Don't unnecessarily read unresolved symlinks

[fmeum]

Get the target path from the InputMetadataProvider instead and avoid I/O that way.

[gemini-code-assist]

Summary of Changes

Hello @fmeum, 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 optimization in how symlink targets are resolved within the system. By integrating an InputMetadataProvider into key areas like SourceManifestAction and MerkleTreeComputer, the system can now efficiently retrieve symlink target paths from cached metadata. This change is designed to enhance performance by minimizing direct file system interactions and ensuring a more consistent approach to handling symbolic links.

Highlights

• Optimized Symlink Resolution: The pull request modifies how symlink targets are resolved by prioritizing the use of an InputMetadataProvider to retrieve the unresolved symlink target, rather than performing direct file system reads.
• Reduced I/O Operations: By leveraging cached metadata from the InputMetadataProvider, the change aims to prevent unnecessary and potentially costly file system reads, especially for unresolved symlinks.
• Refactoring SourceManifestAction: The writeFile method in SourceManifestAction has been updated to accept an InputMetadataProvider, and its call sites now pass the provider when available, enabling the optimized symlink resolution.
• Merkle Tree Generation Improvement: The process of building Merkle trees now also utilizes the InputMetadataProvider to obtain symlink targets, ensuring consistency and efficiency across different components.

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[gemini-code-assist]

Code Review

This pull request introduces an optimization to avoid unnecessary filesystem reads for unresolved symlinks. Instead of reading the symlink target from the filesystem, the change leverages the InputMetadataProvider to get the cached target path. This is applied in SourceManifestAction and MerkleTreeComputer.

The changes look good and align with the goal of improving performance. However, I've found a potential NullPointerException in SourceManifestAction where the result of getInputMetadata is not checked for null before being used. I've added a comment with a suggestion to fix this, making the code more robust and consistent with the changes in MerkleTreeComputer.

[fmeum]

@bazel-io fork 9.0.0