[3.8] bpo-39778: Don't traverse weak-reference lists OrderedDict's tp_traverse and tp_clear (GH-18749) (GH-18756)

Objects do not own weak references to them directly through the __weakref__ list so these
do not need to be traversed by the GC.

(cherry picked from commit 0c2b509)

1 files changed, 20 insertions, 0 deletions

diff --git a/Lib/test/test_ordered_dict.py b/Lib/test/test_ordered_dict.py
index 148a9bd..085e5f6 100644
--- a/Lib/test/test_ordered_dict.py
+++ b/Lib/test/test_ordered_dict.py
@@ -753,6 +753,26 @@ class CPythonOrderedDictTests(OrderedDictTests, unittest.TestCase):
                     self.assertEqual(list(unpickled), expected)
                     self.assertEqual(list(it), expected)
 
+    @support.cpython_only
+    def test_weakref_list_is_not_traversed(self):
+        # Check that the weakref list is not traversed when collecting
+        # OrderedDict objects. See bpo-39778 for more information.
+
+        gc.collect()
+
+        x = self.OrderedDict()
+        x.cycle = x
+
+        cycle = []
+        cycle.append(cycle)
+
+        x_ref = weakref.ref(x)
+        cycle.append(x_ref)
+
+        del x, cycle, x_ref
+
+        gc.collect()
+
 
 class PurePythonOrderedDictSubclassTests(PurePythonOrderedDictTests):