closes bpo-31608: Fix a crash in methods of a subclass of _collections.deque with a bad __new__(). (GH-3788)

(cherry picked from commit 24bd50bdcc)

Co-authored-by: Oren Milman <orenmn@gmail.com>

Lib/test/test_deque.py

@@ -892,6 +892,21 @@ class TestSubclass(unittest.TestCase):
         d1 == d2   # not clear if this is supposed to be True or False,
                    # but it used to give a SystemError
 
+    @support.cpython_only
+    def test_bug_31608(self):
+        # The interpreter used to crash in specific cases where a deque
+        # subclass returned a non-deque.
+        class X(deque):
+            pass
+        d = X()
+        def bad___new__(cls, *args, **kwargs):
+            return [42]
+        X.__new__ = bad___new__
+        with self.assertRaises(TypeError):
+            d * 42  # shouldn't crash
+        with self.assertRaises(TypeError):
+            d + deque([1, 2, 3])  # shouldn't crash
+
 
 class SubclassWithKwargs(deque):
     def __init__(self, newarg=1):

Misc/NEWS.d/next/Library/2017-10-29-10-37-55.bpo-31608.wkp8Nw.rst

@@ -0,0 +1,2 @@
+Raise a ``TypeError`` instead of crashing if a ``collections.deque`` subclass
+returns a non-deque from ``__new__``. Patch by Oren Milman.

Modules/_collectionsmodule.c

@@ -514,6 +514,7 @@ deque_inplace_concat(dequeobject *deque, PyObject *other)
 static PyObject *
 deque_copy(PyObject *deque)
 {
+    PyObject *result;
     dequeobject *old_deque = (dequeobject *)deque;
     if (Py_TYPE(deque) == &deque_type) {
         dequeobject *new_deque;
@@ -538,11 +539,19 @@ deque_copy(PyObject *deque)
         return NULL;
     }
     if (old_deque->maxlen < 0)
-        return PyObject_CallFunctionObjArgs((PyObject *)(Py_TYPE(deque)),
+        result = PyObject_CallFunctionObjArgs((PyObject *)(Py_TYPE(deque)),
                                               deque, NULL);
     else
-        return PyObject_CallFunction((PyObject *)(Py_TYPE(deque)), "Oi",
+        result = PyObject_CallFunction((PyObject *)(Py_TYPE(deque)), "Oi",
                                        deque, old_deque->maxlen, NULL);
+    if (result != NULL && !PyObject_TypeCheck(result, &deque_type)) {
+        PyErr_Format(PyExc_TypeError,
+                     "%.200s() must return a deque, not %.200s",
+                     Py_TYPE(deque)->tp_name, Py_TYPE(result)->tp_name);
+        Py_DECREF(result);
+        return NULL;
+    }
+    return result;
 }
 
 PyDoc_STRVAR(copy_doc, "Return a shallow copy of a deque.");