Fix segfault creating view objects (#1163)

Author: bdraco

CHANGES/1164.bugfix.rst

@@ -0,0 +1,8 @@
+Fixed a segmentation fault when attempting to directly instantiate view objects
+(``multidict._ItemsView``, ``multidict._KeysView``, ``multidict._ValuesView``) -- by :user:`bdraco`.
+
+View objects now raise a proper :exc:`TypeError` with the message "cannot create '...' instances directly"
+when direct instantiation is attempted.
+
+View objects should only be created through the proper methods: :py:meth:`multidict.MultiDict.items`,
+:py:meth:`multidict.MultiDict.keys`, and :py:meth:`multidict.MultiDict.values`.

multidict/_multilib/views.h

@@ -1006,7 +1006,16 @@ static PyMethodDef multidict_itemsview_methods[] = {
     {NULL, NULL}   /* sentinel */
 };
 
+static inline PyObject *
+multidict_view_forbidden_new(PyTypeObject *type, PyObject *args, PyObject *kwargs)
+{
+    PyErr_Format(PyExc_TypeError,
+                 "cannot create '%s' instances directly", type->tp_name);
+    return NULL;
+}
+
 static PyType_Slot multidict_itemsview_slots[] = {
+    {Py_tp_new, multidict_view_forbidden_new},
     {Py_tp_dealloc, multidict_view_dealloc},
     {Py_tp_repr, multidict_itemsview_repr},
 
@@ -1561,6 +1570,7 @@ static PyMethodDef multidict_keysview_methods[] = {
 };
 
 static PyType_Slot multidict_keysview_slots[] = {
+    {Py_tp_new, multidict_view_forbidden_new},
     {Py_tp_dealloc, multidict_view_dealloc},
     {Py_tp_repr, multidict_keysview_repr},
 
@@ -1635,6 +1645,7 @@ multidict_valuesview_repr(_Multidict_ViewObject *self)
 }
 
 static PyType_Slot multidict_valuesview_slots[] = {
+    {Py_tp_new, multidict_view_forbidden_new},
     {Py_tp_dealloc, multidict_view_dealloc},
     {Py_tp_repr, multidict_valuesview_repr},
 

tests/test_multidict.py

@@ -1324,3 +1324,22 @@ class SubclassedMultiDict(any_multidict_class):  # type: ignore[valid-type, misc
     assert d1 == d3
     assert d1 == SubclassedMultiDict([("key", "value1")])
     assert d1 != SubclassedMultiDict([("key", "value2")])
+
+
+@pytest.mark.c_extension
+def test_view_direct_instantiation_segfault() -> None:
+    """Test that view objects cannot be instantiated directly (issue: segfault).
+
+    This test only applies to the C extension implementation.
+    """
+    # Test that _ItemsView cannot be instantiated directly
+    with pytest.raises(TypeError, match="cannot create '.*_ItemsView' instances directly"):
+        multidict._ItemsView()  # type: ignore[attr-defined]
+
+    # Test that _KeysView cannot be instantiated directly
+    with pytest.raises(TypeError, match="cannot create '.*_KeysView' instances directly"):
+        multidict._KeysView()  # type: ignore[attr-defined]
+
+    # Test that _ValuesView cannot be instantiated directly
+    with pytest.raises(TypeError, match="cannot create '.*_ValuesView' instances directly"):
+        multidict._ValuesView()  # type: ignore[attr-defined]