The multithreaded compressor still handled the terminal empty-job case in C,
including the decision to acquire a buffer, emit the final empty block, and
clear the consumed source range. That left a small but stateful branch outside
the Rust job helpers and made allocation-failure behavior difficult to test.
Project the terminal job and result through a stable C ABI, then let Rust own
that branch's assertions, buffer callback, block-header serialization, and
failure-preserving result. C remains responsible for its private job layout
and applies the returned buffer, source, and compressed-size fields exactly as
before.
Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --all-targets -- --test-threads=1` -- 473 passed.
- Native multithreaded cases in `test-fuzzer`, `test-zstream`, `test-cli-tests`, and `test-zstd` -- passed.
- `make -B -C lib -j2 lib ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT=1 ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG=0` -- passed.
- `make -B -C lib -j2 lib ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT=0 ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG=1` -- passed.
ddidderr
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