youknowone · Oct 29, 2026 · Oct 29, 2026
diff --git a/Lib/test/test_free_threading/test_collections.py b/Lib/test/test_free_threading/test_collections.py
@@ -0,0 +1,53 @@
+import unittest
+from collections import deque
+from copy import copy
+from test.support import threading_helper
+
+threading_helper.requires_working_threading(module=True)
+
+
+class TestDeque(unittest.TestCase):
+    def test_copy_race(self):
+        # gh-144809: Test that deque copy is thread safe. It previously
+        # could raise a "deque mutated during iteration" error.
+        d = deque(range(100))
+
+        def mutate():
+            for i in range(1000):
+                d.append(i)
+                if len(d) > 200:
+                    d.popleft()
+
+        def copy_loop():
+            for _ in range(1000):
+                copy(d)
+
+        threading_helper.run_concurrently([mutate, copy_loop])
+
+    def test_index_race_in_ac(self):
+        # gh-150750: There was a c_default specified as `Py_SIZE(self)`,
+        # it was used without a critical section.
+
+        d = deque(range(100))
+
+        def index():
+            for _ in range(10000):
+                try:
+                    d.index(50)
+                except ValueError:
+                    pass
+
+        def mutate():
+            for _ in range(10000):
+                d.append(0)
+                d.clear()
+                d.extend(range(100))
+                d.appendleft(-1)
+
+        threading_helper.run_concurrently(
+            [index, *[mutate for _ in range(3)]],
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()