refactor(gazelle): Generate a modules map per wheel, then merge

[thejcannon]

This change internally splits modules mapping generation to be per-wheel, with a final quick "merge" action at the end.

The idea is to make this process both concurrent and cached (courtesy of Bazel), which can be ideal for codebases with a large set of requirements (as many monorepos end up doing)

Note that the generator.py interface changed. This seemed internal, so I didn't mark it breaking (but this change could actually just leave the generator alone, since the current implementation is fine with 1 wheel).

I ran this on the work repo and saw no change in output (but as I edited a single requirement, the overall process was fast ⚡ )

[gemini-code-assist]

Summary of Changes

Hello @thejcannon, 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 introduces a significant refactoring to how Gazelle generates its modules mapping. The core change involves splitting the mapping generation into a per-wheel process, where each Python wheel is processed independently to create its own module map. These individual maps are then consolidated by a new merging utility. This approach is intended to enhance build performance by taking advantage of Bazel's inherent concurrency and caching mechanisms, which is particularly beneficial for projects with extensive dependency trees.

Highlights

• Refactored Module Mapping Generation: The process for generating Gazelle's modules mapping has been refactored to process each Python wheel individually, rather than all wheels at once. This change is designed to leverage Bazel's caching and concurrency capabilities.
• New Merger Component: A new Python script, merger.py, and its corresponding Bazel rule have been introduced. This component is responsible for combining the individual modules mapping JSON files generated for each wheel into a single, final modules mapping.
• Generator Interface Update: The generator.py script has been updated to accept a single wheel as input, aligning with the new per-wheel processing strategy. Its argument parsing was also simplified by removing the @ prefix for parameter files.
• Improved Performance Potential: This architectural change aims to significantly improve the performance of modules mapping generation in large monorepos by enabling more efficient caching and concurrent execution of tasks within Bazel.

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[gemini-code-assist]

Code Review

This pull request refactors the module mapping generation to be per-wheel, which is a great improvement for performance and caching in large projects. The approach of generating intermediate files for each wheel and then merging them is sound and well-implemented in Bazel. The new merger.py script is clean and efficient.

I have a couple of suggestions to improve the code further. One is to make the new test for the merger more robust by using a better pattern for temporary files. The other is a minor suggestion to improve the naming of the intermediate files for better readability and debugging.

Overall, this is a solid refactoring that leverages Bazel's strengths effectively.

[dougthor42]

Nice! This looks like a great speed increase.