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gh-160295: Fix race in test_forkserver_sigkill by devdavidejesus · Pull Request #160296 · python/cpython · GitHub

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gh-160295: Fix race in test_forkserver_sigkill - #160296

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gpshead merged 3 commits into
python:mainfrom
devdavidejesus:fix-forkserver-sigkill-race
Oct 23, 2026
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gh-160295: Fix race in test_forkserver_sigkill#160296
gpshead merged 3 commits into
python:mainfrom
devdavidejesus:fix-forkserver-sigkill-race

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devdavidejesus commented Oct 22, 2026 •
edited by gpshead
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After the forkserver is killed with SIGKILL, proc.join() returns as soon as the status pipe is closed (exit code 255), possibly before the orphaned child has run. Wait for the event instead of checking it right away.

Test-only change, so no NEWS entry.

Tested with pydebug builds of main:

  • The reproducer from the issue (child stopped with SIGSTOP for 3 seconds) fails without the change and passes with it, on macOS 27.0.1 (M3) and Linux.
  • test_forkserver_sigkill 20 times with the change while the machine was loaded (load average up to 120 on macOS): no failures. Without the change the failure did not show up in 20 runs either, so the SIGSTOP reproducer is the real check.
  • ./python.exe -m test test_multiprocessing_forkserver.test_processes passes on macOS; -m 'test_forkserver_sig*' passes on Linux.

Fixes #160295.

pythongh-160295: Fix race in test_forkserver_sigkill
b6ae309 
After a SIGKILL, the forkserver can no longer report the first child's
exit status, so proc.join() returns with exit code 255 while the child
may still be running. Wait for the event instead of checking it right
after join().

python-cla-bot Bot commented Oct 22, 2026 •
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All commit authors signed the Contributor License Agreement.

bedevere-app Bot mentioned this pull request Oct 22, 2026
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bedevere-app Bot added the tests Tests in the Lib/test dir label Oct 22, 2026

bedevere-app Bot commented Oct 22, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

devdavidejesus mentioned this pull request Oct 22, 2026
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devdavidejesus commented Oct 22, 2026

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The ASan failure is unrelated: it is test_sampling_with_collapsed_export in test_profiling, reported as gh-160334. test_multiprocessing_forkserver is skipped under ASan, so this change is not exercised there.

bedevere-app Bot commented Oct 23, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

gpshead added skip news needs backport to 3.14 bugs and security fixes needs backport to 3.15 bugs and security fixes labels Oct 23, 2026
gpshead enabled auto-merge (squash) October 23, 2026 20:14
gpshead merged commit 40c8edd into python:main Oct 23, 2026
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miss-islington-app Bot commented Oct 23, 2026

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Thanks @devdavidejesus for the PR, and @gpshead for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14, 3.15.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

bedevere-app Bot commented Oct 23, 2026

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GH-160508 is a backport of this pull request to the 3.15 branch.

bedevere-app Bot commented Oct 23, 2026

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GH-160509 is a backport of this pull request to the 3.14 branch.

bedevere-app Bot removed needs backport to 3.15 bugs and security fixes needs backport to 3.14 bugs and security fixes labels Oct 23, 2026
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test_forkserver_sigkill: join() can return before the child sets the event

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