[libcxx-commits] [libcxx] [libcxx] Update testing documentation about CI container images. (PR #149192)

[Louis Dionne via libcxx-commits]

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@@ -139,6 +139,43 @@ Also note that the Docker container shares the same filesystem as your local mac
 modifying files on your local machine will also modify what the Docker container sees.
 This is useful for editing source files as you're testing your code in the Docker container.
 
+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"
+or "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, their current values, and
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ldionne wrote:

I agree, we should strive to have a single source of truth (and have that source of truth be in the monorepo).

https://github.com/llvm/llvm-project/pull/149192