From 2f8919ee3c152d6249a0a0dc14c86c0547bfe4b6 Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Mon, 8 Jul 2024 19:09:54 +0200
Subject: [PATCH] [3.13] GH-121012: Set index to -1 when list iterators become
 exhausted in tier 2 (GH-121483) (GH-121494)

---
 Lib/test/test_list.py                                    | 9 +++++++++
 .../2024-07-08-10-31-08.gh-issue-121012.M5hHk-.rst       | 2 ++
 Python/bytecodes.c                                       | 5 ++++-
 Python/executor_cases.c.h                                | 7 +++++--
 4 files changed, 20 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2024-07-08-10-31-08.gh-issue-121012.M5hHk-.rst

diff --git a/Lib/test/test_list.py b/Lib/test/test_list.py
index 4d2d54705fc894..ad7accf2099f43 100644
--- a/Lib/test/test_list.py
+++ b/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()
diff --git a/Misc/NEWS.d/next/Core and Builtins/2024-07-08-10-31-08.gh-issue-121012.M5hHk-.rst b/Misc/NEWS.d/next/Core and Builtins/2024-07-08-10-31-08.gh-issue-121012.M5hHk-.rst
new file mode 100644
index 00000000000000..7b04eb68b03752
--- /dev/null
+++ b/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.
diff --git a/Python/bytecodes.c b/Python/bytecodes.c
index 0dc60bbbb6a579..a6e9a73cb0e57f 100644
--- a/Python/bytecodes.c
+++ b/Python/bytecodes.c
@@ -2695,7 +2695,10 @@ dummy_func(
             assert(Py_TYPE(iter) == &PyListIter_Type);
             PyListObject *seq = it->it_seq;
             EXIT_IF(seq == NULL);
-            EXIT_IF((size_t)it->it_index >= (size_t)PyList_GET_SIZE(seq));
+            if ((size_t)it->it_index >= (size_t)PyList_GET_SIZE(seq)) {
+                it->it_index = -1;
+                EXIT_IF(1);
+            }
         }
 
         op(_ITER_NEXT_LIST, (iter -- iter, next)) {
diff --git a/Python/executor_cases.c.h b/Python/executor_cases.c.h
index 4e0f73f7e23a0a..8080d20915f1d9 100644
--- a/Python/executor_cases.c.h
+++ b/Python/executor_cases.c.h
@@ -2675,8 +2675,11 @@
                 JUMP_TO_JUMP_TARGET();
             }
             if ((size_t)it->it_index >= (size_t)PyList_GET_SIZE(seq)) {
-                UOP_STAT_INC(uopcode, miss);
-                JUMP_TO_JUMP_TARGET();
+                it->it_index = -1;
+                if (1) {
+                    UOP_STAT_INC(uopcode, miss);
+                    JUMP_TO_JUMP_TARGET();
+                }
             }
             break;
         }