From 0c2b509f9d1d3a9065bc62c2407e1dc2ed70e9c2 Mon Sep 17 00:00:00 2001
From: Pablo Galindo <Pablogsal@gmail.com>
Date: Mon, 2 Mar 2020 23:12:54 +0000
Subject: [PATCH] bpo-39778: Don't traverse weak-reference lists OrderedDict's
 tp_traverse and tp_clear (GH-18749)

Objects do not own weak references to them directly through the __weakref__ list so these
do not need to be traversed by the GC.
---
 Lib/test/test_ordered_dict.py                 | 20 +++++++++++++++++++
 .../2020-03-02-19-21-21.bpo-39778._YGLEc.rst  |  2 ++
 Objects/odictobject.c                         |  2 --
 3 files changed, 22 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2020-03-02-19-21-21.bpo-39778._YGLEc.rst

diff --git a/Lib/test/test_ordered_dict.py b/Lib/test/test_ordered_dict.py
index f337be8eb85..eb0a8f4ba0d 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 00000000000..dc495121673
--- /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 f412220e8cc..6813cddfddc 100644
--- a/Objects/odictobject.c
+++ b/Objects/odictobject.c
@@ -1454,7 +1454,6 @@ 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));
     }
@@ -1467,7 +1466,6 @@ 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;