bpo-39590: make deque.__contains__ and deque.count hold strong references (GH-18421)

Lib/test/test_deque.py

@@ -183,6 +183,18 @@ class TestBasic(unittest.TestCase):
         with self.assertRaises(RuntimeError):
             n in d
 
+    def test_contains_count_stop_crashes(self):
+        class A:
+            def __eq__(self, other):
+                d.clear()
+                return NotImplemented
+        d = deque([A(), A()])
+        with self.assertRaises(RuntimeError):
+            _ = 3 in d
+        d = deque([A(), A()])
+        with self.assertRaises(RuntimeError):
+            _ = d.count(3)
+
     def test_extend(self):
         d = deque('a')
         self.assertRaises(TypeError, d.extend, 1)

Misc/NEWS.d/next/Library/2020-02-09-05-51-05.bpo-39590.rf98GU.rst

@@ -0,0 +1 @@
+Collections.deque now holds strong references during deque.__contains__ and deque.count, fixing crashes.

Modules/_collectionsmodule.c

@@ -965,7 +965,9 @@ deque_count(dequeobject *deque, PyObject *v)
     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 NULL;
         count += cmp;
@@ -1002,7 +1004,9 @@ deque_contains(dequeobject *deque, PyObject *v)
     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) {
             return cmp;
         }