Move provider cache to home directory and refactor Transactions

Major stuff:

- Created a FileCache for managing user cache files in Spack.  Currently just
  handles virtuals.

- Moved virtual cache from the repository to the home directory so that users do
  not need write access to Spack repositories to use them.

- Refactored `Transaction` class in `database.py` -- moved it to
  `LockTransaction` in `lock.py` and made it reusable by other classes.

Other additions:

- Added tests for file cache and transactions.

- Added a few more tests for database

- Fixed bug in DB where writes could happen even if exceptions were raised
  during a transaction.

- `spack uninstall` now attempts to repair the database when it discovers that a
  prefix doesn't exist but a DB record does.

18 files changed, 600 insertions, 113 deletions

diff --git a/lib/spack/llnl/util/lock.py b/lib/spack/llnl/util/lock.py
index 479a1b0167..e1f5b4878a 100644
--- a/lib/spack/llnl/util/lock.py
+++ b/lib/spack/llnl/util/lock.py
@@ -28,6 +28,9 @@ import errno
 import time
 import socket
 
+__all__ = ['Lock', 'LockTransaction', 'WriteTransaction', 'ReadTransaction',
+           'LockError']
+
 # Default timeout in seconds, after which locks will raise exceptions.
 _default_timeout = 60
 
@@ -63,7 +66,9 @@ class Lock(object):
 
                 fcntl.lockf(self._fd, op | fcntl.LOCK_NB)
                 if op == fcntl.LOCK_EX:
-                    os.write(self._fd, "pid=%s,host=%s" % (os.getpid(), socket.getfqdn()))
+                    os.write(
+                        self._fd,
+                        "pid=%s,host=%s" % (os.getpid(), socket.getfqdn()))
                 return
 
             except IOError as error:
@@ -170,6 +175,66 @@ class Lock(object):
             return False
 
 
+class LockTransaction(object):
+    """Simple nested transaction context manager that uses a file lock.
+
+    This class can trigger actions when the lock is acquired for the
+    first time and released for the last.
+
+    If the acquire_fn returns a value, it is used as the return value for
+    __enter__, allowing it to be passed as the `as` argument of a `with`
+    statement.
+
+    If acquire_fn returns a context manager, *its* `__enter__` function will be
+    called in `__enter__` after acquire_fn, and its `__exit__` funciton will be
+    called before `release_fn` in `__exit__`, allowing you to nest a context
+    manager to be used along with the lock.
+
+    Timeout for lock is customizable.
+
+    """
+
+    def __init__(self, lock, acquire_fn=None, release_fn=None,
+                 timeout=_default_timeout):
+        self._lock = lock
+        self._timeout = timeout
+        self._acquire_fn = acquire_fn
+        self._release_fn = release_fn
+        self._as = None
+
+    def __enter__(self):
+        if self._enter() and self._acquire_fn:
+            self._as = self._acquire_fn()
+            if hasattr(self._as, '__enter__'):
+                return self._as.__enter__()
+            else:
+                return self._as
+
+    def __exit__(self, type, value, traceback):
+        if self._exit():
+            if self._as and hasattr(self._as, '__exit__'):
+                self._as.__exit__(type, value, traceback)
+            if self._release_fn:
+                self._release_fn(type, value, traceback)
+        if value:
+            raise value
+
+
+class ReadTransaction(LockTransaction):
+    def _enter(self):
+        return self._lock.acquire_read(self._timeout)
+
+    def _exit(self):
+        return self._lock.release_read()
+
+
+class WriteTransaction(LockTransaction):
+    def _enter(self):
+        return self._lock.acquire_write(self._timeout)
+
+    def _exit(self):
+        return self._lock.release_write()
+
+
 class LockError(Exception):
     """Raised when an attempt to acquire a lock times out."""
-    pass
diff --git a/lib/spack/spack/__init__.py b/lib/spack/spack/__init__.py
index d67585aac4..a6e21987c8 100644
--- a/lib/spack/spack/__init__.py
+++ b/lib/spack/spack/__init__.py
@@ -50,8 +50,15 @@ repos_path     = join_path(var_path, "repos")
 share_path     = join_path(spack_root, "share", "spack")
 cache_path     = join_path(var_path, "cache")
 
