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overseer主要完成了三部分功能：

1、连接的无损关闭，2、连接的平滑重启，3、文件变更的自动重启。

下面依次讲一下：

一、连接的无损关闭

golang官方的net包是不支持连接的无损关闭的，当主监听协程退出时，并不会等待各个实际work协程的处理完成。

以下是golang官方代码：

Go/src/net/http/server.go

func (srv *Server) Serve(l net.Listener) error {if fn := testHookServerServe; fn != nil {fn(srv, l) // call hook with unwrapped listener}origListener := ll = &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 failurectx := 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 := ctxif cc := srv.ConnContext; cc != nil {connCtx = cc(connCtx, rw)if connCtx == nil {panic("ConnContext returned nil")}}tempDelay = 0c := srv.newConn(rw)c.setState(c.rwc, StateNew, runHooks) // before Serve can returngo c.serve(connCtx)}
}

当监听套接字关闭，l.Accept()退出循环时，并不会等待go c.serve(connCtx)协程的处理完成。

overseer的处理方式是，包装了golang的监听套接字和连接套接字，通过sync.WaitGroup提供了对主协程异步等待work协程处理完成的支持。

overseer代码如下：

overseer-v1.1.6\graceful.go

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.tcpKeepAliveListenerconn.SetKeepAlivePeriod(3 * time.Minute) // see http.tcpKeepAliveListeneruconn := overseerConn{Conn:   conn,wg:     &l.wg,closed: make(chan bool),}go func() {//connection watcherselect {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 fdl.closeError = l.Listener.Close()//start timer, close by force if deadline not metwaited := 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
}

在(l *overseerListener) Accept函数中，每生成一个work连接，执行l.wg.Add(1)，在(o overseerConn) Close函数中，每关闭一个work连接，执行o.wg.Done()。

在异步关闭模式(l *overseerListener) release函数中和在同步关闭模式(l *overseerListener) Close函数中都会调用l.wg.Wait()以等待work协程的处理完成。

监听套接字关闭流程：

1、work进程收到重启信号，或者master进程收到重启信号然后转发到work进程。

2、work进程的信号处理里包含对(l *overseerListener) release的调用。

3、在(l *overseerListener) release里关闭监听套接字，并异步l.wg.Wait()。

4、在官方包net/http/server.go的 (srv *Server) Serve里l.Accept()出错返回，退出监听循环，然后执行defer l.Close()，即(l *overseerListener) Close。

5、在(l *overseerListener) Close里同步执行l.wg.Wait()，等待work连接处理完成。

6、work连接处理完成时，会调用(o overseerConn) Close()，进而调用o.wg.Done()。

7、所有work连接处理完成后，向master进程发送SIGUSR1信号。

8、master进程收到SIGUSR1信号后，将true写入mp.descriptorsReleased管道。

9、master进程的(mp *master) fork里，收到mp.descriptorsReleased后，结束本次fork，进入下一次fork。

二、连接的平滑重启

所谓平滑重启，就是重启不会造成客户端的断连，对客户端无感知，比如原有的排队连接不会被丢弃，所以监听套接字通过master进程在新旧work进程间传递，而不是新启的work进程重新创建监听连接。

监听套接字由master进程创建：

overseer-v1.1.6/proc_master.go

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
}

从mp.Config.Addresses中拿到地址，建立监听连接，最后把文件句柄存入mp.slaveExtraFiles。

在这个过程中调用了(l *TCPListener) Close，但其实对work进程无影响，影响的只是master进程自己不能读写监听套接字。

这里引用下对网络套接字close和shutdown的区别：

close ---- 关闭本进程的socket id，但连接还是开着的，用这个socket id的其它进程还能用这个连接，能读或写这个socket id。
shutdown ---- 则破坏了socket 连接，读的时候可能侦探到EOF结束符，写的时候可能会收到一个SIGPIPE信号，这个信号可能直到socket buffer被填充了才收到，shutdown还有一个关闭方式的参数，0 不能再读，1不能再写，2 读写都不能。

将mp.slaveExtraFiles传递给子进程即work进程：

overseer-v1.1.6/proc_master.go

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 = cmdmp.slaveID++//provide the slave process with some statee := 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/stdfilescmd.Args = os.Argscmd.Stdin = os.Stdincmd.Stdout = os.Stdoutcmd.Stderr = os.Stderr//include socket filescmd.ExtraFiles = mp.slaveExtraFilesif err := cmd.Start(); err != nil {return fmt.Errorf("Failed to start slave process: %s", err)}//was scheduled to restart, notify successif mp.restarting {mp.restartedAt = time.Now()mp.restarting = falsemp.restarted <- true}//convert wait into channelcmdwait := make(chan error)go func() {cmdwait <- cmd.Wait()}()//wait....select {case err := <-cmdwait://program exited before releasing descriptors//proxy exit code out to mastercode := 0if err != nil {code = 1if 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 processif 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
}

