bpo-28866: No type cache for types with specialized mro, invalidation is hard. (#13157)

* No type cache for types with specialized mro, invalidation is hard.

* FIX: Don't disable method cache custom types that do not implement mro().

* fixing implem.

* Avoid storing error flags, also decref.

* news entry

* Clear as soon as we're getting an error.

* FIX: Reference leak.

Misc/NEWS.d/next/Core and Builtins/2019-05-08-16-36-51.bpo-28866.qCv_bj.rst

+Avoid caching attributes of classes which type defines mro() to avoid a hard
+cache invalidation problem.

Objects/typeobject.c

@@ -78,6 +78,9 @@ slot_tp_new(PyTypeObject *type, PyObject *args, PyObject *kwds);
 static void
 clear_slotdefs(void);
 
+static PyObject *
+lookup_maybe_method(PyObject *self, _Py_Identifier *attrid, int *unbound);
+
 /*
  * finds the beginning of the docstring's introspection signature.
  * if present, returns a pointer pointing to the first '('.
@@ -287,17 +290,35 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
 
        Unset HAVE_VERSION_TAG and VALID_VERSION_TAG if the type
        has a custom MRO that includes a type which is not officially
-       super type.
+       super type, or if the type implements its own mro() method.
 
        Called from mro_internal, which will subsequently be called on
        each subclass when their mro is recursively updated.
      */
     Py_ssize_t i, n;
-    int clear = 0;
+    int custom = (Py_TYPE(type) != &PyType_Type);
+    int unbound;
+    PyObject *mro_meth = NULL;
+    PyObject *type_mro_meth = NULL;
 
     if (!PyType_HasFeature(type, Py_TPFLAGS_HAVE_VERSION_TAG))
         return;
 
+    if (custom) {
+        _Py_IDENTIFIER(mro);
+        mro_meth = lookup_maybe_method(
+            (PyObject *)type, &PyId_mro, &unbound);
+        if (mro_meth == NULL)
+            goto clear;
+        type_mro_meth = lookup_maybe_method(
+            (PyObject *)&PyType_Type, &PyId_mro, &unbound);
+        if (type_mro_meth == NULL)
+            goto clear;
+        if (mro_meth != type_mro_meth)
+            goto clear;
+        Py_XDECREF(mro_meth);
+        Py_XDECREF(type_mro_meth);
+    }
     n = PyTuple_GET_SIZE(bases);
     for (i = 0; i < n; i++) {
         PyObject *b = PyTuple_GET_ITEM(bases, i);
@@ -308,14 +329,15 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
 
         if (!PyType_HasFeature(cls, Py_TPFLAGS_HAVE_VERSION_TAG) ||
             !PyType_IsSubtype(type, cls)) {
-            clear = 1;
-            break;
+            goto clear;
         }
     }
-
-    if (clear)
-        type->tp_flags &= ~(Py_TPFLAGS_HAVE_VERSION_TAG|
-                            Py_TPFLAGS_VALID_VERSION_TAG);
+    return;
+ clear:
+    Py_XDECREF(mro_meth);
+    Py_XDECREF(type_mro_meth);
+    type->tp_flags &= ~(Py_TPFLAGS_HAVE_VERSION_TAG|
+                        Py_TPFLAGS_VALID_VERSION_TAG);
 }
 
 static int