code: func Listen(network, addr string) (net.Listener, error) {
  	ln, err := cfg.NewListener(network, addr)
  	if err != nil && strings.Contains(err.Error(), "SO_REUSEPORT") {
  		return nil, &ErrNoReusePort{err}
  	}
  	return ln, err
  }
docs: // Listen returns TCP listener with SO_REUSEPORT option set.
  //
  // The returned listener tries enabling the following TCP options, which usually
  // have positive impact on performance:
  //
  // - TCP_DEFER_ACCEPT. This option expects that the server reads from accepted
  //   connections before writing to them.
  //
  // - TCP_FASTOPEN. See https://lwn.net/Articles/508865/ for details.
  //
  // Use https://github.com/valyala/tcplisten if you want customizing
  // these options.
  //
  // Only tcp4 and tcp6 networks are supported.
  //
  // ErrNoReusePort error is returned if the system doesn't support SO_REUSEPORT.