[3.11] pythongh-115243: Fix crash in deque.index() when the deque is concurrently modified (pythonGH-115247) (pythonGH-115466)

(cherry picked from commit 6713601)

Co-authored-by: kcatss <[email protected]>

Author: 2 people

Lib/test/test_deque.py

@@ -184,7 +184,7 @@ def test_contains(self):
         with self.assertRaises(RuntimeError):
             n in d
 
-    def test_contains_count_stop_crashes(self):
+    def test_contains_count_index_stop_crashes(self):
         class A:
             def __eq__(self, other):
                 d.clear()
@@ -196,6 +196,10 @@ def __eq__(self, other):
         with self.assertRaises(RuntimeError):
             _ = d.count(3)
 
+        d = deque([A()])
+        with self.assertRaises(RuntimeError):
+            d.index(0)
+
     def test_extend(self):
         d = deque('a')
         self.assertRaises(TypeError, d.extend, 1)

Modules/_collectionsmodule.c

@@ -1077,7 +1077,9 @@ deque_index(dequeobject *deque, PyObject *const *args, Py_ssize_t nargs)
     while (--n >= 0) {
         CHECK_NOT_END(b);
         item = b->data[index];
+        Py_INCREF(item);
         cmp = PyObject_RichCompareBool(item, v, Py_EQ);
+        Py_DECREF(item);
         if (cmp > 0)
             return PyLong_FromSsize_t(stop - n - 1);
         if (cmp < 0)