// containerd-shim is a small shim that sits in front of a runtime implementation that allows it to be reparented to init and handle reattach from the caller.
// the cwd of the shim should be the path to the state directory where the shim can locate fifos and other information\
// Arg0: id of the container
// Arg1: bundle path
// Arg2: runtime binary
在啟動容器時，containerd-shim的命令行格式如下所示：
docker-containerd-shim 9decb150527a3b64a86b85cfc6faeb09f786dcb2f4e668611418280c75755539
/var/run/docker/libcontainerd/9decb150527a3b64a86b85cfc6faeb09f786dcb2f4e668611418280c75755539 docker-runc
1、containerd/containerd-shim/main.go
func main()
（1）、創建log文件，f, err := os.OpenFile(filepath.Join(cwd, "shim-log.json") ...)
（2）、調用err := start(f)函數，若err不為nil，當err為errRuntime的時候，直接關閉f並返回，否則將錯誤記錄到shim-log.json中
2、containerd/containerd-shim/main.go
// start handling signals as soon as possible so that things are properly reaped
// or if runtime exits before we hit the handler
func start(log *os.File) error
（1）、set the shim as the subreaper for all orphaned processes created by the container，err := osutils.SetSubreaper(1)
（2）、打開exit pipe和control pipe
（3）、調用p, err := newProcess(flag.Arg(0), flag.Arg(1), flag.Arg(2))，加載process實例，再調用p.create()
（4）、msgC := make(chan controlMesage, 32)，創建一個goroutine，從control pipe中不斷讀取controlMessage
（5）、最后，一個無限for循環，對來自signal的信號和controlMessage進行處理
（6）、當從signal中獲得的信號為SIGCHLD時，當退出的進程為runtime時，退出shim
（7）、對來此control pipe的controlMessage進行處理，當msg的Type為0時，關閉stdin，當Type為1時，且p.console不為nil，則調整tty的窗口大小
3、containerd/containerd-shim/process.go
func newProcess(id, bundle, runtimeName string)
（1）、p := &process{id: id, bundle: bundle, runtime: runtimeName}
（2）、調用s, err := loadProcess()，從process.json文件中加載state，p.state = s
（3）、調用p.openIO()，最后，返回p
4、containerd/containerd-shim/process.go
// openIO opens the pre-created fifo's for use with the container in RDWR so that they remain open if the other side stops listening
func (p *process) openIO() error
（1）、先創建一個goroutine，打開p.state.Stdin, p.stdinCloser = os.openFile(p.state.Stdin, syscall.O_WRONLY)
// NewConsole returns an initialized console that can be used within a container by copying bytes from the master side to the slave that is attached as the tty for the container's init process
（2）、如果p.state.Terminal為true，調用master, console, err := newConsole(uid, gid)，p.console = master, p.consolePath = console，再打開p.state.Stdin和p.state.Stdout，最后調用io.Copy將stdin/stdout與master相連
（3）、對於非tty的情況，調用i, err := p.initializeIO(uid)，p.shimIO = i，再打開p.state.Stdout和p.state.Stderr（方式為可讀寫）分別與i.Stdou和i.Stderr相連。接着打開p.state.Stdin為只讀模式，再將i.Stdin和p.state.Stdin相連
5、containerd/containerd-shim/process.go
func (p *process) initializeIO(rootuid int)
該函數生成三個os.Pipe()並分別將一端賦給i = &IO{}，再將另一端賦給p.stdio
6、containerd/containerd-shim/process.go
func (p *process) create() error
（1）、獲取當前目錄，創建logPath := filepath.Join(cwd, "log.json")，再擴展args := append([]string{"--log", logPath, "--log-format", "json"}, p.state.RuntimeArgs...)
（2）、對exec，checkpoint，create進行不同的處理，這里只討論create的情況，繼續擴展args = append(args, "create", "--bundle", p.bundle, "--console", p.consolepath)
（3）、擴展參數--pid-file，cmd := exec.Command(p.runtime, args...)，並且將cmd的stdio設置為p.stdio
（4）、調用cmd.Start()，再調用p.stdio.stdout.Close()和p.stdio.stderr.Close()(why????)
（5）、cmd.Wait()，從pid文件中讀出pid，並且將p.containerPid 設置為pid