Preserve callable addresses in ctypes function pointer casts (#8965)

Read the stored executable address when casting a function pointer and accept function pointer types as cast destinations. The existing ownership dictionary retains the original callback through a cast roundtrip.

Extracted independently on the unchanged Python 3.14 target, without typed callback argument or broader py_object transport changes. One unique callback-address/ownership regression extends the existing ctypes snippet; canonical coverage was checked for duplicates.

Assisted-by: Codex:model-version-unavailable

crates/vm/src/stdlib/_ctypes/function.rs

@@ -509,7 +509,10 @@ fn cast_check_pointertype(ctype: &PyObject, vm: &VirtualMachine) -> bool {
         return true;
     }
 
-    // PyCFuncPtrTypeObject_Check - TODO
+    // PyCFuncPtrTypeObject_Check
+    if ctype.class().fast_issubclass(PyCFuncPtrType::static_type()) {
+        return true;
+    }
 
     // simple pointer types via StgInfo.proto (c_void_p, c_char_p, etc.)
     if let Ok(type_attr) = ctype.get_attr("_type_", vm)
@@ -562,6 +565,9 @@ pub(super) fn cast_impl(
         // Simple type (c_void_p, c_char_p, etc.) → value from buffer
         let buffer = simple.0.buffer.read();
         rustpython_host_env::ctypes::read_pointer_from_buffer(&buffer)
+    } else if let Some(funcptr) = obj.downcast_ref::<PyCFuncPtr>() {
+        // A function pointer stores the executable address in its buffer.
+        funcptr.get_func_ptr()
     } else if let Some(cdata) = obj.downcast_ref::<PyCData>() {
         // Array, Structure, Union → buffer address (b_ptr)
         cdata.buffer.read().as_ptr() as usize

extra_tests/snippets/stdlib_ctypes.py

@@ -1,4 +1,5 @@
 import ctypes
+import gc
 import itertools
 import os as _os
 import sys as _sys
@@ -458,4 +459,14 @@ except MemoryError:
 else:
     raise AssertionError("an unallocatable array was created")
 
+# A function-pointer cast retains the callback and its executable address.
+Unary = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_int)
+callback = Unary(lambda value: value + 1)
+address = ctypes.cast(callback, ctypes.c_void_p)
+roundtrip = ctypes.cast(address, Unary)
+assert roundtrip._objects[id(callback)] is callback
+del callback, address
+gc.collect()
+assert roundtrip(41) == 42
+
 print("done")