GH-91636: Clear weakrefs created by finalizers. (GH-136401)

Weakrefs to unreachable garbage that are created during running of
finalizers need to be cleared.  This avoids exposing objects that
have `tp_clear` called on them to Python-level code.

Python/gc.c

@@ -870,7 +870,7 @@ move_legacy_finalizer_reachable(PyGC_Head *finalizers)
  * no object in `unreachable` is weakly referenced anymore.
  */
 static int
-handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
+handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old, bool allow_callbacks)
 {
     PyGC_Head *gc;
     PyObject *op;               /* generally FROM_GC(gc) */
@@ -879,7 +879,9 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
     PyGC_Head *next;
     int num_freed = 0;
 
-    gc_list_init(&wrcb_to_call);
+    if (allow_callbacks) {
+        gc_list_init(&wrcb_to_call);
+    }
 
     /* Clear all weakrefs to the objects in unreachable.  If such a weakref
      * also has a callback, move it into `wrcb_to_call` if the callback
@@ -935,6 +937,11 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
             _PyObject_ASSERT((PyObject *)wr, wr->wr_object == op);
             _PyWeakref_ClearRef(wr);
             _PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
+
+            if (!allow_callbacks) {
+                continue;
+            }
+
             if (wr->wr_callback == NULL) {
                 /* no callback */
                 continue;
@@ -987,6 +994,10 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
         }
     }
 
+    if (!allow_callbacks) {
+        return 0;
+    }
+
     /* Invoke the callbacks we decided to honor.  It's safe to invoke them
      * because they can't reference unreachable objects.
      */
@@ -1737,7 +1748,7 @@ gc_collect_region(PyThreadState *tstate,
     }
 
     /* Clear weakrefs and invoke callbacks as necessary. */
-    stats->collected += handle_weakrefs(&unreachable, to);
+    stats->collected += handle_weakrefs(&unreachable, to, true);
     gc_list_validate_space(to, gcstate->visited_space);
     validate_list(to, collecting_clear_unreachable_clear);
     validate_list(&unreachable, collecting_set_unreachable_clear);
@@ -1751,6 +1762,14 @@ gc_collect_region(PyThreadState *tstate,
     gc_list_init(&final_unreachable);
     handle_resurrected_objects(&unreachable, &final_unreachable, to);
 
+    /* Clear weakrefs to objects in the unreachable set.  No Python-level
+     * code must be allowed to access those unreachable objects.  During
+     * delete_garbage(), finalizers outside the unreachable set might run
+     * and create new weakrefs.  If those weakrefs were not cleared, they
+     * could reveal unreachable objects.  Callbacks are not executed.
+     */
+    handle_weakrefs(&final_unreachable, NULL, false);
+
     /* Call tp_clear on objects in the final_unreachable set.  This will cause
     * the reference cycles to be broken.  It may also cause some objects
     * in finalizers to be freed.