通过cmd.ExtraFiles = mp.slaveExtraFiles语句向子进程传递套接字，这个参数最终传递给fork系统调用，传递的fd会被子进程继承。

子进程即work进程处理继承的套接字：

overseer-v1.1.6/proc_slave.go

func (sp *slave) run() error {sp.id = os.Getenv(envSlaveID)sp.debugf("run")sp.state.Enabled = truesp.state.ID = os.Getenv(envBinID)sp.state.StartedAt = time.Now()sp.state.Address = sp.Config.Addresssp.state.Addresses = sp.Config.Addressessp.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 statesp.debugf("start program")sp.Config.Program(sp.state)return nil
}func (sp *slave) initFileDescriptors() error {//inspect file descriptorsnumFDs, 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] = usp.state.Listeners[i] = u}if len(sp.state.Listeners) > 0 {sp.state.Listener = sp.state.Listeners[0]}return nil
}

子进程只是重新包装套接字，并没有新建监听连接，包装成u := newOverseerListener(l)类型，这些监听套接字最后传递给sp.Config.Program(sp.state)，即用户的启动程序：

overseer-v1.1.6/example/main.go

// 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 actionsTerminateTimeout: 10 * time.Minute,})
}

在用户程序中http.Serve(state.Listener, nil)调用：

1、使用的accept方式是包装后的(l *overseerListener) Accept()。

2、defer l.Close()使用也是包装后的(l *overseerListener) Close()。

3、由(l *overseerListener) Accept()创建的work连接也都包装成了overseerConn连接，在关闭时会调用(o overseerConn) Close()

三、文件变更的自动重启

能够自动监视文件变化，有变更时自动触发重启流程。

在master进程启动时检查配置，如果设置了mp.Config.Fetcher则进入fetchLoop：

overseer-v1.1.6/proc_master.go

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

mp.Config.MinFetchInterval默认是1秒，也就是每秒检查一次变更。time.Duration类型，可以设置更小的粒度。

已经支持的fetcher包括：fetcher_file.go、fetcher_github.go、fetcher_http.go、fetcher_s3.go。

以fetcher_file.go为例说明。

1、文件变更的判断：

overseer-v1.1.6/proc_master.go

	//tee off to sha1hash := 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 hashnewHash := hash.Sum(nil)if bytes.Equal(mp.binHash, newHash) {mp.debugf("hash match - skip")return}

通过sha1算法实现，比较新旧hash值，并没有关注文件时间戳。

2、验证是可执行文件，且是支持overseer的：

overseer-v1.1.6/proc_master.go

	tokenIn := token()cmd := exec.Command(tmpBinPath)cmd.Env = append(os.Environ(), []string{envBinCheck + "=" + tokenIn}...)cmd.Args = os.Argsreturned := falsego 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 = trueif 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}

这是通过overseer预埋的代码实现的：

overseer-v1.1.6/overseer.go

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

这段代码在main启动时在overseer.Run里会调用到，传递固定的环境变量，然后命令行输出会原样显示出来即为成功。

3、覆盖旧文件，并触发重启。

overseer-v1.1.6/proc_master.go

	//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 replacedif !mp.Config.NoRestartAfterFetch {mp.triggerRestart()}

由(mp *master) triggerRestart进入重启流程：

overseer-v1.1.6/proc_master.go

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 = truemp.awaitingUSR1 = truemp.signalledAt = time.Now()mp.sendSignal(mp.Config.RestartSignal) //ask nicely to terminateselect {case <-mp.restarted://successmp.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)}
}

向子进程发送mp.Config.RestartSignal信号，子进程收到信号后，关闭监听套接字然后向父进程发送SIGUSR1信号：

overseer-v1.1.6/proc_slave.go

		if len(sp.listeners) > 0 {//perform graceful shutdownfor _, 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...}

父进程收到SIGUSR1信号后，通知mp.descriptorsReleased管道监听套接字已经关闭：

overseer-v1.1.6/proc_master.go

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

最终回到(mp *master) fork函数，fork函数一直在等待mp.descriptorsReleased通知或者cmd.Wait子进程退出，收到管道通知后fork退出，进入下一轮fork循环。

overseer-v1.1.6/proc_master.go

func (mp *master) fork() error {//... ...//... ...//... ...//convert wait into channelcmdwait := make(chan error)go func() {cmdwait <- cmd.Wait()}()//wait....select {case err := <-cmdwait://program exited before releasing descriptors//proxy exit code out to mastercode := 0if err != nil {code = 1if 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 processif 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
}

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