ZSTDMT_compressionJob kept the chunking kernel in C while Rust already owned
most of the MT pool and progress helpers. The loop mixed compression calls
with mutex and condition-variable publication, making it difficult to test
its exact final-chunk and partial-tail behavior independently.
Move only the chunk-processing kernel into Rust. It calls the existing C
compression entry points through an explicit ABI, reports the final block size
or error in a repr(C) result, and invokes a C progress callback after each
successful intermediate chunk. C retains worker setup, serial and raw-sequence
state, synchronization, error publication, cleanup, and final completion
reporting. Mock compressors cover empty, exact, partial, progress, and error
boundaries without mirroring private MT job state in Rust.
Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --lib zstdmt_compress --
--test-threads=1` -- passed (44 tests).
- `cargo clippy --manifest-path rust/Cargo.toml --lib -- -D warnings` -- passed.
- `cargo +nightly fmt --manifest-path rust/Cargo.toml -- --check` -- passed.
- `ZSTREAM_TESTTIME=-T2s make -B -C tests -j2 test-zstream` -- passed,
including multi-threaded dictionary/checksum coverage.
ddidderr
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