Add const to BindStateBase to align RefCounted constness

base/callback_internal.h

Index: base/callback_internal.h
diff --git a/base/callback_internal.h b/base/callback_internal.h
index afed81c843e408c01076d92691f81fb9153298e0..f7501f96c5d737f0a0be70590743c45e6b05329a 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -36,9 +36,9 @@ class BASE_EXPORT BindStateBase {
 
  protected:
   BindStateBase(InvokeFuncStorage polymorphic_invoke,
-                void (*destructor)(BindStateBase*));
+                void (*destructor)(const BindStateBase*));
   BindStateBase(InvokeFuncStorage polymorphic_invoke,
-                void (*destructor)(BindStateBase*),
+                void (*destructor)(const BindStateBase*),
                 bool (*is_cancelled)(const BindStateBase*));
   ~BindStateBase() = default;
 
@@ -51,8 +51,8 @@ class BASE_EXPORT BindStateBase {
     return is_cancelled_(this);
   }
 
-  void AddRef();
-  void Release();
+  void AddRef() const;
+  void Release() const;

[Yuta Kitamura, 2016/09/20 10:04:21]

I'm not sure adding const to these is legit. In my plain understanding
they modify the refcount, so they shouldn't be const.

[tzik, 2016/09/20 10:54:30]

The refcount is not a state of the instance, but a shared state of the strong
references to the instance. Even on a const object, IMO, we should be able to
make a strong reference to it, without modifying the main part of the object.

Also, this is needed to make BindStateBase be a subclass of
RefCountedThreadSafe. That will remove the manual refcount handling in
BindStateBase, and simplify the class.

[Yuta Kitamura, 2016/09/21 06:01:36]

Ah okay, this lets us refcount const objects, right?

 
   // In C++, it is safe to cast function pointers to function pointers of
   // another type. It is not okay to use void*. We create a InvokeFuncStorage
@@ -60,10 +60,10 @@ class BASE_EXPORT BindStateBase {
   // the original type on usage.
   InvokeFuncStorage polymorphic_invoke_;
 
-  AtomicRefCount ref_count_;
+  mutable AtomicRefCount ref_count_;
 
   // Pointer to a function that will properly destroy |this|.
-  void (*destructor_)(BindStateBase*);
+  void (*destructor_)(const BindStateBase*);
   bool (*is_cancelled_)(const BindStateBase*);
 
   DISALLOW_COPY_AND_ASSIGN(BindStateBase);