1 files changed, 170 insertions, 2 deletions
diff --git a/lib/spack/docs/build_systems/rubypackage.rst b/lib/spack/docs/build_systems/rubypackage.rst
index 6310507809..d1deaf6fa7 100644
--- a/lib/spack/docs/build_systems/rubypackage.rst
+++ b/lib/spack/docs/build_systems/rubypackage.rst
@@ -12,5 +12,173 @@ RubyPackage
 Like Perl, Python, and R, Ruby has its own build system for
 installing Ruby gems.
 
-This build system is a work-in-progress. See
-https://github.com/spack/spack/pull/3127 for more information.
+^^^^^^
+Phases
+^^^^^^
+
+The ``RubyPackage`` base class provides the following phases that
+can be overridden:
+
+#. ``build`` - build everything needed to install
+#. ``install`` - install everything from build directory
+
+For packages that come with a ``*.gemspec`` file, these phases run:
+
+.. code-block:: console
+
+   $ gem build *.gemspec
+   $ gem install *.gem
+
+
+For packages that come with a ``Rakefile`` file, these phases run:
+
+.. code-block:: console
+
+   $ rake package
+   $ gem install *.gem
+
+
+For packages that come pre-packaged as a ``*.gem`` file, the build
+phase is skipped and the install phase runs:
+
+.. code-block:: console
+
+   $ gem install *.gem
+
+
+These are all standard ``gem`` commands and can be found by running:
+
+.. code-block:: console
+
+   $ gem help commands
+
+
+For packages that only distribute ``*.gem`` files, these files can be
+downloaded with the ``expand=False`` option in the ``version`` directive.
+The build phase will be automatically skipped.
+
+^^^^^^^^^^^^^^^
+Important files
+^^^^^^^^^^^^^^^
+
+When building from source, Ruby packages can be identified by the
+presence of any of the following files:
+
+* ``*.gemspec``
+* ``Rakefile``
+* ``setup.rb`` (not yet supported)
+
+However, not all Ruby packages are released as source code. Some are only
+released as ``*.gem`` files. These files can be extracted using:
+
+.. code-block:: console
+
+   $ gem unpack *.gem
+
+
+^^^^^^^^^^^
+Description
+^^^^^^^^^^^
+
+The ``*.gemspec`` file may contain something like:
+
+.. code-block:: ruby
+
+   summary = 'An implementation of the AsciiDoc text processor and publishing toolchain'
+   description = 'A fast, open source text processor and publishing toolchain for converting AsciiDoc content to HTML 5, DocBook 5, and other formats.'
+
+
+Either of these can be used for the description of the Spack package.
+
+^^^^^^^^
+Homepage
+^^^^^^^^
+
+The ``*.gemspec`` file may contain something like:
+
+.. code-block:: ruby
+
+   homepage = 'https://asciidoctor.org'
+
+
+This should be used as the official homepage of the Spack package.
+
+^^^^^^^^^^^^^^^^^^^^^^^^^
+Build system dependencies
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+All Ruby packages require Ruby at build and run-time. For this reason,
+the base class contains:
+
+.. code-block:: python
+
+   extends('ruby')
+   depends_on('ruby', type=('build', 'run'))
+
+
+The ``*.gemspec`` file may contain something like:
+
+.. code-block:: ruby
+
+   required_ruby_version = '>= 2.3.0'
+
+
+This can be added to the Spack package using:
+
+.. code-block:: python
+
+   depends_on('ruby@2.3.0:', type=('build', 'run'))
+
+
+^^^^^^^^^^^^^^^^^
+Ruby dependencies
+^^^^^^^^^^^^^^^^^
+
+When you install a package with ``gem``, it reads the ``*.gemspec``
+file in order to determine the dependencies of the package.
+If the dependencies are not yet installed, ``gem`` downloads them
+and installs them for you. This may sound convenient, but Spack
+cannot rely on this behavior for two reasons:
+
+#. Spack needs to be able to install packages on air-gapped networks.
+
+   If there is no internet connection, ``gem`` can't download the
+   package dependencies. By explicitly listing every dependency in
+   the ``package.py``, Spack knows what to download ahead of time.
+
+#. Duplicate installations of the same dependency may occur.
+
+   Spack supports *activation* of Ruby extensions, which involves
+   symlinking the package installation prefix to the Ruby installation
+   prefix. If your package is missing a dependency, that dependency
+   will be installed to the installation directory of the same package.
+   If you try to activate the package + dependency, it may cause a
+   problem if that package has already been activated.
+
+For these reasons, you must always explicitly list all dependencies.
+Although the documentation may list the package's dependencies,
+often the developers assume people will use ``gem`` and won't have to
+worry about it. Always check the ``*.gemspec`` file to find the true
+dependencies.
+
+Check for the following clues in the ``*.gemspec`` file:
+
+* ``add_runtime_dependency``
+
+  These packages are required for installation.
+
+* ``add_dependency``
+
+  This is an alias for ``add_runtime_dependency``
+
+* ``add_development_dependency``
+
+  These packages are optional dependencies used for development.
+  They should not be added as dependencies of the package.
+
+^^^^^^^^^^^^^^^^^^^^^^
+External documentation
+^^^^^^^^^^^^^^^^^^^^^^
+
+For more information on Ruby packaging, see:
+https://guides.rubygems.org/