setting low level socket options in golang (SO_PRIORITY, SO_REUSEADDR)

sysserver.go

package main

import (
	"bufio"
	"fmt"
	"net"
	"os"
	"syscall"
)

const (
	PRIORITY = 2
	PORT     = "6000"
)

func main() {
	ln, err := net.Listen("tcp", "127.0.0.1:"+PORT)
	if err != nil {
		fmt.Println("error in listening on socket:", err)
		os.Exit(1)
	}
	fmt.Println("listening on socket!")

	con, err := ln.Accept()
	if err != nil {
		fmt.Println("unable to accept connection on socket:", err)
		os.Exit(1)
	}
	fmt.Println("accepted connection from:", con.RemoteAddr())
	defer con.Close()

	tcpconn, ok := con.(*net.TCPConn)
	if !ok {
		fmt.Println("error in casting *net.Conn to *net.TCPConn!")
		os.Exit(1)
	}
	file, err := tcpconn.File()
	if err != nil {
		fmt.Println("error in getting file for the connection!")
		os.Exit(1)
	}
	err = syscall.SetsockoptInt(int(file.Fd()), syscall.SOL_SOCKET, syscall.SO_PRIORITY, PRIORITY)
	file.Close()
	if err != nil {
		fmt.Println("error in setting priority option on socket:", err)
		os.Exit(1)
	}

	message, _ := bufio.NewReader(con).ReadString('\n')
	fmt.Print("Message Received: ", string(message))
	fmt.Fprintf(con, "Hello World!\n")
}

I don't think this will work. As described in this article sock opts must be set before the call to Listen(). I honestly have no idea how this is working given you seem to close the socket's file as well.