`test_active_children` multiprocessing test is flaky

[colesbury]

Bug report

Seen in https://github.com/python/cpython/actions/runs/13606431591/job/38038447198?pr=130724

FAIL: test_active_children (test.test_multiprocessing_spawn.test_threads.WithThreadsTestProcess.test_active_children)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\a\cpython\cpython\Lib\test\_test_multiprocessing.py", line 597, in test_active_children
    self.assertIn(p, self.active_children())
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: <DummyProcess(Thread-120 (sleep), stopped daemon 6440)> not found in []

Test succeeded when retried.

Also see the old issue:

• Transient test_multiprocessing failure (test_active_children) #50910

Linked PRs

• gh-130730: Fix flaky test_active_children test #130731
• gh-130730: Fix multiprocessing test_active_children() #130837
• [3.13] gh-130730: Fix multiprocessing test_active_children() (GH-130837) #130845
• [3.12] gh-130730: Fix multiprocessing test_active_children() (GH-130837) #130846

[vstinner]

The test uses a sleep as a synchronization method: it's not reliable. If the process completes before self.assertIn(p, self.active_children()), the test fails.

I wrote #130837 to replace the sleep with an event.