+# User configuration location
+user_config_path = os.path.expanduser('~/.spack')
+
 import spack.fetch_strategy
-cache = spack.fetch_strategy.FsCache(cache_path)
+fetch_cache = spack.fetch_strategy.FsCache(cache_path)
+
+from spack.file_cache import FileCache
+user_cache_path = join_path(user_config_path, 'cache')
+user_cache = FileCache(user_cache_path)
 
 prefix = spack_root
 opt_path       = join_path(prefix, "opt")
diff --git a/lib/spack/spack/cmd/purge.py b/lib/spack/spack/cmd/purge.py
index f4e27a3969..26745810a8 100644
--- a/lib/spack/spack/cmd/purge.py
+++ b/lib/spack/spack/cmd/purge.py
@@ -33,7 +33,11 @@ def setup_parser(subparser):
         '-s', '--stage', action='store_true', default=True,
         help="Remove all temporary build stages (default).")
     subparser.add_argument(
-        '-c', '--cache', action='store_true', help="Remove cached downloads.")
+        '-d', '--downloads', action='store_true',
+        help="Remove cached downloads.")
+    subparser.add_argument(
+        '-u', '--user-cache', action='store_true',
+        help="Remove caches in user home directory. Includes virtual indices.")
     subparser.add_argument(
         '-a', '--all', action='store_true',
         help="Remove all of the above.")
@@ -49,4 +53,6 @@ def purge(parser, args):
     if args.stage or args.all:
         stage.purge()
     if args.cache or args.all:
-        spack.cache.destroy()
+        spack.fetch_cache.destroy()
+    if args.user_cache or args.all:
+        spack.user_cache.destroy()
diff --git a/lib/spack/spack/cmd/test.py b/lib/spack/spack/cmd/test.py
index 36810321ef..2667b42820 100644
--- a/lib/spack/spack/cmd/test.py
+++ b/lib/spack/spack/cmd/test.py
@@ -41,10 +41,10 @@ def setup_parser(subparser):
     subparser.add_argument(
         '-l', '--list', action='store_true', dest='list', help="Show available tests")
     subparser.add_argument(
-        '--createXmlOutput', action='store_true', dest='createXmlOutput', 
+        '--createXmlOutput', action='store_true', dest='createXmlOutput',
         help="Create JUnit XML from test results")
     subparser.add_argument(
-        '--xmlOutputDir', dest='xmlOutputDir', 
+        '--xmlOutputDir', dest='xmlOutputDir',
         help="Nose creates XML files in this directory")
     subparser.add_argument(
         '-v', '--verbose', action='store_true', dest='verbose',
@@ -62,7 +62,7 @@ class MockCache(object):
 class MockCacheFetcher(object):
     def set_stage(self, stage):
         pass
-    
+
     def fetch(self):
         raise FetchError("Mock cache always fails for tests")
 
@@ -82,8 +82,8 @@ def test(parser, args):
                 outputDir = join_path(os.getcwd(), "test-output")
             else:
                 outputDir = os.path.abspath(args.xmlOutputDir)
-            
+
             if not os.path.exists(outputDir):
                 mkdirp(outputDir)
-        spack.cache = MockCache()
+        spack.fetch_cache = MockCache()
         spack.test.run(args.names, outputDir, args.verbose)
diff --git a/lib/spack/spack/cmd/uninstall.py b/lib/spack/spack/cmd/uninstall.py
index a17b7c685c..dbe6cd6584 100644
--- a/lib/spack/spack/cmd/uninstall.py
+++ b/lib/spack/spack/cmd/uninstall.py
@@ -184,7 +184,8 @@ def uninstall(parser, args):
             uninstall_list = list(set(uninstall_list))
 
         if has_error:
-            tty.die('You can use spack uninstall --dependents to uninstall these dependencies as well')  # NOQA: ignore=E501
+            tty.die('You can use spack uninstall --dependents '
+                    'to uninstall these dependencies as well')
 
         if not args.yes_to_all:
             tty.msg("The following packages will be uninstalled : ")
diff --git a/lib/spack/spack/config.py b/lib/spack/spack/config.py
index 31f0eb3a56..a4e274893c 100644
--- a/lib/spack/spack/config.py
+++ b/lib/spack/spack/config.py
@@ -525,7 +525,7 @@ ConfigScope('defaults', os.path.join(spack.etc_path, 'spack', 'defaults'))
 ConfigScope('site', os.path.join(spack.etc_path, 'spack'))
 
