Potential thread unsafety in test_free_threading.test_monitoring

Bug report

Bug description:

It seems that there is some kind of race with test_free_threading.test_monitoring, specifically the sys.settrace tests:
https://github.com/python/cpython/blob/fba5dded6df3c2b1943557afef89a5cb418f65a2/Lib/test/test_free_threading/test_monitoring.py#L143-L166.

This was initially spotted in a buildbot failure here: https://buildbot.python.org/#/builders/1610/builds/1590. There have been a few behaviors noted:

• refleaks causing a test failure
• assertion failures
• presumed deadlocks or other contention causing the tests to run extremely slowly (timing out after 45 minutes)
• tests pass without issue 😬

The variety of failure modes makes me think there is some kind of thread safety issue lurking behind the failures...

After some local debugging (the remote debugger rocks!), I was able to reproduce the extremely long test run of test_freethreading, and narrow down the majority of the time being in the sys.settrace tests. My command line was:

./python -E  -m test --timeout=2700 -R 3:3 -u-cpu -j10 test_free_threading

On my x86 laptop on Ubuntu 24.04/WSL.

Reading over the test, it isn't clear to me what is meant to be tested. sys.settrace only affects the current thread according to the docs (indeed, the C code sets the trace function to the current threadstate), so currently it is only checking that the main thread executes a frame of some sort.

I didn't have time to find the root cause of the weird behavior above, so I'm filing this bug to note down my findings.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Beginne mit Lib/test/test_free_threading/test_monitoring.py Zeilen 143–166 und reproduziere das Problem mit ./python -E -m test --timeout=2700 -R 3:3 -u-cpu -j10 test_free_threading. Untersuche die sys.settrace-Tests zusammen mit dem verknüpften Buildbot-Fehler und ermittle die Ursache des Race-Conditions, der Konkurrenz oder der Leaks. Erledigt ist die Aufgabe, wenn der Fehlermodus verstanden ist und die betroffenen Tests zuverlässig ausgeführt werden.