go zero downtime restarts

main.go

package main

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

func main() {
	fd, _ := strconv.Atoi(os.Getenv("HTTP_LISTEN_FD"))
	serverID, _ := strconv.Atoi(os.Getenv("HTTP_SERVER_ID"))
	fmt.Println("Starting server", serverID)

	// Get or create the listener file
	var err error
	var file *os.File
	if fd == 0 {
		// There is no file descriptor, create a new listener
		listener, err := net.ListenTCP("tcp", &net.TCPAddr{IP: []byte{0, 0, 0, 0}, Port: 12345})
		if err != nil {
			panic(err)
		}
		if file, err = listener.File(); err != nil {
			panic(err)
		}
	} else {
		// Use the file descriptor which our parent passed to us
		file = os.NewFile(uintptr(fd), "apifd")
	}

	// Set up a simple HTTP server to listen on the socket
	mux := http.NewServeMux()
	var i int
	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		i++
		fmt.Println("server " + strconv.Itoa(serverID) + " req " + strconv.Itoa(i))
		time.Sleep(time.Second * 2)
		w.Write([]byte("server " + strconv.Itoa(serverID) + " req " + strconv.Itoa(i) + "\n"))
	})
	listener, err := net.FileListener(file)
	if err != nil {
		panic(err)
	}
	server := &http.Server{Handler: mux}
	go server.Serve(listener)

	// Keep the server alive for 10 seconds
	time.Sleep(time.Second * 10)

	// Restart the process and pass our file descriptor to the child process. fd
	// 0, 1 and 2 are occupied by stdin, stdout and stderr, so our fd will be 3
	executable, _ := os.Executable()
	cmd := exec.Command(executable)
	cmd.Env = []string{"HTTP_LISTEN_FD=3", "HTTP_SERVER_ID=" + strconv.Itoa(serverID+1)}
	cmd.ExtraFiles = []*os.File{file}
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr
	if err = cmd.Start(); err != nil {
		panic(err)
	}

	// Gracefully shut down this server
	ctx, cancel := context.WithTimeout(context.Background(), time.Minute)
	server.Shutdown(ctx)
	cancel()

	if serverID == 0 {
		// This is the parent server. So we'll keep running to listen for OS
		// signals
		sigChan := make(chan os.Signal, 1)
		signal.Notify(sigChan, os.Interrupt, os.Kill, syscall.SIGTERM)
		select {
		case sig := <-sigChan:
			fmt.Println("Caught", sig, "signal, stopping...")
			os.Exit(0)
		}
	}

	fmt.Println("Stopping server", serverID)
}