gh-151722: Move frozendict free-threading regression test into test_dict.py

Author: tonghuaroot

Lib/test/test_free_threading/test_dict.py

@@ -5,7 +5,7 @@
 
 from ast import Or
 from functools import partial
-from threading import Barrier, Thread
+from threading import Barrier, Event, Thread
 from unittest import TestCase
 
 try:
@@ -336,5 +336,50 @@ def reader():
         with threading_helper.start_threads([t1, t2]):
             pass
 
+    def test_racing_frozendict_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()