gh-119004: fix a crash in equality testing between `OrderedDict` (#121329)

Doc/library/collections.rst

@@ -1169,8 +1169,11 @@ Some differences from :class:`dict` still remain:
 In addition to the usual mapping methods, ordered dictionaries also support
 reverse iteration using :func:`reversed`.
 
+.. _collections_OrderedDict__eq__:
+
 Equality tests between :class:`OrderedDict` objects are order-sensitive
-and are implemented as ``list(od1.items())==list(od2.items())``.
+and are roughly equivalent to ``list(od1.items())==list(od2.items())``.
+
 Equality tests between :class:`OrderedDict` objects and other
 :class:`~collections.abc.Mapping` objects are order-insensitive like regular
 dictionaries.  This allows :class:`OrderedDict` objects to be substituted

Lib/test/test_ordered_dict.py

@@ -2,7 +2,9 @@ import builtins
 import contextlib
 import copy
 import gc
+import operator
 import pickle
+import re
 from random import randrange, shuffle
 import struct
 import sys
@@ -740,11 +742,44 @@ class OrderedDictTests:
         # when it's mutated and returned from __next__:
         self.assertTrue(gc.is_tracked(next(it)))
 
+
+class _TriggerSideEffectOnEqual:
+    count = 0   # number of calls to __eq__
+    trigger = 1 # count value when to trigger side effect
+
+    def __eq__(self, other):
+        if self.__class__.count == self.__class__.trigger:
+            self.side_effect()
+        self.__class__.count += 1
+        return True
+
+    def __hash__(self):
+        # all instances represent the same key
+        return -1
+
+    def side_effect(self):
+        raise NotImplementedError
+
 class PurePythonOrderedDictTests(OrderedDictTests, unittest.TestCase):
 
     module = py_coll
     OrderedDict = py_coll.OrderedDict
 
+    def test_issue119004_attribute_error(self):
+        class Key(_TriggerSideEffectOnEqual):
+            def side_effect(self):
+                del dict1[TODEL]
+
+        TODEL = Key()
+        dict1 = self.OrderedDict(dict.fromkeys((0, TODEL, 4.2)))
+        dict2 = self.OrderedDict(dict.fromkeys((0, Key(), 4.2)))
+        # This causes an AttributeError due to the linked list being changed
+        msg = re.escape("'NoneType' object has no attribute 'key'")
+        self.assertRaisesRegex(AttributeError, msg, operator.eq, dict1, dict2)
+        self.assertEqual(Key.count, 2)
+        self.assertDictEqual(dict1, dict.fromkeys((0, 4.2)))
+        self.assertDictEqual(dict2, dict.fromkeys((0, Key(), 4.2)))
+
 
 class CPythonBuiltinDictTests(unittest.TestCase):
     """Builtin dict preserves insertion order.
@@ -765,8 +800,85 @@ for method in (
 del method
 
