gh-125783: Add tests to prevent regressions with the combination of `ctypes` and metaclasses. (GH-125881)

Lib/test/test_ctypes/test_c_simple_type_meta.py

@@ -0,0 +1,87 @@
+import unittest
+import ctypes
+from ctypes import POINTER, c_void_p
+
+from ._support import PyCSimpleType
+
+
+class PyCSimpleTypeAsMetaclassTest(unittest.TestCase):
+    def tearDown(self):
+        # to not leak references, we must clean _pointer_type_cache
+        ctypes._reset_cache()
+
+    def test_creating_pointer_in_dunder_new_1(self):
+        # Test metaclass whose instances are C types; when the type is
+        # created it automatically creates a pointer type for itself.
+        # The pointer type is also an instance of the metaclass.
+        # Such an implementation is used in `IUnknown` of the `comtypes`
+        # project. See gh-124520.
+
+        class ct_meta(type):
+            def __new__(cls, name, bases, namespace):
+                self = super().__new__(cls, name, bases, namespace)
+
+                # Avoid recursion: don't set up a pointer to
+                # a pointer (to a pointer...)
+                if bases == (c_void_p,):
+                    # When creating PtrBase itself, the name
+                    # is not yet available
+                    return self
+                if issubclass(self, PtrBase):
+                    return self
+
+                if bases == (object,):
+                    ptr_bases = (self, PtrBase)
+                else:
+                    ptr_bases = (self, POINTER(bases[0]))
+                p = p_meta(f"POINTER({self.__name__})", ptr_bases, {})
+                ctypes._pointer_type_cache[self] = p
+                return self
+
+        class p_meta(PyCSimpleType, ct_meta):
+            pass
+
+        class PtrBase(c_void_p, metaclass=p_meta):
+            pass
+
+        class CtBase(object, metaclass=ct_meta):
+            pass
+
+        class Sub(CtBase):
+            pass
+
+        class Sub2(Sub):
+            pass
+
+        self.assertIsInstance(POINTER(Sub2), p_meta)
+        self.assertTrue(issubclass(POINTER(Sub2), Sub2))
+        self.assertTrue(issubclass(POINTER(Sub2), POINTER(Sub)))
+        self.assertTrue(issubclass(POINTER(Sub), POINTER(CtBase)))
+
+    def test_creating_pointer_in_dunder_new_2(self):
+        # A simpler variant of the above, used in `CoClass` of the `comtypes`
+        # project.
+
+        class ct_meta(type):
+            def __new__(cls, name, bases, namespace):
+                self = super().__new__(cls, name, bases, namespace)
+                if isinstance(self, p_meta):
+                    return self
+                p = p_meta(f"POINTER({self.__name__})", (self, c_void_p), {})
+                ctypes._pointer_type_cache[self] = p
+                return self
+
+        class p_meta(PyCSimpleType, ct_meta):
+            pass
+
+        class Core(object):
+            pass
+
+        class CtBase(Core, metaclass=ct_meta):
+            pass
+
+        class Sub(CtBase):
+            pass
+
+        self.assertIsInstance(POINTER(Sub), p_meta)
+        self.assertTrue(issubclass(POINTER(Sub), Sub))