Go serveDoor https://stackoverflow.com/questions/71269917

servedoor.go

package main

import (
	"fmt"
	"io"
	"net"
	"os"
	"os/exec"
)

func main() {
	addr, err := net.ResolveTCPAddr("tcp", "localhost:0")
	check(err, "ResolveTCPAddr")

	conn, err := net.ListenTCP("tcp", addr)
	check(err, "ListenTCP")

	fmt.Println("listening on:", conn.Addr())

	go connectAsClient(conn.Addr().(*net.TCPAddr))

	for {
		client, err := conn.AcceptTCP()
		check(err, "AcceptTCP")

		fmt.Println("Accepted from:", client.RemoteAddr())
		go serveDoor(client, "echo", "Hello from client!")
	}
}

func check(err error, msg string) {
	if err != nil {
		fmt.Fprintf(os.Stderr, "%s error: %v\n", msg, err)
		os.Exit(1)
	}
}

func serveDoor(conn *net.TCPConn, name string, args ...string) {
	defer conn.Close()

	cmd := exec.Command(name, args...)
	cmd.Stdin = conn
	cmd.Stdout = conn
	cmd.Stderr = conn

	err := cmd.Run()
	fmt.Println("door finished:", err)
}

func connectAsClient(dst *net.TCPAddr) {
	conn, err := net.DialTCP("tcp", nil, dst)
	check(err, "DialTCP")
	defer conn.Close()

	var data [512]byte

	for {
		n, err := conn.Read(data[:])
		if err != nil {
			if err != io.EOF {
				fmt.Println("client read error:", err)
			}
			break
		}
		fmt.Printf("client got: %q\n", string(data[:n]))
	}
}