gh-154043: Fix a data race when iterating a shared types.GenericAlias iterator in FT build (#154108)

Author: ByteFlowing1337

Misc/NEWS.d/next/Core_and_Builtins/2026-07-19-19-14-39.gh-issue-154043.Kych7F.rst

@@ -0,0 +1,2 @@
+Fix a data race when iterating a shared :class:`types.GenericAlias` iterator
+from multiple threads under the :term:`free-threaded build`.

Objects/genericaliasobject.c

@@ -942,17 +942,23 @@ static PyObject *
 ga_iternext(PyObject *op)
 {
     gaiterobject *gi = (gaiterobject*)op;
-    if (gi->obj == NULL) {
+#ifdef Py_GIL_DISABLED
+    PyObject *obj = _Py_atomic_exchange_ptr(&gi->obj, NULL);
+#else
+    PyObject* obj = gi->obj;
+    gi->obj = NULL;
+#endif
+    if (obj == NULL) {
         PyErr_SetNone(PyExc_StopIteration);
         return NULL;
     }
-    gaobject *alias = (gaobject *)gi->obj;
+    gaobject *alias = (gaobject *)obj;
     PyObject *starred_alias = Py_GenericAlias(alias->origin, alias->args);
+    Py_DECREF(obj);
     if (starred_alias == NULL) {
         return NULL;
     }
     ((gaobject *)starred_alias)->starred = true;
-    Py_SETREF(gi->obj, NULL);
     return starred_alias;
 }