package mainimport (  "context"  "errors"  "flag"  "log"  "net"  "net/http"  "os"  "os/exec"  "os/signal"  "syscall"  "time") var (  server  *http.Server  listener net.Listener  graceful = flag.Bool("graceful", false, "listen on fd open 3 (internal use only)")) func handler(w http.ResponseWriter, r *http.Request) {  time.Sleep(20 * time.Second)  w.Write([]byte("hello world233333!!!!"))} func main() {  flag.Parse()   http.HandleFunc("/hello", handler)  server = &http.Server{Addr: ":9999"}   var err error  if *graceful {    log.Print("main: Listening to existing file descriptor 3.")    // cmd.ExtraFiles: If non-nil, entry i becomes file descriptor 3+i.    // when we put socket FD at the first entry, it will always be 3(0+3)    f := os.NewFile(3, "")    listener, err = net.FileListener(f)  } else {    log.Print("main: Listening on a new file descriptor.")    listener, err = net.Listen("tcp", server.Addr)  }   if err != nil {    log.Fatalf("listener error: %v", err)  }   go func() {    // server.Shutdown() stops Serve() immediately, thus server.Serve() should not be in main goroutine    err = server.Serve(listener)    log.Printf("server.Serve err: %v/n", err)  }()  signalHandler()  log.Printf("signal end")} func reload() error {  tl, ok := listener.(*net.TCPListener)  if !ok {    return errors.New("listener is not tcp listener")  }   f, err := tl.File()  if err != nil {    return err  }   args := []string{"-graceful"}  cmd := exec.Command(os.Args[0], args...)  cmd.Stdout = os.Stdout  cmd.Stderr = os.Stderr  // put socket FD at the first entry  cmd.ExtraFiles = []*os.File{f}  return cmd.Start()} func signalHandler() {  ch := make(chan os.Signal, 1)  signal.Notify(ch, syscall.SIGINT, syscall.SIGTERM, syscall.SIGUSR2)  for {    sig := <-ch    log.Printf("signal: %v", sig)     // timeout context for shutdown    ctx, _ := context.WithTimeout(context.Background(), 20*time.Second)    switch sig {    case syscall.SIGINT, syscall.SIGTERM:      // stop      log.Printf("stop")      signal.Stop(ch)      server.Shutdown(ctx)      log.Printf("graceful shutdown")      return    case syscall.SIGUSR2:      // reload      log.Printf("reload")      err := reload()      if err != nil {        log.Fatalf("graceful restart error: %v", err)      }      server.Shutdown(ctx)      log.Printf("graceful reload")      return    }  }}