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

[eddymul]

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

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[gpshead]

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]

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

[bedevere-app]

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

[bedevere-app]

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

[bedevere-bot]

⚠️⚠️⚠️ 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:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. 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.
4. Check if the failure is related to this commit (44d1849) or if it is a false positive.
5. 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

[marcelotduarte]

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]

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

[marcelotduarte]

@ZeroIntensity @hugovk @gpshead #135726