GH-133261: Make sure that the GC doesn't untrack objects in trashcan

[markshannon]

The trashcan stores pointers in the reference count field. This could make the object appear immortal causing the GC to untrack it.
This PR makes sure that those pointers are stored in such a way as to never appear immortal.

• Issue: test_frame does crash randomly on Linux (x86-64, ppc64le, s390x) #133261

[vstinner]

LGTM.

This change can explain a lot of very weird bugs and crashes that I saw last days on buildbots and GitHub CIs.

[vstinner]

Tests / Hypothesis tests on Ubuntu (pull_request)Failing after 7m

test_external_inspection failed:

FAIL: test_async_remote_stack_trace (test.test_external_inspection.TestGetStackTrace.test_async_remote_stack_trace) (task_factory_variant='asyncio.new_event_loop')

I wrote a fix for this one: #133433.

[vstinner]

Without this change, test_frame crash after a few attempts (between 1 and 3).

With this change, test_frame does no longer crash. I ran ./python -u -m test -j2 -F test_frame for 3 minutes:

0:03:46 load avg: 3.06 [211] test_frame passed
0:03:47 load avg: 3.06 [212] test_frame passed
...

[vstinner]

With this change, #133199 (gh-124715: Fix method_dealloc(): use PyObject_GC_UnTrack()) can also be reverted: test_functools no longer crash.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot x86 Debian Non-Debug with X 3.x (no tier) has failed when building commit f554237.

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/1245/builds/5317) and take a look at the build logs.
4. Check if the failure is related to this commit (f554237) 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/1245/builds/5317

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

==

Click to see traceback logs

Traceback (test.test_exceptions.ExceptionTests.testInvalidTraceback) ... �[32mok�[0m


Traceback (most recent call last):
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.nondebug/build/Lib/test/support/__init__.py"�[0m, line �[35m2876�[0m, in �[35mwrapper�[0m
    return func(*args, **kwargs)
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.nondebug/build/Lib/test/test_exceptions.py"�[0m, line �[35m1518�[0m, in �[35mtest_recursion_normalizing_infinite_exception�[0m
    �[31mself.assertEqual�[0m�[1;31m(rc, 1)�[0m
    �[31m~~~~~~~~~~~~~~~~�[0m�[1;31m^^^^^^^�[0m
�[1;35mAssertionError�[0m: �[35m-6 != 1�[0m


Traceback (test.test_exceptions.ExceptionTests.testWithTraceback) ... �[32mok�[0m


TracebackAttr (test.test_exceptions.ExceptionTests.testNoneClearsTracebackAttr) ... �[32mok�[0m

[vstinner]

Hi! The buildbot x86 Debian Non-Debug with X 3.x (no tier) has failed when building commit f554237.

That's not good, many tests failed with Assertion '!_Py_IsImmortal(op)' failed:

13 tests failed:
    test_call test_capi test_ctypes test_descr test_dict
    test_dictviews test_exception_group test_exceptions test_frame
    test_functools test_isinstance test_json test_list

[markshannon]

That's a 32 bit machine, right?
I'll take a look.