Adds support for content redirects for layer downloads

Includes a delegate implementation which redirects to the URL generated
by the storagedriver, and a cloudfront implementation.
Satisfies proposal #49

docs/app.go

@@ -31,6 +31,8 @@ type App struct {
 
 	tokenProvider tokenProvider
 
+	layerHandler storage.LayerHandler
+
 	accessController auth.AccessController
 }
 
@@ -76,6 +78,16 @@ func NewApp(configuration configuration.Configuration) *App {
 		app.accessController = accessController
 	}
 
+	layerHandlerType := configuration.HTTP.LayerHandler.Type()
+
+	if layerHandlerType != "" {
+		lh, err := storage.GetLayerHandler(layerHandlerType, configuration.HTTP.LayerHandler.Parameters(), driver)
+		if err != nil {
+			panic(fmt.Sprintf("unable to configure layer handler (%s): %v", layerHandlerType, err))
+		}
+		app.layerHandler = lh
+	}
+
 	return app
 }
 

docs/layer.go

@@ -58,5 +58,11 @@ func (lh *layerHandler) GetLayer(w http.ResponseWriter, r *http.Request) {
 	}
 	defer layer.Close()
 
+	handler, err := lh.layerHandler.Resolve(layer)
+	if handler != nil {
+		handler.ServeHTTP(w, r)
+		return
+	}
+
 	http.ServeContent(w, r, layer.Digest().String(), layer.CreatedAt(), layer)
 }