feat: wire up new index feature into pkg/cache/upstream

[kalbasit]

This change integrates the Nix binary cache index protocol into the upstream cache.
It is gated behind an experimental flag --experimental-cache-index.

When enabled, the upstream cache will query the index (manifest, journal, shards)
before making requests for .narinfo files. If the index indicates a definite
miss, the cache returns a NOT FOUND error immediately, avoiding unnecessary
network calls to the upstream binary cache.

Changes:

• Implement nixcacheindex.Fetcher on upstream.Cache.
• Integrate nixcacheindex.Client into upstream.Cache.
• Add --experimental-cache-index flag to serve command.
• Add verification tests in pkg/cache/upstream/cache_index_test.go.

[dosubot]

Related Documentation

Checked 1 published document(s) in 1 knowledge base(s). No updates required.

^{How did I do? Any feedback?}  

[gemini-code-assist]

Summary of Changes

Hello @kalbasit, 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 an experimental optimization to the upstream cache by integrating the Nix binary cache index protocol. The primary goal is to enhance performance by allowing the cache to proactively identify missing .narinfo files through an index lookup. This prevents redundant network requests to the upstream server, leading to faster responses for cache misses. The feature is opt-in via a new command-line flag, ensuring controlled deployment and testing.

Highlights

• Nix Binary Cache Index Integration: The pull request integrates the Nix binary cache index protocol into the upstream cache mechanism. This allows the system to query an index (manifest, journal, shards) to determine the presence of .narinfo files before making direct network requests.
• Performance Optimization for Cache Misses: When the experimental feature is enabled, if the index indicates a 'definite miss' for a .narinfo file, the cache will immediately return a 'NOT FOUND' error. This avoids unnecessary network calls to the upstream binary cache, improving efficiency for non-existent items.
• Experimental Feature Flag: This new functionality is gated behind an experimental flag, --experimental-cache-index, which must be explicitly enabled when running the serve command.
• Fetcher Implementation: The upstream.Cache now implements the nixcacheindex.Fetcher interface, providing the necessary method to fetch index components.
• Verification Tests: New tests have been added in pkg/cache/upstream/cache_index_test.go to verify that the experimental cache index correctly identifies definite misses and prevents redundant upstream .narinfo requests.

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[kalbasit]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: Implement Nix Binary Cache Index Protocol #555
• feat: wire up new index feature into pkg/cache/upstream #554 👈 (View in Graphite)
• feat: implement delta logic for binary cache index protocol #553
• feat(nixcacheindex): implement RFC 0195 binary cache index protocol #552
• feat: introduce golomb-rice encoding package #564
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[gemini-code-assist]

Code Review

This pull request effectively integrates the new Nix binary cache index feature into the upstream cache, gated by the --experimental-cache-index flag. The implementation correctly queries the index before fetching .narinfo files and handles definite misses by returning early, which should improve performance by avoiding unnecessary network requests. The new tests in pkg/cache/upstream/cache_index_test.go properly verify this new behavior.

My review includes two main points for improvement:

1. Context Propagation: The new Fetch method uses context.Background(). For better observability and cancellation, the request context should be propagated through the call chain.
2. Code Duplication: There's an opportunity to refactor duplicated logic for querying the cache index in GetNarInfo and HasNarInfo into a shared helper function to improve maintainability.

All original comments have been retained as they are valid and do not contradict any of the provided repository-specific rules.

[kalbasit]

/gemini review

[gemini-code-assist]

Code Review

This pull request successfully integrates the Nix binary cache index protocol as an experimental feature. The implementation correctly uses the index to avoid unnecessary network requests for .narinfo files when a definite miss is detected. The changes are well-structured and include a good verification test.

I've identified a race condition in the manifest loading logic within the nixcacheindex.Client that could cause issues under concurrent load. While this is a significant concern for concurrent environments, repository guidelines suggest that for single-process applications, such fixes can be deferred if documented as technical debt. I've also pointed out some leftover comments in a test file that should be cleaned up.

[kalbasit]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces an experimental feature to use a Nix binary cache index, which can optimize away requests for paths that are known to be missing. The implementation correctly wires up the new feature through a command-line flag and integrates the index client into the upstream cache logic. My review focuses on a critical issue in the error handling within the nixcacheindex.Client. The current implementation doesn't properly distinguish between 'not found' errors and other network or server errors, which could cause transient failures to be misinterpreted as definite misses. I've provided detailed suggestions to correct this behavior and make the feature more robust.

[kalbasit]

/gemini review

[gemini-code-assist]

Code Review

This pull request integrates the Nix binary cache index protocol as an experimental feature. The implementation looks solid, introducing a check against the index before fetching narinfo files to avoid unnecessary network requests on a definite miss. I've identified a couple of areas for improvement: one is a potential concurrency issue in the index client related to context handling, and the other is a refactoring opportunity for code consistency, aligning with the project's existing patterns. The new tests are well-written and cover the primary success case for the definite miss optimization.