Fix potentially use of wrong method cache

#### Example (with `MRB_METHOD_CACHE`)

  ```ruby
  GC.start
  c = Class.new
  p c            #=> #<Class:0x7fd6a180e790>
  c.new          #=> cache `c.new`
  c = nil
  GC.start       #=> `c` is GCed
  r = Range.dup
  p r            #=> #<Class:0x7fd6a180e790>
                 #   [same pointer as `c`]
  r.new(2, 3)    #=> ArgumentError: 'initialize':
                 #   wrong number of arguments (2 for 0)
                 #   [`c.new` is called instead of `r.new`]
  ```

#### Cause

  An entry of method cache is identified by class pointer and method
  id. However, reusing memory after GC may create a class with the same
  pointer as the cached class.

#### Treatment

  Cleared method caches of the class when the class is GCed.

1 files changed, 6 insertions, 0 deletions

diff --git a/include/mruby/class.h b/include/mruby/class.h
index c15633e83..cbf96fef2 100644
--- a/include/mruby/class.h
+++ b/include/mruby/class.h
@@ -95,6 +95,12 @@ void mrb_gc_mark_mt(mrb_state*, struct RClass*);
 size_t mrb_gc_mark_mt_size(mrb_state*, struct RClass*);
 void mrb_gc_free_mt(mrb_state*, struct RClass*);
 
+#ifdef MRB_METHOD_CACHE
+void mrb_mc_clear_by_class(mrb_state *mrb, struct RClass* c);
+#else
+#define mrb_mc_clear_by_class(mrb,c)
+#endif
+
 MRB_END_DECL
 
 #endif  /* MRUBY_CLASS_H */