gh-151722: Add free-threading regression test for frozendict read race

Author: tonghuaroot

Lib/test/test_free_threading/test_frozendict.py

@@ -0,0 +1,57 @@
+import gc
+import unittest
+from threading import Event, Thread
+
+from test.support import threading_helper
+
+
+@threading_helper.requires_working_threading()
+class TestFrozenDict(unittest.TestCase):
+    def test_racing_reads_during_construction(self):
+        # gh-151722: a frozendict is GC-tracked before it is fully
+        # populated, so a half-built instance is observable from other
+        # threads.  Reading it with len()/repr()/hash() must not race
+        # with the construction-time table and ma_used writes.
+        NUM_KEYS = 8192
+        NUM_ROUNDS = 40
+
+        latest = [frozendict()]  # main -> reader handoff, never empty
+        done = Event()
+
+        class Evil:
+            def keys(self):
+                return [f"k{i}" for i in range(NUM_KEYS)]
+
+            def __getitem__(self, key):
+                if latest[0] is empty:
+                    for obj in gc.get_objects():
+                        if (isinstance(obj, frozendict)
+                                and 0 < len(obj) < NUM_KEYS):
+                            latest[0] = obj  # leak the half-built object
+                            break
+                return 1
+
+        empty = latest[0]
+
+        def reader():
+            while not done.is_set():
+                fd = latest[0]
+                len(fd)
+                repr(fd)
+                hash(fd)
+
+        readers = [Thread(target=reader) for _ in range(3)]
+        for t in readers:
+            t.start()
+        try:
+            for _ in range(NUM_ROUNDS):
+                latest[0] = empty
+                frozendict(Evil())
+        finally:
+            done.set()
+            for t in readers:
+                t.join()
+
+
+if __name__ == "__main__":
+    unittest.main()