gh-98552: flush std streams in the multiprocessing forkserver before fork by gpshead · Pull Request #141849 · python/cpython · GitHub

A minor bug discovered by multiple authors working on a fix for the original bug in the linked issue.

Issue: multiprocessing forkserver does not flush output before fork (was: preloading '__main__' with forkserver has been broken for a long time) #98552

Thanks @gpshead for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

…efore fork (pythonGH-141849) * flush std streams in the multiprocessing forkserver before fork * NEWS (cherry picked from commit 58badb1) Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>

GH-141851 is a backport of this pull request to the 3.14 branch.

…efore fork (pythonGH-141849) * flush std streams in the multiprocessing forkserver before fork * NEWS (cherry picked from commit 58badb1) Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>

GH-141852 is a backport of this pull request to the 3.13 branch.

…before fork (GH-141849) (#141851) gh-98552: flush std streams in the multiprocessing forkserver before fork (GH-141849) * flush std streams in the multiprocessing forkserver before fork * NEWS (cherry picked from commit 58badb1) Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>

…before fork (GH-141849) (#141852) gh-98552: flush std streams in the multiprocessing forkserver before fork (GH-141849) * flush std streams in the multiprocessing forkserver before fork * NEWS (cherry picked from commit 58badb1) Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>

looks like this was already done for #135335, just earlier in the code from where I had been looking. reverting.

…streams in the multiprocessing forkserver before fork (python#141849)" This reverts commit 58badb1.

…s in the multiprocessing forkserver before fork (#141849)" (#141871) Revert (unneeded, already done elsewhere) "gh-98552: flush std streams in the multiprocessing forkserver before fork (#141849)" This reverts commit 58badb1.

…streams in the multiprocessing forkserver before fork (pythonGH-141849)" (pythonGH-141871) Revert (unneeded, already done elsewhere) "pythongh-98552: flush std streams in the multiprocessing forkserver before fork (pythonGH-141849)" This reverts commit 58badb1. (cherry picked from commit 614a28b) Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>

… streams in the multiprocessing forkserver before fork (GH-141849)" (GH-141871) (#141873) gh-98552: Revert (unneeded, already done elsewhere) "flush std streams in the multiprocessing forkserver before fork (GH-141849)" (GH-141871) Revert (unneeded, already done elsewhere) "gh-98552: flush std streams in the multiprocessing forkserver before fork (GH-141849)" This reverts commit 58badb1. (cherry picked from commit 614a28b) Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>

… streams in the multiprocessing forkserver before fork (GH-141849)" (GH-141871) (#141872) gh-98552: Revert (unneeded, already done elsewhere) "flush std streams in the multiprocessing forkserver before fork (GH-141849)" (GH-141871) Revert (unneeded, already done elsewhere) "gh-98552: flush std streams in the multiprocessing forkserver before fork (GH-141849)" This reverts commit 58badb1. (cherry picked from commit 614a28b) Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Debian root 3.13 (tier-1) has failed when building commit e2d320b.

What do you need to do:

Don't panic.
Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1441/builds/1592) and take a look at the build logs.
Check if the failure is related to this commit (e2d320b) or if it is a false positive.
If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1441/builds/1592

Failed tests:

test_os

Failed subtests:

test_timerfd_ns_poll - test.test_os.TimerfdTests.test_timerfd_ns_poll

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/root/buildarea/3.13.angelico-debian-amd64/build/Lib/test/test_os.py", line 4342, in test_timerfd_ns_poll
    self.check_timerfd_poll(True)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/root/buildarea/3.13.angelico-debian-amd64/build/Lib/test/test_os.py", line 4327, in check_timerfd_poll
    self.assertEqual(self.read_count_signaled(fd), 1)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 4 != 1


Traceback (most recent call last):
  File "/root/buildarea/3.13.angelico-debian-amd64/build/Lib/test/test_os.py", line 4342, in test_timerfd_ns_poll
    self.check_timerfd_poll(True)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/root/buildarea/3.13.angelico-debian-amd64/build/Lib/test/test_os.py", line 4327, in check_timerfd_poll
    self.assertEqual(self.read_count_signaled(fd), 1)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 2 != 1

…efore fork (python#141849) * flush std streams in the multiprocessing forkserver before fork * NEWS

…streams in the multiprocessing forkserver before fork (python#141849)" (python#141871) Revert (unneeded, already done elsewhere) "pythongh-98552: flush std streams in the multiprocessing forkserver before fork (python#141849)" This reverts commit 58badb1.

