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fallback: return available bytes on short feed-based reads - #752
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methane
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Oct 27, 2026
`Unpacker.read_bytes()` does not reject negative lengths. The C extension raises an internal `SystemError` from `PyBytes_FromStringAndSize`; the Python implementation accepts the count and can return an unintended slice of buffered data. Raise `ValueError` for a negative byte count before reading or updating the stream position in either implementation. Four regression cases cover feed-based and file-based unpackers with `-1` and a larger negative count. They also check that the failed call leaves the position unchanged and that zero-length reads and subsequent reads/unpacking still work. On Windows/Python 3.13, all four cases fail with each unchanged implementation. With the fix, the complete C-extension suite has 164 passed and 1 skipped; the Python suite has 161 passed and 4 skipped. Both retain three existing unknown-marker warnings for `thread_unsafe` when the free-threaded test plugin is absent. Repository-wide Ruff lint and format checks pass. Other OS/Python and free-threaded matrices were not run locally. Related: #742 validates negative sizes in the packer header APIs; this change covers the unpacker's raw-byte reader. #752 concerns short feed-based reads, which is a separate case. AI assistance: OpenAI Codex investigated, generated and self-reviewed the patch, and ran the local checks. No separate human review is claimed.
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With a feed-based
Unpacker,read_bytes(5)raisesOutOfDatain the pure-Python fallback when onlyb"ab"is buffered. The C extension returnsb"ab", and both implementations already return short reads for file-backed input.Honor
raise_outofdata=Falsein the feed path so raw reads consume and return the available bytes. Normal unpacking still rolls back and raisesOutOfDatafor incomplete objects. The regression covers empty and short buffers, reads after a decoded object, stream offsets, and feeding the next object afterward.Validation on macOS arm64, CPython 3.12:
git diff --check, pass.The existing
thread_unsafemarker warning also occurs on the base. Other platforms and PyPy have not been tested locally.