练习 8.4： 修改reverb2服务器，在每一个连接中使用sync.WaitGroup来计数活跃的echo goroutine。当计数减为零时，关闭TCP连接的写入，像练习8.3中一样。验证一下你的修改版netcat3客户端会一直等待所有的并发“喊叫”完成，即使是在标准输入流已经关闭的情况下。

 

重点：等待所有的goroutine执行完成后再关闭TCP连接

package main

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

func main() {
        listener, err := net.Listen("tcp", ":8040")
        if err != nil {
                log.Fatal(err)
        }   

        for {
                conn, err := listener.Accept()
                if err != nil {
                        log.Print(err) // e.g., connection aborted
                        continue
                }   
                go handleConn(conn) //新建goroutines处理连接
        }   
}

func handleConn(c net.Conn) {
        input := bufio.NewScanner(c)
        var wg sync.WaitGroup
        //var  ch =make(chan struct{})
        for input.Scan() {
                wg.Add(1)
                go func(c net.Conn, shout string, delay time.Duration) {
                        defer wg.Done()
                        fmt.Fprintln(c, "\t", strings.ToUpper(shout))
                        time.Sleep(delay)
                        fmt.Fprintln(c, "\t", shout)
                        time.Sleep(delay)
                        fmt.Fprintln(c, "\t", strings.ToLower(shout))
                        //ch<-struct{}{}

                }(c, input.Text(), 1*time.Second)
        }   
        wg.Wait()
                //cw := c.(*net.TCPConn)
          //cw.CloseWrite()
        
        c.Close()
}