class Fluent::PluginHelper::Server::EventHandler::TCPServer

def data(&callback)
  raise "data callback can be registered just once, but registered twice" if self.singleton_methods.include?(:on_read)
  @data_callback = callback
  on_read_impl = case callback.arity
                 when 1 then :on_read_without_connection
                 when 2 then :on_read_with_connection
                 else
                   raise "BUG: callback block must have 1 or 2 arguments"
                 end
  self.define_singleton_method(:on_read, method(on_read_impl))
end