement.external:
+                    if (spec.dependencies):
+                        changed = True
+                        spec.dependencies = DependencyMap()
+                    replacement.dependencies = DependencyMap()
+
+                # TODO: could this and the stuff in _dup be cleaned up?
+                def feq(cfield, sfield):
+                    return (not cfield) or (cfield == sfield)
+
+                if replacement is spec or (feq(replacement.name, spec.name) and
+                    feq(replacement.versions, spec.versions) and
+                    feq(replacement.compiler, spec.compiler) and
+                    feq(replacement.architecture, spec.architecture) and
+                    feq(replacement.dependencies, spec.dependencies) and
+                    feq(replacement.variants, spec.variants) and
+                    feq(replacement.external, spec.external)):
+                    continue
+
+                # Refine this spec to the candidate. This uses
+                # replace_with AND dup so that it can work in
+                # place. TODO: make this more efficient.
+                if spec.virtual:
+                    spec._replace_with(replacement)
+                    changed = True
+                if spec._dup(replacement, deps=False, cleardeps=False):
+                    changed = True
 
-            for spec in virtuals:
-                providers = spack.repo.providers_for(spec)
-                concrete = spack.concretizer.choose_provider(spec, providers)
-                concrete = concrete.copy()
-                spec._replace_with(concrete)
-                changed = True
+                self_index.update(spec)
+                done=False
+                break
 
-            # If there are duplicate providers or duplicate provider deps, this
-            # consolidates them and merge constraints.
-            changed |= self.normalize(force=True)
+        return changed
 
 
     def concretize(self):
@@ -837,6 +928,7 @@ class Spec(object):
            with requirements of its pacakges.  See flatten() and normalize() for
            more details on this.
         """
+
         if self._concrete:
             return
 
@@ -844,7 +936,7 @@ class Spec(object):
         force = False
 
         while changed:
-            changes = (self.normalize(force=force),
+            changes = (self.normalize(force),
                        self._expand_virtual_packages(),
                        self._concretize_helper())
             changed = any(changes)
@@ -970,8 +1062,8 @@ class Spec(object):
 
     def _find_provider(self, vdep, provider_index):
         """Find provider for a virtual spec in the provider index.
-        Raise an exception if there is a conflicting virtual
-        dependency already in this spec.
+           Raise an exception if there is a conflicting virtual
+           dependency already in this spec.
         """
         assert(vdep.virtual)
         providers = provider_index.providers_for(vdep)
@@ -1012,17 +1104,14 @@ class Spec(object):
         """
         changed = False
 
-        # If it's a virtual dependency, try to find a provider and
-        # merge that.
+        # If it's a virtual dependency, try to find an existing
+        # provider in the spec, and merge that.
         if dep.virtual:
             visited.add(dep.name)
             provider = self._find_provider(dep, provider_index)
             if provider:
                 dep = provider
-
         else:
-            # if it's a real dependency, check whether it provides
-            # something already required in the spec.
             index = ProviderIndex([dep], restrict=True)
             for vspec in (v for v in spec_deps.values() if v.virtual):
                 if index.providers_for(vspec):
@@ -1069,7 +1158,7 @@ class Spec(object):
 
         # if we descend into a virtual spec, there's nothing more
         # to normalize.  Concretize will finish resolving it later.
-        if self.virtual:
+        if self.virtual or self.external:
             return False
 
         # Combine constraints from package deps with constraints from
@@ -1119,13 +1208,14 @@ class Spec(object):
         # Get all the dependencies into one DependencyMap
         spec_deps = self.flat_dependencies(copy=False)
 
-        # Initialize index of virtual dependency providers
-        index = ProviderIndex(spec_deps.values(), restrict=True)
+        # Initialize index of virtual dependency providers if
+        # concretize didn't pass us one already
+        provider_index = ProviderIndex(spec_deps.values(), restrict=True)
 
         # traverse the package DAG and fill out dependencies according
         # to package files & their 'when' specs
         visited = set()
-        any_change = self._normalize_helper(visited, spec_deps, index)
+        any_change = self._normalize_helper(visited, spec_deps, provider_index)
 
         # If there are deps specified but not visited, they're not
         # actually deps of this package.  Raise an error.
@@ -1163,7 +1253,7 @@ class Spec(object):
 
             # Ensure that variants all exist.
             for vname, variant in spec.variants.items():
-                if vname not in spec.package.variants:
+                if vname not in spec.package_class.variants:
                     raise UnknownVariantError(spec.name, vname)
 
 
@@ -1404,15 +1494,25 @@ class Spec(object):
                Whether deps should be copied too.  Set to false to copy a
                spec but not its dependencies.
         """
+        # We don't count dependencies as changes here
+        changed = True
+        if hasattr(self, 'name'):
+            changed = (self.name != other.name and self.versions != other.versions and
+                       self.architecture != other.architecture and self.compiler != other.compiler and
+                       self.variants != other.variants and self._normal != other._normal and
+                       self.concrete != other.concrete and self.external != other.external)
+
         # Local node attributes get copied first.
         self.name = other.name
         self.versions = other.versions.copy()
         self.architecture = other.architecture
         self.compiler = other.compiler.copy() if other.compiler else None
-        self.dependents = DependencyMap()
-        self.dependencies = DependencyMap()
+        if kwargs.get('cleardeps', True):
+            self.dependents = DependencyMap()
+            self.dependencies = DependencyMap()
         self.variants = other.variants.copy()
         self.variants.spec = self
+        self.external = other.external
         self.namespace = other.namespace
 
         # If we copy dependencies, preserve DAG structure in the new spec
@@ -1431,6 +1531,8 @@ class Spec(object):
         # Since we preserved structure, we can copy _normal safely.
         self._normal = other._normal
         self._concrete = other._concrete
+        self.external = other.external
+        return changed
 
 
     def copy(self, **kwargs):
@@ -1571,14 +1673,28 @@ class Spec(object):
 
                $_   Package name
                $.   Full package name (with namespace)
-               $@   Version
-               $%   Compiler
-               $%@  Compiler & compiler version
+               $@   Version with '@' prefix
+               $%   Compiler with '%' prefix
+               $%@  Compiler with '%' prefix & compiler version with '@' prefix
                $+   Options
-               $=   Architecture
-               $#   7-char prefix of DAG hash
+               $=   Architecture with '=' prefix
+               $#   7-char prefix of DAG hash with '-' prefix
                $$   $
 
+               You can also use full-string versions, which leave off the prefixes:
+
+               ${PACKAGE}       Package name
+               ${VERSION}       Version
+               ${COMPILER}      Full compiler string
+               ${COMPILERNAME}  Compiler name
+               ${COMPILERVER}   Compiler version
+               ${OPTIONS}       Options
+               ${ARCHITECTURE}  Architecture
+               ${SHA1}          Dependencies 8-char sha1 prefix
+
+               ${SPACK_ROOT}    The spack root directory
+               ${SPACK_INSTALL} The default spack install directory, ${SPACK_PREFIX}/opt
+
            Optionally you can provide a width, e.g. $20_ for a 20-wide name.
            Like printf, you can provide '-' for left justification, e.g.
            $-20_ for a left-justified name.
@@ -1594,7 +1710,8 @@ class Spec(object):
         color    = kwargs.get('color', False)
         length = len(format_string)
         out = StringIO()
-        escape = compiler = False
+        named = escape = compiler = False
+        named_str = fmt = ''
 
         def write(s, c):
             if color:
@@ -1636,9 +1753,12 @@ class Spec(object):
                 elif c == '#':
                     out.write('-' + fmt % (self.dag_hash(7)))
                 elif c == '$':
-                    if fmt != '':
+                    if fmt != '%s':
                         raise ValueError("Can't use format width with $$.")
                     out.write('$')
+                elif c == '{':
+                    named = True
+                    named_str = ''
                 escape = False
 
             elif compiler:
@@ -1652,6 +1772,43 @@ class Spec(object):
                     out.write(c)
                 compiler = False
 
+            elif named:
+                if not c == '}':
+                    if i == length - 1:
+                        raise ValueError("Error: unterminated ${ in format: '%s'"
+                                         % format_string)
+                    named_str += c
+                    continue;
+                if named_str == 'PACKAGE':
+                    write(fmt % self.name, '@')
+                if named_str == 'VERSION':
+                    if self.versions and self.versions != _any_version:
+                        write(fmt % str(self.versions), '@')
+                elif named_str == 'COMPILER':
+                    if self.compiler:
+                        write(fmt % self.compiler, '%')
+                elif named_str == 'COMPILERNAME':
+                    if self.compiler:
+                        write(fmt % self.compiler.name, '%')
+                elif named_str == 'COMPILERVER':
+                    if self.compiler:
+                        write(fmt % self.compiler.versions, '%')
+                elif named_str == 'OPTIONS':
+                    if self.variants:
+                        write(fmt % str(self.variants), '+')
+                elif named_str == 'ARCHITECTURE':
+                    if self.architecture:
+                        write(fmt % str(self.architecture), '=')
+                elif named_str == 'SHA1':
+                    if self.dependencies:
+                        out.write(fmt % str(self.dag_hash(7)))
+                elif named_str == 'SPACK_ROOT':
+                    out.write(fmt % spack.prefix)
+                elif named_str == 'SPACK_INSTALL':
+                    out.write(fmt % spack.install_path)
+
+                named = False
+
             elif c == '$':
                 escape = True
                 if i == length - 1:
@@ -1782,6 +1939,7 @@ class SpecParser(spack.parse.Parser):
         spec.variants = VariantMap(spec)
         spec.architecture = None
         spec.compiler = None
+        spec.external = None
         spec.dependents   = DependencyMap()
         spec.dependencies = DependencyMap()
         spec.namespace = spec_namespace
diff --git a/lib/spack/spack/stage.py b/lib/spack/spack/stage.py
index 79c9030e20..f88f82fc2d 100644
--- a/lib/spack/spack/stage.py
+++ b/lib/spack/spack/stage.py
@@ -23,7 +23,7 @@
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
 import os
-import re
+import errno
 import shutil
 import tempfile
 from urlparse import urljoin
@@ -31,46 +31,65 @@ from urlparse import urljoin
 import llnl.util.tty as tty
 from llnl.util.filesystem import *
 
+import spack.util.pattern as pattern
+
 import spack
 import spack.config
 import spack.fetch_strategy as fs
 import spack.error
 
