gh-120608: Make reversed iterator work with free-threading (#120971)

Co-authored-by: Sam Gross <colesbury@gmail.com>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/test/test_free_threading/test_reversed.py

@@ -0,0 +1,39 @@
+import unittest
+from threading import Barrier, Thread
+from test.support import threading_helper
+
+threading_helper.requires_working_threading(module=True)
+
+class TestReversed(unittest.TestCase):
+
+    @threading_helper.reap_threads
+    def test_reversed(self):
+        # Iterating over the iterator with multiple threads should not
+        # emit TSAN warnings
+        number_of_iterations = 10
+        number_of_threads = 10
+        size = 1_000
+
+        barrier = Barrier(number_of_threads)
+        def work(r):
+            barrier.wait()
+            while True:
+                try:
+                     l = r.__length_hint__()
+                     next(r)
+                except StopIteration:
+                    break
+                assert 0 <= l <= size
+        x = tuple(range(size))
+
+        for _ in range(number_of_iterations):
+            r = reversed(x)
+            worker_threads = []
+            for _ in range(number_of_threads):
+                worker_threads.append(Thread(target=work, args=[r]))
+            with threading_helper.start_threads(worker_threads):
+                pass
+            barrier.reset()
+
+if __name__ == "__main__":
+    unittest.main()