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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:mainfrom
hyongtao-code:fix-mem-leak
Jan 13, 2026
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gh-143249: Fix buffer leak when overlapped operation fails to start (OS-Windows)#143250
kumaraditya303 merged 8 commits intopython:mainfrom
hyongtao-code:fix-mem-leak

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@hyongtao-code hyongtao-code commented Dec 28, 2025 •
edited by picnixz
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On Windows, some overlapped I/O operations leak buffers when they fail to start, notably in WSASendTo(), WSARecvFrom(), and WSARecvFromInto().

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 commit 5485085, which fixed related overlapped cleanup problems.

Python script for tracking leaks

import sys
import _overlapped

def total():
    return sys.gettotalrefcount()

if not hasattr(sys, "gettotalrefcount"):
    raise SystemExit("need a debug build (Py_DEBUG) for gettotalrefcount")

before = total()

N = 20000
STEP = 1000

for i in range(1, N + 1):
    ov = _overlapped.Overlapped()
    buf = memoryview(bytearray(4096))
    try:
        ov.WSASendTo(0x1234, buf, 0, ("127.0.0.1", 1))
    except OSError:
        pass

    if i % 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 = +79906
done: 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 = +10
done: N=20000, totalrefcount delta = +9

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picnixz
picnixz previously approved these changes Dec 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

@picnixz picnixz dismissed their stale review December 28, 2025 14:41

wrong button

@picnixz picnixz requested a review from zooba December 28, 2025 14:44
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hyongtao-code commented Dec 28, 2025

and some tests as well

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

picnixz
picnixz reviewed Dec 28, 2025
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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
@serhiy-storchaka serhiy-storchaka added needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes labels Dec 29, 2025
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LGTM. 👍

@kumaraditya303 kumaraditya303 merged commit 103a384 into python:main Jan 13, 2026
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miss-islington-app bot commented Jan 13, 2026

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 request Jan 13, 2026
@hyongtao-code @miss-islington
pythongh-143249: Fix buffer leak when overlapped operation fails to s…
b2833d0 
…tart on windows (pythonGH-143250)

(cherry picked from commit 103a384)

Co-authored-by: Yongtao Huang <yongtaoh2022@gmail.com>
miss-islington pushed a commit to miss-islington/cpython that referenced this pull request Jan 13, 2026
@hyongtao-code @miss-islington
pythongh-143249: Fix buffer leak when overlapped operation fails to s…
0891f44 
…tart on windows (pythonGH-143250)

(cherry picked from commit 103a384)

Co-authored-by: Yongtao Huang <yongtaoh2022@gmail.com>
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bedevere-app bot commented Jan 13, 2026

GH-143795 is a backport of this pull request to the 3.14 branch.

@bedevere-app bedevere-app bot removed the needs backport to 3.14 bugs and security fixes label Jan 13, 2026
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bedevere-app bot commented Jan 13, 2026

GH-143796 is a backport of this pull request to the 3.13 branch.

@bedevere-app bedevere-app bot removed the needs backport to 3.13 bugs and security fixes label Jan 13, 2026
kumaraditya303 pushed a commit that referenced this pull request Jan 13, 2026
@miss-islington @hyongtao-code
[3.13] gh-143249: Fix buffer leak when overlapped operation fails to …
be602fc 
…start on windows (GH-143250) (#143796)

gh-143249: Fix buffer leak when overlapped operation fails to start on windows (GH-143250)
(cherry picked from commit 103a384)

Co-authored-by: Yongtao Huang <yongtaoh2022@gmail.com>
kumaraditya303 pushed a commit that referenced this pull request Jan 13, 2026
@miss-islington @hyongtao-code
[3.14] gh-143249: Fix buffer leak when overlapped operation fails to …
4a191f9 
…start on windows (GH-143250) (#143795)

gh-143249: Fix buffer leak when overlapped operation fails to start on windows (GH-143250)
(cherry picked from commit 103a384)

Co-authored-by: Yongtao Huang <yongtaoh2022@gmail.com>
@hyongtao-code hyongtao-code deleted the fix-mem-leak branch January 25, 2026 12:44
thunder-coding pushed a commit to thunder-coding/cpython that referenced this pull request Feb 15, 2026
@hyongtao-code @thunder-coding
pythongh-143249: Fix buffer leak when overlapped operation fails to s…
13595cb 
…tart on windows (python#143250)
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