gh-80334: fix multiprocessing.freeze_support for other spawn platforms by eddymul · Pull Request #134462 · python/cpython

eddymul · 2025-05-21T19:09:07Z

multiprocessing.freeze_support() now checks for work on any "spawn"
start method platform rather than only on Windows.

Issue: Freeze support documentation is misleading. #80334

📚 Documentation preview 📚: https://cpython-previews--134462.org.readthedocs.build/en/134462/library/multiprocessing.html#multiprocessing.freeze_support

…ge to specify spawn method instead of platform

python-cla-bot · 2025-05-21T19:09:11Z

All commit authors signed the Contributor License Agreement.

Doc/library/multiprocessing.rst

… chars

…, not just win32.

gpshead · 2025-05-24T03:22:51Z

reading things over, i'm convinced you are correct that this needs to not just use a win32 check. i moved my #134617 changes over into this PR.

miss-islington-app · 2025-05-24T03:50:23Z

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

…atforms (pythonGH-134462) Doc/library/multiprocessing.rst: freeze_support: Change to specify spawn method instead of platform Have multiprocessing.freeze_support() enable on spawn, not just win32. --------- (cherry picked from commit 80284b5) Co-authored-by: Eddy Mulyono <eddymul@gmail.com> Co-authored-by: Gregory P. Smith <greg@krypto.org>

bedevere-app · 2025-05-24T03:50:32Z

GH-134619 is a backport of this pull request to the 3.14 branch.

bedevere-app · 2025-05-24T03:50:37Z

GH-134620 is a backport of this pull request to the 3.13 branch.

…latforms (GH-134462) (#134620) gh-80334: fix multiprocessing.freeze_support for other spawn platforms (GH-134462) Doc/library/multiprocessing.rst: freeze_support: Change to specify spawn method instead of platform Have multiprocessing.freeze_support() enable on spawn, not just win32. --------- (cherry picked from commit 80284b5) Co-authored-by: Eddy Mulyono <eddymul@gmail.com> Co-authored-by: Gregory P. Smith <greg@krypto.org>

…latforms (GH-134462) (#134619) gh-80334: fix multiprocessing.freeze_support for other spawn platforms (GH-134462) Doc/library/multiprocessing.rst: freeze_support: Change to specify spawn method instead of platform Have multiprocessing.freeze_support() enable on spawn, not just win32. --------- (cherry picked from commit 80284b5) Co-authored-by: Eddy Mulyono <eddymul@gmail.com> Co-authored-by: Gregory P. Smith <greg@krypto.org>

bedevere-bot · 2025-05-24T17:58:25Z

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Windows Server 2022 NoGIL 3.13 (tier-1) has failed when building commit 44d1849.

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/1488/builds/1340) and take a look at the build logs.
Check if the failure is related to this commit (44d1849) 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/1488/builds/1340

Failed tests:

test_launcher

Failed subtests:

test_search_major_3 - test.test_launcher.TestLauncher.test_search_major_3
test_search_major_2 - test.test_launcher.TestLauncher.test_search_major_2

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

==

Click to see traceback logs

Traceback (most recent call last):
  File "C:\Users\Administrator\buildarea\3.13.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\test\test_launcher.py", line 445, in test_search_major_3
    self.assertEqual("PythonCore", data["env.company"])
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 'PythonCore' != 'PythonTestSuite'
- PythonCore
+ PythonTestSuite


Traceback (most recent call last):
  File "C:\Users\Administrator\buildarea\3.13.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\test\test_launcher.py", line 465, in test_search_major_2
    self.assertEqual("PythonCore", data["env.company"])
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 'PythonCore' != 'PythonTestSuite'
- PythonCore
+ PythonTestSuite

… other spawn platforms (pythonGH-134462) (python#134619)" This reverts commit 00fd544.

marcelotduarte · 2025-06-19T06:59:02Z

This patch breaks set_start_method for use in Python (not frozen) and also breaks it for cx_Freeze.
A simple sample using 'spaw', but also breaks with 'fork' or 'forkserver':

import multiprocessing

def foo(q):
    q.put("Hello from cx_Freeze")

if __name__ == "__main__":
    multiprocessing.freeze_support()
    multiprocessing.set_start_method('spawn')
    q = multiprocessing.SimpleQueue()
    p = multiprocessing.Process(target=foo, args=(q,))
    p.start()
    print(q.get())
    p.join()

This sample worked before in windows and linux for python 3.9 until python 3.13.3.
For cx_Freeze I made a temporary fix.

