gh-134679: Fix assertion failure in QSBR

[colesbury]

This is the same underlying bug as gh-130519. The destructor may call arbitrary code, changing the tstate->qsbr pointer and invalidating the old _qsbr_thread_state.

• Issue: Assertion failed: Python/qsbr.c in _Py_qsbr_poll #134679

[colesbury]

Tested by running ./python -m test test_free_threading.test_dict in a loop with TSan enabled. TSan seems to help reproduce the bug, probably because it slows a lot of things down.

[miss-islington-app]

Thanks @colesbury for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-134814 is a backport of this pull request to the 3.14 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 CentOS9 NoGIL Refleaks 3.x (tier-1) has failed when building commit a4d37f8.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1610/builds/1486) and take a look at the build logs.
4. Check if the failure is related to this commit (a4d37f8) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1610/builds/1486

Failed tests:

• test.test_concurrent_futures.test_thread_pool

Failed subtests:

• test_free_reference - test.test_concurrent_futures.test_thread_pool.ThreadPoolExecutorTest.test_free_reference

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.itamaro-centos-aws.refleak.nogil/build/Lib/test/test_concurrent_futures/executor.py", line 220, in test_free_reference
    for _ in support.sleeping_retry(support.SHORT_TIMEOUT):
             ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.x.itamaro-centos-aws.refleak.nogil/build/Lib/test/support/__init__.py", line 2590, in sleeping_retry
    for _ in busy_retry(timeout, err_msg, error=error):
             ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.x.itamaro-centos-aws.refleak.nogil/build/Lib/test/support/__init__.py", line 2560, in busy_retry
    raise AssertionError(msg)
AssertionError: timeout (68.3 seconds)