 """User configuration can override both spack defaults and site config."""
-ConfigScope('user', os.path.expanduser('~/.spack'))
+ConfigScope('user', spack.user_config_path)
 
 
 def highest_precedence_scope():
diff --git a/lib/spack/spack/database.py b/lib/spack/spack/database.py
index 317b0d5784..5ce42b2e67 100644
--- a/lib/spack/spack/database.py
+++ b/lib/spack/spack/database.py
@@ -165,11 +165,11 @@ class Database(object):
 
     def write_transaction(self, timeout=_db_lock_timeout):
         """Get a write lock context manager for use in a `with` block."""
-        return WriteTransaction(self, self._read, self._write, timeout)
+        return WriteTransaction(self.lock, self._read, self._write, timeout)
 
     def read_transaction(self, timeout=_db_lock_timeout):
         """Get a read lock context manager for use in a `with` block."""
-        return ReadTransaction(self, self._read, None, timeout)
+        return ReadTransaction(self.lock, self._read, timeout=timeout)
 
     def _write_to_yaml(self, stream):
         """Write out the databsae to a YAML file.
@@ -352,12 +352,22 @@ class Database(object):
                     "Invalid ref_count: %s: %d (expected %d), in DB %s" %
                     (key, found, expected, self._index_path))
 
-    def _write(self):
+    def _write(self, type, value, traceback):
         """Write the in-memory database index to its file path.
 
-        Does no locking.
+        This is a helper function called by the WriteTransaction context
+        manager. If there is an exception while the write lock is active,
+        nothing will be written to the database file, but the in-memory database
+        *may* be left in an inconsistent state.  It will be consistent after the
+        start of the next transaction, when it read from disk again.
+
+        This routine does no locking.
 
         """
+        # Do not write if exceptions were raised
+        if type is not None:
+            return
+
         temp_file = self._index_path + (
             '.%s.%s.temp' % (socket.getfqdn(), os.getpid()))
 
@@ -589,49 +599,6 @@ class Database(object):
             return key in self._data and not self._data[key].installed
 
