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gh-143249: Fix buffer leak when overlapped operation fails to start (OS-Windows)#143250

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kumaraditya303 merged 8 commits intopython:mainfromhyongtao-code:fix-mem-leak
Jan 13, 2026

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@hyongtao-codehyongtao-code commentedDec 28, 2025•
edited by picnixz
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On Windows, some overlapped I/O operations leak buffers when they fail to start, notably inWSASendTo(),WSARecvFrom(), andWSARecvFromInto().

This change ensures that buffers are released on failure by clearing the overlapped state, following the same pattern used in other overlapped error paths. This issue is similar in nature to commit5485085, which fixed related overlapped cleanup problems.

Python script for tracking leaks

importsysimport_overlappeddeftotal():returnsys.gettotalrefcount()ifnothasattr(sys,"gettotalrefcount"):raiseSystemExit("need a debug build (Py_DEBUG) for gettotalrefcount")before=total()N=20000STEP=1000foriinrange(1,N+1):ov=_overlapped.Overlapped()buf=memoryview(bytearray(4096))try:ov.WSASendTo(0x1234,buf,0, ("127.0.0.1",1))exceptOSError:passifi%STEP==0:now=total()print(f"{i:6d}: totalrefcount delta ={now-before:+d}")after=total()print(f"done: N={N}, totalrefcount delta ={after-before:+d}")

Result without the patch

d:\MyCode\cpython\PCbuild\amd64>python_d.exe py_overlapped_leak.py  1000: totalrefcount delta = +4007  2000: totalrefcount delta = +8009  3000: totalrefcount delta = +11906  4000: totalrefcount delta = +15906  5000: totalrefcount delta = +19906...... 19000: totalrefcount delta = +75906 20000: totalrefcount delta = +79906done: N=20000, totalrefcount delta = +79905

Result with the patch

d:\MyCode\cpython\PCbuild\amd64>python_d.exe py_overlapped_leak.py  1000: totalrefcount delta = +10  2000: totalrefcount delta = +10  3000: totalrefcount delta = +10  4000: totalrefcount delta = +10  5000: totalrefcount delta = +10...... 19000: totalrefcount delta = +10 20000: totalrefcount delta = +10done: N=20000, totalrefcount delta = +9

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picnixz
picnixz previously approved these changesDec 28, 2025
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You'll need a NEWS entry for that and some tests as well. Please add them so that we can run them under the refleak bot as well.

cc@zooba

@picnixzpicnixz dismissed theirstale reviewDecember 28, 2025 14:41

wrong button

@picnixzpicnixz requested a review fromzoobaDecember 28, 2025 14:44
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and some tests as well

Thanks for the review. I will add a suitable test case.

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
@serhiy-storchakaserhiy-storchaka added needs backport to 3.13bugs and security fixes needs backport to 3.14bugs and security fixes labelsDec 29, 2025
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LGTM. 👍

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@kumaraditya303kumaraditya303 merged commit103a384 intopython:mainJan 13, 2026
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Thanks@hyongtao-code for the PR, and@kumaraditya303 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

miss-islington pushed a commit to miss-islington/cpython that referenced this pull requestJan 13, 2026
…tart on windows (pythonGH-143250)(cherry picked from commit103a384)Co-authored-by: Yongtao Huang <yongtaoh2022@gmail.com>
miss-islington pushed a commit to miss-islington/cpython that referenced this pull requestJan 13, 2026
…tart on windows (pythonGH-143250)(cherry picked from commit103a384)Co-authored-by: Yongtao Huang <yongtaoh2022@gmail.com>
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GH-143795 is a backport of this pull request to the3.14 branch.

@bedevere-appbedevere-appbot removed the needs backport to 3.14bugs and security fixes labelJan 13, 2026
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GH-143796 is a backport of this pull request to the3.13 branch.

@bedevere-appbedevere-appbot removed the needs backport to 3.13bugs and security fixes labelJan 13, 2026
kumaraditya303 pushed a commit that referenced this pull requestJan 13, 2026
…start on windows (GH-143250) (#143796)gh-143249: Fix buffer leak when overlapped operation fails to start on windows (GH-143250)(cherry picked from commit103a384)Co-authored-by: Yongtao Huang <yongtaoh2022@gmail.com>
kumaraditya303 pushed a commit that referenced this pull requestJan 13, 2026
…start on windows (GH-143250) (#143795)gh-143249: Fix buffer leak when overlapped operation fails to start on windows (GH-143250)(cherry picked from commit103a384)Co-authored-by: Yongtao Huang <yongtaoh2022@gmail.com>
@hyongtao-codehyongtao-code deleted the fix-mem-leak branchJanuary 25, 2026 12:44
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