gh-135552: Add tests that check if weakref for tp_subclasses cleared after finalization

Lib/test/test_gc.py

@@ -1125,6 +1125,37 @@ def test_something(self):
         """)
         assert_python_ok("-c", source)
 
+    def test_do_not_cleanup_type_subclasses_before_finalization(self):
+        #  See https://github.com/python/cpython/issues/135552
+        # If we cleanup weakrefs for tp_subclasses before calling
+        # the finalizer (__del__) then the line `fail = BaseNode.next.next`
+        # should fail because we are trying to access a subclass
+        # attribute. But subclass type cache was not properly invalidated.
+        code = """
+            class BaseNode:
+                def __del__(self):
+                    BaseNode.next = BaseNode.next.next
+                    fail = BaseNode.next.next
+
+            class Node(BaseNode):
+                pass
+
+            BaseNode.next = Node()
+            BaseNode.next.next = Node()
+        """
+        # this test checks garbage collection while interp
+        # finalization
+        assert_python_ok("-c", textwrap.dedent(code))
+
+        code_inside_function = textwrap.dedent(F"""
+            def test():
+                {textwrap.indent(code, '    ')}
+
+            test()
+        """)
+        # this test checks regular garbage collection
+        assert_python_ok("-c", code_inside_function)
+
 
 class IncrementalGCTests(unittest.TestCase):
     @unittest.skipIf(_testinternalcapi is None, "requires _testinternalcapi")

Lib/test/test_weakref.py

@@ -1044,6 +1044,38 @@ def callback(obj):
         stderr = res.err.decode("ascii", "backslashreplace")
         self.assertNotRegex(stderr, "_Py_Dealloc: Deallocator of type 'TestObj'")
 
+    def test_clearing_weakrefs_in_gc(self):
+        # This test checks that:
+        # 1. weakrefs for types with callbacks are cleared before the
+        # finalizer is called
+        # 2. weakrefs for types without callbacks are cleared after the
+        # finalizer is called
+        # 3. other weakrefs cleared before the finalizer is called
+        errors = []
+        def test():
+            class Class:
+                def __init__(self):
+                    self._self = self
+                    self.wr1 = weakref.ref(Class, lambda x: None)
+                    self.wr2 = weakref.ref(Class)
+                    self.wr3 = weakref.ref(self)
+
+                def __del__(self):
+                    # we can't use assert* here, because gc will swallow
+                    # exceptions
+                    if self.wr1() is not None:
+                        errors.append("Type weakref with callback is not None")
+                    if self.wr2() is not Class:
+                        errors.append("Type weakref is not Class")
+                    if self.wr3() is not None:
+                        errors.append("Instance weakref is not None")
+
+            Class()
+
+        test()
+        gc.collect()
+        self.assertEqual(errors, [])
+
 
 class SubclassableWeakrefTestCase(TestBase):