Move frame->previous=NULL

Author: savannahostrowski

Lib/test/test_generators.py

@@ -134,6 +134,20 @@ def gen():
         self.assertEqual(len(resurrected), 1)
         self.assertIsInstance(resurrected[0].gi_code, types.CodeType)
 
+    def test_exhausted_generator_frame_cycle(self):
+        # gh-100964: Ensure that exhausted generator frames don't create
+        # reference cycles through f_back when the frame is accessed.
+        def g():
+            yield
+
+        generator = g()
+        frame = generator.gi_frame
+        self.assertIsNone(frame.f_back)
+        next(generator)
+        self.assertIsNone(frame.f_back)
+        next(generator, None)
+        self.assertIsNone(frame.f_back)
+
 
 class GeneratorTest(unittest.TestCase):
 

Python/ceval.c

@@ -1842,9 +1842,9 @@ clear_gen_frame(PyThreadState *tstate, _PyInterpreterFrame * frame)
     tstate->exc_info = gen->gi_exc_state.previous_item;
     gen->gi_exc_state.previous_item = NULL;
     assert(frame->frame_obj == NULL || frame->frame_obj->f_frame == frame);
+    frame->previous = NULL;
     _PyFrame_ClearExceptCode(frame);
     _PyErr_ClearExcState(&gen->gi_exc_state);
-    frame->previous = NULL;
 }
 
 void