code: func ReusableListen(proto, addr string) (net.Listener, error) {
  	backlogOnce.Do(func() {
  		backlog = maxListenerBacklog()
  	})
  
  	saddr, typ, err := sockaddr(proto, addr)
  	if err != nil {
  		return nil, err
  	}
  
  	fd, err := syscall.Socket(typ, syscall.SOCK_STREAM, syscall.IPPROTO_TCP)
  	if err != nil {
  		return nil, err
  	}
  
  	if err := setSockopt(fd); err != nil {
  		return nil, err
  	}
  
  	if err := syscall.Bind(fd, saddr); err != nil {
  		return nil, err
  	}
  
  	if err := syscall.Listen(fd, backlog); err != nil {
  		return nil, err
  	}
  
  	f := os.NewFile(uintptr(fd), proto+":"+addr)
  	l, err := net.FileListener(f)
  	if err != nil {
  		return nil, err
  	}
  
  	if err := f.Close(); err != nil {
  		l.Close()
  		return nil, err
  	}
  
  	return l, nil
  }
docs: // ReusableListen returns a TCP listener with SO_REUSEPORT and keepalives
  // enabled.