gh-121464: Make concurrent iteration over enumerate safe under free-threading (#125734)

Lib/test/test_free_threading/test_enumerate.py

@@ -0,0 +1,38 @@
+import unittest
+import sys
+from threading import Thread, Barrier
+
+from test.support import threading_helper
+
+threading_helper.requires_working_threading(module=True)
+
+class EnumerateThreading(unittest.TestCase):
+
+    @threading_helper.reap_threads
+    def test_threading(self):
+        number_of_threads = 10
+        number_of_iterations = 8
+        n = 100
+        start = sys.maxsize - 40
+        barrier = Barrier(number_of_threads)
+        def work(enum):
+            barrier.wait()
+            while True:
+                try:
+                    _ = next(enum)
+                except StopIteration:
+                    break
+
+        for it in range(number_of_iterations):
+            enum = enumerate(tuple(range(start, start + n)))
+            worker_threads = []
+            for ii in range(number_of_threads):
+                worker_threads.append(
+                    Thread(target=work, args=[enum]))
+            with threading_helper.start_threads(worker_threads):
+                pass
+
+            barrier.reset()
+
+if __name__ == "__main__":
+    unittest.main()