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

storage/layerhandler.go

@@ -0,0 +1,50 @@
+package storage
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/docker/distribution/storagedriver"
+)
+
+// LayerHandler provides middleware for serving the contents of a Layer.
+type LayerHandler interface {
+	// Resolve returns an http.Handler which can serve the contents of a given
+	// Layer if possible, or nil and an error when unsupported. This may
+	// directly serve the contents of the layer or issue a redirect to another
+	// URL hosting the content.
+	Resolve(layer Layer) (http.Handler, error)
+}
+
+// LayerHandlerInitFunc is the type of a LayerHandler factory function and is
+// used to register the contsructor for different LayerHandler backends.
+type LayerHandlerInitFunc func(storageDriver storagedriver.StorageDriver, options map[string]interface{}) (LayerHandler, error)
+
+var layerHandlers map[string]LayerHandlerInitFunc
+
+// RegisterLayerHandler is used to register an LayerHandlerInitFunc for
+// a LayerHandler backend with the given name.
+func RegisterLayerHandler(name string, initFunc LayerHandlerInitFunc) error {
+	if layerHandlers == nil {
+		layerHandlers = make(map[string]LayerHandlerInitFunc)
+	}
+	if _, exists := layerHandlers[name]; exists {
+		return fmt.Errorf("name already registered: %s", name)
+	}
+
+	layerHandlers[name] = initFunc
+
+	return nil
+}
+
+// GetLayerHandler constructs a LayerHandler
+// with the given options using the named backend.
+func GetLayerHandler(name string, options map[string]interface{}, storageDriver storagedriver.StorageDriver) (LayerHandler, error) {
+	if layerHandlers != nil {
+		if initFunc, exists := layerHandlers[name]; exists {
+			return initFunc(storageDriver, options)
+		}
+	}
+
+	return nil, fmt.Errorf("no layer handler registered with name: %s", name)
+}