[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)

3 files changed, 22 insertions, 2 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):
 
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-03-02-19-21-21.bpo-39778._YGLEc.rst b/Misc/NEWS.d/next/Core and Builtins/2020-03-02-19-21-21.bpo-39778._YGLEc.rst
new file mode 100644
index 0000000..dc49512
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-03-02-19-21-21.bpo-39778._YGLEc.rst
@@ -0,0 +1,2 @@
+Fixed a crash due to incorrect handling of weak references in
+``collections.OrderedDict`` classes. Patch by Pablo Galindo.
diff --git a/Objects/odictobject.c b/Objects/odictobject.c
index 4c9ae3b..ac0da9b 100644
--- a/Objects/odictobject.c
+++ b/Objects/odictobject.c
@@ -1453,7 +1453,6 @@ odict_traverse(PyODictObject *od, visitproc visit, void *arg)
     _ODictNode *node;
 
     Py_VISIT(od->od_inst_dict);
-    Py_VISIT(od->od_weakreflist);
     _odict_FOREACH(od, node) {
         Py_VISIT(_odictnode_KEY(node));
     }
@@ -1466,7 +1465,6 @@ static int
 odict_tp_clear(PyODictObject *od)
 {
     Py_CLEAR(od->od_inst_dict);
-    Py_CLEAR(od->od_weakreflist);
     PyDict_Clear((PyObject *)od);
     _odict_clear_nodes(od);
     return 0;