-
 STAGE_PREFIX = 'spack-stage-'
 
 
 class Stage(object):
-    """A Stage object manaages a directory where some source code is
-       downloaded and built before being installed.  It handles
-       fetching the source code, either as an archive to be expanded
-       or by checking it out of a repository.  A stage's lifecycle
-       looks like this:
-
-       Stage()
-         Constructor creates the stage directory.
-       fetch()
-         Fetch a source archive into the stage.
-       expand_archive()
-         Expand the source archive.
-       <install>
-         Build and install the archive.  This is handled by the Package class.
-       destroy()
-         Remove the stage once the package has been installed.
-
-       If spack.use_tmp_stage is True, spack will attempt to create stages
-       in a tmp directory.  Otherwise, stages are created directly in
-       spack.stage_path.
-
-       There are two kinds of stages: named and unnamed.  Named stages can
-       persist between runs of spack, e.g. if you fetched a tarball but
-       didn't finish building it, you won't have to fetch it again.
-
-       Unnamed stages are created using standard mkdtemp mechanisms or
-       similar, and are intended to persist for only one run of spack.
+    """Manages a temporary stage directory for building.
+
+    A Stage object is a context manager that handles a directory where
+    some source code is downloaded and built before being installed.
+    It handles fetching the source code, either as an archive to be
+    expanded or by checking it out of a repository.  A stage's
+    lifecycle looks like this:
+
+    ```
+    with Stage() as stage:      # Context manager creates and destroys the stage directory
+        stage.fetch()           # Fetch a source archive into the stage.
+        stage.expand_archive()  # Expand the source archive.
+        <install>               # Build and install the archive. (handled by user of Stage)
+    ```
+
+    When used as a context manager, the stage is automatically
+    destroyed if no exception is raised by the context. If an
+    excpetion is raised, the stage is left in the filesystem and NOT
+    destroyed, for potential reuse later.
+
+    You can also use the stage's create/destroy functions manually,
+    like this:
+
+    ```
+    stage = Stage()
+    try:
+        stage.create()          # Explicitly create the stage directory.
+        stage.fetch()           # Fetch a source archive into the stage.
+        stage.expand_archive()  # Expand the source archive.
+        <install>               # Build and install the archive. (handled by user of Stage)
+    finally:
+        stage.destroy()         # Explicitly destroy the stage directory.
+    ```
+
+    If spack.use_tmp_stage is True, spack will attempt to create
+    stages in a tmp directory.  Otherwise, stages are created directly
+    in spack.stage_path.
+
+    There are two kinds of stages: named and unnamed.  Named stages
+    can persist between runs of spack, e.g. if you fetched a tarball
+    but didn't finish building it, you won't have to fetch it again.
+
+    Unnamed stages are created using standard mkdtemp mechanisms or
+    similar, and are intended to persist for only one run of spack.
     """
 
-    def __init__(self, url_or_fetch_strategy, **kwargs):
+    def __init__(self, url_or_fetch_strategy,
+                 name=None, mirror_path=None, keep=False):
         """Create a stage object.
            Parameters:
              url_or_fetch_strategy
@@ -82,6 +101,17 @@ class Stage(object):
                  and will persist between runs (or if you construct another
                  stage object later).  If name is not provided, then this
                  stage will be given a unique name automatically.
+
+             mirror_path
+                 If provided, Stage will search Spack's mirrors for
+                 this archive at the mirror_path, before using the
+                 default fetch strategy.
+
+             keep
+                 By default, when used as a context manager, the Stage
+                 is deleted on exit when no exceptions are raised.
+                 Pass True to keep the stage intact even if no
+                 exceptions are raised.
         """
         # TODO: fetch/stage coupling needs to be reworked -- the logic
         # TODO: here is convoluted and not modular enough.
@@ -93,24 +123,52 @@ class Stage(object):
             raise ValueError("Can't construct Stage without url or fetch strategy")
         self.fetcher.set_stage(self)
         self.default_fetcher = self.fetcher  # self.fetcher can change with mirrors.
-        self.skip_checksum_for_mirror = True # used for mirrored archives of repositories.
-
-        self.name = kwargs.get('name')
-        self.mirror_path = kwargs.get('mirror_path')
+        self.skip_checksum_for_mirror = True  # used for mirrored archives of repositories.
+
+        # TODO : this uses a protected member of tempfile, but seemed the only way to get a temporary name
+        # TODO : besides, the temporary link name won't be the same as the temporary stage area in tmp_root
+        self.name = name
+        if name is None:
+            self.name = STAGE_PREFIX + next(tempfile._get_candidate_names())
+        self.mirror_path = mirror_path
         self.tmp_root = find_tmp_root()
 
-        self.path = None
-        self._setup()
+        # Try to construct here a temporary name for the stage directory
+        # If this is a named stage, then construct a named path.
+        self.path = join_path(spack.stage_path, self.name)
 
+        # Flag to decide whether to delete the stage folder on exit or not
+        self.keep = keep
 
-    def _cleanup_dead_links(self):
-        """Remove any dead links in the stage directory."""
-        for file in os.listdir(spack.stage_path):
-            path = join_path(spack.stage_path, file)
-            if os.path.islink(path):
-                real_path = os.path.realpath(path)
-                if not os.path.exists(path):
-                    os.unlink(path)
+
+    def __enter__(self):
+        """
+        Entering a stage context will create the stage directory
+
+        Returns:
+            self
+        """
+        self.create()
+        return self
+
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        """
+        Exiting from a stage context will delete the stage directory unless:
+        - it was explicitly requested not to do so
+        - an exception has been raised
+
+        Args:
+            exc_type: exception type
+            exc_val: exception value
+            exc_tb: exception traceback
+
+        Returns:
+            Boolean
+        """
+        # Delete when there are no exceptions, unless asked to keep.
+        if exc_type is None and not self.keep:
+            self.destroy()
 
 
     def _need_to_create_path(self):
@@ -130,11 +188,11 @@ class Stage(object):
         # Path looks ok, but need to check the target of the link.
         if os.path.islink(self.path):
             real_path = os.path.realpath(self.path)
-            real_tmp  = os.path.realpath(self.tmp_root)
+            real_tmp = os.path.realpath(self.tmp_root)
 
             if spack.use_tmp_stage:
-                # If we're using a tmp dir, it's a link, and it points at the right spot,
-                # then keep it.
+                # If we're using a tmp dir, it's a link, and it points at the
+                # right spot, then keep it.
                 if (real_path.startswith(real_tmp) and os.path.exists(real_path)):
                     return False
                 else:
@@ -149,56 +207,6 @@ class Stage(object):
 
         return False
 
-
-    def _setup(self):
-        """Creates the stage directory.
-           If spack.use_tmp_stage is False, the stage directory is created
-           directly under spack.stage_path.
-
-           If spack.use_tmp_stage is True, this will attempt to create a
-           stage in a temporary directory and link it into spack.stage_path.
-           Spack will use the first writable location in spack.tmp_dirs to
-           create a stage.  If there is no valid location in tmp_dirs, fall
-           back to making the stage inside spack.stage_path.
-        """
-        # Create the top-level stage directory
-        mkdirp(spack.stage_path)
-        self._cleanup_dead_links()
-
-        # If this is a named stage, then construct a named path.
-        if self.name is not None:
-            self.path = join_path(spack.stage_path, self.name)
-
-        # If this is a temporary stage, them make the temp directory
-        tmp_dir = None
-        if self.tmp_root:
-            if self.name is None:
-                # Unnamed tmp root.  Link the path in
-                tmp_dir = tempfile.mkdtemp('', STAGE_PREFIX, self.tmp_root)
-                self.name = os.path.basename(tmp_dir)
-                self.path = join_path(spack.stage_path, self.name)
-                if self._need_to_create_path():
-                    os.symlink(tmp_dir, self.path)
-
-            else:
-                if self._need_to_create_path():
-                    tmp_dir = tempfile.mkdtemp('', STAGE_PREFIX, self.tmp_root)
-                    os.symlink(tmp_dir, self.path)
-
-        # if we're not using a tmp dir, create the stage directly in the
-        # stage dir, rather than linking to it.
-        else:
-            if self.name is None:
-                self.path = tempfile.mkdtemp('', STAGE_PREFIX, spack.stage_path)
-                self.name = os.path.basename(self.path)
-            else:
-                if self._need_to_create_path():
-                    mkdirp(self.path)
-
-        # Make sure we can actually do something with the stage we made.
-        ensure_access(self.path)
-
-
     @property
     def archive_file(self):
         """Path to the source archive within this stage directory."""
@@ -215,29 +223,35 @@ class Stage(object):
         else:
             return None
 
-
     @property
     def source_path(self):
-        """Returns the path to the expanded/checked out source code
-           within this fetch strategy's path.
+        """Returns the path to the expanded/checked out source code.
 
-           This assumes nothing else is going ot be put in the
-           FetchStrategy's path.  It searches for the first
-           subdirectory of the path it can find, then returns that.
+        To find the source code, this method searches for the first
+        subdirectory of the stage that it can find, and returns it.
+        This assumes nothing besides the archive file will be in the
+        stage path, but it has the advantage that we don't need to
+        know the name of the archive or its contents.
+
+        If the fetch strategy is not supposed to expand the downloaded
+        file, it will just return the stage path. If the archive needs
+        to be expanded, it will return None when no archive is found.
         """
+        if isinstance(self.fetcher, fs.URLFetchStrategy):
+            if not self.fetcher.expand_archive:
+                return self.path
+
         for p in [os.path.join(self.path, f) for f in os.listdir(self.path)]:
             if os.path.isdir(p):
                 return p
         return None
 
-
     def chdir(self):
         """Changes directory to the stage path.  Or dies if it is not set up."""
         if os.path.isdir(self.path):
             os.chdir(self.path)
         else:
-            tty.die("Setup failed: no such directory: " + self.path)
-
+            raise ChdirError("Setup failed: no such directory: " + self.path)
 
     def fetch(self, mirror_only=False):
         """Downloads an archive or checks out code from a repository."""
@@ -282,7 +296,7 @@ class Stage(object):
                 self.fetcher = fetcher
                 self.fetcher.fetch()
                 break
-            except spack.error.SpackError, e:
+            except spack.error.SpackError as e:
                 tty.msg("Fetching from %s failed." % fetcher)
                 tty.debug(e)
                 continue
@@ -291,7 +305,6 @@ class Stage(object):
             self.fetcher = self.default_fetcher
             raise fs.FetchError(errMessage, None)
 
-
     def check(self):
         """Check the downloaded archive against a checksum digest.
            No-op if this stage checks code out of a repository."""
@@ -305,14 +318,17 @@ class Stage(object):
         else:
             self.fetcher.check()
 
-
     def expand_archive(self):
         """Changes to the stage directory and attempt to expand the downloaded
            archive.  Fail if the stage is not set up or if the archive is not yet
            downloaded.
         """
-        self.fetcher.expand()
-
+        archive_dir = self.source_path
+        if not archive_dir:
+            self.fetcher.expand()
+            tty.msg("Created stage in %s" % self.path)
+        else:
+            tty.msg("Already staged %s in %s" % (self.name, self.path))
 
     def chdir_to_source(self):
         """Changes directory to the expanded archive directory.
@@ -326,16 +342,41 @@ class Stage(object):
             if not os.listdir(path):
                 tty.die("Archive was empty for %s" % self.name)
 
-
     def restage(self):
         """Removes the expanded archive path if it exists, then re-expands
            the archive.
         """
         self.fetcher.reset()
 
