[libcxx] Update testing documentation about CI container images. (llvm#149192)

Add information to the libcxx testing documentation, about the names of
the new CI libcxx runner sets, their current values, and how to change
the values or the runner set being used.

Author: cmtice

libcxx/docs/Contributing.rst

@@ -174,10 +174,11 @@ Pre-commit CI
 Introduction
 ------------
 
-Unlike most parts of the LLVM project, libc++ uses a pre-commit CI [#]_. This
-CI is hosted on `Buildkite <https://buildkite.com/llvm-project/libcxx-ci>`__ and
-the build results are visible in the review on GitHub. Please make sure
-the CI is green before committing a patch.
+Unlike most parts of the LLVM project, libc++ uses a pre-commit CI [#]_. Some of
+this CI is hosted on `Buildkite <https://buildkite.com/llvm-project/libcxx-ci>`__,
+but some has migrated to the LLVM CI infrastructure. The build results are
+visible in the review on GitHub. Please make sure the CI is green before
+committing a patch.
 
 The CI tests libc++ for all :ref:`supported platforms <SupportedPlatforms>`.
 The build is started for every commit added to a Pull Request. A complete CI
@@ -246,21 +247,89 @@ Below is a short description of the most interesting CI builds [#]_:
 Infrastructure
 --------------
 
-All files of the CI infrastructure are in the directory ``libcxx/utils/ci``.
-Note that quite a bit of this infrastructure is heavily Linux focused. This is
-the platform used by most of libc++'s Buildkite runners and developers.
+The files for the CI infrastructure are split between the llvm-project
+and the llvm-zorg repositories. All files of the CI infrastructure in
+the llvm-project are in the directory ``libcxx/utils/ci``. Note that
+quite a bit of this infrastructure is heavily Linux focused. This is
+the platform used by most of libc++'s Buildkite runners and
+developers.
 
-Dockerfile
-~~~~~~~~~~
+Dockerfile/Container Images
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Contains the Docker image for the Ubuntu CI. Because the same Docker image is
 used for the ``main`` and ``release`` branch, it should contain no hard-coded
-versions.  It contains the used versions of Clang, various clang-tools,
+versions. It contains the used versions of Clang, various clang-tools,
 GCC, and CMake.
 
 .. note:: This image is pulled from Docker hub and not rebuild when changing
    the Dockerfile.
 
+Updating the CI testing container images
+----------------------------------------
+
+The libcxx linux premerge testing can run on one of three sets of runner
+groups. The three runner group names are "llvm-premerge-libcxx-runners",
+"llvm-premerge-libcxx-release-runners" and "llvm-premerge-libcxx-next-runners".
+Which runner set to use is controlled by the contents of
+https://github.com/llvm/llvm-project/blob/main/.github/workflows/libcxx-build-and-test.yaml.
+By default, it uses "llvm-premerge-libcxx-runners". To switch to one of the
+other runner sets, just replace all uses of "llvm-premerge-libcxx-runners" in
+the yaml file with the desired runner set.
+
+Which container image is used by these three runner sets is controlled
+and set by the variable values in
+https://github.com/llvm/llvm-zorg/blob/main/premerge/premerge_resources/variables.tf.
+The table below shows the variable names and
+the runner sets to which they correspond. To see their values, follow the
+link above (to variables.tf in llvm-zorg).
+
++------------------------------------+---------------------------+
+|Runner Set                          |Variable                   |
++====================================+===========================+
+|llvm-premerge-libcxx-runners        |libcxx_runner_image        |
++------------------------------------+---------------------------+
+|llvm-premerge-libcxx-release-runners|libcxx_release_runner_image|
++------------------------------------+---------------------------+
+|llvm-premerge-libcxx-next-runners   |libcxx_next_runner_image   |
++------------------------------------+---------------------------+
+
+
+When updating the container image you can either update just the
+runner binary (the part the connects to Github), or you can update
+everything (tools, etc.). Whether to update just the runner or to update
+everything is controlled by the value of ``ACTIONS_BASE_IMAGE``, under
+``actions-builder`` in ``libcxx/utils/ci/docker-compose.yml``.
+
+To update just the runner binary, change the value of ``ACTIONS_BASE_IMAGE``
+to be a modified version of one of the libcxx runner variable images from
+https://github.com/llvm/llvm-zorg/blob/main/premerge/premerge_resources/variables.tf,
+as follows: Find the libcxx runner image name you want to use from the
+variables.tf file. The name will be something like
+``ghcr.io/llvm/libcxx-linux-builder:<some-commit-SHA>``. Replace
+``libcxx-linux-builder`` with ``libcxx-linux-builder-base``. Use this new image
+name as the value you assign to ``ACTIONS_BASE_IMAGE``.
+
+To update the entire container image, set the value of ``ACTIONS_BASE_IMAGE``
+to ``builder-base``. If the value is already ``builder-base`` (there
+have been no just-the-runner updates since the last complete update), then you
+need to find the line containing ``RUN echo "Last forced update executed on``
+in ``libcxx/utils/ci/Dockerfile`` and update the date to be the current date.
+
+Once you have created and merged a PR with those changes, a new image
+will be created, and a link to it can be found at
+https://github.com/llvm/llvm-project/pkgs/container/libcxx-linux-builder,
+where the actual image name should be
+``ghcr.io/llvm/libcxx-linux-builder:<SHA-of-committed-change-from-PR>``.
+
+Lastly you need to create a PR in the llvm-zorg repository,
+updating the the value of the appropriate libcxx runner variable in
+the variables.tf file mentioned above to the name of your newly created
+image (see above paragraph about finding the image name). Once that change
+has been merged, an LLVM premerge maintainer (a Google employee) must use
+terraform to apply the change to the running GKE cluster.
+
+
 run-buildbot-container
 ~~~~~~~~~~~~~~~~~~~~~~