 
-class _Transaction(object):
-    """Simple nested transaction context manager that uses a file lock.
-
-    This class can trigger actions when the lock is acquired for the
-    first time and released for the last.
-
-    Timeout for lock is customizable.
-    """
-
-    def __init__(self, db,
-                 acquire_fn=None,
-                 release_fn=None,
-                 timeout=_db_lock_timeout):
-        self._db = db
-        self._timeout = timeout
-        self._acquire_fn = acquire_fn
-        self._release_fn = release_fn
-
-    def __enter__(self):
-        if self._enter() and self._acquire_fn:
-            self._acquire_fn()
-
-    def __exit__(self, type, value, traceback):
-        if self._exit() and self._release_fn:
-            self._release_fn()
-
-
-class ReadTransaction(_Transaction):
-    def _enter(self):
-        return self._db.lock.acquire_read(self._timeout)
-
-    def _exit(self):
-        return self._db.lock.release_read()
-
-
-class WriteTransaction(_Transaction):
-    def _enter(self):
-        return self._db.lock.acquire_write(self._timeout)
-
-    def _exit(self):
-        return self._db.lock.release_write()
-
-
 class CorruptDatabaseError(SpackError):
     def __init__(self, path, msg=''):
         super(CorruptDatabaseError, self).__init__(
diff --git a/lib/spack/spack/file_cache.py b/lib/spack/spack/file_cache.py
new file mode 100644
index 0000000000..2124df9c9c
--- /dev/null
+++ b/lib/spack/spack/file_cache.py
@@ -0,0 +1,181 @@
+##############################################################################
+# Copyright (c) 2013-2016, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Created 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 Lesser General Public License (as
+# published by the Free Software Foundation) version 2.1, 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 Lesser 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 shutil
+
+from llnl.util.filesystem import *
+from llnl.util.lock import *
+
+import spack
+from spack.error import SpackError
+
+
+class FileCache(object):
+    """This class manages cached data in the filesystem.
+
+    - Cache files are fetched and stored by unique keys.  Keys can be relative
+      paths, so that thre can be some hierarchy in the cache.
+
+    - The FileCache handles locking cache files for reading and writing, so
+      client code need not manage locks for cache entries.
+
+    """
+    def __init__(self, root):
+        """Create a file cache object.
+
+        This will create the cache directory if it does not exist yet.
+
+        """
+        self.root = root.rstrip(os.path.sep)
+        if not os.path.exists(self.root):
+            mkdirp(self.root)
+
+        self._locks = {}
+
+    def purge(self):
+        """Remove all files under the cache root."""
+        for f in os.listdir(self.root):
+            path = join_path(self.root, f)
+            shutil.rmtree(f)
+
+    def cache_path(self, key):
+        """Path to the file in the cache for a particular key."""
+        return join_path(self.root, key)
+
+    def _lock_path(self, key):
+        """Path to the file in the cache for a particular key."""
+        keyfile = os.path.basename(key)
+        keydir = os.path.dirname(key)
+
+        return join_path(self.root, keydir, '.' + keyfile + '.lock')
+
+    def _get_lock(self, key):
+        """Create a lock for a key, if necessary, and return a lock object."""
+        if key not in self._locks:
+            lock_file = self._lock_path(key)
+            if not os.path.exists(lock_file):
+                touch(lock_file)
+            self._locks[key] = Lock(lock_file)
+        return self._locks[key]
+
+    def init_entry(self, key):
+        """Ensure we can access a cache file. Create a lock for it if needed.
+
+        Return whether the cache file exists yet or not.
+        """
+        cache_path = self.cache_path(key)
+
+        exists = os.path.exists(cache_path)
+        if exists:
+            if not os.path.isfile(cache_path):
+                raise CacheError("Cache file is not a file: %s" % cache_path)
+
+            if not os.access(cache_path, os.R_OK|os.W_OK):
+                raise CacheError("Cannot access cache file: %s" % cache_path)
+        else:
