module AWS::Core::AsyncHandle

successfully, it will call the callback immediately.
If this is called when the task has already completed

handle.on_success { puts "It worked!" }

the task.
Registers a callback to be called on successful completion of

def on_success(&block)
  if @_async_status == :success
    block.call
  else
    (@_async_callbacks ||= []) << { :success => block }
  end
end