Make bases/mro accesses thread safe

Author: DinoV

Modules/_abc.c

@@ -8,7 +8,6 @@
 #include "pycore_object.h"        // _PyType_GetSubclasses()
 #include "pycore_runtime.h"       // _Py_ID()
 #include "pycore_setobject.h"     // _PySet_NextEntry()
-#include "pycore_typeobject.h"    // _PyType_GetMRO()
 #include "pycore_weakref.h"       // _PyWeakref_GET_REF()
 #include "clinic/_abc.c.h"
 
@@ -744,18 +743,12 @@ _abc__abc_subclasscheck_impl(PyObject *module, PyObject *self,
     Py_DECREF(ok);
 
     /* 4. Check if it's a direct subclass. */
-    PyObject *mro = _PyType_GetMRO((PyTypeObject *)subclass);
-    assert(PyTuple_Check(mro));
-    for (pos = 0; pos < PyTuple_GET_SIZE(mro); pos++) {
-        PyObject *mro_item = PyTuple_GET_ITEM(mro, pos);
-        assert(mro_item != NULL);
-        if ((PyObject *)self == mro_item) {
-            if (_add_to_weak_set(&impl->_abc_cache, subclass) < 0) {
-                goto end;
-            }
-            result = Py_True;
+    if (PyType_IsSubtype((PyTypeObject *)subclass, (PyTypeObject *)self)) {
+        if (_add_to_weak_set(&impl->_abc_cache, subclass) < 0) {
             goto end;
         }
+        result = Py_True;
+        goto end;
     }
 
     /* 5. Check if it's a subclass of a registered class (recursive). */