I suspect that a correct fix is change the line 148 from:
if self.get_start_method() == 'spawn' and getattr(sys, 'frozen', False):
to a simple:
if getattr(sys, 'frozen', False):

ZeroIntensity · 2025-06-19T11:55:34Z

@marcelotduarte Please make a new issue so we can track this.

marcelotduarte · 2025-06-19T18:00:04Z

@ZeroIntensity @hugovk @gpshead #135726

…atforms (pythonGH-134462) Doc/library/multiprocessing.rst: freeze_support: Change to specify spawn method instead of platform Have multiprocessing.freeze_support() enable on spawn, not just win32. --------- Co-authored-by: Gregory P. Smith <greg@krypto.org>

pythongh-80334: Doc/library/multiprocessing.rst: freeze_support: Chan…

1814cda

…ge to specify spawn method instead of platform

eddymul requested a review from gpshead as a code owner May 21, 2025 19:09

bedevere-app bot added docs Documentation in the Doc dir skip news labels May 21, 2025

bedevere-app bot mentioned this pull request May 21, 2025

Freeze support documentation is misleading. #80334

Open

bedevere-app bot added the awaiting review label May 21, 2025

github-project-automation bot added this to Docs PRs May 21, 2025

github-project-automation bot moved this to Todo in Docs PRs May 21, 2025

eddymul force-pushed the main branch from 70f32b3 to 1814cda Compare May 21, 2025 20:30

Merge branch 'main' into main

3eb159e

ZeroIntensity added topic-multiprocessing needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes labels May 22, 2025

ZeroIntensity reviewed May 22, 2025

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Doc/library/multiprocessing.rst: freeze_support: Change to wrap at 80…

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… chars

gpshead mentioned this pull request May 24, 2025

GH-80334: make multiprocessing freeze_support() usable on all spawn default platforms, not just windows #134617

Closed

gpshead added 2 commits May 24, 2025 03:20

pythongh-80334: Have multiprocessing.freeze_support() enable on spawn…

2ffa860

…, not just win32.

NEWS

da15203

gpshead changed the title ~~gh-80334: Doc/library/multiprocessing.rst: freeze_support: Change to …~~ gh-80334: fix multiprocessing.freeze_support for other spawn platforms May 24, 2025

gpshead self-assigned this May 24, 2025

gpshead added type-bug An unexpected behavior, bug, or error and removed skip news labels May 24, 2025

gpshead enabled auto-merge (squash) May 24, 2025 03:31

gpshead merged commit 80284b5 into python:main May 24, 2025
43 checks passed

github-project-automation bot moved this from Todo to Done in Docs PRs May 24, 2025

bedevere-app bot removed the awaiting review label May 24, 2025

bedevere-app bot removed the needs backport to 3.14 bugs and security fixes label May 24, 2025

bedevere-app bot removed the needs backport to 3.13 bugs and security fixes label May 24, 2025

hugovk added a commit to hugovk/cpython that referenced this pull request May 25, 2025

Revert "[3.14] pythongh-80334: fix multiprocessing.freeze_support for…

0978de2

… other spawn platforms (pythonGH-134462) (python#134619)" This reverts commit 00fd544.

marcelotduarte mentioned this pull request Jun 19, 2025

hooks: multiprocessing - fix Python 3.13.4+ bug introduced by gh-80334 on Windows marcelotduarte/cx_Freeze#3009

Merged

marcelotduarte mentioned this pull request Jun 19, 2025

Regression in multiprocessing.set_start_method introduced in Python 3.13.4 #135726

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gh-80334: fix multiprocessing.freeze_support for other spawn platforms#134462

gh-80334: fix multiprocessing.freeze_support for other spawn platforms#134462
gpshead merged 5 commits intopython:mainfrom
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