pythongh-137238: Fix data race in _Py_slot_tp_getattr_hook (pythongh-137240)

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

@@ -10585,7 +10585,10 @@ _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;
@@ -1259,12 +1258,6 @@ do_specialize_instance_load_attr(PyObject* owner, _Py_CODEUNIT* instr, PyObject*
             return -1;
         case GETATTRIBUTE_IS_PYTHON_FUNCTION:
         {
-            #ifndef Py_GIL_DISABLED
-            // In free-threaded builds it's possible for tp_getattro to change
-            // after the call to analyze_descriptor. That is fine: the version
-            // guard will fail.
-            assert(type->tp_getattro == _Py_slot_tp_getattro);
-            #endif
             assert(Py_IS_TYPE(descr, &PyFunction_Type));
             _PyLoadMethodCache *lm_cache = (_PyLoadMethodCache *)(instr + 1);
             if (!function_check_args(descr, 2, LOAD_ATTR)) {

Tools/tsan/suppressions_free_threading.txt

@@ -43,6 +43,5 @@ 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:set_tp_bases
 race_top:type_set_bases_unlocked