+            # if the file is hierarchical, make parent directories
+            parent = os.path.dirname(cache_path)
+            if parent.rstrip(os.path.sep) != self.root:
+                mkdirp(parent)
+
+            if not os.access(parent, os.R_OK|os.W_OK):
+                raise CacheError("Cannot access cache directory: %s" % parent)
+
+            # ensure lock is created for this key
+            self._get_lock(key)
+        return exists
+
+    def read_transaction(self, key):
+        """Get a read transaction on a file cache item.
+
+        Returns a ReadTransaction context manager and opens the cache file for
+        reading.  You can use it like this:
+
+           with spack.user_cache.read_transaction(key) as cache_file:
+               cache_file.read()
+
+        """
+        return ReadTransaction(
+            self._get_lock(key), lambda: open(self.cache_path(key)))
+
+    def write_transaction(self, key):
+        """Get a write transaction on a file cache item.
+
+        Returns a WriteTransaction context manager that opens a temporary file
+        for writing.  Once the context manager finishes, if nothing went wrong,
+        moves the file into place on top of the old file atomically.
+
+        """
+        class WriteContextManager(object):
+            def __enter__(cm):
+                cm.orig_filename = self.cache_path(key)
+                cm.orig_file = None
+                if os.path.exists(cm.orig_filename):
+                    cm.orig_file = open(cm.orig_filename, 'r')
+
+                cm.tmp_filename = self.cache_path(key) + '.tmp'
+                cm.tmp_file = open(cm.tmp_filename, 'w')
+
+                return cm.orig_file, cm.tmp_file
+
+            def __exit__(cm, type, value, traceback):
+                if cm.orig_file:
+                    cm.orig_file.close()
+                cm.tmp_file.close()
+
+                if value:
+                    # remove tmp on exception & raise it
+                    shutil.rmtree(cm.tmp_filename, True)
+                    raise value
+                else:
+                    os.rename(cm.tmp_filename, cm.orig_filename)
+
+        return WriteTransaction(self._get_lock(key), WriteContextManager)
+
+
+    def mtime(self, key):
+        """Return modification time of cache file, or 0 if it does not exist.
+
+        Time is in units returned by os.stat in the mtime field, which is
+        platform-dependent.
+
+        """
+        if not self.init_entry(key):
+            return 0
+        else:
+            sinfo = os.stat(self.cache_path(key))
+            return sinfo.st_mtime
+
+
+    def remove(self, key):
+        lock = self._get_lock(key)
+        try:
+            lock.acquire_write()
+            os.unlink(self.cache_path(key))
+        finally:
+            lock.release_write()
+        os.unlink(self._lock_path(key))
+
+class CacheError(SpackError): pass
diff --git a/lib/spack/spack/modules.py b/lib/spack/spack/modules.py
index 8701a31c49..8ac6a77d13 100644
--- a/lib/spack/spack/modules.py
+++ b/lib/spack/spack/modules.py
@@ -520,7 +520,8 @@ class Dotkit(EnvModule):
 
     def prerequisite(self, spec):
         tty.warn('prerequisites:  not supported by dotkit module files')
-        tty.warn('\tYou may want to check  ~/.spack/modules.yaml')
+        tty.warn('\tYou may want to check %s/modules.yaml'
+                 % spack.user_config_path)
         return ''
 
 
diff --git a/lib/spack/spack/package.py b/lib/spack/spack/package.py
index 43aefbf65e..475155937c 100644
--- a/lib/spack/spack/package.py
+++ b/lib/spack/spack/package.py
@@ -1198,7 +1198,15 @@ class Package(object):
 
     def do_uninstall(self, force=False):
         if not self.installed:
-            raise InstallError(str(self.spec) + " is not installed.")
+            # prefix may not exist, but DB may be inconsistent. Try to fix by
+            # removing, but omit hooks.
+            specs = spack.installed_db.query(self.spec, installed=True)
+            if specs:
+                spack.installed_db.remove(specs[0])
+                tty.msg("Removed stale DB entry for %s" % self.spec.short_spec)
+                return
+            else:
+                raise InstallError(str(self.spec) + " is not installed.")
 
