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[pull] main from bazel-contrib:main - #747
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## Summary `PyWriteBuildData` is dirty on every build with a cold Skyframe when outputs are not downloaded. That is, once per `py_binary` and `py_test` in the build. When stamping is off, the output is a constant, so the action spawn is pure overhead. ## Root Cause Analysis The output is declared with `_py_builtins.declare_constant_metadata_file`. Constant metadata always reports itself as unchanged, so Bazel cannot validate it from the action cache. It stats the output on disk instead. Under `--remote_download_outputs=minimal` the output is not on disk. The stat fails and the action is dirty. It re-executes, hits the remote cache, is still not downloaded, and is dirty again on the next cold-server build. Setting `--remote_download_regex` to match the file avoids this, at the cost of downloading it, which confirms the stat is the trigger. The use of a constant metadata file is justified (as per the comment in the code). With stamping on, the action reads `volatile-status.txt`, which changes every build. The build data file is in the binary's runfiles, so without it its consumers would invalidate on every build due to timestamps. However, with stamping off, there is no volatile input. There is nothing to shield. Unfortunately, the current implementation keeps the _additional_ cost (constant metadata being always unchanged plus failed `stat` syscalls) for none of the benefits. ## Change When `is_stamping_enabled(ctx)` is false, the content of the file is known at analysis time: ``` TARGET <label> STAMPED FALSE ``` So declare an ordinary output and write it with `ctx.actions.write`. No spawn, and no constant metadata output. Note this is broader than `--nostamp`. `is_stamping_enabled` is also false for the exec configuration and for `stamp = 0`, so tool-config binaries benefit from the fix even when `--stamp=true` is set at the top level. The stamped path is unchanged and keeps its constant metadata output. ## Compatibility The output is byte-identical. The label is written with `str(ctx.label)` to match `"TARGET": str(ctx.label)` in the spawn's env, which yields the canonical `@@//pkg:target` form. `ctx.actions.args().add(ctx.label)` renders the display form and drops the `@@`. After the change, the tests in `tests/build_data/build_data_test.py` pass without any modifications. ## Alternative considered Keeping the spawn and declaring an ordinary output would also fix the dirtiness. Writing at analysis time is lighter, since it needs no subprocess. ## Environment rules_python 2.2.0, Bazel 9.2.0, Linux. Reproduce with `--remote_download_outputs=minimal` against a warm remote cache, then `bazel shutdown` and rebuild.
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