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seekable() returns True on pipe objects in Windows #86768
Description
| BPO | 42602 |
|---|---|
| Nosy | @pfmoore, @vstinner, @tjguk, @zware, @eryksun, @zooba, @izbyshev |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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GitHub fields:
assignee = None
closed_at = None
created_at = <Date 2020-12-08.15:55:37.151>
labels = ['type-bug', '3.8', '3.9', 'expert-IO', '3.10', 'OS-windows']
title = 'seekable() returns True on pipe objects in Windows'
updated_at = <Date 2020-12-08.17:47:01.347>
user = 'https://bugs.python.org/nedsociety'bugs.python.org fields:
activity = <Date 2020-12-08.17:47:01.347>
actor = 'izbyshev'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Windows', 'IO']
creation = <Date 2020-12-08.15:55:37.151>
creator = 'nedsociety'
dependencies = []
files = []
hgrepos = []
issue_num = 42602
keywords = []
message_count = 2.0
messages = ['382746', '382753']
nosy_count = 8.0
nosy_names = ['paul.moore', 'vstinner', 'tim.golden', 'zach.ware', 'eryksun', 'steve.dower', 'izbyshev', 'nedsociety']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42602'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']
Activity
nedsociety commentedon Dec 8, 2020
This happens on Windows. Consequently seek() works for these objects but they seems to be no-op. This may confuse libraries that depend on seeking.
The named pipe objects (via CreateNamedPipe -> open_osfhandle -> open()) exhibit the same behavior.
izbyshev commentedon Dec 8, 2020
Yes, despite that MSVCRT knows the type of the file descriptor because it calls GetFileType() on its creation, it doesn't check it in lseek() implementation and simply calls SetFilePointer(), which spuriously succeeds for pipes. MSDN says the following[1]:
Calling the SetFilePointer function with a handle to a non-seeking device such as a pipe or a communications device is not supported, even though the SetFilePointer function may not return an error. The behavior of the SetFilePointer function in this case is undefined.
[1] https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-setfilepointer
PeterJCLaw commentedon Jun 7, 2022
Would a possible fix here be to check the file type within the (Windows) implementation of
seekandseekable? Or is the necessary information not available by that point?vstinner commentedon Jun 8, 2022
Maybe Python should get the handle from the file descriptor to call SetFilePointer() on it? Since lseek(fd) seems to silently ignores an error.
gh-86768: Raise OSError when seeking a pipe on Windows (GH-133137)
pythongh-86768: Raise OSError when seeking a pipe on Windows (pythonG…