         if not force:
             dependents = self.installed_dependents
diff --git a/lib/spack/spack/repository.py b/lib/spack/spack/repository.py
index 58747ba25d..a0904a2cde 100644
--- a/lib/spack/spack/repository.py
+++ b/lib/spack/spack/repository.py
@@ -41,6 +41,7 @@ import llnl.util.tty as tty
 from llnl.util.lock import Lock
 from llnl.util.filesystem import *
 
+import spack
 import spack.error
 import spack.config
 import spack.spec
@@ -414,17 +415,6 @@ class Repo(object):
         check(os.path.isdir(self.packages_path),
               "No directory '%s' found in '%s'" % (repo_config_name, root))
 
-        self.index_file = join_path(self.root, repo_index_name)
-        check(not os.path.exists(self.index_file) or
-              (os.path.isfile(self.index_file) and os.access(self.index_file, os.R_OK|os.W_OK)),
-              "Cannot access repository index file in %s" % root)
-
-        # lock file for reading/writing the index
-        self._lock_path = join_path(self.root, 'lock')
-        if not os.path.exists(self._lock_path):
-            touch(self._lock_path)
-        self._lock = Lock(self._lock_path)
-
         # Read configuration and validate namespace
         config = self._read_config()
         check('namespace' in config, '%s must define a namespace.'
@@ -461,6 +451,8 @@ class Repo(object):
         # make sure the namespace for packages in this repo exists.
         self._create_namespace()
 
+        # Unique filename for cache of virtual dependency providers
+        self._cache_file = 'providers/%s-index.yaml' % self.namespace
 
     def _create_namespace(self):
         """Create this repo's namespace module and insert it into sys.modules.
@@ -658,21 +650,15 @@ class Repo(object):
                 self._provider_index = ProviderIndex.from_yaml(f)
 
         # Read the old ProviderIndex, or make a new one.
-        index_existed = os.path.isfile(self.index_file)
+        key = self._cache_file
+        index_existed = spack.user_cache.init_entry(key)
         if index_existed and not self._needs_update:
-            self._lock.acquire_read()
-            try:
-                read()
-            finally:
-                self._lock.release_read()
-
+            with spack.user_cache.read_transaction(key) as f:
+                self._provider_index = ProviderIndex.from_yaml(f)
         else:
-            tmp = self.index_file + '.tmp'
-            self._lock.acquire_write()
-            try:
-                if index_existed:
-                    with open(self.index_file) as f:
-                        self._provider_index = ProviderIndex.from_yaml(f)
+            with spack.user_cache.write_transaction(key) as (old, new):
+                if old:
+                    self._provider_index = ProviderIndex.from_yaml(old)
                 else:
                     self._provider_index = ProviderIndex()
 
@@ -681,17 +667,7 @@ class Repo(object):
                     self._provider_index.remove_provider(namespaced_name)
                     self._provider_index.update(namespaced_name)
 
-
-                with open(tmp, 'w') as f:
-                    self._provider_index.to_yaml(f)
-                os.rename(tmp, self.index_file)
-
-            except:
-                shutil.rmtree(tmp, ignore_errors=True)
-                raise
-
-            finally:
-                self._lock.release_write()
+                self._provider_index.to_yaml(new)
 
 
     @property
@@ -745,7 +721,7 @@ class Repo(object):
 
 
     def _fast_package_check(self):
-        """List packages in the repo and cehck whether index is up to date.
+        """List packages in the repo and check whether index is up to date.
 
         Both of these opreations require checking all `package.py`
         files so we do them at the same time.  We list the repo
@@ -763,10 +739,7 @@ class Repo(object):
             self._all_package_names = []
 
             # Get index modification time.
-            index_mtime = 0
-            if os.path.exists(self.index_file):
-                sinfo = os.stat(self.index_file)
-                index_mtime = sinfo.st_mtime
+            index_mtime = spack.user_cache.mtime(self._cache_file)
 
             for pkg_name in os.listdir(self.packages_path):
                 # Skip non-directories in the package root.
@@ -774,8 +747,9 @@ class Repo(object):
 
