TCPOptions      TCPOptions    // all the configurable TCP conn specific configurable options.
	ShutdownTimeout time.Duration // timeout used for graceful server shutdown.
	listenerMutex   sync.Mutex    // to guard 'listener' field.
	listener        *httpListener // HTTP listener for all 'Addrs' field.
	inShutdown      uint32        // indicates whether the server is in shutdown or not