Is the test_threading on python PR easy to failed?

On my computer it success, but the test on CI failed. And I found that many PR at that time failed on this test with the same exception message: exit code is 66

The message here:

0:14:59 load avg: 5.06 [74/76/1] test_threading failed (1 failure) (1 min 45 sec) -- running (2): test_socket (2 min 37 sec), test_free_threading (1 min 19 sec)
test test_threading failed -- Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython/Lib/test/test_threading.py", line 1797, in test_daemon_threads_fatal_error
    assert_python_ok("-c", script)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "/home/runner/work/cpython/cpython/Lib/test/support/script_helper.py", line 182, in assert_python_ok
    return _assert_python(True, *args, **env_vars)
  File "/home/runner/work/cpython/cpython/Lib/test/support/script_helper.py", line 167, in _assert_python
    res.fail(cmd_line)
    ~~~~~~~~^^^^^^^^^^
  File "/home/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 66
command line: ['/home/runner/work/cpython/cpython/python', '-X', 'faulthandler', '-I', '-c', "if 1:\n            import _testcapi\n\n            _testcapi.run_in_subinterp('if 1:\\n            import os\\n            import threading\\n            import time\\n\\n            def f():\\n                # Make sure the daemon thread is still running when\\n                # Py_EndInterpreter is called.\\n                time.sleep(30.0)\\n            threading.Thread(target=f, daemon=True).start()\\n            ')\n            "]

stdout:
---

---

stderr:
---

---

Then there is probably a difference in platform and or build configuration. Are you configuring with the same options, if so, does matching the ones in the CI fix the failure? Are you on the same platform?

Different. However, it seems that many PRs may failed on this same test with same message. Is it a flaky failure?

Yes, the TSan (free-threading) runner has been flaky today.

Ok. #140138 show the real reason.