Prefetch Bazel files in the remote repo contents cache

[fmeum]

BUILD and .bzl files are typically small, but can form deep DAGs that require a large number of sequential cache requests to fetch lazily. By prefetching them (as well as REPO.bazel files) eagerly, the wall time of one particular fully cached cold --nobuild build of Bazel itself decreased by a factor of 10.

Along the way, make remote repo contents cache failures non-fatal, matching the behavior of the remote cache.

[gemini-code-assist[bot]]

Summary of Changes

Hello @fmeum, I'm Gemini Code Assist[^1]! 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 targeted performance optimization for remote repository handling by implementing eager prefetching of .bzl files. The change aims to mitigate the performance overhead associated with the lazy fetching of these files, which are critical for build logic and can lead to numerous sequential cache requests due to their interconnected nature. By fetching them proactively, the system can reduce latency and improve overall build efficiency.

Highlights

• Prefetching .bzl files: Implemented logic to proactively identify and prefetch .bzl files during remote repository injection and materialization processes. This targets files with the .bzl extension for eager fetching.
• Performance Optimization: The primary goal of this change is to reduce the number of sequential cache requests for .bzl files. These files, though small, often form deep dependency DAGs, and prefetching them can significantly improve performance by avoiding lazy, sequential fetches.
• Refactoring of Prefetching Logic: Extracted the common file prefetching mechanism into a dedicated prefetch helper method, improving code organization and reusability. The injectRecursively method was also updated to collect files for prefetching.

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[fmeum]

@bazel-io fork 9.0.0