boost::future set_wait_callback thread safety issues

[ronag89@…]

See, ​http://stackoverflow.com/questions/9355110/boostfuture-is-wait-callback-guaranteed-to-only-be-invoked-once/13891873#13891873.

[viboes]

The issue is more complex than it seems at a first glance. From the documentation it seems that the callback should be called each time a wait function is called.

"Effects: Store a copy of f with the asynchronous result associated with *this as a
wait callback. This will replace any existing wait callback store alongside that
result. If a thread subsequently calls one of the wait functions on a future or
boost::shared_future associated with this result, and the result is not ready, f(*this)
shall be invoked."

[viboes]

I have just added a lock to protect the callback setting. Committed in trunk [81975].