gh-125859: Fix crash when `gc.get_objects` is called during GC (#125882)

This fixes a crash when `gc.get_objects()` or `gc.get_referrers()` is
called during a GC in the free threading build.

Switch to `_PyObjectStack` to avoid corrupting the `struct worklist`
linked list maintained by the GC. Also, don't return objects that are frozen
(`gc.freeze()`) or in the process of being collected to more closely match
the behavior of the default build.

2 files changed, 84 insertions, 0 deletions

diff --git a/Lib/test/test_free_threading/test_gc.py b/Lib/test/test_free_threading/test_gc.py
new file mode 100644
index 0000000..401067f
--- /dev/null
+++ b/Lib/test/test_free_threading/test_gc.py
@@ -0,0 +1,61 @@
+import unittest
+
+import threading
+from threading import Thread
+from unittest import TestCase
+import gc
+
+from test.support import threading_helper
+
+
+class MyObj:
+    pass
+
+
+@threading_helper.requires_working_threading()
+class TestGC(TestCase):
+    def test_get_objects(self):
+        event = threading.Event()
+
+        def gc_thread():
+            for i in range(100):
+                o = gc.get_objects()
+            event.set()
+
+        def mutator_thread():
+            while not event.is_set():
+                o1 = MyObj()
+                o2 = MyObj()
+                o3 = MyObj()
+                o4 = MyObj()
+
+        gcs = [Thread(target=gc_thread)]
+        mutators = [Thread(target=mutator_thread) for _ in range(4)]
+        with threading_helper.start_threads(gcs + mutators):
+            pass
+
+    def test_get_referrers(self):
+        event = threading.Event()
+
+        obj = MyObj()
+
+        def gc_thread():
+            for i in range(100):
+                o = gc.get_referrers(obj)
+            event.set()
+
+        def mutator_thread():
+            while not event.is_set():
+                d1 = { "key": obj }
+                d2 = { "key": obj }
+                d3 = { "key": obj }
+                d4 = { "key": obj }
+
+        gcs = [Thread(target=gc_thread) for _ in range(2)]
+        mutators = [Thread(target=mutator_thread) for _ in range(4)]
+        with threading_helper.start_threads(gcs + mutators):
+            pass
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Lib/test/test_gc.py b/Lib/test/test_gc.py
index bb7df1f..cc2b4fa 100644
--- a/Lib/test/test_gc.py
+++ b/Lib/test/test_gc.py
@@ -1065,6 +1065,29 @@ class GCTests(unittest.TestCase):
         self.assertEqual(len(gc.get_referents(untracked_capsule)), 0)
         gc.get_referents(tracked_capsule)
 
+    @cpython_only
+    def test_get_objects_during_gc(self):
+        # gh-125859: Calling gc.get_objects() or gc.get_referrers() during a
+        # collection should not crash.
+        test = self
+        collected = False
+
+        class GetObjectsOnDel:
+            def __del__(self):
+                nonlocal collected
+                collected = True
+                objs = gc.get_objects()
+                # NB: can't use "in" here because some objects override __eq__
+                for obj in objs:
+                    test.assertTrue(obj is not self)
+                test.assertEqual(gc.get_referrers(self), [])
+
+        obj = GetObjectsOnDel()
+        obj.cycle = obj
+        del obj
+
+        gc.collect()
+        self.assertTrue(collected)
 
 
 class IncrementalGCTests(unittest.TestCase):