should decref shared string body on gc_free

include/mruby/string.h

@@ -49,6 +49,7 @@ struct RString {
 #define RSTRING_END(s)    (RSTRING(s)->buf + RSTRING(s)->len)
 #define MRB_STR_SHARED      256
 
+void mrb_str_decref(mrb_state*, struct mrb_shared_string*);
 mrb_value mrb_str_literal(mrb_state*, mrb_value);
 void mrb_str_concat(mrb_state*, mrb_value, mrb_value);
 mrb_value mrb_obj_to_str(mrb_state*, mrb_value);

src/gc.c

@@ -460,7 +460,9 @@ obj_free(mrb_state *mrb, struct RBasic *obj)
     break;
 
   case MRB_TT_STRING:
-    if (!(obj->flags & MRB_STR_SHARED))
+    if (obj->flags & MRB_STR_SHARED)
+      mrb_str_decref(mrb, ((struct RString*)obj)->aux.shared);
+    else
       mrb_free(mrb, ((struct RString*)obj)->buf);
     break;
 

src/string.c

@@ -43,6 +43,16 @@ static mrb_value mrb_str_subseq(mrb_state *mrb, mrb_value str, int beg, int len)
       s->aux.capa = capacity;\
 } while (0)
 
+void
+mrb_str_decref(mrb_state *mrb, struct mrb_shared_string *shared)
+{
+  shared->refcnt--;
+  if (shared->refcnt == 0) {
+    mrb_free(mrb, shared->buf);
+    mrb_free(mrb, shared);
+  }
+}
+
 static void
 str_modify(mrb_state *mrb, struct RString *s)
 {
@@ -63,11 +73,7 @@ str_modify(mrb_state *mrb, struct RString *s)
     s->aux.capa = len;
     s->flags &= ~MRB_STR_SHARED;
 
-    shared->refcnt--;
-    if (shared->refcnt == 0) {
-      mrb_free(mrb, shared->buf);
-      mrb_free(mrb, shared);
-    }
+    mrb_str_decref(mrb, shared);
   }
 }