test_multiprocessing_fork: test_nested_startmethod() fails randomly.

[vstinner]

test_nested_startmethod() fails randomly. Since there is no synchronization, putting items in the queue is not really ordered.

I suggest to either accept [1, 2] and [2, 1] in the test, or add some kind of synchronization to ensure that events happen in the expected order. Here I don't think that order matters.

The test was added by PR #108568 of issue gh-108520.

FAIL: test_nested_startmethod (test.test_multiprocessing_fork.test_misc.TestStartMethod.test_nested_startmethod)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL7-ppc64le.lto/build/Lib/test/_test_multiprocessing.py", line 5475, in test_nested_startmethod
    self.assertEqual(results, [2, 1])
AssertionError: Lists differ: [1, 2] != [2, 1]

First differing element 0:
1
2

- [1, 2]
+ [2, 1]

build: https://buildbot.python.org/all/#/builders/503/builds/3941

The failure can be reproduced by stressing the test:

$ ./python -m test test_multiprocessing_fork.test_misc -m test_nested_startmethod -v -j50 --forever
(...)
0:00:24 load avg: 20.03 [157/1] test_multiprocessing_fork.test_misc failed (1 failure)
test_nested_startmethod (test.test_multiprocessing_fork.test_misc.TestStartMethod.test_nested_startmethod) ... FAIL

======================================================================
FAIL: test_nested_startmethod (test.test_multiprocessing_fork.test_misc.TestStartMethod.test_nested_startmethod)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/3.11/Lib/test/_test_multiprocessing.py", line 5399, in test_nested_startmethod
    self.assertEqual(results, [2, 1])
AssertionError: Lists differ: [1, 2] != [2, 1]

First differing element 0:
1
2

- [1, 2]
+ [2, 1]

----------------------------------------------------------------------
Ran 1 test in 2.150s

FAILED (failures=1)
test test_multiprocessing_fork.test_misc failed

Linked PRs

• gh-109706: Fix multiprocessing test_nested_startmethod() #109707
• [3.12] gh-109706: Fix multiprocessing test_nested_startmethod() (GH-109707) #109762
• [3.11] gh-109706: Fix multiprocessing test_nested_startmethod() (GH-109707) #109763