[3.12] gh-111777: Fix assertion errors on incorrectly still-tracked GC object destruction (GH-111778) (#111989)

gh-111777: Fix assertion errors on incorrectly still-tracked GC object destruction (GH-111778)

In PyObject_GC_Del, in Py_DEBUG mode, when warning about GC objects that
were not properly untracked before starting destruction, take care to
untrack the object _before_ warning, to avoid triggering a GC run and
causing the problem the code tries to warn about. Also make sure to save and
restore any pending exceptions, which the warning would otherwise clobber or
trigger an assertion error on.
(cherry picked from commit ce6a533)

Co-authored-by: T. Wouters <[email protected]>

Author: miss-islington

Modules/gcmodule.c

@@ -2381,14 +2381,16 @@ PyObject_GC_Del(void *op)
     size_t presize = _PyType_PreHeaderSize(((PyObject *)op)->ob_type);
     PyGC_Head *g = AS_GC(op);
     if (_PyObject_GC_IS_TRACKED(op)) {
+        gc_list_remove(g);
 #ifdef Py_DEBUG
+        PyObject *exc = PyErr_GetRaisedException();
         if (PyErr_WarnExplicitFormat(PyExc_ResourceWarning, "gc", 0,
                                      "gc", NULL, "Object of type %s is not untracked before destruction",
                                      ((PyObject*)op)->ob_type->tp_name)) {
             PyErr_WriteUnraisable(NULL);
         }
+        PyErr_SetRaisedException(exc);
 #endif
-        gc_list_remove(g);
     }
     GCState *gcstate = get_gc_state();
     if (gcstate->generations[0].count > 0) {