gh-86768: Raise OSError when seeking a pipe on Windows (GH-133137) · python/cpython@d65bf51 · GitHub
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gh-86768: Raise OSError when seeking a pipe on Windows (GH-133137)
Previously os.lseek() and file seek() silently succeeded for pipes, and seekable() wrongly returned True. Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
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‎Doc/whatsnew/3.16.rst‎

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‎Lib/test/test_os/test_os.py‎

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@@ -2993,6 +2993,14 @@ def test_ftruncate(self):
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def test_lseek(self):
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self.check(os.lseek, 0, 0)
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@unittest.skipUnless(hasattr(os, 'lseek'), 'test needs os.lseek()')
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@unittest.skipUnless(hasattr(os, 'pipe'), "need os.pipe()")
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def test_lseek_on_pipe(self):
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rfd, wfd = os.pipe()
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self.addCleanup(os.close, rfd)
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self.addCleanup(os.close, wfd)
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self.assertRaises(OSError, os.lseek, rfd, 123, os.SEEK_END)
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@unittest.skipUnless(hasattr(os, 'read'), 'test needs os.read()')
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def test_read(self):
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self.check(os.read, 1)

‎Lib/test/test_winapi.py‎

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@@ -152,7 +152,7 @@ def test_namedpipe(self):
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# Pipe instance is available, so this passes
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_winapi.WaitNamedPipe(pipe_name, 0)
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with open(pipe_name, 'w+b') as pipe2:
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with open(pipe_name, 'w+b', buffering=0) as pipe2:
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# No instances available, so this times out
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# (WinError 121 does not get mapped to TimeoutError)
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with self.assertRaises(OSError):

‎Misc/NEWS.d/next/Windows/2025-04-29-17-55-55.gh-issue-86768.uIDTHc.rst‎

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:func:`os.lseek` and :meth:`~io.IOBase.seek` of file objects now raise
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:exc:`OSError` for pipes on Windows, and :meth:`~io.IOBase.seekable` now
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returns ``False`` for them. Previously seeking a pipe silently appeared to
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succeed. As a consequence, opening a pipe in a read-write binary mode
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(``'r+b'`` or ``'w+b'``) now raises :exc:`io.UnsupportedOperation` unless
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buffering is disabled.

‎Modules/_io/fileio.c‎

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@@ -992,7 +992,14 @@ portable_lseek(fileio *self, PyObject *posobj, int whence, bool suppress_pipe_er
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Py_BEGIN_ALLOW_THREADS
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_Py_BEGIN_SUPPRESS_IPH
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#ifdef MS_WINDOWS
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res = _lseeki64(fd, pos, whence);
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HANDLE h = (HANDLE)_get_osfhandle(fd);
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if (h != INVALID_HANDLE_VALUE && GetFileType(h) == FILE_TYPE_PIPE) {
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res = -1;
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errno = ESPIPE;
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}
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else {
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res = _lseeki64(fd, pos, whence);
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}
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#else
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res = lseek(fd, pos, whence);
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#endif

‎Modules/posixmodule.c‎

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