bpo-25750: fix refcounts in type_getattro() (GH-6118)

When calling tp_descr_get(self, obj, type), make sure that
we own a strong reference to "self".

Misc/NEWS.d/next/Core and Builtins/2018-03-14-21-42-17.bpo-25750.lxgkQz.rst

+Fix rare Python crash due to bad refcounting in ``type_getattro()`` if a
+descriptor deletes itself from the class. Patch by Jeroen Demeyer.

Objects/typeobject.c

@@ -3145,6 +3145,7 @@ type_getattro(PyTypeObject *type, PyObject *name)
     PyTypeObject *metatype = Py_TYPE(type);
     PyObject *meta_attribute, *attribute;
     descrgetfunc meta_get;
+    PyObject* res;
 
     if (!PyUnicode_Check(name)) {
         PyErr_Format(PyExc_TypeError,
@@ -3166,6 +3167,7 @@ type_getattro(PyTypeObject *type, PyObject *name)
     meta_attribute = _PyType_Lookup(metatype, name);
 
     if (meta_attribute != NULL) {
+        Py_INCREF(meta_attribute);
         meta_get = Py_TYPE(meta_attribute)->tp_descr_get;
 
         if (meta_get != NULL && PyDescr_IsData(meta_attribute)) {
@@ -3173,10 +3175,11 @@ type_getattro(PyTypeObject *type, PyObject *name)
              * writes. Assume the attribute is not overridden in
              * type's tp_dict (and bases): call the descriptor now.
              */
-            return meta_get(meta_attribute, (PyObject *)type,
-                            (PyObject *)metatype);
+            res = meta_get(meta_attribute, (PyObject *)type,
+                           (PyObject *)metatype);
+            Py_DECREF(meta_attribute);
+            return res;
         }
-        Py_INCREF(meta_attribute);
     }
 
     /* No data descriptor found on metatype. Look in tp_dict of this
@@ -3184,6 +3187,7 @@ type_getattro(PyTypeObject *type, PyObject *name)
     attribute = _PyType_Lookup(type, name);
     if (attribute != NULL) {
         /* Implement descriptor functionality, if any */
+        Py_INCREF(attribute);
         descrgetfunc local_get = Py_TYPE(attribute)->tp_descr_get;
 
         Py_XDECREF(meta_attribute);
@@ -3191,11 +3195,12 @@ type_getattro(PyTypeObject *type, PyObject *name)
         if (local_get != NULL) {
             /* NULL 2nd argument indicates the descriptor was
              * found on the target object itself (or a base)  */
-            return local_get(attribute, (PyObject *)NULL,
-                             (PyObject *)type);
+            res = local_get(attribute, (PyObject *)NULL,
+                            (PyObject *)type);
+            Py_DECREF(attribute);
+            return res;
         }
 
-        Py_INCREF(attribute);
         return attribute;
     }