$ go build restart.go
$ ./restart &
[1] 95147
$ Created listener file descriptor for :8080.$ curl http://localhost:8080/hello
Hello from 95147!

$ kill -SIGUSR2 95147
user defined signal 2 signal received.
Forked child 95170.
$ Imported listener file descriptor for :8080.$ curl http://localhost:8080/hello
Hello from 95170!
$ curl http://localhost:8080/hello
Hello from 95147!

$ kill -SIGTERM 95147
signal: killed
[1]+  Exit 1                  go run restart.go
$ curl http://localhost:8080/hello
Hello from 95170!
$ curl http://localhost:8080/hello
Hello from 95170!

$ kill -SIGTERM 95170
$ curl http://localhost:8080/hello
curl: (7) Failed to connect to localhost port 8080: Connection refused

package mainimport ("context""encoding/json""flag""fmt""net""net/http""os""os/signal""path/filepath""syscall""time"
)type listener struct {Addr     string `json:"addr"`FD       int    `json:"fd"`Filename string `json:"filename"`
}func importListener(addr string) (net.Listener, error) {// 从环境变量中抽离出被编码的 listener 的元数据。listenerEnv := os.Getenv("LISTENER")if listenerEnv == "" {return nil, fmt.Errorf("unable to find LISTENER environment variable")}// 解码 listener 的元数据。var l listenererr := json.Unmarshal([]byte(listenerEnv), &l)if err != nil {return nil, err}if l.Addr != addr {return nil, fmt.Errorf("unable to find listener for %v", addr)}// 文件已经被传入到这个进程中，从元数据中抽离文件描述符和名字，为 listener 重建/发现 *os.filelistenerFile := os.NewFile(uintptr(l.FD), l.Filename)if listenerFile == nil {return nil, fmt.Errorf("unable to create listener file: %v", err)}defer listenerFile.Close()// Create a net.Listener from the *os.File.ln, err := net.FileListener(listenerFile)if err != nil {return nil, err}return ln, nil
}func createListener(addr string) (net.Listener, error) {ln, err := net.Listen("tcp", addr)if err != nil {return nil, err}return ln, nil
}func createOrImportListener(addr string) (net.Listener, error) {// 尝试为地址导入一个 listener, 如果导入成功，则使用。ln, err := importListener(addr)if err == nil {fmt.Printf("Imported listener file descriptor for %v.n", addr)return ln, nil}// 没有 listener 被导入，这就意味着进程必须自己创建一个。ln, err = createListener(addr)if err != nil {return nil, err}fmt.Printf("Created listener file descriptor for %v.n", addr)return ln, nil
}func getListenerFile(ln net.Listener) (*os.File, error) {switch t := ln.(type) {case *net.TCPListener:return t.File()case *net.UnixListener:return t.File()}return nil, fmt.Errorf("unsupported listener: %T", ln)
}func forkChild(addr string, ln net.Listener) (*os.Process, error) {// 从 listener 中获取文件描述符，在环境变量编码在传递给这个子进程的元数据。lnFile, err := getListenerFile(ln)if err != nil {return nil, err}defer lnFile.Close()l := listener{Addr:     addr,FD:       3,Filename: lnFile.Name(),}listenerEnv, err := json.Marshal(l)if err != nil {return nil, err}// 将 stdin, stdout, stderr 和 listener 传入子进程。// 译注: 以上四个文件描述符分别为 0,1,2,3files := []*os.File{os.Stdin,os.Stdout,os.Stderr,lnFile,}// 获取当前环境变量，并且传入子进程。environment := append(os.Environ(), "LISTENER="+string(listenerEnv))// 获取当前进程名和工作目录execName, err := os.Executable()if err != nil {return nil, err}execDir := filepath.Dir(execName)// 生成子进程p, err := os.StartProcess(execName, []string{execName}, &os.ProcAttr{Dir:   execDir,Env:   environment,Files: files,Sys:   &syscall.SysProcAttr{},})if err != nil {return nil, err}return p, nil
}func waitForSignals(addr string, ln net.Listener, server *http.Server) error {signalCh := make(chan os.Signal, 1024)signal.Notify(signalCh, syscall.SIGHUP, syscall.SIGUSR2, syscall.SIGINT, syscall.SIGQUIT)for {select {case s := <-signalCh:fmt.Printf("%v signal received.n", s)switch s {case syscall.SIGHUP:// Fork 一个子进程。p, err := forkChild(addr, ln)if err != nil {fmt.Printf("Unable to fork child: %v.n", err)continue}fmt.Printf("Forked child %v.n", p.Pid)// 创建一个在 5 秒钟过去的 Context, 使用这个超时定时器关闭。ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)defer cancel()// 返回关闭过程中发生的任何错误。return server.Shutdown(ctx)case syscall.SIGUSR2:// Fork 一个子进程。p, err := forkChild(addr, ln)if err != nil {fmt.Printf("Unable to fork child: %v.n", err)continue}// 输出被 fork 的子进程的 PID，并等待更多的信号。fmt.Printf("Forked child %v.n", p.Pid)case syscall.SIGINT, syscall.SIGQUIT:// 创建一个在 5 秒钟过去的 Context, 使用这个超时定时器关闭。ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)defer cancel()// 返回关闭过程中发生的任何错误。return server.Shutdown(ctx)}}}
}func handler(w http.ResponseWriter, r *http.Request) {fmt.Fprintf(w, "Hello from %v!n", os.Getpid())
}func startServer(addr string, ln net.Listener) *http.Server {http.HandleFunc("/hello", handler)httpServer := &http.Server{Addr: addr,}go httpServer.Serve(ln)return httpServer
}func main() {// Parse command line flags for the address to listen on.var addr stringflag.StringVar(&addr, "addr", ":8080", "Address to listen on.")// Create (or import) a net.Listener and start a goroutine that runs// a HTTP server on that net.Listener.ln, err := createOrImportListener(addr)if err != nil {fmt.Printf("Unable to create or import a listener: %v.n", err)os.Exit(1)}server := startServer(addr, ln)// 等待复制或结束的信号err = waitForSignals(addr, ln, server)if err != nil {fmt.Printf("Exiting: %vn", err)return}fmt.Printf("Exiting.n")
}