func (srv *Server) Serve(l net.Listener) error {
	if fn := testHookServerServe; fn != nil {
		fn(srv, l) // call hook with unwrapped listener
	}
 
	origListener := l
	l = &onceCloseListener{Listener: l}
	defer l.Close()
 
	if err := srv.setupHTTP2_Serve(); err != nil {
		return err
	}
 
	if !srv.trackListener(&l, true) {
		return ErrServerClosed
	}
	defer srv.trackListener(&l, false)
 
	baseCtx := context.Background()
	if srv.BaseContext != nil {
		baseCtx = srv.BaseContext(origListener)
		if baseCtx == nil {
			panic("BaseContext returned a nil context")
		}
	}
 
	var tempDelay time.Duration // how long to sleep on accept failure
 
	ctx := context.WithValue(baseCtx, ServerContextKey, srv)
	for {
		rw, err := l.Accept()
		if err != nil {
			if srv.shuttingDown() {
				return ErrServerClosed
			}
			if ne, ok := err.(net.Error); ok && ne.Temporary() {
				if tempDelay == 0 {
					tempDelay = 5 * time.Millisecond
				} else {
					tempDelay *= 2
				}
				if max := 1 * time.Second; tempDelay > max {
					tempDelay = max
				}
				srv.logf("http: Accept error: %v; retrying in %v", err, tempDelay)
				time.Sleep(tempDelay)
				continue
			}
			return err
		}
		connCtx := ctx
		if cc := srv.ConnContext; cc != nil {
			connCtx = cc(connCtx, rw)
			if connCtx == nil {
				panic("ConnContext returned nil")
			}
		}
		tempDelay = 0
		c := srv.newConn(rw)
		c.setState(c.rwc, StateNew, runHooks) // before Serve can return
		go c.serve(connCtx)
	}
}

func (l *overseerListener) Accept() (net.Conn, error) {
	conn, err := l.Listener.(*net.TCPListener).AcceptTCP()
	if err != nil {
		return nil, err
	}
	conn.SetKeepAlive(true)                  // see http.tcpKeepAliveListener
	conn.SetKeepAlivePeriod(3 * time.Minute) // see http.tcpKeepAliveListener
	uconn := overseerConn{
		Conn:   conn,
		wg:     &l.wg,
		closed: make(chan bool),
	}
	go func() {
		//connection watcher
		select {
		case <-l.closeByForce:
			uconn.Close()
		case <-uconn.closed:
			//closed manually
		}
	}()
	l.wg.Add(1)
	return uconn, nil
}
 
//non-blocking trigger close
func (l *overseerListener) release(timeout time.Duration) {
	//stop accepting connections - release fd
	l.closeError = l.Listener.Close()
	//start timer, close by force if deadline not met
	waited := make(chan bool)
	go func() {
		l.wg.Wait()
		waited <- true
	}()
	go func() {
		select {
		case <-time.After(timeout):
			close(l.closeByForce)
		case <-waited:
			//no need to force close
		}
	}()
}
 
//blocking wait for close
func (l *overseerListener) Close() error {
	l.wg.Wait()
	return l.closeError
}
 
func (o overseerConn) Close() error {
	err := o.Conn.Close()
	if err == nil {
		o.wg.Done()
		o.closed <- true
	}
	return err
}

func (mp *master) retreiveFileDescriptors() error {
	mp.slaveExtraFiles = make([]*os.File, len(mp.Config.Addresses))
	for i, addr := range mp.Config.Addresses {
		a, err := net.ResolveTCPAddr("tcp", addr)
		if err != nil {
			return fmt.Errorf("Invalid address %s (%s)", addr, err)
		}
		l, err := net.ListenTCP("tcp", a)
		if err != nil {
			return err
		}
		f, err := l.File()
		if err != nil {
			return fmt.Errorf("Failed to retreive fd for: %s (%s)", addr, err)
		}
		if err := l.Close(); err != nil {
			return fmt.Errorf("Failed to close listener for: %s (%s)", addr, err)
		}
		mp.slaveExtraFiles[i] = f
	}
	return nil
}