                 # Warn about invalid names that look like packages.
                 if not valid_module_name(pkg_name):
-                    tty.warn("Skipping package at %s. '%s' is not a valid Spack module name."
-                             % (pkg_dir, pkg_name))
+                    msg = ("Skipping package at %s. "
+                           "'%s' is not a valid Spack module name.")
+                    tty.warn(msg % (pkg_dir, pkg_name))
                     continue
 
                 # construct the file name from the directory
diff --git a/lib/spack/spack/stage.py b/lib/spack/spack/stage.py
index 8fcc331482..7676cb9ab6 100644
--- a/lib/spack/spack/stage.py
+++ b/lib/spack/spack/stage.py
@@ -315,7 +315,8 @@ class Stage(object):
             # Add URL strategies for all the mirrors with the digest
             for url in urls:
                 fetchers.insert(0, fs.URLFetchStrategy(url, digest))
-            fetchers.insert(0, spack.cache.fetcher(self.mirror_path, digest))
+            fetchers.insert(0, spack.fetch_cache.fetcher(self.mirror_path,
+                                                         digest))
 
             # Look for the archive in list_url
             package_name = os.path.dirname(self.mirror_path)
@@ -365,7 +366,7 @@ class Stage(object):
             self.fetcher.check()
 
     def cache_local(self):
-        spack.cache.store(self.fetcher, self.mirror_path)
+        spack.fetch_cache.store(self.fetcher, self.mirror_path)
 
     def expand_archive(self):
         """Changes to the stage directory and attempt to expand the downloaded
diff --git a/lib/spack/spack/test/__init__.py b/lib/spack/spack/test/__init__.py
index 7795cb59c7..4969081e63 100644
--- a/lib/spack/spack/test/__init__.py
+++ b/lib/spack/spack/test/__init__.py
@@ -49,6 +49,7 @@ test_names = [
     'database',
     'directory_layout',
     'environment',
+    'file_cache',
     'git_fetch',
     'hg_fetch',
     'install',
diff --git a/lib/spack/spack/test/database.py b/lib/spack/spack/test/database.py
index e1322f2081..a2f09450bc 100644
--- a/lib/spack/spack/test/database.py
+++ b/lib/spack/spack/test/database.py
@@ -273,3 +273,37 @@ class DatabaseTest(MockDatabase):
         # mpich ref count updated properly.
         mpich_rec = self.installed_db.get_record('mpich')
         self.assertEqual(mpich_rec.ref_count, 0)
+
+    def test_100_no_write_with_exception_on_remove(self):
+        def fail_while_writing():
+            with self.installed_db.write_transaction():
+                self._mock_remove('mpileaks ^zmpi')
+                raise Exception()
+
+        with self.installed_db.read_transaction():
+            self.assertEqual(
+                len(self.installed_db.query('mpileaks ^zmpi', installed=any)), 1)
+
+        self.assertRaises(Exception, fail_while_writing)
+
+        # reload DB and make sure zmpi is still there.
+        with self.installed_db.read_transaction():
+            self.assertEqual(
+                len(self.installed_db.query('mpileaks ^zmpi', installed=any)), 1)
+
+    def test_110_no_write_with_exception_on_install(self):
+        def fail_while_writing():
+            with self.installed_db.write_transaction():
+                self._mock_install('cmake')
+                raise Exception()
+
+        with self.installed_db.read_transaction():
+            self.assertEqual(
+                self.installed_db.query('cmake', installed=any), [])
+
+        self.assertRaises(Exception, fail_while_writing)
+
+        # reload DB and make sure cmake was not written.
+        with self.installed_db.read_transaction():
+            self.assertEqual(
+                self.installed_db.query('cmake', installed=any), [])
diff --git a/lib/spack/spack/test/file_cache.py b/lib/spack/spack/test/file_cache.py
new file mode 100644
index 0000000000..6142b135eb
--- /dev/null
+++ b/lib/spack/spack/test/file_cache.py
@@ -0,0 +1,84 @@
+##############################################################################
+# Copyright (c) 2013-2016, Lawrence Livermore National Security, LLC.
+# Produced at the Lawrence Livermore National Laboratory.
+#
+# This file is part of Spack.
+# Created 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 Lesser General Public License (as
+# published by the Free Software Foundation) version 2.1, 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 Lesser 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 Spack's FileCache.
+"""
+import os
+import shutil
+import tempfile
+import unittest
+
+import spack
+from spack.file_cache import FileCache
+
+
+class FileCacheTest(unittest.TestCase):
+    """Ensure that a file cache can properly write to a file and recover its
+       contents."""
+
+    def setUp(self):
+        self.scratch_dir = tempfile.mkdtemp()
+        self.cache = FileCache(self.scratch_dir)
+
+    def tearDown(self):
+        shutil.rmtree(self.scratch_dir)
+
+    def test_write_and_read_cache_file(self):
+        """Test writing then reading a cached file."""
+        with self.cache.write_transaction('test.yaml') as (old, new):
+            self.assertTrue(old is None)
+            self.assertTrue(new is not None)
+            new.write("foobar\n")
+
+        with self.cache.read_transaction('test.yaml') as stream:
+            text = stream.read()
+            self.assertEqual("foobar\n", text)
+
+    def test_remove(self):
+        """Test removing an entry from the cache."""
+        self.test_write_and_write_cache_file()
+
+        self.cache.remove('test.yaml')
+
+        self.assertFalse(os.path.exists(self.cache.cache_path('test.yaml')))
+        self.assertFalse(os.path.exists(self.cache._lock_path('test.yaml')))
+
+    def test_write_and_write_cache_file(self):
+        """Test two write transactions on a cached file."""
+        with self.cache.write_transaction('test.yaml') as (old, new):
+            self.assertTrue(old is None)
+            self.assertTrue(new is not None)
+            new.write("foobar\n")
+
+        with self.cache.write_transaction('test.yaml') as (old, new):
+            self.assertTrue(old is not None)
+            text = old.read()
+            self.assertEqual("foobar\n", text)
+            self.assertTrue(new is not None)
+            new.write("barbaz\n")
+
+        with self.cache.read_transaction('test.yaml') as stream:
+            text = stream.read()
+            self.assertEqual("barbaz\n", text)
diff --git a/lib/spack/spack/test/lock.py b/lib/spack/spack/test/lock.py
index 0e9f6daf4d..aaf573241b 100644
--- a/lib/spack/spack/test/lock.py
+++ b/lib/spack/spack/test/lock.py
@@ -187,7 +187,6 @@ class LockTest(unittest.TestCase):
             barrier.wait() # ---------------------------------------- 13
             lock.release_read()
 
