Dedup (1.1): Freeze FastCDC chunk boundaries - #887
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WalkthroughThe PR adds a public chunked-storage module with FastCDC v2020 chunking, stable chunker identifiers, structured errors, and tests for boundaries, reconstruction, deduplication stability, and reader failures. ChangesChunked storage
Estimated code review effort: 3 (Moderate) | ~25 minutes Merge Risk: 🟡 Moderate · up to The PR freezes chunk boundaries under chunker ID 1, but its FastCDC dependency range may allow a future compatible version to change those boundaries without changing the ID, which could break deduplication compatibility. Merge should wait for the dependency to be pinned or for this risk to be explicitly accepted by the owner. Sequence Diagram(s)sequenceDiagram
participant Reader
participant FastCdc2020Chunker
participant ChunkedError
Reader->>FastCdc2020Chunker: stream input bytes
FastCdc2020Chunker->>FastCdc2020Chunker: produce RawChunk values
FastCdc2020Chunker->>ChunkedError: convert reader failure to ChunkRead
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Inline comments:
In `@Cargo.toml`:
- Line 70: Update the fastcdc dependency declaration to pin exactly version
3.2.1, preserving the StreamCDC implementation used by
ChunkerId::GENERIC_FASTCDC_V1.
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Cargo.tomlcrates/liboxen/Cargo.tomlcrates/liboxen/src/error.rscrates/liboxen/src/storage.rscrates/liboxen/src/storage/chunked.rscrates/liboxen/src/storage/chunked/chunker.rscrates/liboxen/src/storage/chunked/error.rscrates/liboxen/src/storage/chunked/registry.rs
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In `@crates/liboxen/src/error.rs`:
- Around line 304-307: Update is_fatal_for_retry to classify the
UnknownChunkerId variant from ChunkedError as fatal, preventing retries for
unsupported storage formats or corrupt metadata while preserving existing
classifications for other errors.
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First piece of block level dedup.
This does two important things
Chunker::chunktakes a reader and yieldsRawChunk { offset, data }in file order. Concatenating the chunks reproduces the input exactly. Chunking is sync and streams with bounded memory (at most one maximum chunk plus a window).Same bytes with the same chunker id must always produce the same boundaries or deduplication would fail without any errors.
golden_fastcdc_boundariestests exact offset and length pairs to ensure boundaries never shift unintentionally. If boundaries ever move due to an intentional chunker update or a fastcdc upgrade or any other reason, we must set a new chunker ID. Also, ID 0 is permanently reserved, so that zeroed or corrupt metadata cannot point to an actual chunker.chunker()which maps chunker id to implementation has no callers yet. Added here to ensure that unknown chunkers fail loudly.Tests ensure (a) exact tiling, where offsets are contiguous, (b) every chunk except the last sits between the min and max size, (c) every chunk's bytes match the input at its offset, for these inputs:
This also introduces
ChunkedErroron the theory that dedup will have its own category of errors. If we want to handle errors differenty, I'm open to ideas.Completes ENG-1693