filterx-istype: add FilterXRef safety check

FilterXRef can mimic the functionality of its referenced value, but only
through virtual methods.

filterx_object_is_type() is usually followed by an implicit type cast,
which is impossible to implement transparently (without the unwrap() call),
so this safety check has been added. This is far from ideal, but doing
downcasts is the original and real problem here.

Signed-off-by: László Várady <laszlo.varady@anno.io>

lib/filterx/filterx-object-istype.h

@@ -31,7 +31,7 @@
 #include "filterx/filterx-ref.h"
 
 static inline gboolean
-filterx_object_is_type(FilterXObject *object, FilterXType *type)
+_filterx_object_is_type(FilterXObject *object, FilterXType *type)
 {
   FilterXType *self_type = object->type;
   while (self_type)
@@ -42,3 +42,14 @@ filterx_object_is_type(FilterXObject *object, FilterXType *type)
     }
   return FALSE;
 }
+
+static inline gboolean
+filterx_object_is_type(FilterXObject *object, FilterXType *type)
+{
+#if SYSLOG_NG_ENABLE_DEBUG
+  g_assert(!(_filterx_type_is_referenceable(type) && _filterx_object_is_type(object, &FILTERX_TYPE_NAME(ref)))
+           && "filterx_ref_unwrap() must be used before comparing to mutable types");
+#endif
+
+  return _filterx_object_is_type(object, type);
+}