func (mp *master) fork() error {
	mp.debugf("starting %s", mp.binPath)
	cmd := exec.Command(mp.binPath)
	//mark this new process as the "active" slave process.
	//this process is assumed to be holding the socket files.
	mp.slaveCmd = cmd
	mp.slaveID++
	//provide the slave process with some state
	e := os.Environ()
	e = append(e, envBinID+"="+hex.EncodeToString(mp.binHash))
	e = append(e, envBinPath+"="+mp.binPath)
	e = append(e, envSlaveID+"="+strconv.Itoa(mp.slaveID))
	e = append(e, envIsSlave+"=1")
	e = append(e, envNumFDs+"="+strconv.Itoa(len(mp.slaveExtraFiles)))
	cmd.Env = e
	//inherit master args/stdfiles
	cmd.Args = os.Args
	cmd.Stdin = os.Stdin
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr
	//include socket files
	cmd.ExtraFiles = mp.slaveExtraFiles
	if err := cmd.Start(); err != nil {
		return fmt.Errorf("Failed to start slave process: %s", err)
	}
	//was scheduled to restart, notify success
	if mp.restarting {
		mp.restartedAt = time.Now()
		mp.restarting = false
		mp.restarted <- true
	}
	//convert wait into channel
	cmdwait := make(chan error)
	go func() {
		cmdwait <- cmd.Wait()
	}()
	//wait....
	select {
	case err := <-cmdwait:
		//program exited before releasing descriptors
		//proxy exit code out to master
		code := 0
		if err != nil {
			code = 1
			if exiterr, ok := err.(*exec.ExitError); ok {
				if status, ok := exiterr.Sys().(syscall.WaitStatus); ok {
					code = status.ExitStatus()
				}
			}
		}
		mp.debugf("prog exited with %d", code)
		//if a restarts are disabled or if it was an
		//unexpected crash, proxy this exit straight
		//through to the main process
		if mp.NoRestart || !mp.restarting {
			os.Exit(code)
		}
	case <-mp.descriptorsReleased:
		//if descriptors are released, the program
		//has yielded control of its sockets and
		//a parallel instance of the program can be
		//started safely. it should serve state.Listeners
		//to ensure downtime is kept at <1sec. The previous
		//cmd.Wait() will still be consumed though the
		//result will be discarded.
	}
	return nil
}

func (sp *slave) run() error {
	sp.id = os.Getenv(envSlaveID)
	sp.debugf("run")
	sp.state.Enabled = true
	sp.state.ID = os.Getenv(envBinID)
	sp.state.StartedAt = time.Now()
	sp.state.Address = sp.Config.Address
	sp.state.Addresses = sp.Config.Addresses
	sp.state.GracefulShutdown = make(chan bool, 1)
	sp.state.BinPath = os.Getenv(envBinPath)
	if err := sp.watchParent(); err != nil {
		return err
	}
	if err := sp.initFileDescriptors(); err != nil {
		return err
	}
	sp.watchSignal()
	//run program with state
	sp.debugf("start program")
	sp.Config.Program(sp.state)
	return nil
}
 
func (sp *slave) initFileDescriptors() error {
	//inspect file descriptors
	numFDs, err := strconv.Atoi(os.Getenv(envNumFDs))
	if err != nil {
		return fmt.Errorf("invalid %s integer", envNumFDs)
	}
	sp.listeners = make([]*overseerListener, numFDs)
	sp.state.Listeners = make([]net.Listener, numFDs)
	for i := 0; i < numFDs; i++ {
		f := os.NewFile(uintptr(3+i), "")
		l, err := net.FileListener(f)
		if err != nil {
			return fmt.Errorf("failed to inherit file descriptor: %d", i)
		}
		u := newOverseerListener(l)
		sp.listeners[i] = u
		sp.state.Listeners[i] = u
	}
	if len(sp.state.Listeners) > 0 {
		sp.state.Listener = sp.state.Listeners[0]
	}
	return nil
}

// convert your 'main()' into a 'prog(state)'
// 'prog()' is run in a child process
func prog(state overseer.State) {
	fmt.Printf("app#%s (%s) listening...\n", BuildID, state.ID)
	http.Handle("/", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		d, _ := time.ParseDuration(r.URL.Query().Get("d"))
		time.Sleep(d)
		fmt.Fprintf(w, "app#%s (%s) %v says hello\n", BuildID, state.ID, state.StartedAt)
	}))
	http.Serve(state.Listener, nil)
	fmt.Printf("app#%s (%s) exiting...\n", BuildID, state.ID)
}
 
// then create another 'main' which runs the upgrades
// 'main()' is run in the initial process
func main() {
	overseer.Run(overseer.Config{
		Program:          prog,
		Address:          ":5001",
		Fetcher:          &fetcher.File{Path: "my_app_next"},
		Debug:            true, //display log of overseer actions
		TerminateTimeout: 10 * time.Minute,
	})
}

// fetchLoop is run in a goroutine
func (mp *master) fetchLoop() {
	min := mp.Config.MinFetchInterval
	time.Sleep(min)
	for {
		t0 := time.Now()
		mp.fetch()
		//duration fetch of fetch
		diff := time.Now().Sub(t0)
		if diff < min {
			delay := min - diff
			//ensures at least MinFetchInterval delay.
			//should be throttled by the fetcher!
			time.Sleep(delay)
		}
	}
}