+    def create(self):
+        """
+        Creates the stage directory
+
+        If self.tmp_root evaluates to False, the stage directory is
+        created directly under spack.stage_path, otherwise this will
+        attempt to create a stage in a temporary directory and link it
+        into spack.stage_path.
+
+        Spack will use the first writable location in spack.tmp_dirs
+        to create a stage. If there is no valid location in tmp_dirs,
+        fall back to making the stage inside spack.stage_path.
+        """
+        # Create the top-level stage directory
+        mkdirp(spack.stage_path)
+        remove_dead_links(spack.stage_path)
+        # If a tmp_root exists then create a directory there and then link it in the stage area,
+        # otherwise create the stage directory in self.path
+        if self._need_to_create_path():
+            if self.tmp_root:
+                tmp_dir = tempfile.mkdtemp('', STAGE_PREFIX, self.tmp_root)
+                os.symlink(tmp_dir, self.path)
+            else:
+                mkdirp(self.path)
+        # Make sure we can actually do something with the stage we made.
+        ensure_access(self.path)
 
     def destroy(self):
-        """Remove this stage directory."""
+        """Removes this stage directory."""
         remove_linked_tree(self.path)
 
         # Make sure we don't end up in a removed directory
@@ -345,8 +386,82 @@ class Stage(object):
             os.chdir(os.path.dirname(self.path))
 
 
+class ResourceStage(Stage):
+    def __init__(self, url_or_fetch_strategy, root, resource, **kwargs):
+        super(ResourceStage, self).__init__(url_or_fetch_strategy, **kwargs)
+        self.root_stage = root
+        self.resource = resource
+
+    def expand_archive(self):
+        super(ResourceStage, self).expand_archive()
+        root_stage = self.root_stage
+        resource = self.resource
+        placement = os.path.basename(self.source_path) if resource.placement is None else resource.placement
+        if not isinstance(placement, dict):
+            placement = {'': placement}
+        # Make the paths in the dictionary absolute and link
+        for key, value in placement.iteritems():
+            target_path = join_path(root_stage.source_path, resource.destination)
+            destination_path = join_path(target_path, value)
+            source_path = join_path(self.source_path, key)
+
+            try:
+                os.makedirs(target_path)
+            except OSError as err:
+                if err.errno == errno.EEXIST and os.path.isdir(target_path):
+                    pass
+                else:
+                    raise
+
+            if not os.path.exists(destination_path):
+                # Create a symlink
+                tty.info('Moving resource stage\n\tsource : {stage}\n\tdestination : {destination}'.format(
+                    stage=source_path, destination=destination_path
+                ))
+                shutil.move(source_path, destination_path)
+
+
+@pattern.composite(method_list=['fetch', 'create', 'check', 'expand_archive',  'restage', 'destroy'])
+class StageComposite:
+    """
+    Composite for Stage type objects. The first item in this composite is considered to be the root package, and
+    operations that return a value are forwarded to it.
+    """
+    #
+    # __enter__ and __exit__ delegate to all stages in the composite.
+    #
+    def __enter__(self):
+        for item in self:
+            item.__enter__()
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        for item in reversed(self):
+            item.keep = getattr(self, 'keep', False)
+            item.__exit__(exc_type, exc_val, exc_tb)
+
+    #
+    # Below functions act only on the *first* stage in the composite.
+    #
+    @property
+    def source_path(self):
+        return self[0].source_path
+
+    @property
+    def path(self):
+        return self[0].path
+
+    def chdir_to_source(self):
+        return self[0].chdir_to_source()
+
+    @property
+    def archive_file(self):
+        return self[0].archive_file
+
+
 class DIYStage(object):
     """Simple class that allows any directory to be a spack stage."""
+
     def __init__(self, path):
         self.archive_file = None
         self.path = path
@@ -356,12 +471,16 @@ class DIYStage(object):
         if os.path.isdir(self.path):
             os.chdir(self.path)
         else:
-            tty.die("Setup failed: no such directory: " + self.path)
+            raise ChdirError("Setup failed: no such directory: " + self.path)
+
+    # DIY stages do nothing as context managers.
+    def __enter__(self): pass
+    def __exit__(self, exc_type, exc_val, exc_tb): pass
 
     def chdir_to_source(self):
         self.chdir()
 
-    def fetch(self):
+    def fetch(self, mirror_only):
         tty.msg("No need to fetch for DIY.")
 
     def check(self):
@@ -384,26 +503,12 @@ def _get_mirrors():
     return [val for name, val in config.iteritems()]
 
 
-
 def ensure_access(file=spack.stage_path):
     """Ensure we can access a directory and die with an error if we can't."""
     if not can_access(file):
         tty.die("Insufficient permissions for %s" % file)
 
 
-def remove_linked_tree(path):
-    """Removes a directory and its contents.  If the directory is a symlink,
-       follows the link and reamoves the real directory before removing the
-       link.
-    """
-    if os.path.exists(path):
-        if os.path.islink(path):
-            shutil.rmtree(os.path.realpath(path), True)
-            os.unlink(path)
-        else:
-            shutil.rmtree(path, True)
-
-
 def purge():
     """Remove all build directories in the top-level stage path."""
     if os.path.isdir(spack.stage_path):
@@ -432,19 +537,15 @@ def find_tmp_root():
 
 
 class StageError(spack.error.SpackError):
-    def __init__(self, message, long_message=None):
-        super(self, StageError).__init__(message, long_message)
+    """"Superclass for all errors encountered during staging."""
 
 
 class RestageError(StageError):
-    def __init__(self, message, long_msg=None):
-        super(RestageError, self).__init__(message, long_msg)
+    """"Error encountered during restaging."""
 
 
 class ChdirError(StageError):
-    def __init__(self, message, long_msg=None):
-        super(ChdirError, self).__init__(message, long_msg)
-
+    """Raised when Spack can't change directories."""
 
 # Keep this in namespace for convenience
 FailedDownloadError = fs.FailedDownloadError
diff --git a/lib/spack/spack/test/__init__.py b/lib/spack/spack/test/__init__.py
index a569cbbf35..d5d8b64765 100644
--- a/lib/spack/spack/test/__init__.py
+++ b/lib/spack/spack/test/__init__.py
@@ -48,6 +48,7 @@ test_names = ['versions',
               'package_sanity',
               'config',
               'directory_layout',
+              'pattern',
               'python_version',
               'git_fetch',
               'svn_fetch',
@@ -64,7 +65,8 @@ test_names = ['versions',
               'lock',
               'database',
               'namespace_trie',
-              'yaml']
+              'yaml',
+              'sbang']
 
 
 def list_tests():
@@ -86,20 +88,20 @@ def run(names, outputDir, verbose=False):
                           "Valid names are:")
                 colify(sorted(test_names), indent=4)
                 sys.exit(1)
-    
+
     tally = Tally()
     for test in names:
         module = 'spack.test.' + test
         print module
-        
+
         tty.msg("Running test: %s" % test)
-        
+
         runOpts = ["--with-%s" % spack.test.tally_plugin.Tally.name]
-        
+
         if outputDir:
             xmlOutputFname = "unittests-{0}.xml".format(test)
             xmlOutputPath = join_path(outputDir, xmlOutputFname)
-            runOpts += ["--with-xunit", 
+            runOpts += ["--with-xunit",
                 "--xunit-file={0}".format(xmlOutputPath)]
         argv = [""] + runOpts + [module]
         result = nose.run(argv=argv, addplugins=[tally])
diff --git a/lib/spack/spack/test/cc.py b/lib/spack/spack/test/cc.py
index 905af28a06..f3f6d4a22e 100644
--- a/lib/spack/spack/test/cc.py
+++ b/lib/spack/spack/test/cc.py
@@ -39,11 +39,11 @@ test_command = [
     'arg1',
     '-Wl,--start-group',
     'arg2',
-    '-Wl,-rpath=/first/rpath', 'arg3', '-Wl,-rpath', '-Wl,/second/rpath',
+    '-Wl,-rpath,/first/rpath', 'arg3', '-Wl,-rpath', '-Wl,/second/rpath',
     '-llib1', '-llib2',
     'arg4',
     '-Wl,--end-group',
-    '-Xlinker,-rpath', '-Xlinker,/third/rpath', '-Xlinker,-rpath=/fourth/rpath',
+    '-Xlinker', '-rpath', '-Xlinker', '/third/rpath', '-Xlinker', '-rpath', '-Xlinker', '/fourth/rpath',
     '-llib3', '-llib4',
     'arg5', 'arg6']
 
@@ -95,13 +95,13 @@ class CompilerTest(unittest.TestCase):
     def test_ccld_mode(self):
         self.check_cc('dump-mode', [], "ccld")
         self.check_cc('dump-mode', ['foo.c', '-o', 'foo'], "ccld")
-        self.check_cc('dump-mode', ['foo.c', '-o', 'foo', '-Wl,-rpath=foo'], "ccld")
-        self.check_cc('dump-mode', ['foo.o', 'bar.o', 'baz.o', '-o', 'foo', '-Wl,-rpath=foo'], "ccld")
+        self.check_cc('dump-mode', ['foo.c', '-o', 'foo', '-Wl,-rpath,foo'], "ccld")
+        self.check_cc('dump-mode', ['foo.o', 'bar.o', 'baz.o', '-o', 'foo', '-Wl,-rpath,foo'], "ccld")
 
 
     def test_ld_mode(self):
         self.check_ld('dump-mode', [], "ld")
-        self.check_ld('dump-mode', ['foo.o', 'bar.o', 'baz.o', '-o', 'foo', '-Wl,-rpath=foo'], "ld")
+        self.check_ld('dump-mode', ['foo.o', 'bar.o', 'baz.o', '-o', 'foo', '-Wl,-rpath,foo'], "ld")
 
 
     def test_includes(self):
diff --git a/lib/spack/spack/test/concretize.py b/lib/spack/spack/test/concretize.py
index 7f2938aec5..f264faf17a 100644
--- a/lib/spack/spack/test/concretize.py
+++ b/lib/spack/spack/test/concretize.py
@@ -22,10 +22,9 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import unittest
-
 import spack
 from spack.spec import Spec, CompilerSpec
+from spack.concretize import find_spec
 from spack.test.mock_packages_test import *
 
 class ConcretizeTest(MockPackagesTest):
@@ -143,6 +142,34 @@ class ConcretizeTest(MockPackagesTest):
                                 for spec in spack.repo.providers_for('mpi@3')))
 
