Add NULL check to FOR_ITER

Author: markshannon

Python/bytecodes.c

@@ -3155,7 +3155,14 @@ dummy_func(
         replaced op(_FOR_ITER, (iter -- iter, next)) {
             /* before: [iter]; after: [iter, iter()] *or* [] (and jump over END_FOR.) */
             PyObject *iter_o = PyStackRef_AsPyObjectBorrow(iter);
-            PyObject *next_o = (*Py_TYPE(iter_o)->tp_iternext)(iter_o);
+            iternextfunc func = Py_TYPE(iter_o)->tp_iternext;
+            if (func == NULL) {
+                _PyErr_Format(tstate, PyExc_TypeError,
+                              "'%.100s' object is not an iterator",
+                              Py_TYPE(iter_o)->tp_name);
+                ERROR_NO_POP();
+            }
+            PyObject *next_o = func(iter_o);
             if (next_o == NULL) {
                 if (_PyErr_Occurred(tstate)) {
                     int matches = _PyErr_ExceptionMatches(tstate, PyExc_StopIteration);
@@ -3179,7 +3186,14 @@ dummy_func(
         op(_FOR_ITER_TIER_TWO, (iter -- iter, next)) {
             /* before: [iter]; after: [iter, iter()] *or* [] (and jump over END_FOR.) */
             PyObject *iter_o = PyStackRef_AsPyObjectBorrow(iter);
-            PyObject *next_o = (*Py_TYPE(iter_o)->tp_iternext)(iter_o);
+            iternextfunc func = Py_TYPE(iter_o)->tp_iternext;
+            if (func == NULL) {
+                _PyErr_Format(tstate, PyExc_TypeError,
+                              "'%.100s' object is not an iterator",
+                              Py_TYPE(iter_o)->tp_name);
+                ERROR_NO_POP();
+            }
+            PyObject *next_o = func(iter_o);
             if (next_o == NULL) {
                 if (_PyErr_Occurred(tstate)) {
                     int matches = _PyErr_ExceptionMatches(tstate, PyExc_StopIteration);
@@ -3202,7 +3216,14 @@ dummy_func(
 
         inst(INSTRUMENTED_FOR_ITER, (unused/1, iter -- iter, next)) {
             PyObject *iter_o = PyStackRef_AsPyObjectBorrow(iter);
-            PyObject *next_o = (*Py_TYPE(iter_o)->tp_iternext)(iter_o);
+            iternextfunc func = Py_TYPE(iter_o)->tp_iternext;
+            if (func == NULL) {
+                _PyErr_Format(tstate, PyExc_TypeError,
+                              "'%.100s' object is not an iterator",
+                              Py_TYPE(iter_o)->tp_name);
+                ERROR_NO_POP();
+            }
+            PyObject *next_o = func(iter_o);
             if (next_o != NULL) {
                 next = PyStackRef_FromPyObjectSteal(next_o);
                 INSTRUMENTED_JUMP(this_instr, next_instr, PY_MONITORING_EVENT_BRANCH_LEFT);