Documentation and new method for CachedCMakePackage build system (#22706)

Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>

lib/spack/docs/build_systems.rst

@@ -39,6 +39,7 @@ on these ideas for each distinct build system that Spack supports:
 
    build_systems/autotoolspackage
    build_systems/cmakepackage
+   build_systems/cachedcmakepackage
    build_systems/mesonpackage
    build_systems/qmakepackage
    build_systems/sippackage

lib/spack/docs/build_systems/cachedcmakepackage.rst

@@ -0,0 +1,123 @@
+.. Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+   Spack Project Developers. See the top-level COPYRIGHT file for details.
+
+   SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+.. _cachedcmakepackage:
+
+------------------
+CachedCMakePackage
+------------------
+
+The CachedCMakePackage base class is used for CMake-based workflows
+that create a CMake cache file prior to running ``cmake``. This is
+useful for packages with arguments longer than the system limit, and
+for reproducibility.
+
+The documentation for this class assumes that the user is familiar with
+the ``CMakePackage`` class from which it inherits. See the documentation
+for :ref:`CMakePackage <cmakepackage>`.
+
+^^^^^^
+Phases
+^^^^^^
+
+The ``CachedCMakePackage`` base class comes with the following phases:
+
+#. ``initconfig`` - generate the CMake cache file
+#. ``cmake`` - generate the Makefile
+#. ``build`` - build the package
+#. ``install`` - install the package
+
+By default, these phases run:
+
+.. code-block:: console
+
+   $ mkdir spack-build
+   $ cd spack-build
+   $ cat << EOF > name-arch-compiler@version.cmake
+   # Write information on compilers and dependencies
+   # includes information on mpi and cuda if applicable
+   $ cmake .. -DCMAKE_INSTALL_PREFIX=/path/to/installation/prefix -C name-arch-compiler@version.cmake
+   $ make
+   $ make test  # optional
+   $ make install
+
+The ``CachedCMakePackage`` class inherits from the ``CMakePackage``
+class, and accepts all of the same options and adds all of the same
+flags to the ``cmake`` command. Similar to the ``CMakePAckage`` class,
+you may need to add a few arguments yourself, and the
+``CachedCMakePackage`` provides the same interface to add those
+flags.
+
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Adding entries to the CMake cache
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In addition to adding flags to the ``cmake`` command, you may need to
+add entries to the CMake cache in the ``initconfig`` phase. This can
+be done by overriding one of four methods:
+
+#. ``CachedCMakePackage.initconfig_compiler_entries``
+#. ``CachedCMakePackage.initconfig_mpi_entries``
+#. ``CachedCMakePackage.initconfig_hardware_entries``
+#. ``CachedCMakePackage.initconfig_package_entries``
+
+Each of these methods returns a list of CMake cache strings. The
+distinction between these methods is merely to provide a
+well-structured and legible cmake cache file -- otherwise, entries
+from each of these methods are handled identically.
+
+Spack also provides convenience methods for generating CMake cache
+entries. These methods are available at module scope in every Spack
+package. Because CMake parses boolean options, strings, and paths
+differently, there are three such methods:
+
+#. ``cmake_cache_option``
+#. ``cmake_cache_string``
+#. ``cmake_cache_path``
+
+These methods each accept three parameters -- the name of the CMake
+variable associated with the entry, the value of the entry, and an
+optional comment -- and return strings in the appropriate format to be
+returned from any of the ``initconfig*`` methods. Additionally, these
+methods may return comments beginning with the ``#`` character.
+
+A typical usage of these methods may look something like this:
+
+.. code-block:: python
+
+   def initconfig_mpi_entries(self)
+       # Get existing MPI configurations
+       entries = super(self, Foo).initconfig_mpi_entries()
+
+       # The existing MPI configurations key on whether ``mpi`` is in the spec
+       # This spec has an MPI variant, and we need to enable MPI when it is on.
+       # This hypothetical package controls MPI with the ``FOO_MPI`` option to
+       # cmake.
+       if '+mpi' in self.spec:
+           entries.append(cmake_cache_option('FOO_MPI', True, "enable mpi"))
+       else:
+           entries.append(cmake_cache_option('FOO_MPI', False, "disable mpi"))
+
+   def initconfig_package_entries(self):
+       # Package specific options
+       entries = []
+
+       entries.append('#Entries for build options')
+
+       bar_on = '+bar' in self.spec
+       entries.append(cmake_cache_option('FOO_BAR', bar_on, 'toggle bar'))
+
+       entries.append('#Entries for dependencies')
+
+       if self.spec['blas'].name == 'baz':  # baz is our blas provider
+       entries.append(cmake_cache_string('FOO_BLAS', 'baz', 'Use baz'))
+       entries.append(cmake_cache_path('BAZ_PREFIX', self.spec['baz'].prefix))
+
+^^^^^^^^^^^^^^^^^^^^^^
+External documentation
+^^^^^^^^^^^^^^^^^^^^^^
+
+For more information on CMake cache files, see:
+https://cmake.org/cmake/help/latest/manual/cmake.1.html

lib/spack/spack/build_systems/cached_cmake.py

@@ -210,6 +210,10 @@ def std_initconfig_entries(self):
             "#------------------{0}\n".format("-" * 60),
         ]
 
+    def initconfig_package_entries(self):
+        """This method is to be overwritten by the package"""
+        return []
+
     def initconfig(self, spec, prefix):
         cache_entries = (self.std_initconfig_entries() +
                          self.initconfig_compiler_entries() +