gh-150858: fix data race while changing __qualname__ of a type object(#150859)

Author: KowalskiThomas

Misc/NEWS.d/next/Core_and_Builtins/2026-06-03-16-27-00.gh-issue-150858.j2dSkD.rst

@@ -0,0 +1 @@
+Fix a data race while changing ``__qualname__`` of a type concurrently on free-threaded builds.

Objects/typeobject.c

@@ -1594,7 +1594,12 @@ type_set_qualname(PyObject *tp, PyObject *value, void *context)
     }
 
     et = (PyHeapTypeObject*)type;
-    Py_SETREF(et->ht_qualname, Py_NewRef(value));
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    _PyEval_StopTheWorld(interp);
+    PyObject *old_qualname = et->ht_qualname;
+    et->ht_qualname = Py_NewRef(value);
+    _PyEval_StartTheWorld(interp);
+    Py_DECREF(old_qualname);
     return 0;
 }