gh-95782: Fix io.BufferedReader.tell etc. being able to return offsets < 0
#99709
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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.
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Thanks @6t8k for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12. |
miss-islington
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Feb 17, 2024
…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>
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GH-115599 is a backport of this pull request to the 3.12 branch. |
miss-islington
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Feb 17, 2024
…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>
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GH-115600 is a backport of this pull request to the 3.11 branch. |
serhiy-storchaka
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Feb 17, 2024
…rn offsets < 0 (GH-99709) (GH-115600) 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>
serhiy-storchaka
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Feb 17, 2024
…rn offsets < 0 (GH-99709) (GH-115599) 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>
woodruffw
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Mar 4, 2024
…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.
diegorusso
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Apr 17, 2024
…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.
LukasWoodtli
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Jan 22, 2025
…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.
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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.BufferedReaderthat wraps such a file,BufferedReaderreads ahead, filling its buffer, creating a discrepancy between the number of bytes read and the internaltell()always returning 0, which results in e.g.BufferedReader.tell()orBufferedReader.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.
Other possible solutions that I've decided against and why (aside from the aspect that they wouldn't reflect the underlying file's behavior):
io.BufferedWriter.telldoes for bytes written/dev/urandometc. would be detected as unseekable_io.BufferedReader.tell#95782