GH-97002: Prevent _PyInterpreterFrames from backing more than one PyFrameObject (GH-97996)

Lib/test/test_frame.py

@@ -1,3 +1,4 @@
+import gc
 import re
 import sys
 import textwrap
@@ -261,5 +262,69 @@ class TestIncompleteFrameAreInvisible(unittest.TestCase):
         """)
         assert_python_ok("-c", code)
 
+    @support.cpython_only
+    def test_sneaky_frame_object(self):
+
+        def trace(frame, event, arg):
+            """
+            Don't actually do anything, just force a frame object to be created.
+            """
+
+        def callback(phase, info):
+            """
+            Yo dawg, I heard you like frames, so I'm allocating a frame while
+            you're allocating a frame, so you can have a frame while you have a
+            frame!
+            """
+            nonlocal sneaky_frame_object
+            sneaky_frame_object = sys._getframe().f_back
+            # We're done here:
+            gc.callbacks.remove(callback)
+
+        def f():
+            while True:
+                yield
+
+        old_threshold = gc.get_threshold()
+        old_callbacks = gc.callbacks[:]
+        old_enabled = gc.isenabled()
+        old_trace = sys.gettrace()
+        try:
+            # Stop the GC for a second while we set things up:
+            gc.disable()
+            # Create a paused generator:
+            g = f()
+            next(g)
+            # Move all objects to the oldest generation, and tell the GC to run
+            # on the *very next* allocation:
+            gc.collect()
+            gc.set_threshold(1, 0, 0)
+            # Okay, so here's the nightmare scenario:
+            # - We're tracing the resumption of a generator, which creates a new
+            #   frame object.
+            # - The allocation of this frame object triggers a collection
+            #   *before* the frame object is actually created.
+            # - During the collection, we request the exact same frame object.
+            #   This test does it with a GC callback, but in real code it would
+            #   likely be a trace function, weakref callback, or finalizer.
+            # - The collection finishes, and the original frame object is
+            #   created. We now have two frame objects fighting over ownership
+            #   of the same interpreter frame!
+            sys.settrace(trace)
+            gc.callbacks.append(callback)
+            sneaky_frame_object = None
+            gc.enable()
+            next(g)
+            # g.gi_frame should be the the frame object from the callback (the
+            # one that was *requested* second, but *created* first):
+            self.assertIs(g.gi_frame, sneaky_frame_object)
+        finally:
+            gc.set_threshold(*old_threshold)
+            gc.callbacks[:] = old_callbacks
+            sys.settrace(old_trace)
+            if old_enabled:
+                gc.enable()
+
+
 if __name__ == "__main__":
     unittest.main()