fix: github actions' bazel cache

[leonm1]

GitHub's actions cache appears to be empty, even with recent pushes to main. This config has been working well in Google's github.com/GoogleCloudPlatform/service-extensions repo [1], which should be building all the same things as this repo. Using --disk_cache avoids putting a bunch of junk in the cache that the previous xdg cache dir entry had to manually strip out.

[mpwarres]

LGTM, but can you elaborate on "Using --disk_cache avoids putting a bunch of junk in the cache that the previous xdg cache dir entry had to manually strip out"? Is --disk_cache more selective in what it caches?

[leonm1]

Yes! The --disk_cache option only caches action outputs (https://bazel.build/remote/caching), whereas ~/.cache/bazel contains the entire bazel output root (https://bazel.build/remote/output-directories#layout).

Some things which exist in the bazel output root which are completely unnecessary to be cached:

• Uncompressed java binaries and scripts used by the bazel runtime
• Sandbox root working directory
• Debug output for each command run
• Source and temporary files for all repos referenced in the WORKSPACE file (this will be big with the addition of hermetic_llvm)

[mpwarres]

Yes! The --disk_cache option only caches action outputs (https://bazel.build/remote/caching), whereas ~/.cache/bazel contains the entire bazel output root (https://bazel.build/remote/output-directories#layout).

Great--thanks!