add support for container ID files (a la pidfile)

commands.go

@@ -1341,6 +1341,20 @@ func (cli *DockerCli) CmdRun(args ...string) error {
 	for _, warning := range runResult.Warnings {
 		fmt.Fprintf(cli.err, "WARNING: %s\n", warning)
 	}
+	if len(hostConfig.ContainerIDFile) > 0 {
+		if _, err := ioutil.ReadFile(hostConfig.ContainerIDFile); err == nil {
+			return fmt.Errorf("cid file found, make sure the other container isn't running or delete %s", hostConfig.ContainerIDFile)
+		}
+		file, err := os.Create(hostConfig.ContainerIDFile)
+		if err != nil {
+			return fmt.Errorf("failed to create the container ID file: %s", err)
+		}
+
+		defer file.Close()
+		if _, err = file.WriteString(runResult.ID); err != nil {
+			return fmt.Errorf("failed to write the container ID to the file: %s", err)
+		}
+	}
 
 	//start the container
 	if _, _, err = cli.call("POST", "/containers/"+runResult.ID+"/start", hostConfig); err != nil {

container.go

@@ -80,7 +80,8 @@ type Config struct {
 }
 
 type HostConfig struct {
-	Binds []string
+	Binds           []string
+	ContainerIDFile string
 }
 
 type BindMap struct {
@@ -103,6 +104,7 @@ func ParseRun(args []string, capabilities *Capabilities) (*Config, *HostConfig,
 	flStdin := cmd.Bool("i", false, "Keep stdin open even if not attached")
 	flTty := cmd.Bool("t", false, "Allocate a pseudo-tty")
 	flMemory := cmd.Int64("m", 0, "Memory limit (in bytes)")
+	flContainerIDFile := cmd.String("cidfile", "", "Write the container ID to the file")
 
 	if capabilities != nil && *flMemory > 0 && !capabilities.MemoryLimit {
 		//fmt.Fprintf(stdout, "WARNING: Your kernel does not support memory limit capabilities. Limitation discarded.\n")
@@ -190,7 +192,8 @@ func ParseRun(args []string, capabilities *Capabilities) (*Config, *HostConfig,
 		Entrypoint:   entrypoint,
 	}
 	hostConfig := &HostConfig{
-		Binds: binds,
+		Binds:           binds,
+		ContainerIDFile: *flContainerIDFile,
 	}
 
 	if capabilities != nil && *flMemory > 0 && !capabilities.SwapLimit {