feat: implement delta logic for binary cache index protocol

[kalbasit]

This implements the differential update (delta) logic as specified in Section 10 of the HLSSI protocol (RFC 0195).

The implementation includes:

• DeltaOp and DeltaEntry for representing atomic additions and deletions.
• ParseDelta and WriteDelta for handling the line-delimited .delta file format.
• GenerateDeltas for computing the difference between two sorted lists of hashes.
• ApplyDelta for updating a shard with a sequence of delta operations.
• ChecksumFile and ShardChecksum structures for epoch-level verification.

These changes enable bandwidth-efficient synchronization for clients by allowing them to download only the changes between epochs rather than full shard files.

[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 the foundational components for differential updates within the binary cache index protocol. By implementing delta generation, parsing, writing, and application logic, it significantly enhances the efficiency of cache synchronization. This allows clients to update their local cache indices by only fetching the changes between epochs, rather than full index files, leading to substantial bandwidth savings.

Highlights

• HLSSI Protocol Implementation: Implements the differential update (delta) logic as specified in Section 10 of the HLSSI protocol (RFC 0195), enabling efficient synchronization of binary cache indices.
• Core Delta Structures: Introduces DeltaOp (representing add or delete operations) and DeltaEntry (combining an operation with a 32-character Nix base32 hash) to define atomic changes within the cache index.
• Delta File I/O: Adds ParseDelta and WriteDelta functions to handle the line-delimited .delta file format, including validation for hash length and ensuring delta entries are sorted by hash during parsing.
• Delta Generation Logic: Provides GenerateDeltas to compute the necessary DeltaEntry operations required to transform an oldHashes list into a newHashes list, both of which must be sorted unique lists of hashes.
• Delta Application Logic: Implements ApplyDelta to update an existing sorted list of hashes by applying a sequence of DeltaEntry operations, returning the new sorted list and handling potential errors like trying to delete a non-existent hash.
• Epoch Verification Structures: Defines ChecksumFile and ShardChecksum structures to support epoch-level verification, including details like epoch number, algorithm, checksums, item counts, and size bytes for individual shards.
• Bandwidth Efficiency: The overall implementation enables bandwidth-efficient synchronization for clients by allowing them to download only the changes (deltas) between epochs rather than full shard files, significantly reducing data transfer.

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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
• feat: implement delta logic for binary cache index protocol #553 👈 (View in Graphite)
• 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 introduces the delta logic for the binary cache index protocol, which is a significant feature for efficient updates. The implementation of GenerateDeltas and ApplyDelta is mostly correct and follows an efficient two-pointer approach. The associated tests provide good coverage for the new functionality.

My review focuses on improving the robustness and performance of the ApplyDelta function. I've identified several critical issues where invalid delta operations are not handled correctly, potentially leading to silent data corruption. I've proposed changes to use switch statements for explicit and safe handling of operations. Additionally, I've included a medium-severity suggestion to pre-allocate a slice to optimize memory allocations.

[kalbasit]

/gemini review

[gemini-code-assist]

Code Review

This pull request successfully implements the delta update logic for the binary cache index protocol, as described in RFC 0195. This includes parsing, writing, generating, and applying deltas, along with necessary data structures and tests. The client is also updated to handle zstd-compressed shards. My review includes suggestions to improve resource management by fixing a potential file handle leak, optimizing slice allocations for better performance, and using more idiomatic Go for improved code clarity and maintainability.

[kalbasit]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces the delta update logic for the binary cache index protocol, which is a significant feature for improving client synchronization efficiency. The implementation is well-structured, with clear separation of concerns in the new pkg/nixcacheindex/delta.go file. The core logic for generating and applying deltas appears correct and handles various cases appropriately. The addition of comprehensive unit tests in pkg/nixcacheindex/delta_test.go is excellent and covers many scenarios, including edge cases and error conditions. The related change in the client to handle zstd-compressed shards is also correct and properly tested. I have one minor suggestion to improve the robustness of the delta parsing logic. Overall, this is a high-quality contribution.

[kalbasit]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces delta logic for the binary cache index protocol, which is a significant feature for enabling efficient updates. The implementation looks solid, covering parsing, generation, and application of deltas, along with comprehensive tests. My review includes a few suggestions to improve diagnostics, code formatting, and test coverage.