Initial implementation of Layer API

The http API has its first set of endpoints to implement the core aspects of
fetching and uploading layers. Uploads can be started and completed in a single
chunk and the content can be fetched via tarsum. Most proposed error conditions
should be represented but edge cases likely remain.

In this version, note that the layers are still called layers, even though the
routes are pointing to blobs. This will change with backend refactoring over
the next few weeks.

The unit tests are a bit of a shamble but these need to be carefully written
along with the core specification process. As the the client-server interaction
solidifies, we can port this into a verification suite for registry providers.

layer.go

@@ -3,17 +3,28 @@ package registry
 import (
 	"net/http"
 
+	"github.com/docker/docker-registry/digest"
+	"github.com/docker/docker-registry/storage"
 	"github.com/gorilla/handlers"
+	"github.com/gorilla/mux"
 )
 
 // layerDispatcher uses the request context to build a layerHandler.
 func layerDispatcher(ctx *Context, r *http.Request) http.Handler {
-	layerHandler := &layerHandler{
-		Context: ctx,
-		TarSum:  ctx.vars["tarsum"],
+	dgst, err := digest.ParseDigest(ctx.vars["digest"])
+
+	if err != nil {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			ctx.Errors.Push(ErrorCodeInvalidDigest, err)
+		})
 	}
 
-	layerHandler.log = layerHandler.log.WithField("tarsum", layerHandler.TarSum)
+	layerHandler := &layerHandler{
+		Context: ctx,
+		Digest:  dgst,
+	}
+
+	layerHandler.log = layerHandler.log.WithField("digest", dgst)
 
 	return handlers.MethodHandler{
 		"GET":  http.HandlerFunc(layerHandler.GetLayer),
@@ -25,11 +36,44 @@ func layerDispatcher(ctx *Context, r *http.Request) http.Handler {
 type layerHandler struct {
 	*Context
 
-	TarSum string
+	Digest digest.Digest
 }
 
 // GetLayer fetches the binary data from backend storage returns it in the
 // response.
 func (lh *layerHandler) GetLayer(w http.ResponseWriter, r *http.Request) {
+	layers := lh.services.Layers()
 
+	layer, err := layers.Fetch(lh.Name, lh.Digest)
+
+	if err != nil {
+		switch err {
+		case storage.ErrLayerUnknown:
+			w.WriteHeader(http.StatusNotFound)
+			lh.Errors.Push(ErrorCodeUnknownLayer,
+				map[string]interface{}{
+					"unknown": FSLayer{BlobSum: lh.Digest},
+				})
+			return
+		default:
+			lh.Errors.Push(ErrorCodeUnknown, err)
+			return
+		}
+	}
+	defer layer.Close()
+
+	http.ServeContent(w, r, layer.Digest().String(), layer.CreatedAt(), layer)
+}
+
+func buildLayerURL(router *mux.Router, r *http.Request, layer storage.Layer) (string, error) {
+	route := clonedRoute(router, routeNameBlob)
+
+	layerURL, err := route.Schemes(r.URL.Scheme).Host(r.Host).
+		URL("name", layer.Name(),
+		"digest", layer.Digest().String())
+	if err != nil {
+		return "", err
+	}
+
+	return layerURL.String(), nil
 }