 
+    def test_concretize_two_virtuals(self):
+        """Test a package with multiple virtual dependencies."""
+        s = Spec('hypre').concretize()
+
+
+    def test_concretize_two_virtuals_with_one_bound(self):
+        """Test a package with multiple virtual dependencies and one preset."""
+        s = Spec('hypre ^openblas').concretize()
+
+
+    def test_concretize_two_virtuals_with_two_bound(self):
+        """Test a package with multiple virtual dependencies and two of them preset."""
+        s = Spec('hypre ^openblas ^netlib-lapack').concretize()
+
+
+    def test_concretize_two_virtuals_with_dual_provider(self):
+        """Test a package with multiple virtual dependencies and force a provider
+           that provides both."""
+        s = Spec('hypre ^openblas-with-lapack').concretize()
+
+
+    def test_concretize_two_virtuals_with_dual_provider_and_a_conflict(self):
+        """Test a package with multiple virtual dependencies and force a provider
+           that provides both, and another conflicting package that provides one."""
+        s = Spec('hypre ^openblas-with-lapack ^netlib-lapack')
+        self.assertRaises(spack.spec.MultipleProviderError, s.concretize)
+
+
     def test_virtual_is_fully_expanded_for_callpath(self):
         # force dependence on fake "zmpi" by asking for MPI 10.0
         spec = Spec('callpath ^mpi@10.0')
@@ -192,3 +219,93 @@ class ConcretizeTest(MockPackagesTest):
         # TODO: not exactly the syntax I would like.
         self.assertTrue(spec['libdwarf'].compiler.satisfies('clang'))
         self.assertTrue(spec['libelf'].compiler.satisfies('clang'))
+
+
+    def test_external_package(self):
+        spec = Spec('externaltool')
+        spec.concretize()
+
+        self.assertEqual(spec['externaltool'].external, '/path/to/external_tool')
+        self.assertFalse('externalprereq' in spec)
+        self.assertTrue(spec['externaltool'].compiler.satisfies('gcc'))
+
+
+    def test_nobuild_package(self):
+        got_error = False
+        spec = Spec('externaltool%clang')
+        try:
+            spec.concretize()
+        except spack.concretize.NoBuildError:
+            got_error = True
+        self.assertTrue(got_error)
+
+
+    def test_external_and_virtual(self):
+        spec = Spec('externaltest')
+        spec.concretize()
+        self.assertEqual(spec['externaltool'].external, '/path/to/external_tool')
+        self.assertEqual(spec['stuff'].external, '/path/to/external_virtual_gcc')
+        self.assertTrue(spec['externaltool'].compiler.satisfies('gcc'))
+        self.assertTrue(spec['stuff'].compiler.satisfies('gcc'))
+
+
+    def test_find_spec_parents(self):
+        """Tests the spec finding logic used by concretization. """
+        s = Spec('a +foo',
+                 Spec('b +foo',
+                      Spec('c'),
+                      Spec('d +foo')),
+                 Spec('e +foo'))
+
+        self.assertEqual('a', find_spec(s['b'], lambda s: '+foo' in s).name)
+
+
+    def test_find_spec_children(self):
+        s = Spec('a',
+                 Spec('b +foo',
+                      Spec('c'),
+                      Spec('d +foo')),
+                 Spec('e +foo'))
+        self.assertEqual('d', find_spec(s['b'], lambda s: '+foo' in s).name)
+        s = Spec('a',
+                 Spec('b +foo',
+                      Spec('c +foo'),
+                      Spec('d')),
+                 Spec('e +foo'))
+        self.assertEqual('c', find_spec(s['b'], lambda s: '+foo' in s).name)
+
+
+    def test_find_spec_sibling(self):
+        s = Spec('a',
+                 Spec('b +foo',
+                      Spec('c'),
+                      Spec('d')),
+                 Spec('e +foo'))
+        self.assertEqual('e', find_spec(s['b'], lambda s: '+foo' in s).name)
+        self.assertEqual('b', find_spec(s['e'], lambda s: '+foo' in s).name)
+
+        s = Spec('a',
+                 Spec('b +foo',
+                      Spec('c'),
+                      Spec('d')),
+                 Spec('e',
+                      Spec('f +foo')))
+        self.assertEqual('f', find_spec(s['b'], lambda s: '+foo' in s).name)
+
+
+    def test_find_spec_self(self):
+        s = Spec('a',
+                 Spec('b +foo',
+                      Spec('c'),
+                      Spec('d')),
+                 Spec('e'))
+        self.assertEqual('b', find_spec(s['b'], lambda s: '+foo' in s).name)
+
+
+    def test_find_spec_none(self):
+        s = Spec('a',
+                 Spec('b',
+                      Spec('c'),
+                      Spec('d')),
+                 Spec('e'))
+        self.assertEqual(None, find_spec(s['b'], lambda s: '+foo' in s))
diff --git a/lib/spack/spack/test/config.py b/lib/spack/spack/test/config.py
index d8be5a855b..0562d2d620 100644
--- a/lib/spack/spack/test/config.py
+++ b/lib/spack/spack/test/config.py
@@ -22,13 +22,13 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import unittest
-import shutil
 import os
+import shutil
 from tempfile import mkdtemp
-from ordereddict_backport import OrderedDict
+
 import spack
 import spack.config
+from ordereddict_backport import OrderedDict
 from spack.test.mock_packages_test import *
 
 # Some sample compiler config data
diff --git a/lib/spack/spack/test/configure_guess.py b/lib/spack/spack/test/configure_guess.py
index a4e8565b62..2440d120e5 100644
--- a/lib/spack/spack/test/configure_guess.py
+++ b/lib/spack/spack/test/configure_guess.py
@@ -23,20 +23,15 @@
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
 import os
-import unittest
 import shutil
 import tempfile
+import unittest
 
 from llnl.util.filesystem import *
-
 from spack.cmd.create import ConfigureGuesser
 from spack.stage import Stage
-
-from spack.fetch_strategy import URLFetchStrategy
-from spack.directory_layout import YamlDirectoryLayout
-from spack.util.executable import which
 from spack.test.mock_packages_test import *
-from spack.test.mock_repo import MockArchive
+from spack.util.executable import which
 
 
 class InstallTest(unittest.TestCase):
@@ -52,8 +47,6 @@ class InstallTest(unittest.TestCase):
 
     def tearDown(self):
         shutil.rmtree(self.tmpdir, ignore_errors=True)
-        if self.stage:
-            self.stage.destroy()
         os.chdir(self.orig_dir)
 
 
@@ -64,12 +57,12 @@ class InstallTest(unittest.TestCase):
 
         url = 'file://' + join_path(os.getcwd(), 'archive.tar.gz')
         print url
-        self.stage = Stage(url)
-        self.stage.fetch()
+        with Stage(url) as stage:
+            stage.fetch()
 
-        guesser = ConfigureGuesser()
-        guesser(self.stage)
-        self.assertEqual(system, guesser.build_system)
+            guesser = ConfigureGuesser()
+            guesser(stage)
+            self.assertEqual(system, guesser.build_system)
 
 
     def test_python(self):
diff --git a/lib/spack/spack/test/database.py b/lib/spack/spack/test/database.py
index 0205f4b8ce..9a57e1f03e 100644
--- a/lib/spack/spack/test/database.py
+++ b/lib/spack/spack/test/database.py
@@ -26,19 +26,18 @@
 These tests check the database is functioning properly,
 both in memory and in its file
 """
-import tempfile
-import shutil
 import multiprocessing
-
-from llnl.util.lock import *
-from llnl.util.filesystem import join_path
+import shutil
+import tempfile
 
 import spack
+from llnl.util.filesystem import join_path
+from llnl.util.lock import *
+from llnl.util.tty.colify import colify
 from spack.database import Database
 from spack.directory_layout import YamlDirectoryLayout
 from spack.test.mock_packages_test import *
 
-from llnl.util.tty.colify import colify
 
 def _print_ref_counts():
     """Print out all ref counts for the graph used here, for debugging"""
diff --git a/lib/spack/spack/test/directory_layout.py b/lib/spack/spack/test/directory_layout.py
index 925cb648ed..8ad8f1a360 100644
--- a/lib/spack/spack/test/directory_layout.py
+++ b/lib/spack/spack/test/directory_layout.py
@@ -25,20 +25,17 @@
 """\
 This test verifies that the Spack directory layout works properly.
 """
-import unittest
-import tempfile
-import shutil
 import os
-
-from llnl.util.filesystem import *
+import shutil
+import tempfile
 
 import spack
-from spack.spec import Spec
-from spack.repository import RepoPath
+from llnl.util.filesystem import *
 from spack.directory_layout import YamlDirectoryLayout
+from spack.repository import RepoPath
+from spack.spec import Spec
 from spack.test.mock_packages_test import *
 
-
 # number of packages to test (to reduce test time)
 max_packages = 10
 
@@ -69,6 +66,9 @@ class DirectoryLayoutTest(MockPackagesTest):
         packages = list(spack.repo.all_packages())[:max_packages]
 
         for pkg in packages:
+            if pkg.name.startswith('external'):
+                #External package tests cannot be installed
+                continue            
             spec = pkg.spec
 
             # If a spec fails to concretize, just skip it.  If it is a
@@ -174,6 +174,9 @@ class DirectoryLayoutTest(MockPackagesTest):
         # Create install prefixes for all packages in the list
         installed_specs = {}
         for pkg in packages:
+            if pkg.name.startswith('external'):
+                #External package tests cannot be installed
+                continue
             spec = pkg.spec.concretized()
             installed_specs[spec.name] = spec
             self.layout.create_install_directory(spec)
diff --git a/lib/spack/spack/test/git_fetch.py b/lib/spack/spack/test/git_fetch.py
index d84433176a..3578044116 100644
--- a/lib/spack/spack/test/git_fetch.py
+++ b/lib/spack/spack/test/git_fetch.py
@@ -23,19 +23,12 @@
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
 import os
-import unittest
-import shutil
-import tempfile
-
-from llnl.util.filesystem import *
 
 import spack
-from spack.version import ver
-from spack.stage import Stage
-from spack.util.executable import which
-
+from llnl.util.filesystem import *
 from spack.test.mock_packages_test import *
 from spack.test.mock_repo import MockGitRepo
+from spack.version import ver
 
 
 class GitFetchTest(MockPackagesTest):
@@ -52,19 +45,15 @@ class GitFetchTest(MockPackagesTest):
         spec.concretize()
         self.pkg = spack.repo.get(spec, new=True)
 
-
     def tearDown(self):
         """Destroy the stage space used by this test."""
         super(GitFetchTest, self).tearDown()
         self.repo.destroy()
-        self.pkg.do_clean()
-
 
     def assert_rev(self, rev):
         """Check that the current git revision is equal to the supplied rev."""
         self.assertEqual(self.repo.rev_hash('HEAD'), self.repo.rev_hash(rev))
 
-
     def try_fetch(self, rev, test_file, args):
         """Tries to:
            1. Fetch the repo using a fetch strategy constructed with
@@ -76,26 +65,27 @@ class GitFetchTest(MockPackagesTest):
         """
         self.pkg.versions[ver('git')] = args
 
-        self.pkg.do_stage()
-        self.assert_rev(rev)
+        with self.pkg.stage:
+            self.pkg.do_stage()
+            self.assert_rev(rev)
 
-        file_path = join_path(self.pkg.stage.source_path, test_file)
-        self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
-        self.assertTrue(os.path.isfile(file_path))
+            file_path = join_path(self.pkg.stage.source_path, test_file)
+            self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
+            self.assertTrue(os.path.isfile(file_path))
 
-        os.unlink(file_path)
-        self.assertFalse(os.path.isfile(file_path))
+            os.unlink(file_path)
+            self.assertFalse(os.path.isfile(file_path))
 
-        untracked_file = 'foobarbaz'
-        touch(untracked_file)
-        self.assertTrue(os.path.isfile(untracked_file))
-        self.pkg.do_restage()
-        self.assertFalse(os.path.isfile(untracked_file))
+            untracked_file = 'foobarbaz'
+            touch(untracked_file)
+            self.assertTrue(os.path.isfile(untracked_file))
+            self.pkg.do_restage()
+            self.assertFalse(os.path.isfile(untracked_file))
 
