Fix a data race when iterating a shared types.GenericAlias iterator

Author: ByteFlowing1337

Lib/test/test_free_threading/test_genericalias.py

@@ -0,0 +1,31 @@
+import threading
+import unittest
+
+from test.support import threading_helper
+
+
+NTHREADS = 10
+
+
+@threading_helper.requires_working_threading()
+class TestGenericAlias(unittest.TestCase):
+    def worker(self, it, barrier):
+        barrier.wait()
+        try:
+            next(it)
+        except StopIteration:
+            pass
+
+    def test_shared_generic_alias_iter(self):
+        # See https://github.com/python/cpython/issues/154043
+        bar = threading.Barrier(NTHREADS)
+        number_of_iterations = 50
+        for _ in range(number_of_iterations):
+            shared = iter(list[int])
+            threading_helper.run_concurrently(
+                worker_func=self.worker, nthreads=NTHREADS, args=(shared, bar)
+            )
+
+
+if __name__ == "__main__":
+    unittest.main()

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

@@ -939,17 +939,24 @@ 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);
     if (starred_alias == NULL) {
+        Py_DECREF(obj);
         return NULL;
     }
     ((gaobject *)starred_alias)->starred = true;
-    Py_SETREF(gi->obj, NULL);
+    Py_SETREF(obj, NULL);
     return starred_alias;
 }