`test_frame.test_repr_deep()` fails randomly on Windows11 Bigmem buildbot: _PyObject_GC_UNTRACK() assertion error

[StanFromIreland]

What happened?

Assertion error in _PyObject_GC_UNTRACK() when running test_frame.test_repr_deep() on AMD64 Windows11 Bigmem 3.x buildbot.

AMD64 Windows11 Bigmem 3.x: https://buildbot.python.org/#/builders/1079/builds/6431

test_frame
(...)
test_repr_deep (test.test_frame.FrameLocalsProxyMappingTests.test_repr_deep) ...

R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Objects\listobject.c:3997: _PyObject_GC_UNTRACK: Assertion "_PyObject_GC_IS_TRACKED(((PyObject*)(op)))" failed: object not tracked by the garbage collector
Enable tracemalloc to get the memory block allocation traceback
object address : 0000016FA81B1090
object refcount : 0
object type : 00007FFB97E7EEB0
object type name: list_iterator
object repr : <refcnt 0 at 0000016FA81B1090>
Fatal Python error: _PyObject_AssertFailed: _PyObject_AssertFailed
Python runtime state: initialized

Warning -- Unraisable exception
Exception ignored in the internal traceback machinery:
Traceback (most recent call last):
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\traceback.py", line 143, in _print_exception_bltin
 return print_exception(exc, limit=BUILTIN_EXCEPTION_LIMIT, file=file, colorize=colorize)
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\traceback.py", line 134, in print_exception
 te.print(file=file, chain=chain, colorize=colorize)
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\traceback.py", line 1577, in print
 for line in self.format(chain=chain, colorize=colorize):
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\traceback.py", line 1514, in format
 yield from _ctx.emit(exc.format_exception_only(colorize=colorize))
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\traceback.py", line 1001, in emit
 for text in text_gen:
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\traceback.py", line 1268, in format_exception_only
 isinstance(self.__notes__, collections.abc.Sequence)
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\abc.py", line 119, in __instancecheck__
 return _abc_instancecheck(cls, instance)
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\abc.py", line 123, in __subclasscheck__
 return _abc_subclasscheck(cls, subclass)
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\abc.py", line 123, in __subclasscheck__
 return _abc_subclasscheck(cls, subclass)
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\abc.py", line 123, in __subclasscheck__
 return _abc_subclasscheck(cls, subclass)
RecursionError: Stack overflow (used 11719 kB)

Current thread 0x00000408 (most recent call first):
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\unittest\case.py", line 247 in handle
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\unittest\case.py", line 813 in assertRaises
 File "R:\buildarea\pull_request.ambv-bb-win11.bigmem\build\Lib\test\mapping_tests.py", line 634 in test_repr_deep
 (...)
Extension modules: _testinternalcapi, _testcapi (total: 2)

See also #133261 (test_frame crash).

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Output from running 'python -VV' on the command line:

Python 3.14

[vstinner]

@StanFromIreland: I don't see the assertion error in your output. "Fatal Python error: Segmentation fault" looks more like #133261.

Update: I modified the issue first message to focus on the assertion error.

[vstinner]

It should be fixed by #133431. Reopen the issue if the bug is still there.