-        self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
-        self.assertTrue(os.path.isfile(file_path))
+            self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
+            self.assertTrue(os.path.isfile(file_path))
 
-        self.assert_rev(rev)
+            self.assert_rev(rev)
 
 
     def test_fetch_master(self):
diff --git a/lib/spack/spack/test/hg_fetch.py b/lib/spack/spack/test/hg_fetch.py
index bbcb64e4c1..b8a0c1ec46 100644
--- a/lib/spack/spack/test/hg_fetch.py
+++ b/lib/spack/spack/test/hg_fetch.py
@@ -23,16 +23,12 @@
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
 import os
-import unittest
-
-from llnl.util.filesystem import *
-
 import spack
+
 from spack.version import ver
-from spack.stage import Stage
-from spack.util.executable import which
-from spack.test.mock_packages_test import *
 from spack.test.mock_repo import MockHgRepo
+from llnl.util.filesystem import *
+from spack.test.mock_packages_test import *
 
 
 class HgFetchTest(MockPackagesTest):
@@ -49,13 +45,10 @@ class HgFetchTest(MockPackagesTest):
         spec.concretize()
         self.pkg = spack.repo.get(spec, new=True)
 
-
     def tearDown(self):
         """Destroy the stage space used by this test."""
         super(HgFetchTest, self).tearDown()
         self.repo.destroy()
-        self.pkg.do_clean()
-
 
     def try_fetch(self, rev, test_file, args):
         """Tries to:
@@ -68,26 +61,27 @@ class HgFetchTest(MockPackagesTest):
         """
         self.pkg.versions[ver('hg')] = args
 
-        self.pkg.do_stage()
-        self.assertEqual(self.repo.get_rev(), rev)
+        with self.pkg.stage:
+            self.pkg.do_stage()
+            self.assertEqual(self.repo.get_rev(), rev)
 
-        file_path = join_path(self.pkg.stage.source_path, test_file)
-        self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
-        self.assertTrue(os.path.isfile(file_path))
+            file_path = join_path(self.pkg.stage.source_path, test_file)
+            self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
+            self.assertTrue(os.path.isfile(file_path))
 
-        os.unlink(file_path)
-        self.assertFalse(os.path.isfile(file_path))
+            os.unlink(file_path)
+            self.assertFalse(os.path.isfile(file_path))
 
-        untracked = 'foobarbaz'
-        touch(untracked)
-        self.assertTrue(os.path.isfile(untracked))
-        self.pkg.do_restage()
-        self.assertFalse(os.path.isfile(untracked))
+            untracked = 'foobarbaz'
+            touch(untracked)
+            self.assertTrue(os.path.isfile(untracked))
+            self.pkg.do_restage()
+            self.assertFalse(os.path.isfile(untracked))
 
-        self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
-        self.assertTrue(os.path.isfile(file_path))
+            self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
+            self.assertTrue(os.path.isfile(file_path))
 
-        self.assertEqual(self.repo.get_rev(), rev)
+            self.assertEqual(self.repo.get_rev(), rev)
 
 
     def test_fetch_default(self):
diff --git a/lib/spack/spack/test/install.py b/lib/spack/spack/test/install.py
index c09bd24c8e..8297893f01 100644
--- a/lib/spack/spack/test/install.py
+++ b/lib/spack/spack/test/install.py
@@ -22,18 +22,13 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import os
-import unittest
 import shutil
 import tempfile
 
-from llnl.util.filesystem import *
-
 import spack
-from spack.stage import Stage
-from spack.fetch_strategy import URLFetchStrategy
+from llnl.util.filesystem import *
 from spack.directory_layout import YamlDirectoryLayout
-from spack.util.executable import which
+from spack.fetch_strategy import URLFetchStrategy, FetchStrategyComposite
 from spack.test.mock_packages_test import *
 from spack.test.mock_repo import MockArchive
 
@@ -79,7 +74,10 @@ class InstallTest(MockPackagesTest):
         pkg = spack.repo.get(spec)
 
         # Fake the URL for the package so it downloads from a file.
-        pkg.fetcher = URLFetchStrategy(self.repo.url)
+
+        fetcher = FetchStrategyComposite()
+        fetcher.append(URLFetchStrategy(self.repo.url))
+        pkg.fetcher = fetcher
 
         try:
             pkg.do_install()
diff --git a/lib/spack/spack/test/link_tree.py b/lib/spack/spack/test/link_tree.py
index 886b7ef4c5..ee37e765c7 100644
--- a/lib/spack/spack/test/link_tree.py
+++ b/lib/spack/spack/test/link_tree.py
@@ -24,8 +24,6 @@
 ##############################################################################
 import os
 import unittest
-import shutil
-import tempfile
 
 from llnl.util.filesystem import *
 from llnl.util.link_tree import LinkTree
@@ -38,6 +36,7 @@ class LinkTreeTest(unittest.TestCase):
 
     def setUp(self):
         self.stage = Stage('link-tree-test')
+        self.stage.create()
 
         with working_dir(self.stage.path):
             touchp('source/1')
@@ -51,10 +50,8 @@ class LinkTreeTest(unittest.TestCase):
         source_path = os.path.join(self.stage.path, 'source')
         self.link_tree = LinkTree(source_path)
 
-
     def tearDown(self):
-        if self.stage:
-            self.stage.destroy()
+        self.stage.destroy()
 
 
     def check_file_link(self, filename):
diff --git a/lib/spack/spack/test/lock.py b/lib/spack/spack/test/lock.py
index bc68df01db..3b11d18da4 100644
--- a/lib/spack/spack/test/lock.py
+++ b/lib/spack/spack/test/lock.py
@@ -25,15 +25,13 @@
 """
 These tests ensure that our lock works correctly.
 """
-import unittest
-import os
-import tempfile
 import shutil
+import tempfile
+import unittest
 from multiprocessing import Process
 
-from llnl.util.lock import *
 from llnl.util.filesystem import join_path, touch
-
+from llnl.util.lock import *
 from spack.util.multiproc import Barrier
 
 # This is the longest a failed test will take, as the barriers will
diff --git a/lib/spack/spack/test/make_executable.py b/lib/spack/spack/test/make_executable.py
index d568a28d44..a2606acf19 100644
--- a/lib/spack/spack/test/make_executable.py
+++ b/lib/spack/spack/test/make_executable.py
@@ -28,13 +28,13 @@ Tests for Spack's built-in parallel make support.
 This just tests whether the right args are getting passed to make.
 """
 import os
-import unittest
-import tempfile
 import shutil
+import tempfile
+import unittest
 
 from llnl.util.filesystem import *
-from spack.util.environment import path_put_first
 from spack.build_environment import MakeExecutable
+from spack.util.environment import path_put_first
 
 
 class MakeExecutableTest(unittest.TestCase):
diff --git a/lib/spack/spack/test/mirror.py b/lib/spack/spack/test/mirror.py
index 046ec56604..e707adfe9d 100644
--- a/lib/spack/spack/test/mirror.py
+++ b/lib/spack/spack/test/mirror.py
@@ -23,11 +23,10 @@
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
 import os
-from filecmp import dircmp
-
 import spack
 import spack.mirror
-from spack.util.compression import decompressor_for
+
+from filecmp import dircmp
 from spack.test.mock_packages_test import *
 from spack.test.mock_repo import *
 
@@ -74,14 +73,14 @@ class MirrorTest(MockPackagesTest):
 
 
     def check_mirror(self):
-        stage = Stage('spack-mirror-test')
-        mirror_root = join_path(stage.path, 'test-mirror')
+        with Stage('spack-mirror-test') as stage:
+            mirror_root = join_path(stage.path, 'test-mirror')
+
+            # register mirror with spack config
+            mirrors = { 'spack-mirror-test' : 'file://' + mirror_root }
+            spack.config.update_config('mirrors', mirrors)
 
-        # register mirror with spack config
-        mirrors = { 'spack-mirror-test' : 'file://' + mirror_root }
-        spack.config.update_config('mirrors', mirrors)
 
-        try:
             os.chdir(stage.path)
             spack.mirror.create(
                 mirror_root, self.repos, no_checksum=True)
@@ -97,37 +96,28 @@ class MirrorTest(MockPackagesTest):
                 files = os.listdir(subdir)
                 self.assertEqual(len(files), 1)
 
-            # Now try to fetch each package.
-            for name, mock_repo in self.repos.items():
-                spec = Spec(name).concretized()
-                pkg = spec.package
-
-                saved_checksum_setting = spack.do_checksum
-                try:
-                    # Stage the archive from the mirror and cd to it.
-                    spack.do_checksum = False
-                    pkg.do_stage(mirror_only=True)
-
-                    # Compare the original repo with the expanded archive
-                    original_path = mock_repo.path
-                    if 'svn' in name:
-                        # have to check out the svn repo to compare.
-                        original_path = join_path(mock_repo.path, 'checked_out')
-                        svn('checkout', mock_repo.url, original_path)
-
-                    dcmp = dircmp(original_path, pkg.stage.source_path)
-
-                    # make sure there are no new files in the expanded tarball
-                    self.assertFalse(dcmp.right_only)
-
-                    # and that all original files are present.
-                    self.assertTrue(all(l in exclude for l in dcmp.left_only))
-
-                finally:
-                    spack.do_checksum = saved_checksum_setting
-                    pkg.do_clean()
-        finally:
-            stage.destroy()
+                # Now try to fetch each package.
+                for name, mock_repo in self.repos.items():
+                    spec = Spec(name).concretized()
+                    pkg = spec.package
+
+                    saved_checksum_setting = spack.do_checksum
+                    with pkg.stage:
+                        # Stage the archive from the mirror and cd to it.
+                        spack.do_checksum = False
+                        pkg.do_stage(mirror_only=True)
+                        # Compare the original repo with the expanded archive
+                        original_path = mock_repo.path
+                        if 'svn' in name:
+                            # have to check out the svn repo to compare.
+                            original_path = join_path(mock_repo.path, 'checked_out')
+                            svn('checkout', mock_repo.url, original_path)
+                        dcmp = dircmp(original_path, pkg.stage.source_path)
+                        # make sure there are no new files in the expanded tarball
+                        self.assertFalse(dcmp.right_only)
+                        # and that all original files are present.
+                        self.assertTrue(all(l in exclude for l in dcmp.left_only))
+                        spack.do_checksum = saved_checksum_setting
 
 
     def test_git_mirror(self):
diff --git a/lib/spack/spack/test/mock_packages_test.py b/lib/spack/spack/test/mock_packages_test.py
index e9f1f95df5..6d24a84150 100644
--- a/lib/spack/spack/test/mock_packages_test.py
+++ b/lib/spack/spack/test/mock_packages_test.py
@@ -22,17 +22,15 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import sys
 import os
 import shutil
-import unittest
 import tempfile
-from ordereddict_backport import OrderedDict
-
-from llnl.util.filesystem import mkdirp
+import unittest
 
 import spack
 import spack.config
+from llnl.util.filesystem import mkdirp
+from ordereddict_backport import OrderedDict
 from spack.repository import RepoPath
 from spack.spec import Spec
 
