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[3.12] gh-95782: Fix io.BufferedReader.tell() etc. being able to return offsets < 0 (GH-99709)
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…n offsets < 0 (pythonGH-99709) lseek() always returns 0 for character pseudo-devices like `/dev/urandom` (for other non-regular files, e.g. `/dev/stdin`, it always returns -1, to which CPython reacts by raising appropriate exceptions). They are thus technically seekable despite not having seek semantics. When calling read() on e.g. an instance of `io.BufferedReader` that wraps such a file, `BufferedReader` reads ahead, filling its buffer, creating a discrepancy between the number of bytes read and the internal `tell()` always returning 0, which previously resulted in e.g. `BufferedReader.tell()` or `BufferedReader.seek()` being able to return positions < 0 even though these are supposed to be always >= 0. Invariably keep the return value non-negative by returning max(former_return_value, 0) instead, and add some corresponding tests. (cherry picked from commit 26800cf) Co-authored-by: 6t8k <58048945+6t8k@users.noreply.github.com>
This was referenced Feb 17, 2024
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lseek() always returns 0 for character pseudo-devices like
/dev/urandom(for other non-regular files, e.g./dev/stdin, italways returns -1, to which CPython reacts by raising appropriate
exceptions). They are thus technically seekable despite not having seek
semantics.
When calling read() on e.g. an instance of
io.BufferedReaderthatwraps such a file,
BufferedReaderreads ahead, filling its buffer,creating a discrepancy between the number of bytes read and the internal
tell()always returning 0, which previously resulted in e.g.BufferedReader.tell()orBufferedReader.seek()being able to returnpositions < 0 even though these are supposed to be always >= 0.
Invariably keep the return value non-negative by returning
max(former_return_value, 0) instead, and add some corresponding tests.
(cherry picked from commit 26800cf)
Co-authored-by: 6t8k 58048945+6t8k@users.noreply.github.com
_io.BufferedReader.tell#95782