Set index to -1 when list iterators become exhausted in tier 2

Author: markshannon

Lib/test/test_list.py

@@ -299,6 +299,15 @@ def __eq__(self, other):
         lst = [X(), X()]
         X() in lst
 
+    def test_tier2_invalidates_iterator(self):
+        # GH-121012
+        for _ in range(100):
+            a = [1, 2, 3]
+            it = iter(a)
+            for _ in it:
+                pass
+            a.append(4)
+            self.assertEqual(list(it), [])
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2024-07-08-10-31-08.gh-issue-121012.M5hHk-.rst

@@ -0,0 +1,2 @@
+Tier 2 execution now ensures that list iterators remain exhausted, once they
+become exhausted.

Python/bytecodes.c

@@ -2975,9 +2975,11 @@ dummy_func(
             _PyListIterObject *it = (_PyListIterObject *)iter_o;
             assert(Py_TYPE(iter_o) == &PyListIter_Type);
             PyListObject *seq = it->it_seq;
+            Py_ssize_t index = it->it_index;
             assert(seq);
-            assert(it->it_index < PyList_GET_SIZE(seq));
-            next = PyStackRef_FromPyObjectNew(PyList_GET_ITEM(seq, it->it_index++));
+            assert(index < PyList_GET_SIZE(seq));
+            next = PyStackRef_FromPyObjectNew(PyList_GET_ITEM(seq, index));
+            it->it_index = index + 1 == PyList_GET_SIZE(seq) ? -1 : index + 1;
         }
 
         macro(FOR_ITER_LIST) =