GH-100942: Fix incorrect cast in property_copy(). (#100965)

Author: rhettinger

Lib/test/test_property.py

@@ -214,6 +214,23 @@ def test_property_set_name_incorrect_args(self):
             ):
                 p.__set_name__(*([0] * i))
 
+    def test_property_setname_on_property_subclass(self):
+        # https://github.com/python/cpython/issues/100942
+        # Copy was setting the name field without first
+        # verifying that the copy was an actual property
+        # instance.  As a result, the code below was
+        # causing a segfault.
+
+        class pro(property):
+            def __new__(typ, *args, **kwargs):
+                return "abcdef"
+
+        class A:
+            pass
+
+        p = property.__new__(pro)
+        p.__set_name__(A, 1)
+        np = p.getter(lambda self: 1)
 
 # Issue 5890: subclasses of property do not preserve method __doc__ strings
 class PropertySub(property):

Misc/NEWS.d/next/Core and Builtins/2023-01-11-22-52-19.gh-issue-100942.ontOy_.rst

@@ -0,0 +1,2 @@
+Fixed segfault in property.getter/setter/deleter that occurred when a property
+subclass overrode the ``__new__`` method to return a non-property instance.

Objects/descrobject.c

@@ -1712,7 +1712,9 @@ property_copy(PyObject *old, PyObject *get, PyObject *set, PyObject *del)
     if (new == NULL)
         return NULL;
 
-    Py_XSETREF(((propertyobject *) new)->prop_name, Py_XNewRef(pold->prop_name));
+    if (PyObject_TypeCheck((new), &PyProperty_Type)) {
+        Py_XSETREF(((propertyobject *) new)->prop_name, Py_XNewRef(pold->prop_name));
+    }
     return new;
 }