-
         def p2(barrier):
             lock = Lock(self.lock_path)
 
@@ -224,7 +223,6 @@ class LockTest(unittest.TestCase):
             barrier.wait() # ---------------------------------------- 13
             lock.release_read()
 
-
         def p3(barrier):
             lock = Lock(self.lock_path)
 
@@ -262,3 +260,164 @@ class LockTest(unittest.TestCase):
             lock.release_read()
 
         self.multiproc_test(p1, p2, p3)
+
+    def test_transaction(self):
+        def enter_fn():
+            vals['entered'] = True
+
+        def exit_fn(t, v, tb):
+            vals['exited'] = True
+            vals['exception'] = (t or v or tb)
+
+        lock = Lock(self.lock_path)
+        vals = {'entered': False, 'exited': False, 'exception': False }
+        with ReadTransaction(lock, enter_fn, exit_fn): pass
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertFalse(vals['exception'])
+
+        vals = {'entered': False, 'exited': False, 'exception': False }
+        with WriteTransaction(lock, enter_fn, exit_fn): pass
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertFalse(vals['exception'])
+
+    def test_transaction_with_exception(self):
+        def enter_fn():
+            vals['entered'] = True
+
+        def exit_fn(t, v, tb):
+            vals['exited'] = True
+            vals['exception'] = (t or v or tb)
+
+        lock = Lock(self.lock_path)
+
+        def do_read_with_exception():
+            with ReadTransaction(lock, enter_fn, exit_fn):
+                raise Exception()
+
+        def do_write_with_exception():
+            with WriteTransaction(lock, enter_fn, exit_fn):
+                raise Exception()
+
+        vals = {'entered': False, 'exited': False, 'exception': False }
+        self.assertRaises(Exception, do_read_with_exception)
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertTrue(vals['exception'])
+
+        vals = {'entered': False, 'exited': False, 'exception': False }
+        self.assertRaises(Exception, do_write_with_exception)
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertTrue(vals['exception'])
+
+    def test_transaction_with_context_manager(self):
+        class TestContextManager(object):
+            def __enter__(self):
+                vals['entered'] = True
+
+            def __exit__(self, t, v, tb):
+                vals['exited'] = True
+                vals['exception'] = (t or v or tb)
+
+        def exit_fn(t, v, tb):
+            vals['exited_fn'] = True
+            vals['exception_fn'] = (t or v or tb)
+
+        lock = Lock(self.lock_path)
+
+        vals = {'entered': False, 'exited': False, 'exited_fn': False,
+                'exception': False, 'exception_fn': False }
+        with ReadTransaction(lock, TestContextManager, exit_fn): pass
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertFalse(vals['exception'])
+        self.assertTrue(vals['exited_fn'])
+        self.assertFalse(vals['exception_fn'])
+
+        vals = {'entered': False, 'exited': False, 'exited_fn': False,
+                'exception': False, 'exception_fn': False }
+        with ReadTransaction(lock, TestContextManager): pass
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertFalse(vals['exception'])
+        self.assertFalse(vals['exited_fn'])
+        self.assertFalse(vals['exception_fn'])
+
+        vals = {'entered': False, 'exited': False, 'exited_fn': False,
+                'exception': False, 'exception_fn': False }
+        with WriteTransaction(lock, TestContextManager, exit_fn): pass
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertFalse(vals['exception'])
+        self.assertTrue(vals['exited_fn'])
+        self.assertFalse(vals['exception_fn'])
+
+        vals = {'entered': False, 'exited': False, 'exited_fn': False,
+                'exception': False, 'exception_fn': False }
+        with WriteTransaction(lock, TestContextManager): pass
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertFalse(vals['exception'])
+        self.assertFalse(vals['exited_fn'])
+        self.assertFalse(vals['exception_fn'])
+
+    def test_transaction_with_context_manager_and_exception(self):
+        class TestContextManager(object):
+            def __enter__(self):
+                vals['entered'] = True
+
+            def __exit__(self, t, v, tb):
+                vals['exited'] = True
+                vals['exception'] = (t or v or tb)
+
+        def exit_fn(t, v, tb):
+            vals['exited_fn'] = True
+            vals['exception_fn'] = (t or v or tb)
+
+        lock = Lock(self.lock_path)
+
+        def do_read_with_exception(exit_fn):
+            with ReadTransaction(lock, TestContextManager, exit_fn):
+                raise Exception()
+
+        def do_write_with_exception(exit_fn):
+            with WriteTransaction(lock, TestContextManager, exit_fn):
+                raise Exception()
+
+        vals = {'entered': False, 'exited': False, 'exited_fn': False,
+                'exception': False, 'exception_fn': False }
+        self.assertRaises(Exception, do_read_with_exception, exit_fn)
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertTrue(vals['exception'])
+        self.assertTrue(vals['exited_fn'])
+        self.assertTrue(vals['exception_fn'])
+
+        vals = {'entered': False, 'exited': False, 'exited_fn': False,
+                'exception': False, 'exception_fn': False }
+        self.assertRaises(Exception, do_read_with_exception, None)
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertTrue(vals['exception'])
+        self.assertFalse(vals['exited_fn'])
+        self.assertFalse(vals['exception_fn'])
+
+        vals = {'entered': False, 'exited': False, 'exited_fn': False,
+                'exception': False, 'exception_fn': False }
+        self.assertRaises(Exception, do_write_with_exception, exit_fn)
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertTrue(vals['exception'])
+        self.assertTrue(vals['exited_fn'])
+        self.assertTrue(vals['exception_fn'])
+
+        vals = {'entered': False, 'exited': False, 'exited_fn': False,
+                'exception': False, 'exception_fn': False }
+        self.assertRaises(Exception, do_write_with_exception, None)
+        self.assertTrue(vals['entered'])
+        self.assertTrue(vals['exited'])
+        self.assertTrue(vals['exception'])
+        self.assertFalse(vals['exited_fn'])
+        self.assertFalse(vals['exception_fn'])
diff --git a/lib/spack/spack/test/mock_database.py b/lib/spack/spack/test/mock_database.py
index b1194f2451..da01e82bfa 100644
--- a/lib/spack/spack/test/mock_database.py
+++ b/lib/spack/spack/test/mock_database.py
@@ -95,8 +95,10 @@ class MockDatabase(MockPackagesTest):
             self._mock_install('mpileaks ^zmpi')
 
     def tearDown(self):
-        for spec in spack.installed_db.query():
-            spec.package.do_uninstall(spec)
+        with spack.installed_db.write_transaction():
+            for spec in spack.installed_db.query():
+                spec.package.do_uninstall(spec)
+
         super(MockDatabase, self).tearDown()
         shutil.rmtree(self.install_path)
         spack.install_path = self.spack_install_path
diff --git a/lib/spack/spack/test/provider_index.py b/lib/spack/spack/test/provider_index.py
index 7d5f997b0a..861814e0ae 100644
--- a/lib/spack/spack/test/provider_index.py
+++ b/lib/spack/spack/test/provider_index.py
@@ -94,7 +94,3 @@ class ProviderIndexTest(MockPackagesTest):
         p = ProviderIndex(spack.repo.all_package_names())
         q = p.copy()
         self.assertEqual(p, q)
-
-
-    def test_copy(self):
-        pass