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gh-150175: Fix ThreadingMock call_count race condition#150176

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cjw296 merged 1 commit into
python:mainfrom
saisneha196:fix-threadingmock-call-count-race
May 21, 2026
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gh-150175: Fix ThreadingMock call_count race condition#150176
cjw296 merged 1 commit into
python:mainfrom
saisneha196:fix-threadingmock-call-count-race

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@saisneha196 saisneha196 commented May 21, 2026

Problem

ThreadingMock._increment_mock_call() is not thread-safe.
Multiple threads calling the mock simultaneously lose increments
due to race conditions on call_count and other attributes.

This causes intermittent test failures like:
AssertionError: 983 != 1000

Reported in Yocto Project bug:
https://bugzilla.yoctoproject.org/show_bug.cgi?id=16213

Fixes gh-150175

Root Cause

In the base Mock class, _increment_mock_call does:
self.called = True
self.call_count += 1 # not atomic!
self.call_args = _call
self.call_args_list.append(_call)

These are not protected by any lock in ThreadingMock.

Fix

Override _increment_mock_call in ThreadingMixin and wrap it
with the existing _mock_calls_events_lock:

def _increment_mock_call(self, /, *args, **kwargs):
    with self._mock_calls_events_lock:
        super()._increment_mock_call(*args, **kwargs)

Testing

Verified on both x86 and ARM64 (qemuarm64):

  • Before fix: 23/30 failures with 50 threads x 10000 calls
  • After fix: 0/30 failures with 50 threads x 10000 calls

@saisneha196 saisneha196 requested a review from cjw296 as a code owner May 21, 2026 05:31
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python-cla-bot Bot commented May 21, 2026 •
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bedevere-app Bot commented May 21, 2026

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bedevere-app Bot commented May 21, 2026

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@saisneha196
pythongh-150175: Fix ThreadingMock call_count race condition
8e08b9c 
ThreadingMock._increment_mock_call() was not thread-safe.
Multiple threads calling the mock simultaneously could lose
increments due to race conditions on call_count and other
attributes.

Fix by overriding _increment_mock_call in ThreadingMixin
and wrapping it with the existing _mock_calls_events_lock.
@saisneha196 saisneha196 force-pushed the fix-threadingmock-call-count-race branch from d444e47 to 8e08b9c Compare May 21, 2026 06:32
@cjw296 cjw296 merged commit 388e023 into python:main May 21, 2026
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@cjw296 cjw296 added 3.13 bugs and security fixes 3.14 bugs and security fixes needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes 3.15 pre-release feature fixes, bugs and security fixes 3.16 new features, bugs and security fixes needs backport to 3.15 pre-release feature fixes, bugs and security fixes labels May 21, 2026
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miss-islington-app Bot commented May 21, 2026

Thanks @saisneha196 for the PR, and @cjw296 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

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miss-islington-app Bot commented May 21, 2026

Thanks @saisneha196 for the PR, and @cjw296 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

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miss-islington-app Bot commented May 21, 2026

Thanks @saisneha196 for the PR, and @cjw296 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

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bedevere-app Bot commented May 21, 2026

GH-150180 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 May 21, 2026
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bedevere-app Bot commented May 21, 2026

GH-150181 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 May 21, 2026
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bedevere-app Bot commented May 21, 2026

GH-150182 is a backport of this pull request to the 3.15 branch.

@bedevere-app bedevere-app Bot removed the needs backport to 3.15 pre-release feature fixes, bugs and security fixes label May 21, 2026
@cjw296 cjw296 mentioned this pull request May 21, 2026
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ThreadingMock.call_count is not thread-safe (race condition)

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