@@ -51,6 +49,19 @@ compilers:
       fc: /path/to/gfortran
 """
 
+mock_packages_config = """\
+packages:
+  externaltool:
+    buildable: False
+    paths:
+      externaltool@1.0%gcc@4.5.0: /path/to/external_tool
+  externalvirtual:
+    buildable: False
+    paths:
+      externalvirtual@2.0%clang@3.3: /path/to/external_virtual_clang
+      externalvirtual@1.0%gcc@4.5.0: /path/to/external_virtual_gcc
+"""
+
 class MockPackagesTest(unittest.TestCase):
     def initmock(self):
         # Use the mock packages database for these tests.  This allows
@@ -68,9 +79,10 @@ class MockPackagesTest(unittest.TestCase):
         self.mock_user_config = os.path.join(self.temp_config, 'user')
         mkdirp(self.mock_site_config)
         mkdirp(self.mock_user_config)
-        comp_yaml = os.path.join(self.mock_site_config, 'compilers.yaml')
-        with open(comp_yaml, 'w') as f:
-            f.write(mock_compiler_config)
+        for confs in [('compilers.yaml', mock_compiler_config), ('packages.yaml', mock_packages_config)]:
+            conf_yaml = os.path.join(self.mock_site_config, confs[0])
+            with open(conf_yaml, 'w') as f:
+                f.write(confs[1])
 
         # TODO: Mocking this up is kind of brittle b/c ConfigScope
         # TODO: constructor modifies config_scopes.  Make it cleaner.
diff --git a/lib/spack/spack/test/mock_repo.py b/lib/spack/spack/test/mock_repo.py
index ed94023b0e..a8bdfb5571 100644
--- a/lib/spack/spack/test/mock_repo.py
+++ b/lib/spack/spack/test/mock_repo.py
@@ -26,13 +26,9 @@ import os
 import shutil
 
 from llnl.util.filesystem import *
-
-import spack
-from spack.version import ver
 from spack.stage import Stage
 from spack.util.executable import which
 
-
 #
 # VCS Systems used by mock repo code.
 #
diff --git a/lib/spack/spack/test/multimethod.py b/lib/spack/spack/test/multimethod.py
index 7bf4ff0a0a..2d4b8cd584 100644
--- a/lib/spack/spack/test/multimethod.py
+++ b/lib/spack/spack/test/multimethod.py
@@ -25,14 +25,11 @@
 """
 Test for multi_method dispatch.
 """
-import unittest
 
 import spack
 from spack.multimethod import *
-from spack.version import *
-from spack.spec import Spec
-from spack.multimethod import when
 from spack.test.mock_packages_test import *
+from spack.version import *
 
 
 class MultiMethodTest(MockPackagesTest):
diff --git a/lib/spack/spack/test/namespace_trie.py b/lib/spack/spack/test/namespace_trie.py
index d0d809004d..2023ba6d96 100644
--- a/lib/spack/spack/test/namespace_trie.py
+++ b/lib/spack/spack/test/namespace_trie.py
@@ -6,7 +6,7 @@
 # Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
 # LLNL-CODE-647188
 #
-# For details, see https://llnl.github.io/spack
+# For details, see https://software.llnl.gov/spack
 # Please also see the LICENSE file for our notice and the LGPL.
 #
 # This program is free software; you can redistribute it and/or modify
@@ -23,6 +23,7 @@
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
 import unittest
+
 from spack.util.naming import NamespaceTrie
 
 
diff --git a/lib/spack/spack/test/optional_deps.py b/lib/spack/spack/test/optional_deps.py
index ebd7281999..55f35ea4c9 100644
--- a/lib/spack/spack/test/optional_deps.py
+++ b/lib/spack/spack/test/optional_deps.py
@@ -22,10 +22,8 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import unittest
 
-import spack
-from spack.spec import Spec, CompilerSpec
+from spack.spec import Spec
 from spack.test.mock_packages_test import *
 
 class ConcretizeTest(MockPackagesTest):
diff --git a/lib/spack/spack/test/packages.py b/lib/spack/spack/test/packages.py
index 83984dc5f6..f0b5e05f3b 100644
--- a/lib/spack/spack/test/packages.py
+++ b/lib/spack/spack/test/packages.py
@@ -22,14 +22,12 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import unittest
-
-from llnl.util.filesystem import join_path
 
 import spack
+from llnl.util.filesystem import join_path
 from spack.repository import Repo
-from spack.util.naming import mod_to_class
 from spack.test.mock_packages_test import *
+from spack.util.naming import mod_to_class
 
 
 class PackagesTest(MockPackagesTest):
diff --git a/lib/spack/spack/test/pattern.py b/lib/spack/spack/test/pattern.py
new file mode 100644
index 0000000000..6c783c6a5f
--- /dev/null
+++ b/lib/spack/spack/test/pattern.py
@@ -0,0 +1,104 @@
+##############################################################################
+# Copyright (c) 2013, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/llnl/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+
+import unittest
+
+import spack.util.pattern as pattern
+
+
+class CompositeTest(unittest.TestCase):
+
+    def setUp(self):
+        class Base:
+            counter = 0
+
+            def add(self):
+                raise NotImplemented('add not implemented')
+
+            def subtract(self):
+                raise NotImplemented('subtract not implemented')
+
+        class One(Base):
+            def add(self):
+                Base.counter += 1
+
+            def subtract(self):
+                Base.counter -= 1
+
+        class Two(Base):
+            def add(self):
+                Base.counter += 2
+
+            def subtract(self):
+                Base.counter -= 2
+
+        self.Base = Base
+        self.One = One
+        self.Two = Two
+
+    def test_composite_from_method_list(self):
+
+        @pattern.composite(method_list=['add', 'subtract'])
+        class CompositeFromMethodList:
+            pass
+
+        composite = CompositeFromMethodList()
+        composite.append(self.One())
+        composite.append(self.Two())
+        composite.add()
+        self.assertEqual(self.Base.counter, 3)
+        composite.pop()
+        composite.subtract()
+        self.assertEqual(self.Base.counter, 2)
+
+    def test_composite_from_interface(self):
+
+        @pattern.composite(interface=self.Base)
+        class CompositeFromInterface:
+            pass
+
+        composite = CompositeFromInterface()
+        composite.append(self.One())
+        composite.append(self.Two())
+        composite.add()
+        self.assertEqual(self.Base.counter, 3)
+        composite.pop()
+        composite.subtract()
+        self.assertEqual(self.Base.counter, 2)
+
+    def test_error_conditions(self):
+
+        def wrong_container():
+            @pattern.composite(interface=self.Base, container=2)
+            class CompositeFromInterface:
+                pass
+
+        def no_methods():
+            @pattern.composite()
+            class CompositeFromInterface:
+                pass
+
+        self.assertRaises(TypeError, wrong_container)
+        self.assertRaises(TypeError, no_methods)
diff --git a/lib/spack/spack/test/python_version.py b/lib/spack/spack/test/python_version.py
index d74d3b9b7d..4294975304 100644
--- a/lib/spack/spack/test/python_version.py
+++ b/lib/spack/spack/test/python_version.py
@@ -28,12 +28,11 @@ This test ensures that all Spack files are Python version 2.6 or less.
 Spack was originally 2.7, but enough systems in 2014 are still using
 2.6 on their frontend nodes that we need 2.6 to get adopted.
 """
-import unittest
 import os
 import re
+import unittest
 
 import llnl.util.tty as tty
-
 import pyqver2
 import spack
 
diff --git a/lib/spack/spack/test/sbang.py b/lib/spack/spack/test/sbang.py
new file mode 100644
index 0000000000..825bc4be98
--- /dev/null
+++ b/lib/spack/spack/test/sbang.py
@@ -0,0 +1,93 @@
+##############################################################################
+# Copyright (c) 2013-2015, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/llnl/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+"""\
+Test that Spack's shebang filtering works correctly.
+"""
+import os
+import unittest
+import tempfile
+import shutil
+
+from llnl.util.filesystem import *
+from spack.hooks.sbang import filter_shebangs_in_directory
+import spack
+
+short_line = "#!/this/is/short/bin/bash\n"
+long_line  = "#!/this/" + ('x' * 200) + "/is/long\n"
+sbang_line = '#!/bin/bash %s/bin/sbang\n' % spack.spack_root
+last_line = "last!\n"
+
+class SbangTest(unittest.TestCase):
+    def setUp(self):
+        self.tempdir = tempfile.mkdtemp()
+
+        # make sure we can ignore non-files
+        directory = os.path.join(self.tempdir, 'dir')
+        mkdirp(directory)
+
+        # Script with short shebang
+        self.short_shebang = os.path.join(self.tempdir, 'short')
+        with open(self.short_shebang, 'w') as f:
+            f.write(short_line)
+            f.write(last_line)
+
+        # Script with long shebang
+        self.long_shebang = os.path.join(self.tempdir, 'long')
+        with open(self.long_shebang, 'w') as f:
+            f.write(long_line)
+            f.write(last_line)
+
+        # Script already using sbang.
+        self.has_shebang = os.path.join(self.tempdir, 'shebang')
+        with open(self.has_shebang, 'w') as f:
+            f.write(sbang_line)
+            f.write(long_line)
+            f.write(last_line)
+
+
+    def tearDown(self):
+         shutil.rmtree(self.tempdir, ignore_errors=True)
+
+
+
+    def test_shebang_handling(self):
+        filter_shebangs_in_directory(self.tempdir)
+
+        # Make sure this is untouched
+        with open(self.short_shebang, 'r') as f:
+            self.assertEqual(f.readline(), short_line)
+            self.assertEqual(f.readline(), last_line)
+
+        # Make sure this got patched.
+        with open(self.long_shebang, 'r') as f:
+            self.assertEqual(f.readline(), sbang_line)
+            self.assertEqual(f.readline(), long_line)
+            self.assertEqual(f.readline(), last_line)
+
+        # Make sure this is untouched
+        with open(self.has_shebang, 'r') as f:
+            self.assertEqual(f.readline(), sbang_line)
+            self.assertEqual(f.readline(), long_line)
+            self.assertEqual(f.readline(), last_line)
diff --git a/lib/spack/spack/test/spec_dag.py b/lib/spack/spack/test/spec_dag.py
index 632f777cde..5e6162b6e6 100644
--- a/lib/spack/spack/test/spec_dag.py
+++ b/lib/spack/spack/test/spec_dag.py
@@ -31,8 +31,6 @@ You can find the dummy packages here::
 import spack
 import spack.package
 
-from llnl.util.lang import list_modules
-
 from spack.spec import Spec
 from spack.test.mock_packages_test import *
 
diff --git a/lib/spack/spack/test/spec_semantics.py b/lib/spack/spack/test/spec_semantics.py
index 44a09cbd7f..8c33d1ff6e 100644
--- a/lib/spack/spack/test/spec_semantics.py
+++ b/lib/spack/spack/test/spec_semantics.py
@@ -22,7 +22,6 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import unittest
 from spack.spec import *
 from spack.test.mock_packages_test import *
 
diff --git a/lib/spack/spack/test/spec_syntax.py b/lib/spack/spack/test/spec_syntax.py
index 1daaa4be8f..6e08e30e13 100644
--- a/lib/spack/spack/test/spec_syntax.py
+++ b/lib/spack/spack/test/spec_syntax.py
@@ -23,9 +23,10 @@
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
 import unittest
+
 import spack.spec
-from spack.spec import *
 from spack.parse import Token
+from spack.spec import *
 
