perf(py_wheel): defer depset expansion to execution time

[bd-dstodolsky]

This is the py_wheel counterpart to the analysis-time performance work done
for py_binary/py_test in #3381 and #3442, deferring depset
expansion to execution time. Tested under Bazel 8.5.1 and 9.0.0.

[gemini-code-assist]

Summary of Changes

Hello @bd-dstodolsky, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the performance of py_wheel targets in Bazel by optimizing how large dependency sets are processed. It refactors the _py_wheel_impl rule to defer depset expansion from the analysis phase to the execution phase, eliminating a major bottleneck that caused quadratic scaling with increasing dependencies. This change results in substantial improvements in analysis time, particularly for projects with extensive dependency trees, and is supported by new performance and correctness tests.

Highlights

• Performance Improvement for py_wheel: Replaced an analysis-time depset.to_list() loop with Args.add_all(map_each=...) in _py_wheel_impl to defer depset expansion to execution time, addressing a significant performance bottleneck.
• Elimination of Quadratic Scaling: The previous implementation caused near-quadratic scaling for py_wheel targets with large dependency trees due to expensive to_list() and string concatenation during the analysis phase, which is now resolved.
• Significant Analysis Time Reduction: Benchmarks demonstrate a reduction in analysis time ratio from 3.05x (quadratic) to 1.55x (linear) for 10k vs 5k dependencies, with a reported 18-second improvement for 18k files in a production monorepo.
• Comprehensive Testing and Validation: New performance tests, correctness verification for wheel contents, and byte-identical output confirmation have been added, along with profiling to validate the effectiveness of the fix.

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Changelog

• CHANGELOG.md
  • (wheel) py_wheel no longer expands the input depset during analysis, improving analysis performance for targets with large dependency trees.

Activity

• Pull request created by bd-dstodolsky.

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[gemini-code-assist]

Code Review

This pull request introduces a significant performance improvement to the py_wheel rule by deferring depset expansion to execution time. The implementation correctly replaces an analysis-time depset.to_list() loop with Args.add_all(map_each=...), which is the recommended approach for handling large dependency sets in Bazel. The change is well-supported by a comprehensive new test suite that includes both correctness and scaling performance tests, ensuring the change is both safe and effective. The pull request is well-documented and the changes are excellent.

[rickeylev]

Thanks for the optmization, +1 to it in general.

This looks entirely AI generated, so I'm not looking at it particularly deeply.

Trim the PR description. It becomes the commit description, so it needs to be concise and relevant for future people reading through the commit log. If there's information to keep, post it as a comment.

Remove the tests that rely on timing. CI is not a reliable environment for performance tests.

Do not create shell tests that invoke bazel as a subprocess. Such integration tests are expensive and difficult to debug.

If any test is created, it should be an analysis test, using rules_testing, that verifies the args are passed to the action as a manifest file.

I'm fine with no tests here; how the content is passed is an implementation detail, so existing tests should verify correctness.

[bd-dstodolsky]

Thanks. Tests removed and comments shortened. Note the performance test was tagged manual so it would not run in CI, but happy to remove; I had originally done it just as a benchmark but then decided to convert to a test, but happy to eliminate. Quick review appreciated!

[rickeylev]

Thanks, looks good!