gh-137238: Fix data race in _Py_slot_tp_getattr_hook

Replacing the slot isn't thread-safe if the GIL is disabled. Don't
require that the slot has been replaced when specializing.

Author: colesbury

Objects/typeobject.c

@@ -10581,7 +10581,10 @@ _Py_slot_tp_getattr_hook(PyObject *self, PyObject *name)
     getattr = _PyType_LookupRef(tp, &_Py_ID(__getattr__));
     if (getattr == NULL) {
         /* No __getattr__ hook: use a simpler dispatcher */
+#ifndef Py_GIL_DISABLED
+        // Replacing the slot is only thread-safe if there is a GIL.
         tp->tp_getattro = _Py_slot_tp_getattro;
+#endif
         return _Py_slot_tp_getattro(self, name);
     }
     /* speed hack: we could use lookup_maybe, but that would resolve the

Python/specialize.c

@@ -935,8 +935,7 @@ analyze_descriptor_load(PyTypeObject *type, PyObject *name, PyObject **descr, un
         PyObject *getattr = _PyType_Lookup(type, &_Py_ID(__getattr__));
         has_getattr = getattr != NULL;
         if (has_custom_getattribute) {
-            if (getattro_slot == _Py_slot_tp_getattro &&
-                !has_getattr &&
+            if (!has_getattr &&
                 Py_IS_TYPE(getattribute, &PyFunction_Type)) {
                 *descr = getattribute;
                 *tp_version = ga_version;

Tools/tsan/suppressions_free_threading.txt

@@ -43,7 +43,6 @@ race:list_inplace_repeat_lock_held
 race:PyObject_Realloc
 
 # gh-133467.  Some of these could be hard to trigger.
-race_top:_Py_slot_tp_getattr_hook
 race_top:slot_tp_descr_get
 race_top:type_set_name
 race_top:set_tp_bases