Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)

Initial Implementation of Garbage Collection Algorithm in wal3 (CPU-side, Non-IO Path)

This PR introduces the core CPU-side algorithm and data structures for garbage collection (GC) in the Rust wal3 write-ahead log (WAL) library. It defines the Garbage and GarbageAction structures to model what data to collect, implements the CPU logic to compute GC effects on the in-memory manifest/snapshot data, adds tracking for collected setsums, provides methods to verify and apply garbage actions to manifests, and covers the full GC lifecycle with extensive property-based testing. The I/O integration and actual delete operations on persisted data are intentionally left as TODOs for future follow-up.

Key Changes:
• Introduced new gc.rs module with Garbage and GarbageAction types representing GC plans and actions.
• Implemented logic to compute which manifest fragments and snapshots are eligible for collection given a cutoff point, and the recursive setsum tracking for correctness.
• Added apply_garbage, has_collected_garbage, and utility methods to Manifest for applying and verifying the effects of GC.
• Extended the Manifest and Fragment types to track collected setsums for verification.
• Integrated property-based tests (rust/wal3/tests/properties.rs) for the GC logic, covering both fragments and snapshots.
• Updated documentation (README.md) to describe the GC file format, invariants, process, error handling, and performance considerations.
• Added missing workspace dev-deps for property testing (proptest, rand).
• Updated various call-sites and test code to handle new fields (e.g., collected setsum) and new GC logic.

Affected Areas:
• rust/wal3/src/gc.rs
• rust/wal3/src/manifest.rs
• rust/wal3/src/lib.rs
• rust/wal3/tests/properties.rs
• rust/wal3/README.md
• rust/wal3/src/manifest_manager.rs
• rust/wal3/src/copy.rs
• Cargo.lock
• rust/wal3/Cargo.toml
• test code and integration points for log-service

Potential Impact:

Functionality: Adds foundational, rigorously tested manifest/snapshot GC algorithmic support for the WAL, but does not yet delete files from object storage. Manifests now track collected garbage and enable end-to-end garbage collection state transitions and verification.

Performance: Limited immediate impact; actual I/O batching/deletion and any performance changes will come when I/O is wired in. CPU-only logic is property tested and should be efficient.

Security: No sensitive changes, but integrity checks and error handling are enhanced-setsum mismatches and partial collection are detected and explicitly handled.

Scalability: Prepares codebase for efficient, incremental GC even on large logs by supporting manifest-level tracking, batched GC, and partial manifest application.

Review Focus:
• Correctness of the recursive setsum tracking in Garbage and GarbageAction.
• Edge cases in the manifest-snapshot-fragment tree traversal and cutoff logic.
• Property-based test coverage for possible failure scenarios and correctness.
• Backward compatibility for manifest formats and optional collected field.
• Propagation and usage of new collected setsum field in existing code.

This summary was automatically generated by @propel-code-bot