Go语言：如何在HTTP Sever服务端端口监听成功时输出一条日志

像在Nodejs语言里端口监听建立成功时会触发一个事件来执行相应的代码，而在Go里面如果直接使用ListenAndServe函数的话是没办法知道端口监听是何时建立成功的。ListenAndServe包含两步操作：1）监听端口 2）accept以阻塞方式等待接受请求；如果1）执行成功，会立即进入阻塞状态，后面的代码在shutdown之前都无法执行，所以如果想在端口监听成功时输出一条日志，则

需要单独建立net.Listener再传给Serve函数，具体代码如下：

package main

import (
        "context"
        "fmt"
        "log"
        "net"
        "net/http"
        "os"
        "os/signal"
        "syscall"
        "time"
)


//http://pub.funshion.com:1341/service
func handler(w http.ResponseWriter, r *http.Request) {
        fmt.Fprintf(w, "hello")
}

func main() {
        start := time.Now()
        port := "1341"
        srv := http.Server{
                Addr:    ":" + port,
                Handler: http.DefaultServeMux,
        }

        http.HandleFunc("/", handler)

        //gracefully shutdown
        go func() {
                sigint := make(chan os.Signal, 1)
                signal.Notify(sigint, syscall.SIGINT, syscall.SIGTERM)
                data := <-sigint
                log.Printf("received signal: " + data.String())
                log.Printf("start to shutdown...")

                if err := srv.Shutdown(context.Background()); err != nil {
                        log.Fatalf("HTTP server Shutdown: %v", err)
                }
        }()

        //listen
        ln, err := net.Listen("tcp", srv.Addr)
        if err != nil {
                log.Fatalf(err.Error())
                return
        }
        log.Printf("listen on %s", port)
        log.Printf("time elapse: %d ms", time.Now().Sub(start).Milliseconds())

        //serve
        err = srv.Serve(ln)

        if err != nil && err != http.ErrServerClosed {
                log.Fatalf("server error: %v", err)
        } else {
                log.Printf("successfully shutdown: %v", err)
        }
}