[object] Add "writable"

src/hb-object.hh

@@ -194,9 +194,10 @@ struct hb_user_data_array_t
 struct hb_object_header_t
 {
   hb_reference_count_t ref_count;
+  hb_atomic_int_t writable;
   hb_atomic_ptr_t<hb_user_data_array_t> user_data;
 };
-#define HB_OBJECT_HEADER_STATIC {HB_REFERENCE_COUNT_INIT, HB_ATOMIC_PTR_INIT (nullptr)}
+#define HB_OBJECT_HEADER_STATIC {HB_REFERENCE_COUNT_INIT, HB_ATOMIC_INT_INIT (0), HB_ATOMIC_PTR_INIT (nullptr)}
 
 
 /*
@@ -228,6 +229,7 @@ template <typename Type>
 static inline void hb_object_init (Type *obj)
 {
   obj->header.ref_count.init ();
+  obj->header.writable.set_relaxed (true);
   obj->header.user_data.init ();
 }
 template <typename Type>
@@ -241,6 +243,16 @@ static inline bool hb_object_is_valid (const Type *obj)
   return likely (obj->header.ref_count.is_valid ());
 }
 template <typename Type>
+static inline bool hb_object_is_immutable (const Type *obj)
+{
+  return !obj->header.writable.get_relaxed ();
+}
+template <typename Type>
+static inline bool hb_object_make_immutable (const Type *obj)
+{
+  return !obj->header.writable.set_relaxed (false);
+}
+template <typename Type>
 static inline Type *hb_object_reference (Type *obj)
 {
   hb_object_trace (obj, HB_FUNC);