gh-130263: fix list iterators to use atomics for storing index

Lib/test/test_free_threading/test_list.py

@@ -71,6 +71,24 @@ def reader_func():
         for reader in readers:
             reader.join()
 
+    def test_list_iterator_reduce(self):
+        l = list(range(100))
+
+        for it in [iter(l), iter(reversed(l))]:
+
+            def reduce():
+                for i in range(100):
+                    it.__reduce__()
+
+            def setstate():
+                for i in range(100):
+                    it.__setstate__(i)
+
+            t1 = Thread(target=reduce)
+            t2 = Thread(target=setstate)
+
+            with threading_helper.start_threads([t1, t2]):
+                pass
 
 if __name__ == "__main__":
     unittest.main()

Objects/listobject.c

@@ -3988,12 +3988,12 @@ listiter_setstate(PyObject *self, PyObject *state)
     Py_ssize_t index = PyLong_AsSsize_t(state);
     if (index == -1 && PyErr_Occurred())
         return NULL;
-    if (it->it_seq != NULL) {
+    if (FT_ATOMIC_LOAD_SSIZE_RELAXED(it->it_index) >= 0) {
         if (index < -1)
             index = -1;
         else if (index > PyList_GET_SIZE(it->it_seq))
             index = PyList_GET_SIZE(it->it_seq); /* iterator exhausted */
-        it->it_index = index;
+        FT_ATOMIC_STORE_SSIZE_RELAXED(it->it_index, index);
     }
     Py_RETURN_NONE;
 }
@@ -4140,12 +4140,12 @@ listreviter_setstate(PyObject *self, PyObject *state)
     Py_ssize_t index = PyLong_AsSsize_t(state);
     if (index == -1 && PyErr_Occurred())
         return NULL;
-    if (it->it_seq != NULL) {
+    if (FT_ATOMIC_LOAD_SSIZE_RELAXED(it->it_index) >= 0) {
         if (index < -1)
             index = -1;
         else if (index > PyList_GET_SIZE(it->it_seq) - 1)
             index = PyList_GET_SIZE(it->it_seq) - 1;
-        it->it_index = index;
+        FT_ATOMIC_STORE_SSIZE_RELAXED(it->it_index, index);
     }
     Py_RETURN_NONE;
 }