Refactoring cloudfactory layer handler into a more generic storage

middleware concept.

This also breaks the dependency the storage package had on goamz
Signed-off-by: David Lawrence <david.lawrence@docker.com> (github: endophage)

registry/storage/layerreader.go

@@ -1,13 +1,14 @@
 package storage
 
 import (
+	"net/http"
 	"time"
 
 	"github.com/docker/distribution"
 	"github.com/docker/distribution/digest"
 )
 
-// layerReadSeeker implements Layer and provides facilities for reading and
+// LayerRead implements Layer and provides facilities for reading and
 // seeking.
 type layerReader struct {
 	fileReader
@@ -17,6 +18,10 @@ type layerReader struct {
 
 var _ distribution.Layer = &layerReader{}
 
+func (lrs *layerReader) Path() string {
+	return lrs.path
+}
+
 func (lrs *layerReader) Digest() digest.Digest {
 	return lrs.digest
 }
@@ -33,3 +38,12 @@ func (lrs *layerReader) CreatedAt() time.Time {
 func (lrs *layerReader) Close() error {
 	return lrs.closeWithErr(distribution.ErrLayerClosed)
 }
+
+func (lrs *layerReader) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Docker-Content-Digest", lrs.digest.String())
+
+	if url, err := lrs.fileReader.driver.URLFor(lrs.Path(), map[string]interface{}{}); err == nil {
+		http.Redirect(w, r, url, http.StatusTemporaryRedirect)
+	}
+	http.ServeContent(w, r, lrs.Digest().String(), lrs.CreatedAt(), lrs)
+}