 # Sample output for a complex lexing.
 complex_lex = [Token(ID, 'mvapich_foo'),
diff --git a/lib/spack/spack/test/stage.py b/lib/spack/spack/test/stage.py
index c1b2a2a573..ea425127c4 100644
--- a/lib/spack/spack/test/stage.py
+++ b/lib/spack/spack/test/stage.py
@@ -25,15 +25,13 @@
 """\
 Test that the Stage class works correctly.
 """
-import unittest
-import shutil
 import os
-import getpass
+import shutil
+import unittest
 from contextlib import *
 
-from llnl.util.filesystem import *
-
 import spack
+from llnl.util.filesystem import *
 from spack.stage import Stage
 from spack.util.executable import which
 
@@ -192,116 +190,125 @@ class StageTest(unittest.TestCase):
 
     def test_setup_and_destroy_name_with_tmp(self):
         with use_tmp(True):
-            stage = Stage(archive_url, name=stage_name)
-            self.check_setup(stage, stage_name)
-
-            stage.destroy()
+            with Stage(archive_url, name=stage_name) as stage:
+                self.check_setup(stage, stage_name)
             self.check_destroy(stage, stage_name)
 
 
     def test_setup_and_destroy_name_without_tmp(self):
         with use_tmp(False):
-            stage = Stage(archive_url, name=stage_name)
-            self.check_setup(stage, stage_name)
-
-            stage.destroy()
+            with Stage(archive_url, name=stage_name) as stage:
+                self.check_setup(stage, stage_name)
             self.check_destroy(stage, stage_name)
 
 
     def test_setup_and_destroy_no_name_with_tmp(self):
         with use_tmp(True):
-            stage = Stage(archive_url)
-            self.check_setup(stage, None)
-
-            stage.destroy()
+            with Stage(archive_url) as stage:
+                self.check_setup(stage, None)
             self.check_destroy(stage, None)
 
 
     def test_setup_and_destroy_no_name_without_tmp(self):
         with use_tmp(False):
-            stage = Stage(archive_url)
-            self.check_setup(stage, None)
-
-            stage.destroy()
+            with Stage(archive_url) as stage:
+                self.check_setup(stage, None)
             self.check_destroy(stage, None)
 
 
     def test_chdir(self):
-        stage = Stage(archive_url, name=stage_name)
-
-        stage.chdir()
-        self.check_setup(stage, stage_name)
-        self.check_chdir(stage, stage_name)
-
-        stage.destroy()
+        with Stage(archive_url, name=stage_name) as stage:
+            stage.chdir()
+            self.check_setup(stage, stage_name)
+            self.check_chdir(stage, stage_name)
         self.check_destroy(stage, stage_name)
 
 
     def test_fetch(self):
-        stage = Stage(archive_url, name=stage_name)
-
-        stage.fetch()
-        self.check_setup(stage, stage_name)
-        self.check_chdir(stage, stage_name)
-        self.check_fetch(stage, stage_name)
-
-        stage.destroy()
+        with Stage(archive_url, name=stage_name) as stage:
+            stage.fetch()
+            self.check_setup(stage, stage_name)
+            self.check_chdir(stage, stage_name)
+            self.check_fetch(stage, stage_name)
         self.check_destroy(stage, stage_name)
 
 
     def test_expand_archive(self):
-        stage = Stage(archive_url, name=stage_name)
-
-        stage.fetch()
-        self.check_setup(stage, stage_name)
-        self.check_fetch(stage, stage_name)
-
-        stage.expand_archive()
-        self.check_expand_archive(stage, stage_name)
-
-        stage.destroy()
+        with Stage(archive_url, name=stage_name) as stage:
+            stage.fetch()
+            self.check_setup(stage, stage_name)
+            self.check_fetch(stage, stage_name)
+            stage.expand_archive()
+            self.check_expand_archive(stage, stage_name)
         self.check_destroy(stage, stage_name)
 
 
     def test_expand_archive(self):
-        stage = Stage(archive_url, name=stage_name)
+        with Stage(archive_url, name=stage_name) as stage:
+            stage.fetch()
+            self.check_setup(stage, stage_name)
+            self.check_fetch(stage, stage_name)
+            stage.expand_archive()
+            stage.chdir_to_source()
+            self.check_expand_archive(stage, stage_name)
+            self.check_chdir_to_source(stage, stage_name)
+        self.check_destroy(stage, stage_name)
 
-        stage.fetch()
-        self.check_setup(stage, stage_name)
-        self.check_fetch(stage, stage_name)
 
-        stage.expand_archive()
-        stage.chdir_to_source()
-        self.check_expand_archive(stage, stage_name)
-        self.check_chdir_to_source(stage, stage_name)
+    def test_restage(self):
+        with Stage(archive_url, name=stage_name) as stage:
+            stage.fetch()
+            stage.expand_archive()
+            stage.chdir_to_source()
+            self.check_expand_archive(stage, stage_name)
+            self.check_chdir_to_source(stage, stage_name)
+
+            # Try to make a file in the old archive dir
+            with open('foobar', 'w') as file:
+                file.write("this file is to be destroyed.")
+
+            self.assertTrue('foobar' in os.listdir(stage.source_path))
+
+            # Make sure the file is not there after restage.
+            stage.restage()
+            self.check_chdir(stage, stage_name)
+            self.check_fetch(stage, stage_name)
+            stage.chdir_to_source()
+            self.check_chdir_to_source(stage, stage_name)
+            self.assertFalse('foobar' in os.listdir(stage.source_path))
+        self.check_destroy(stage, stage_name)
 
-        stage.destroy()
+
+    def test_no_keep_without_exceptions(self):
+        with Stage(archive_url, name=stage_name, keep=False) as stage:
+            pass
         self.check_destroy(stage, stage_name)
 
 
-    def test_restage(self):
-        stage = Stage(archive_url, name=stage_name)
+    def test_keep_without_exceptions(self):
+        with Stage(archive_url, name=stage_name, keep=True) as stage:
+            pass
+        path = self.get_stage_path(stage, stage_name)
+        self.assertTrue(os.path.isdir(path))
 
-        stage.fetch()
-        stage.expand_archive()
-        stage.chdir_to_source()
-        self.check_expand_archive(stage, stage_name)
-        self.check_chdir_to_source(stage, stage_name)
 
-        # Try to make a file in the old archive dir
-        with open('foobar', 'w') as file:
-            file.write("this file is to be destroyed.")
+    def test_no_keep_with_exceptions(self):
+        try:
+            with Stage(archive_url, name=stage_name, keep=False) as stage:
+                raise Exception()
 
-        self.assertTrue('foobar' in os.listdir(stage.source_path))
+            path = self.get_stage_path(stage, stage_name)
+            self.assertTrue(os.path.isdir(path))
+        except:
+            pass # ignore here.
 
-        # Make sure the file is not there after restage.
-        stage.restage()
-        self.check_chdir(stage, stage_name)
-        self.check_fetch(stage, stage_name)
 
-        stage.chdir_to_source()
-        self.check_chdir_to_source(stage, stage_name)
-        self.assertFalse('foobar' in os.listdir(stage.source_path))
+    def test_keep_exceptions(self):
+        try:
+            with Stage(archive_url, name=stage_name, keep=True) as stage:
+                raise Exception()
 
-        stage.destroy()
-        self.check_destroy(stage, stage_name)
+            path = self.get_stage_path(stage, stage_name)
+            self.assertTrue(os.path.isdir(path))
+        except:
+            pass # ignore here.
diff --git a/lib/spack/spack/test/svn_fetch.py b/lib/spack/spack/test/svn_fetch.py
index 454a7f1d1f..1ee4ee700e 100644
--- a/lib/spack/spack/test/svn_fetch.py
+++ b/lib/spack/spack/test/svn_fetch.py
@@ -24,18 +24,12 @@
 ##############################################################################
 import os
 import re
-import unittest
-import shutil
-import tempfile
-
-from llnl.util.filesystem import *
-
 import spack
+
+from spack.test.mock_repo import svn, MockSvnRepo
 from spack.version import ver
-from spack.stage import Stage
-from spack.util.executable import which
 from spack.test.mock_packages_test import *
-from spack.test.mock_repo import svn, MockSvnRepo
+from llnl.util.filesystem import *
 
 
 class SvnFetchTest(MockPackagesTest):
@@ -51,13 +45,10 @@ class SvnFetchTest(MockPackagesTest):
         spec.concretize()
         self.pkg = spack.repo.get(spec, new=True)
 
-
     def tearDown(self):
         """Destroy the stage space used by this test."""
         super(SvnFetchTest, self).tearDown()
         self.repo.destroy()
-        self.pkg.do_clean()
-
 
     def assert_rev(self, rev):
         """Check that the current revision is equal to the supplied rev."""
@@ -70,7 +61,6 @@ class SvnFetchTest(MockPackagesTest):
                     return match.group(1)
         self.assertEqual(get_rev(), rev)
 
-
     def try_fetch(self, rev, test_file, args):
         """Tries to:
            1. Fetch the repo using a fetch strategy constructed with
@@ -82,26 +72,27 @@ class SvnFetchTest(MockPackagesTest):
         """
         self.pkg.versions[ver('svn')] = args
 
-        self.pkg.do_stage()
-        self.assert_rev(rev)
+        with self.pkg.stage:
+            self.pkg.do_stage()
+            self.assert_rev(rev)
 
-        file_path = join_path(self.pkg.stage.source_path, test_file)
-        self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
-        self.assertTrue(os.path.isfile(file_path))
+            file_path = join_path(self.pkg.stage.source_path, test_file)
+            self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
+            self.assertTrue(os.path.isfile(file_path))
 
-        os.unlink(file_path)
-        self.assertFalse(os.path.isfile(file_path))
+            os.unlink(file_path)
+            self.assertFalse(os.path.isfile(file_path))
 
-        untracked = 'foobarbaz'
-        touch(untracked)
-        self.assertTrue(os.path.isfile(untracked))
-        self.pkg.do_restage()
-        self.assertFalse(os.path.isfile(untracked))
+            untracked = 'foobarbaz'
+            touch(untracked)
+            self.assertTrue(os.path.isfile(untracked))
+            self.pkg.do_restage()
+            self.assertFalse(os.path.isfile(untracked))
 
-        self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
-        self.assertTrue(os.path.isfile(file_path))
+            self.assertTrue(os.path.isdir(self.pkg.stage.source_path))
+            self.assertTrue(os.path.isfile(file_path))
 
