The change added a new functional test. As I expected, it failed on "AMD64 Debian root". I expected it because the previously added functional test failed failed on "AMD64 Debian root", I reported the race condition in 2017 and it's still not fixed: bpo-30849.

The new functional test has two race conditions.

I reopen the issue.


== Race condition 1 ==

AMD64 Debian root 3.x:
https://buildbot.python.org/all/#/builders/345/builds/903

0:15:05 load avg: 2.58 [137/427/1] test_signal failed (env changed) (1 min 13 sec)
Warning -- Unraisable exception
Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/threading.py", line 1067, in _wait_for_tstate_lock
    elif lock.acquire(block, timeout):
OSError: Signal 10 ignored due to race condition


== Race condition 2 ==

By the way, the test also fails when I stress my laptop:

$ ./python -m test --fail-env-changed test_signal -F -j40 -m test_stress_modifying_handlers -v
== CPython 3.10.0a6+ (heads/master:a9c03d7fb7, Mar 10 2021, 12:41:26) [GCC 10.2.1 20201125 (Red Hat 10.2.1-9)]
== Linux-5.10.15-200.fc33.x86_64-x86_64-with-glibc2.32 little-endian
== cwd: /home/vstinner/python/master/build/test_python_223315æ
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
0:00:00 load avg: 4.17 Run tests in parallel using 40 child processes
(...)
0:00:05 load avg: 7.52 [  5/1] test_signal failed
test_stress_modifying_handlers (test.test_signal.StressTest) ... FAIL

======================================================================
FAIL: test_stress_modifying_handlers (test.test_signal.StressTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/master/Lib/test/test_signal.py", line 1297, in test_stress_modifying_handlers
    self.assertGreater(num_received_signals, 0)
AssertionError: 0 not greater than 0

----------------------------------------------------------------------

Ran 1 test in 0.039s

FAILED (failures=1)
test test_signal failed
(...)