Move 'auth' to the registry subsystem

This is the first step towards separating the registry subsystem from
the deprecated `Server` object.

* New service `github.com/dotcloud/docker/registry/Service`
* The service is installed by default in `builtins`
* The service only exposes `auth` for now...
* ...Soon to be followed by `pull`, `push` and `search`.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

docs/service.go

@@ -0,0 +1,54 @@
+package registry
+
+import (
+	"github.com/dotcloud/docker/engine"
+)
+
+// Service exposes registry capabilities in the standard Engine
+// interface. Once installed, it extends the engine with the
+// following calls:
+//
+//  'auth': Authenticate against the public registry
+//  'search': Search for images on the public registry (TODO)
+//  'pull': Download images from any registry (TODO)
+//  'push': Upload images to any registry (TODO)
+type Service struct {
+}
+
+// NewService returns a new instance of Service ready to be
+// installed no an engine.
+func NewService() *Service {
+	return &Service{}
+}
+
+// Install installs registry capabilities to eng.
+func (s *Service) Install(eng *engine.Engine) error {
+	eng.Register("auth", s.Auth)
+	return nil
+}
+
+// Auth contacts the public registry with the provided credentials,
+// and returns OK if authentication was sucessful.
+// It can be used to verify the validity of a client's credentials.
+func (s *Service) Auth(job *engine.Job) engine.Status {
+	var (
+		err        error
+		authConfig = &AuthConfig{}
+	)
+
+	job.GetenvJson("authConfig", authConfig)
+	// TODO: this is only done here because auth and registry need to be merged into one pkg
+	if addr := authConfig.ServerAddress; addr != "" && addr != IndexServerAddress() {
+		addr, err = ExpandAndVerifyRegistryUrl(addr)
+		if err != nil {
+			return job.Error(err)
+		}
+		authConfig.ServerAddress = addr
+	}
+	status, err := Login(authConfig, HTTPRequestFactory(nil))
+	if err != nil {
+		return job.Error(err)
+	}
+	job.Printf("%s\n", status)
+	return engine.StatusOK
+}