-        self.assert_rev(rev)
+            self.assert_rev(rev)
 
 
     def test_fetch_default(self):
diff --git a/lib/spack/spack/test/tally_plugin.py b/lib/spack/spack/test/tally_plugin.py
index 9ca898c47c..4163ab95dd 100644
--- a/lib/spack/spack/test/tally_plugin.py
+++ b/lib/spack/spack/test/tally_plugin.py
@@ -6,7 +6,7 @@
 # Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
 # LLNL-CODE-647188
 #
-# For details, see https://scalability-llnl.github.io/spack
+# For details, see https://scalability-software.llnl.gov/spack
 # Please also see the LICENSE file for our notice and the LGPL.
 #
 # This program is free software; you can redistribute it and/or modify
@@ -22,10 +22,10 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-from nose.plugins import Plugin
-
 import os
 
+from nose.plugins import Plugin
+
 class Tally(Plugin):
     name = 'tally'
 
@@ -34,7 +34,7 @@ class Tally(Plugin):
         self.successCount = 0
         self.failCount = 0
         self.errorCount = 0
-    
+
     @property
     def numberOfTestsRun(self):
         """Excludes skipped tests"""
@@ -48,10 +48,10 @@ class Tally(Plugin):
 
     def addSuccess(self, test):
         self.successCount += 1
-        
+
     def addError(self, test, err):
         self.errorCount += 1
-            
+
     def addFailure(self, test, err):
         self.failCount += 1
 
diff --git a/lib/spack/spack/test/unit_install.py b/lib/spack/spack/test/unit_install.py
index ccc409dd60..18615b7efe 100644
--- a/lib/spack/spack/test/unit_install.py
+++ b/lib/spack/spack/test/unit_install.py
@@ -22,10 +22,11 @@
 # along with this program; if not, write to the Free Software Foundation,
 # Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
-import unittest
 import itertools
+import unittest
 
 import spack
+
 test_install = __import__("spack.cmd.test-install",
     fromlist=["BuildId", "create_test_output", "TestResult"])
 
diff --git a/lib/spack/spack/test/url_extrapolate.py b/lib/spack/spack/test/url_extrapolate.py
index 87adf89401..068a335b49 100644
--- a/lib/spack/spack/test/url_extrapolate.py
+++ b/lib/spack/spack/test/url_extrapolate.py
@@ -25,10 +25,7 @@
 """\
 Tests ability of spack to extrapolate URL versions from existing versions.
 """
-import spack
 import spack.url as url
-from spack.spec import Spec
-from spack.version import ver
 from spack.test.mock_packages_test import *
 
 
diff --git a/lib/spack/spack/test/url_parse.py b/lib/spack/spack/test/url_parse.py
index efde7c0c73..561d4658a1 100644
--- a/lib/spack/spack/test/url_parse.py
+++ b/lib/spack/spack/test/url_parse.py
@@ -27,8 +27,8 @@ This file has a bunch of versions tests taken from the excellent version
 detection in Homebrew.
 """
 import unittest
+
 import spack.url as url
-from pprint import pprint
 
 
 class UrlParseTest(unittest.TestCase):
diff --git a/lib/spack/spack/test/url_substitution.py b/lib/spack/spack/test/url_substitution.py
index aec8baf4ea..2be38af0d3 100644
--- a/lib/spack/spack/test/url_substitution.py
+++ b/lib/spack/spack/test/url_substitution.py
@@ -27,7 +27,6 @@ This test does sanity checks on substituting new versions into URLs
 """
 import unittest
 
-import spack
 import spack.url as url
 
 
diff --git a/lib/spack/spack/test/versions.py b/lib/spack/spack/test/versions.py
index 108450e098..2732006eb3 100644
--- a/lib/spack/spack/test/versions.py
+++ b/lib/spack/spack/test/versions.py
@@ -28,6 +28,7 @@ We try to maintain compatibility with RPM's version semantics
 where it makes sense.
 """
 import unittest
+
 from spack.version import *
 
 
diff --git a/lib/spack/spack/test/yaml.py b/lib/spack/spack/test/yaml.py
index 5a357b8e69..b930c022f2 100644
--- a/lib/spack/spack/test/yaml.py
+++ b/lib/spack/spack/test/yaml.py
@@ -26,6 +26,7 @@
 Test Spack's custom YAML format.
 """
 import unittest
+
 import spack.util.spack_yaml as syaml
 
 test_file = """\
diff --git a/lib/spack/spack/url.py b/lib/spack/spack/url.py
index 02c0b83e26..ad551a6ded 100644
--- a/lib/spack/spack/url.py
+++ b/lib/spack/spack/url.py
@@ -225,7 +225,7 @@ def parse_version_offset(path):
         (r'_((\d+\.)+\d+[a-z]?)[.]orig$', stem),
 
         # e.g. http://www.openssl.org/source/openssl-0.9.8s.tar.gz
-        (r'-([^-]+(-alpha|-beta)?)', stem),
+        (r'-v?([^-]+(-alpha|-beta)?)', stem),
 
         # e.g. astyle_1.23_macosx.tar.gz
         (r'_([^_]+(_alpha|_beta)?)', stem),
diff --git a/lib/spack/spack/util/environment.py b/lib/spack/spack/util/environment.py
index cd413dcfbc..ae8e5708be 100644
--- a/lib/spack/spack/util/environment.py
+++ b/lib/spack/spack/util/environment.py
@@ -63,3 +63,10 @@ def pop_keys(dictionary, *keys):
     for key in keys:
         if key in dictionary:
             dictionary.pop(key)
+
+
+def dump_environment(path):
+    """Dump the current environment out to a file."""
+    with open(path, 'w') as env_file:
+        for key,val in sorted(os.environ.items()):
+            env_file.write("%s=%s\n" % (key, val))
diff --git a/lib/spack/spack/util/pattern.py b/lib/spack/spack/util/pattern.py
new file mode 100644
index 0000000000..17a126498b
--- /dev/null
+++ b/lib/spack/spack/util/pattern.py
@@ -0,0 +1,116 @@
+##############################################################################
+# Copyright (c) 2013, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Written by Todd Gamblin, tgamblin@llnl.gov, All rights reserved.
+# LLNL-CODE-647188
+#
+# For details, see https://github.com/llnl/spack
+# Please also see the LICENSE file for our notice and the LGPL.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License (as published by
+# the Free Software Foundation) version 2.1 dated February 1999.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the IMPLIED WARRANTY OF
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the terms and
+# conditions of the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program; if not, write to the Free Software Foundation,
+# Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+##############################################################################
+import inspect
+import collections
+import functools
+
+
+def composite(interface=None, method_list=None, container=list):
+    """
+    Returns a class decorator that patches a class adding all the methods it needs to be a composite for a given
+    interface.
+
+    :param interface: class exposing the interface to which the composite object must conform. Only non-private and
+    non-special methods will be taken into account
+
+    :param method_list: names of methods that should be part of the composite
+
+    :param container: container for the composite object (default = list). Must fulfill the MutableSequence contract.
+    The composite class will expose the container API to manage object composition
+
+    :return: class decorator
+    """
+    # Check if container fulfills the MutableSequence contract and raise an exception if it doesn't
+    # The patched class returned by the decorator will inherit from the container class to expose the
+    # interface needed to manage objects composition
+    if not issubclass(container, collections.MutableSequence):
+        raise TypeError("Container must fulfill the MutableSequence contract")
+
+    # Check if at least one of the 'interface' or the 'method_list' arguments are defined
+    if interface is None and method_list is None:
+        raise TypeError("Either 'interface' or 'method_list' must be defined on a call to composite")
+
+    def cls_decorator(cls):
+        # Retrieve the base class of the composite. Inspect its methods and decide which ones will be overridden
+        def no_special_no_private(x):
+            return inspect.ismethod(x) and not x.__name__.startswith('_')
+
+        # Patch the behavior of each of the methods in the previous list. This is done associating an instance of the
+        # descriptor below to any method that needs to be patched.
+        class IterateOver(object):
+            """
+            Decorator used to patch methods in a composite. It iterates over all the items in the instance containing the
+            associated attribute and calls for each of them an attribute with the same name
+            """
+            def __init__(self, name, func=None):
+                self.name = name
+                self.func = func
+
+            def __get__(self, instance, owner):
+                def getter(*args, **kwargs):
+                    for item in instance:
+                        getattr(item, self.name)(*args, **kwargs)
+                # If we are using this descriptor to wrap a method from an interface, then we must conditionally
+                # use the `functools.wraps` decorator to set the appropriate fields.
+                if self.func is not None:
+                    getter = functools.wraps(self.func)(getter)
+                return getter
+
+        dictionary_for_type_call = {}
+        # Construct a dictionary with the methods explicitly passed as name
+        if method_list is not None:
+            # python@2.7: method_list_dict = {name: IterateOver(name) for name in method_list}
+            method_list_dict = {}
+            for name in method_list:
+                method_list_dict[name] = IterateOver(name)
+            dictionary_for_type_call.update(method_list_dict)
+        # Construct a dictionary with the methods inspected from the interface
+        if interface is not None:
+            ##########
+            # python@2.7: interface_methods = {name: method for name, method in inspect.getmembers(interface, predicate=no_special_no_private)}
+            interface_methods = {}
+            for name, method in inspect.getmembers(interface, predicate=no_special_no_private):
+                interface_methods[name] = method
+            ##########
+            # python@2.7: interface_methods_dict = {name: IterateOver(name, method) for name, method in interface_methods.iteritems()}
+            interface_methods_dict = {}
+            for name, method in interface_methods.iteritems():
+                interface_methods_dict[name] = IterateOver(name, method)
+            ##########
+            dictionary_for_type_call.update(interface_methods_dict)
+        # Get the methods that are defined in the scope of the composite class and override any previous definition
+        ##########
+        # python@2.7: cls_method = {name: method for name, method in inspect.getmembers(cls, predicate=inspect.ismethod)}
+        cls_method = {}
+        for name, method in inspect.getmembers(cls, predicate=inspect.ismethod):
+            cls_method[name] = method
+        ##########
+        dictionary_for_type_call.update(cls_method)
+        # Generate the new class on the fly and return it
+        # FIXME : inherit from interface if we start to use ABC classes?
+        wrapper_class = type(cls.__name__, (cls, container), dictionary_for_type_call)
+        return wrapper_class
+
+    return cls_decorator
diff --git a/lib/spack/spack/util/web.py b/lib/spack/spack/util/web.py
index e26daef296..73f4858b02 100644
--- a/lib/spack/spack/util/web.py
+++ b/lib/spack/spack/util/web.py
@@ -86,12 +86,12 @@ def _spider(args):
 
         if not "Content-type" in resp.headers:
             tty.debug("ignoring page " + url)
-            return pages
+            return pages, links
 
         if not resp.headers["Content-type"].startswith('text/html'):
             tty.debug("ignoring page " + url + " with content type " +
                       resp.headers["Content-type"])
-            return pages
+            return pages, links
 
         # Do the real GET request when we know it's just HTML.
         req.get_method = lambda: "GET"
@@ -173,7 +173,7 @@ def spider(root_url, **kwargs):
        performance over a sequential fetch.
     """
     max_depth = kwargs.setdefault('depth', 1)
-    pages, links =  _spider((root_url, set(), root_url, None, 1, max_depth, False))
+    pages, links = _spider((root_url, set(), root_url, None, 1, max_depth, False))
     return pages, links