Add support for pluggable Docker exec handlers

Add support for pluggable Docker exec handlers. The default handler is
now Docker's native exec API call. The previous default, nsenter, can be
selected by passing --docker-exec-handler=nsenter when starting the
kubelet.

cmd/kubelet/app/server.go

@@ -111,6 +111,7 @@ type KubeletServer struct {
 	SystemContainer                string
 	ConfigureCBR0                  bool
 	MaxPods                        int
+	DockerExecHandlerName          string
 
 	// Flags intended for testing
 
@@ -173,6 +174,7 @@ func NewKubeletServer() *KubeletServer {
 		DockerDaemonContainer:       "/docker-daemon",
 		SystemContainer:             "",
 		ConfigureCBR0:               false,
+		DockerExecHandlerName:       "native",
 	}
 }
 
@@ -233,6 +235,7 @@ func (s *KubeletServer) AddFlags(fs *pflag.FlagSet) {
 	fs.StringVar(&s.SystemContainer, "system-container", s.SystemContainer, "Optional resource-only container in which to place all non-kernel processes that are not already in a container. Empty for no container. Rolling back the flag requires a reboot. (Default: \"\").")
 	fs.BoolVar(&s.ConfigureCBR0, "configure-cbr0", s.ConfigureCBR0, "If true, kubelet will configure cbr0 based on Node.Spec.PodCIDR.")
 	fs.IntVar(&s.MaxPods, "max-pods", 100, "Number of Pods that can run on this Kubelet.")
+	fs.StringVar(&s.DockerExecHandlerName, "docker-exec-handler", s.DockerExecHandlerName, "Handler to use when executing a command in a container. Valid values are 'native' and 'nsenter'. Defaults to 'native'.")
 
 	// Flags intended for testing, not recommended used in production environments.
 	fs.BoolVar(&s.ReallyCrashForTesting, "really-crash-for-testing", s.ReallyCrashForTesting, "If true, when panics occur crash. Intended for testing.")
@@ -306,6 +309,17 @@ func (s *KubeletServer) Run(_ []string) error {
 		mounter = &mount.NsenterMounter{}
 	}
 
+	var dockerExecHandler dockertools.ExecHandler
+	switch s.DockerExecHandlerName {
+	case "native":
+		dockerExecHandler = &dockertools.NativeExecHandler{}
+	case "nsenter":
+		dockerExecHandler = &dockertools.NsenterExecHandler{}
+	default:
+		glog.Warningf("Unknown Docker exec handler %q; defaulting to native", s.DockerExecHandlerName)
+		dockerExecHandler = &dockertools.NativeExecHandler{}
+	}
+
 	kcfg := KubeletConfig{
 		Address:                        s.Address,
 		AllowPrivileged:                s.AllowPrivileged,
@@ -352,6 +366,7 @@ func (s *KubeletServer) Run(_ []string) error {
 		SystemContainer:           s.SystemContainer,
 		ConfigureCBR0:             s.ConfigureCBR0,
 		MaxPods:                   s.MaxPods,
+		DockerExecHandler:         dockerExecHandler,
 	}
 
 	if err := RunKubelet(&kcfg, nil); err != nil {
@@ -518,6 +533,7 @@ func SimpleKubelet(client *client.Client,
 		DockerDaemonContainer:     "/docker-daemon",
 		SystemContainer:           "",
 		MaxPods:                   32,
+		DockerExecHandler:         &dockertools.NativeExecHandler{},
 	}
 	return &kcfg
 }
@@ -655,6 +671,7 @@ type KubeletConfig struct {
 	SystemContainer                string
 	ConfigureCBR0                  bool
 	MaxPods                        int
+	DockerExecHandler              dockertools.ExecHandler
 }
 
 func createAndInitKubelet(kc *KubeletConfig) (k KubeletBootstrap, pc *config.PodConfig, err error) {
@@ -708,7 +725,8 @@ func createAndInitKubelet(kc *KubeletConfig) (k KubeletBootstrap, pc *config.Pod
 		kc.DockerDaemonContainer,
 		kc.SystemContainer,
 		kc.ConfigureCBR0,
-		kc.MaxPods)
+		kc.MaxPods,
+		kc.DockerExecHandler)
 
 	if err != nil {
 		return nil, nil, err