test_start_new_thread_failed flaky: failed on macOS (free threading)

[colesbury]

Bug report

Seen in https://github.com/python/cpython/actions/runs/12082953758/job/33695120037?pr=127399

======================================================================
FAIL: test_start_new_thread_failed (test.test_threading.ThreadTests.test_start_new_thread_failed)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/admin/actions-runner/_work/cpython/cpython/Lib/test/test_threading.py", line 1201, in test_start_new_thread_failed
    _, out, err = assert_python_ok("-u", "-c", code)
                  ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/Users/admin/actions-runner/_work/cpython/cpython/Lib/test/support/script_helper.py", line 182, in assert_python_ok
    return _assert_python(True, *args, **env_vars)
  File "/Users/admin/actions-runner/_work/cpython/cpython/Lib/test/support/script_helper.py", line 167, in _assert_python
    res.fail(cmd_line)
    ~~~~~~~~^^^^^^^^^^
  File "/Users/admin/actions-runner/_work/cpython/cpython/Lib/test/support/script_helper.py", line 80, in fail
    raise AssertionError(f"Process return code is {exitcode}\n"
    ...<10 lines>...
                         f"---")
AssertionError: Process return code is -6 (SIGABRT)
command line: ['/Users/admin/actions-runner/_work/cpython/cpython/python.exe', '-X', 'faulthandler', '-I', '-u', '-c', 'if 1:\n            import resource\n            import _thread\n\n            def f():\n                print("shouldn\'t be printed")\n\n            limits = resource.getrlimit(resource.RLIMIT_NPROC)\n            [_, hard] = limits\n            resource.setrlimit(resource.RLIMIT_NPROC, (0, hard))\n\n            try:\n                _thread.start_new_thread(f, ())\n            except RuntimeError:\n                print(\'ok\')\n            else:\n                print(\'!skip!\')\n        ']

Linked PRs

• gh-127421: Fix race in test_start_new_thread_failed #127549
• [3.13] gh-127421: Fix race in test_start_new_thread_failed (GH-127549) #127574

[serhiy-storchaka]

As a temporary workaround, adding a small sleep in the else branch can help, but there should be better solution. This failure may be a symptom of existing bug.