Issue #21435: Segfault in gc with cyclic trash

Changed the iteration logic in finalize_garbage() to tolerate objects vanishing
from the list as a side effect of executing a finalizer.

Lib/test/test_gc.py

@@ -580,6 +580,38 @@ class GCTests(unittest.TestCase):
             # would be damaged, with an empty __dict__.
             self.assertEqual(x, None)
 
+    def test_bug21435(self):
+        # This is a poor test - its only virtue is that it happened to
+        # segfault on Tim's Windows box before the patch for 21435 was
+        # applied.  That's a nasty bug relying on specific pieces of cyclic
+        # trash appearing in exactly the right order in finalize_garbage()'s
+        # input list.
+        # But there's no reliable way to force that order from Python code,
+        # so over time chances are good this test won't really be testing much
+        # of anything anymore.  Still, if it blows up, there's _some_
+        # problem ;-)
+        gc.collect()
+
+        class A:
+            pass
+
+        class B:
+            def __init__(self, x):
+                self.x = x
+
+            def __del__(self):
+                self.attr = None
+
+        def do_work():
+            a = A()
+            b = B(A())
+
+            a.attr = b
+            b.attr = a
+
+        do_work()
+        gc.collect() # this blows up (bad C pointer) when it fails
+
     @cpython_only
     def test_garbage_at_shutdown(self):
         import subprocess