Allow kubectl expose to be polymorphic to the source of the selector

Allows exposing new resource types to be exposed (OpenShift deployment
controllers, copying services, and other resources that will have
pod label selectors).

Also fixed a bug where the selector wasn't exposed because of parameter
defaulting.

pkg/kubectl/cmd/cmd.go

@@ -45,6 +45,8 @@ const (
 // Factory provides abstractions that allow the Kubectl command to be extended across multiple types
 // of resources and different API sets.
 // TODO: make the functions interfaces
+// TODO: pass the various interfaces on the factory directly into the command constructors (so the
+// commands are decoupled from the factory).
 type Factory struct {
 	clients *clientCache
 	flags   *pflag.FlagSet
@@ -66,6 +68,9 @@ type Factory struct {
 	Resizer func(mapping *meta.RESTMapping) (kubectl.Resizer, error)
 	// Returns a Reaper for gracefully shutting down resources.
 	Reaper func(mapping *meta.RESTMapping) (kubectl.Reaper, error)
+	// PodSelectorForResource returns the pod selector associated with the provided resource name
+	// or an error.
+	PodSelectorForResource func(mapping *meta.RESTMapping, namespace, name string) (string, error)
 	// Returns a schema that can validate objects stored on disk.
 	Validator func() (validation.Schema, error)
 	// Returns the default namespace to use in cases where no other namespace is specified
@@ -128,6 +133,41 @@ func NewFactory(optionalClientConfig clientcmd.ClientConfig) *Factory {
 		Printer: func(mapping *meta.RESTMapping, noHeaders bool) (kubectl.ResourcePrinter, error) {
 			return kubectl.NewHumanReadablePrinter(noHeaders), nil
 		},
+		PodSelectorForResource: func(mapping *meta.RESTMapping, namespace, name string) (string, error) {
+			// TODO: replace with a swagger schema based approach (identify pod selector via schema introspection)
+			client, err := clients.ClientForVersion("")
+			if err != nil {
+				return "", err
+			}
+			switch mapping.Kind {
+			case "ReplicationController":
+				rc, err := client.ReplicationControllers(namespace).Get(name)
+				if err != nil {
+					return "", err
+				}
+				return kubectl.MakeLabels(rc.Spec.Selector), nil
+			case "Pod":
+				rc, err := client.Pods(namespace).Get(name)
+				if err != nil {
+					return "", err
+				}
+				if len(rc.Labels) == 0 {
+					return "", fmt.Errorf("the pod has no labels and cannot be exposed")
+				}
+				return kubectl.MakeLabels(rc.Labels), nil
+			case "Service":
+				rc, err := client.ReplicationControllers(namespace).Get(name)
+				if err != nil {
+					return "", err
+				}
+				if rc.Spec.Selector == nil {
+					return "", fmt.Errorf("the service has no pod selector set")
+				}
+				return kubectl.MakeLabels(rc.Spec.Selector), nil
+			default:
+				return "", fmt.Errorf("it is not possible to get a pod selector from %s", mapping.Kind)
+			}
+		},
 		Resizer: func(mapping *meta.RESTMapping) (kubectl.Resizer, error) {
 			client, err := clients.ClientForVersion(mapping.APIVersion)
 			if err != nil {