`dict` item-iterator: `_PyObject_GC_UNTRACK` on never-tracked iterator under OOM (`dictobject.c`)

Creating a dict item-iterator (iter(d.items()) or reversed(d.items())) while
allocations are failing aborts a debug build with:

Objects/dictobject.c:5532: _PyObject_GC_UNTRACK:
    Assertion "_PyObject_GC_IS_TRACKED(op)" failed:
    object not tracked by the garbage collector
object type name: dict_itemiterator
Fatal Python error: _PyObject_AssertFailed

dictiter_new() allocates the iterator with PyObject_GC_New() but only calls
_PyObject_GC_TRACK(di) at the very end. For item iterators it first allocates
di->di_result via _PyTuple_FromPairSteal(). If that allocation fails (OOM), the
error path runs Py_DECREF(di) on the still-untracked iterator, and
dictiter_dealloc() unconditionally calls _PyObject_GC_UNTRACK(di).

• Debug build: the _PyObject_GC_IS_TRACKED assertion fires → Py_FatalError / abort.
• Release build (NDEBUG): the assert is compiled out and untracking a never-tracked
  object corrupts the GC list, leading to a later segfault in _Py_Dealloc.

Only item iterators are affected (dict_itemiterator / dict_reverseitemiterator) —
they are the only dict iterators whose di_result requires an allocation between
PyObject_GC_New and _PyObject_GC_TRACK.

This was found via allocation-failure fuzzing as OOM-0006 in the umbrella issue
#151763. Filing a focused issue per that issue's process.

Reproducer

Minimal, stdlib-only, deterministic (requires a --with-pydebug interpreter so
_testcapi.set_nomemory is available):

import faulthandler, _strptime
faulthandler.enable()
from _testcapi import set_nomemory
for start in range(60):
    set_nomemory(start)
    try:
        _strptime._strptime("", "")
    except BaseException:
        pass
print("done, no crash")

_strptime._strptime("", "") performs an iter(d.items()) once the size-2 tuple
freelist is drained, so _PyTuple_FromPairSteal() reaches the failing allocator and
drives the dictiter_new() OOM error path. (Reducer by @devdanzin; gist:
https://gist.github.com/devdanzin/c809eb4072c0c787c0c890f54ba1c843)

Backtrace (abbreviated)

_PyObject_AssertFailed                       Objects/object.c
_PyObject_GC_UNTRACK                          Include/internal/pycore_gc.h:254
dictiter_dealloc (self=dict_itemiterator)     Objects/dictobject.c:5532
_Py_Dealloc                                   Objects/object.c
Py_DECREF                                     Include/refcount.h
dictiter_new (itertype=PyDictIterItem_Type)   Objects/dictobject.c   <- Py_DECREF(di) after di_result alloc fails
PyObject_GetIter                              Objects/abstract.c

Suggested fix

Either guard the untracked against the never-tracked error path:

if (_PyObject_GC_IS_TRACKED(di)) {
    _PyObject_GC_UNTRACK(di);
}

or move _PyObject_GC_TRACK(di) ahead of the fallible di_result allocation (after
initializing di->di_result = NULL so traverse/dealloc stay safe). A PR with the first
approach and a regression test is ready.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

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

No response

Linked PRs

• gh-152110

Start in Objects/dictobject.c at dictiter_new() and dictiter_dealloc(), then run the provided _strptime and _testcapi.set_nomemory reproducer on a --with-pydebug build. Check the linked PR for work already underway. Done means the item-iterator OOM path no longer aborts or corrupts the GC, with a regression test covering the failure.