 
+class CPythonOrderedDictSideEffects:
+
+    def check_runtime_error_issue119004(self, dict1, dict2):
+        msg = re.escape("OrderedDict mutated during iteration")
+        self.assertRaisesRegex(RuntimeError, msg, operator.eq, dict1, dict2)
+
+    def test_issue119004_change_size_by_clear(self):
+        class Key(_TriggerSideEffectOnEqual):
+            def side_effect(self):
+                dict1.clear()
+
+        dict1 = self.OrderedDict(dict.fromkeys((0, Key(), 4.2)))
+        dict2 = self.OrderedDict(dict.fromkeys((0, Key(), 4.2)))
+        self.check_runtime_error_issue119004(dict1, dict2)
+        self.assertEqual(Key.count, 2)
+        self.assertDictEqual(dict1, {})
+        self.assertDictEqual(dict2, dict.fromkeys((0, Key(), 4.2)))
+
+    def test_issue119004_change_size_by_delete_key(self):
+        class Key(_TriggerSideEffectOnEqual):
+            def side_effect(self):
+                del dict1[TODEL]
+
+        TODEL = Key()
+        dict1 = self.OrderedDict(dict.fromkeys((0, TODEL, 4.2)))
+        dict2 = self.OrderedDict(dict.fromkeys((0, Key(), 4.2)))
+        self.check_runtime_error_issue119004(dict1, dict2)
+        self.assertEqual(Key.count, 2)
+        self.assertDictEqual(dict1, dict.fromkeys((0, 4.2)))
+        self.assertDictEqual(dict2, dict.fromkeys((0, Key(), 4.2)))
+
+    def test_issue119004_change_linked_list_by_clear(self):
+        class Key(_TriggerSideEffectOnEqual):
+            def side_effect(self):
+                dict1.clear()
+                dict1['a'] = dict1['b'] = 'c'
+
+        dict1 = self.OrderedDict(dict.fromkeys((0, Key(), 4.2)))
+        dict2 = self.OrderedDict(dict.fromkeys((0, Key(), 4.2)))
+        self.check_runtime_error_issue119004(dict1, dict2)
+        self.assertEqual(Key.count, 2)
+        self.assertDictEqual(dict1, dict.fromkeys(('a', 'b'), 'c'))
+        self.assertDictEqual(dict2, dict.fromkeys((0, Key(), 4.2)))
+
+    def test_issue119004_change_linked_list_by_delete_key(self):
+        class Key(_TriggerSideEffectOnEqual):
+            def side_effect(self):
+                del dict1[TODEL]
+                dict1['a'] = 'c'
+
+        TODEL = Key()
+        dict1 = self.OrderedDict(dict.fromkeys((0, TODEL, 4.2)))
+        dict2 = self.OrderedDict(dict.fromkeys((0, Key(), 4.2)))
+        self.check_runtime_error_issue119004(dict1, dict2)
+        self.assertEqual(Key.count, 2)
+        self.assertDictEqual(dict1, {0: None, 'a': 'c', 4.2: None})
+        self.assertDictEqual(dict2, dict.fromkeys((0, Key(), 4.2)))
+
+    def test_issue119004_change_size_by_delete_key_in_dict_eq(self):
+        class Key(_TriggerSideEffectOnEqual):
+            trigger = 0
+            def side_effect(self):
+                del dict1[TODEL]
+
+        TODEL = Key()
+        dict1 = self.OrderedDict(dict.fromkeys((0, TODEL, 4.2)))
+        dict2 = self.OrderedDict(dict.fromkeys((0, Key(), 4.2)))
+        self.assertEqual(Key.count, 0)
+        # the side effect is in dict.__eq__ and modifies the length
+        self.assertNotEqual(dict1, dict2)
+        self.assertEqual(Key.count, 2)
+        self.assertDictEqual(dict1, dict.fromkeys((0, 4.2)))
+        self.assertDictEqual(dict2, dict.fromkeys((0, Key(), 4.2)))
+
+
 @unittest.skipUnless(c_coll, 'requires the C version of the collections module')
-class CPythonOrderedDictTests(OrderedDictTests, unittest.TestCase):
+class CPythonOrderedDictTests(OrderedDictTests,
+                              CPythonOrderedDictSideEffects,
+                              unittest.TestCase):
 
     module = c_coll
     OrderedDict = c_coll.OrderedDict

Misc/NEWS.d/next/Library/2024-07-03-14-23-04.gh-issue-119004.L5MoUu.rst

@@ -0,0 +1,2 @@
+Fix a crash in :ref:`OrderedDict.__eq__ <collections_OrderedDict__eq__>`
+when operands are mutated during the check. Patch by Bénédikt Tran.

Objects/odictobject.c

@@ -796,6 +796,7 @@ _odict_clear_nodes(PyODictObject *od)
         _odictnode_DEALLOC(node);
         node = next;
     }
+    od->od_state++;
 }
 
 /* There isn't any memory management of nodes past this point. */
@@ -806,24 +807,40 @@ _odict_keys_equal(PyODictObject *a, PyODictObject *b)
 {
     _ODictNode *node_a, *node_b;
 
+    // keep operands' state to detect undesired mutations
+    const size_t state_a = a->od_state;
+    const size_t state_b = b->od_state;
+
     node_a = _odict_FIRST(a);
     node_b = _odict_FIRST(b);
     while (1) {
-        if (node_a == NULL && node_b == NULL)
+        if (node_a == NULL && node_b == NULL) {
             /* success: hit the end of each at the same time */
             return 1;
-        else if (node_a == NULL || node_b == NULL)
+        }
+        else if (node_a == NULL || node_b == NULL) {
             /* unequal length */
             return 0;
+        }
         else {
-            int res = PyObject_RichCompareBool(
-                                            (PyObject *)_odictnode_KEY(node_a),
-                                            (PyObject *)_odictnode_KEY(node_b),
-                                            Py_EQ);
-            if (res < 0)
+            PyObject *key_a = Py_NewRef(_odictnode_KEY(node_a));
+            PyObject *key_b = Py_NewRef(_odictnode_KEY(node_b));
+            int res = PyObject_RichCompareBool(key_a, key_b, Py_EQ);
+            Py_DECREF(key_a);
+            Py_DECREF(key_b);
+            if (res < 0) {
                 return res;
-            else if (res == 0)
+            }
+            else if (a->od_state != state_a || b->od_state != state_b) {
+                PyErr_SetString(PyExc_RuntimeError,
+                                "OrderedDict mutated during iteration");
+                return -1;
+            }
+            else if (res == 0) {
+                // This check comes after the check on the state
+                // in order for the exception to be set correctly.
                 return 0;
+            }
 
             /* otherwise it must match, so move on to the next one */
             node_a = _odictnode_NEXT(node_a);