code: func (engine *Engine) RunFd(fd int) (err error) {
  	debugPrint("Listening and serving HTTP on fd@%d", fd)
  	defer func() { debugPrintError(err) }()
  
  	f := os.NewFile(uintptr(fd), fmt.Sprintf("fd@%d", fd))
  	listener, err := net.FileListener(f)
  	if err != nil {
  		return
  	}
  	defer listener.Close()
  	err = http.Serve(listener, engine)
  	return
  }
docs: // RunFd attaches the router to a http.Server and starts listening and serving HTTP requests
  // through the specified file descriptor.
  // Note: this method will block the calling goroutine indefinitely unless an error happens.