First chat server with Go

main.go

package main

import (
	"bufio"
	"fmt"
	"log"
	"net"
	"strings"
	"sync"
)

var welcomeMessage = `Welcome to the chat server!
Please be respectful to others.
`

type Nickname string

func main() {
	// Create a listener to listen on IRC port 6667
	l, err := net.Listen("tcp", ":9091")
	if err != nil {
		log.Fatal(err)
	}
	defer l.Close()

	// Accept TCP connections
	// We need a map to store connections
	// and a mutex to protect it
	var connections = make(map[Nickname]net.Conn)
	var mu sync.Mutex

	for {
		c, err := l.Accept()
		if err != nil {
			log.Println(err)
			continue
		}
		// Handle the connection in a new goroutine
		go func() {
			defer c.Close()
			var nick Nickname // Connection's nickname

			// Send a welcome message
			c.Write([]byte(welcomeMessage))

			// Read one line at a time.
			scanner := bufio.NewScanner(c)

			// Get a nickname from the user
			for validNick := false; !validNick; {
				c.Write([]byte("Enter your nickname: "))
				if scanner.Scan() {
					nick = Nickname(scanner.Text())

					// Check if the nickname is already taken
					mu.Lock()
					_, exists := connections[nick]
					if exists {
						c.Write([]byte("Nickname already in use.  Please choose another"))
					} else {
						validNick = true

						// Set the connection's nickname
						connections[nick] = c

						c.Write([]byte("Welcome, " + string(nick) + "!\r\n"))
						c.Write([]byte("Current users: "))
						var usersList = make([]string, 0, len(connections))
						for k := range connections {
							usersList = append(usersList, string(k))
						}
						usersListStr := fmt.Sprintf("[%s]", strings.Join(usersList, ", "))
						c.Write([]byte(usersListStr + "\r\n"))

						// Announce the new user to all other users
						for _, conn := range connections {
							conn.Write([]byte(string(nick) + " has joined the chat\r\n"))
						}
					}
					mu.Unlock()
				}
			}

			for {
				if scanner.Scan() {
					for _, conn := range connections {
						if conn != c {
							conn.Write([]byte("[" + string(nick) + "] " + scanner.Text() + "\r\n"))
						}
					}
					continue
				}

				// EOF or Error
				mu.Lock()
				delete(connections, nick)
				// Announce the user has left the chat
				for _, conn := range connections {
					conn.Write([]byte(string(nick) + " has left the chat\r\n"))
				}
				mu.Unlock()
				return
			}
		}()
	}
}