Fix a soundness bug with PyClassInitializer

From now you cannot initialize a `PyClassInitializer<SubClass>` with `PyClassInitializer::from(Py<BaseClass>).add_subclass(SubClass)`.

This was out of bounds write. Now it panics. See details at #4452.

newsfragments/4454.fixed.md

@@ -0,0 +1 @@
+Fix a soundness bug with `PyClassInitializer`: from now you cannot initialize a `PyClassInitializer<SubClass>` with `PyClassInitializer::from(Py<BaseClass>).add_subclass(SubClass)`.

src/pyclass_init.rs

@@ -197,11 +197,17 @@ impl<T: PyClass> PyClassInitializer<T> {
     ///     })
     /// }
     /// ```
+    #[track_caller]
+    #[inline]
     pub fn add_subclass<S>(self, subclass_value: S) -> PyClassInitializer<S>
     where
         S: PyClass<BaseType = T>,
         S::BaseType: PyClassBaseType<Initializer = Self>,
     {
+        if matches!(self.0, PyClassInitializerImpl::Existing { .. }) {
+            // This is unsound; see https://github.com/PyO3/pyo3/issues/4452.
+            panic!("you cannot add a subclass to an existing value");
+        }
         PyClassInitializer::new(subclass_value, self)
     }
 

tests/add_subclass_to_py.rs

@@ -0,0 +1,20 @@
+//! See https://github.com/PyO3/pyo3/issues/4452.
+
+use pyo3::prelude::*;
+
+#[pyclass(subclass)]
+struct BaseClass {}
+
+#[pyclass(extends=BaseClass)]
+struct SubClass {
+    _data: i32,
+}
+
+#[test]
+#[should_panic]
+fn add_subclass_to_py_is_unsound() {
+    Python::with_gil(|py| {
+        let base = Py::new(py, BaseClass {}).unwrap();
+        let _subclass = PyClassInitializer::from(base).add_subclass(SubClass { _data: 42 });
+    });
+}