add gc callback mutation race regression test

Author: dzaramelcone

Lib/test/test_free_threading/test_gc.py

@@ -2,6 +2,7 @@
 
 import threading
 from threading import Thread
+import time
 from unittest import TestCase
 import gc
 
@@ -94,6 +95,27 @@ def evil():
         thread.start()
         thread.join()
 
+    def test_gc_callbacks_race_with_mutation(self):
+        def collect():
+            b.wait()
+            while not stop.is_set():
+                gc.collect()
+
+        def mutate():
+            b.wait()
+            while not stop.is_set():
+                gc.callbacks[:] = [lambda *_: _ for _ in range(16)]
+                time.sleep(0)
+                gc.callbacks.clear()
+
+        threads = [threading.Thread(target=f) for f in (collect, mutate) * 4]
+        b = threading.Barrier(len(threads) + 1)
+        stop = threading.Event()
+
+        with threading_helper.start_threads(threads, stop.set):
+            b.wait()
+            time.sleep(0.2)
+
 
 if __name__ == "__main__":
     unittest.main()