…efore fork (python#141849) * flush std streams in the multiprocessing forkserver before fork * NEWS

…streams in the multiprocessing forkserver before fork (python#141849)" (python#141871) Revert (unneeded, already done elsewhere) "pythongh-98552: flush std streams in the multiprocessing forkserver before fork (python#141849)" This reverts commit 58badb1.

flush std streams in the multiprocessing forkserver before fork

77f6c4e

bedevere-app Bot mentioned this pull request Nov 22, 2025

multiprocessing forkserver does not flush output before fork (was: preloading '__main__' with forkserver has been broken for a long time) #98552

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NEWS

04f523d

gpshead marked this pull request as ready for review November 22, 2025 18:01

bedevere-app Bot added the awaiting core review label Nov 22, 2025

gpshead added needs backport to 3.13 only security fixes needs backport to 3.14 bugs and security fixes labels Nov 22, 2025

gpshead enabled auto-merge (squash) November 22, 2025 18:07

gpshead merged commit 58badb1 into python:main Nov 22, 2025
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bedevere-app Bot removed the awaiting core review label Nov 22, 2025

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Nov 22, 2025

bedevere-app Bot removed the needs backport to 3.13 only security fixes label Nov 22, 2025

gpshead deleted the mp-forkserver-flush-b4-fork branch November 23, 2025 09:20

gpshead added a commit to gpshead/cpython that referenced this pull request Nov 23, 2025

Revert (unneeded, already done elsewhere) "pythongh-98552 : flush std …

355f816

…streams in the multiprocessing forkserver before fork (python#141849)" This reverts commit 58badb1.

miss-islington mentioned this pull request Nov 23, 2025

[3.14] gh-98552: Revert (unneeded, already done elsewhere) "flush std streams in the multiprocessing forkserver before fork (GH-141849)" (GH-141871) #141872

Merged

miss-islington mentioned this pull request Nov 23, 2025

[3.13] gh-98552: Revert (unneeded, already done elsewhere) "flush std streams in the multiprocessing forkserver before fork (GH-141849)" (GH-141871) #141873

Merged

StanFromIreland pushed a commit to StanFromIreland/cpython that referenced this pull request Dec 6, 2025

pythongh-98552 : flush std streams in the multiprocessing forkserver b…

8169441

…efore fork (python#141849) * flush std streams in the multiprocessing forkserver before fork * NEWS

ashm-dev pushed a commit to ashm-dev/cpython that referenced this pull request Dec 8, 2025

pythongh-98552 : flush std streams in the multiprocessing forkserver b…

293a740

…efore fork (python#141849) * flush std streams in the multiprocessing forkserver before fork * NEWS

gh-98552: flush std streams in the multiprocessing forkserver before fork by gpshead · Pull Request #141849 · python/cpython · GitHub

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gh-98552: flush std streams in the multiprocessing forkserver before fork - #141849

gh-98552: flush std streams in the multiprocessing forkserver before fork#141849
gpshead merged 2 commits into
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gpshead:mp-forkserver-flush-b4-fork

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gh-98552: flush std streams in the multiprocessing forkserver before fork by gpshead · Pull Request #141849 · python/cpython · GitHub

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gh-98552: flush std streams in the multiprocessing forkserver before fork - #141849

gh-98552: flush std streams in the multiprocessing forkserver before fork#141849gpshead merged 2 commits intopython:mainpython/cpython:mainfrom gpshead:mp-forkserver-flush-b4-forkgpshead/cpython:mp-forkserver-flush-b4-forkCopy head branch name to clipboard

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gh-98552: flush std streams in the multiprocessing forkserver before fork#141849
gpshead merged 2 commits into
python:mainfrom
gpshead:mp-forkserver-flush-b4-fork

gpshead commented Nov 22, 2025 •
edited by bedevere-app Bot
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