Fix INSTRUMENT_FOR_ITER for list/tuple

Author: markshannon

Lib/test/test_monitoring.py

@@ -2157,6 +2157,21 @@ def callback(code, instruction_offset):
         sys.monitoring.restart_events()
         sys.monitoring.set_events(0, 0)
 
+    def test_134879(self):
+        # gh-134789
+        # Specialized FOR_ITER not incrementing index
+        def foo():
+            t = 0
+            for i in [1,2,3,4]:
+                t += i
+            self.assertEqual(t, 10)
+
+        sys.monitoring.use_tool_id(0, "test")
+        self.addCleanup(sys.monitoring.free_tool_id, 0)
+        sys.monitoring.set_local_events(0, foo.__code__, E.BRANCH_LEFT | E.BRANCH_RIGHT)
+        foo()
+        sys.monitoring.set_local_events(0, foo.__code__, 0)
+
 
 class TestOptimizer(MonitoringTestBase, unittest.TestCase):
 

Python/bytecodes.c

@@ -3130,7 +3130,6 @@ dummy_func(
             if (PyStackRef_IsTaggedInt(null_or_index)) {
                 next = _PyForIter_NextWithIndex(iter_o, null_or_index);
                 if (PyStackRef_IsNull(next)) {
-                    null_or_index = PyStackRef_TagInt(-1);
                     JUMPBY(oparg + 1);
                     DISPATCH();
                 }
@@ -3191,6 +3190,7 @@ dummy_func(
                     JUMPBY(oparg + 1);
                     DISPATCH();
                 }
+                null_or_index = PyStackRef_IncrementTaggedIntNoOverflow(null_or_index);
                 INSTRUMENTED_JUMP(this_instr, next_instr, PY_MONITORING_EVENT_BRANCH_LEFT);
             }
             else {