chat.go

// Chat is a TCP server that connects couples so they can talk.
// Adapted from https://go.dev/talks/2012/chat.slide
package main

import (
	"fmt"
	"io"
	"log"
	"net"
)

const addr = ":1234"

func main() {
	ln, err := net.Listen("tcp", addr)
	if err != nil {
		log.Fatal(err)
	}
	for {
		conn, err := ln.Accept()
		if err != nil {
			log.Print(err)
			continue
		}
		go match(conn)
	}
}

var partner = make(chan io.ReadWriteCloser)

// match simultaneously tries to send and receive a connection on a channel.
func match(conn net.Conn) {
	log.Printf("connection from %s", conn.RemoteAddr())
	fmt.Fprint(conn, "Waiting for a partner...")
	select {
	case partner <- conn:
		// now handled by the other goroutine
	case p := <-partner:
		// start chat session between the two connections
		chat(p, conn)
	}
}

// chat copies data between the two connections.
func chat(a, b io.ReadWriteCloser) {
	fmt.Fprintln(a, "Found one! Say hi.")
	fmt.Fprintln(b, "Found one! Say hi.")
	errc := make(chan error, 1)
	go cp(a, b, errc)
	go cp(b, a, errc)
	if err := <-errc; err != nil {
		log.Print(err)
	}
	a.Close()
	b.Close()
}

// cp copies data with error handling.
func cp(w io.Writer, r io.Reader, errc chan<- error) {
	_, err := io.Copy(w, r)
	errc <- err
}