	//tee off to sha1
	hash := sha1.New()
	reader = io.TeeReader(reader, hash)
	//write to a temp file
	_, err = io.Copy(tmpBin, reader)
	if err != nil {
		mp.warnf("failed to write temp binary: %s", err)
		return
	}
	//compare hash
	newHash := hash.Sum(nil)
	if bytes.Equal(mp.binHash, newHash) {
		mp.debugf("hash match - skip")
		return
	}

	tokenIn := token()
	cmd := exec.Command(tmpBinPath)
	cmd.Env = append(os.Environ(), []string{envBinCheck + "=" + tokenIn}...)
	cmd.Args = os.Args
	returned := false
	go func() {
		time.Sleep(5 * time.Second)
		if !returned {
			mp.warnf("sanity check against fetched executable timed-out, check overseer is running")
			if cmd.Process != nil {
				cmd.Process.Kill()
			}
		}
	}()
	tokenOut, err := cmd.CombinedOutput()
	returned = true
	if err != nil {
		mp.warnf("failed to run temp binary: %s (%s) output \"%s\"", err, tmpBinPath, tokenOut)
		return
	}
	if tokenIn != string(tokenOut) {
		mp.warnf("sanity check failed")
		return
	}

//sanityCheck returns true if a check was performed
func sanityCheck() bool {
	//sanity check
	if token := os.Getenv(envBinCheck); token != "" {
		fmt.Fprint(os.Stdout, token)
		return true
	}
	//legacy sanity check using old env var
	if token := os.Getenv(envBinCheckLegacy); token != "" {
		fmt.Fprint(os.Stdout, token)
		return true
	}
	return false
}

	//overwrite!
	if err := overwrite(mp.binPath, tmpBinPath); err != nil {
		mp.warnf("failed to overwrite binary: %s", err)
		return
	}
	mp.debugf("upgraded binary (%x -> %x)", mp.binHash[:12], newHash[:12])
	mp.binHash = newHash
	//binary successfully replaced
	if !mp.Config.NoRestartAfterFetch {
		mp.triggerRestart()
	}

func (mp *master) triggerRestart() {
	if mp.restarting {
		mp.debugf("already graceful restarting")
		return //skip
	} else if mp.slaveCmd == nil || mp.restarting {
		mp.debugf("no slave process")
		return //skip
	}
	mp.debugf("graceful restart triggered")
	mp.restarting = true
	mp.awaitingUSR1 = true
	mp.signalledAt = time.Now()
	mp.sendSignal(mp.Config.RestartSignal) //ask nicely to terminate
	select {
	case <-mp.restarted:
		//success
		mp.debugf("restart success")
	case <-time.After(mp.TerminateTimeout):
		//times up mr. process, we did ask nicely!
		mp.debugf("graceful timeout, forcing exit")
		mp.sendSignal(os.Kill)
	}
}

		if len(sp.listeners) > 0 {
			//perform graceful shutdown
			for _, l := range sp.listeners {
				l.release(sp.Config.TerminateTimeout)
			}
			//signal release of held sockets, allows master to start
			//a new process before this child has actually exited.
			//early restarts not supported with restarts disabled.
			if !sp.NoRestart {
				sp.masterProc.Signal(SIGUSR1)
			}
			//listeners should be waiting on connections to close...
		}

	//**during a restart** a SIGUSR1 signals
	//to the master process that, the file
	//descriptors have been released
	if mp.awaitingUSR1 && s == SIGUSR1 {
		mp.debugf("signaled, sockets ready")
		mp.awaitingUSR1 = false
		mp.descriptorsReleased <- true
	} else

func (mp *master) fork() error {
    //... ...
    //... ...
    //... ...
	//convert wait into channel
	cmdwait := make(chan error)
	go func() {
		cmdwait <- cmd.Wait()
	}()
	//wait....
	select {
	case err := <-cmdwait:
		//program exited before releasing descriptors
		//proxy exit code out to master
		code := 0
		if err != nil {
			code = 1
			if exiterr, ok := err.(*exec.ExitError); ok {
				if status, ok := exiterr.Sys().(syscall.WaitStatus); ok {
					code = status.ExitStatus()
				}
			}
		}
		mp.debugf("prog exited with %d", code)
		//if a restarts are disabled or if it was an
		//unexpected crash, proxy this exit straight
		//through to the main process
		if mp.NoRestart || !mp.restarting {
			os.Exit(code)
		}
	case <-mp.descriptorsReleased:
		//if descriptors are released, the program
		//has yielded control of its sockets and
		//a parallel instance of the program can be
		//started safely. it should serve state.Listeners
		//to ensure downtime is kept at <1sec. The previous
		//cmd.Wait() will still be consumed though the
		//result will be discarded.
	}
	return nil
}