gh-111178: fix UBSan failures in Objects/capsule.c (GH-128239)

fix UBSan failures for `PyCapsule`

Author: picnixz

Objects/capsule.c

@@ -18,6 +18,8 @@ typedef struct {
 } PyCapsule;
 
 
+#define _PyCapsule_CAST(op)     ((PyCapsule *)(op))
+
 
 static int
 _is_legal_capsule(PyObject *op, const char *invalid_capsule)
@@ -284,7 +286,7 @@ PyCapsule_Import(const char *name, int no_block)
 static void
 capsule_dealloc(PyObject *op)
 {
-    PyCapsule *capsule = (PyCapsule *)op;
+    PyCapsule *capsule = _PyCapsule_CAST(op);
     PyObject_GC_UnTrack(op);
     if (capsule->destructor) {
         capsule->destructor(op);
@@ -296,7 +298,7 @@ capsule_dealloc(PyObject *op)
 static PyObject *
 capsule_repr(PyObject *o)
 {
-    PyCapsule *capsule = (PyCapsule *)o;
+    PyCapsule *capsule = _PyCapsule_CAST(o);
     const char *name;
     const char *quote;
 
@@ -314,28 +316,27 @@ capsule_repr(PyObject *o)
 
 
 static int
-capsule_traverse(PyCapsule *capsule, visitproc visit, void *arg)
+capsule_traverse(PyObject *self, visitproc visit, void *arg)
 {
     // Capsule object is only tracked by the GC
     // if _PyCapsule_SetTraverse() is called, but
     // this can still be manually triggered by gc.get_referents()
-
+    PyCapsule *capsule = _PyCapsule_CAST(self);
     if (capsule->traverse_func != NULL) {
-        return capsule->traverse_func((PyObject*)capsule, visit, arg);
+        return capsule->traverse_func(self, visit, arg);
     }
-
     return 0;
 }
 
 
 static int
-capsule_clear(PyCapsule *capsule)
+capsule_clear(PyObject *self)
 {
     // Capsule object is only tracked by the GC
     // if _PyCapsule_SetTraverse() is called
+    PyCapsule *capsule = _PyCapsule_CAST(self);
     assert(capsule->clear_func != NULL);
-
-    return capsule->clear_func((PyObject*)capsule);
+    return capsule->clear_func(self);
 }
 
 
@@ -358,8 +359,8 @@ PyTypeObject PyCapsule_Type = {
     .tp_dealloc = capsule_dealloc,
     .tp_repr = capsule_repr,
     .tp_doc = PyCapsule_Type__doc__,
-    .tp_traverse = (traverseproc)capsule_traverse,
-    .tp_clear = (inquiry)capsule_clear,
+    .tp_traverse = capsule_traverse,
+    .tp_clear = capsule_clear,
 };