Move registry package out of repo root

Since the repo is no longer just the registry, we are moving the registry web
application package out of the repo root into a sub-package. We may break down
the registry package further to separate webapp components and bring the client
package under it. This change accomplishes the task of freeing up the repo root
for a distribution-oriented package. A stub doc.go file is left in place to
declare intent.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/layer.go

@@ -0,0 +1,62 @@
+package registry
+
+import (
+	"net/http"
+
+	"github.com/docker/distribution/api/v2"
+	"github.com/docker/distribution/digest"
+	"github.com/docker/distribution/storage"
+	"github.com/gorilla/handlers"
+)
+
+// layerDispatcher uses the request context to build a layerHandler.
+func layerDispatcher(ctx *Context, r *http.Request) http.Handler {
+	dgst, err := digest.ParseDigest(ctx.vars["digest"])
+
+	if err != nil {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			ctx.Errors.Push(v2.ErrorCodeDigestInvalid, err)
+		})
+	}
+
+	layerHandler := &layerHandler{
+		Context: ctx,
+		Digest:  dgst,
+	}
+
+	layerHandler.log = layerHandler.log.WithField("digest", dgst)
+
+	return handlers.MethodHandler{
+		"GET":  http.HandlerFunc(layerHandler.GetLayer),
+		"HEAD": http.HandlerFunc(layerHandler.GetLayer),
+	}
+}
+
+// layerHandler serves http layer requests.
+type layerHandler struct {
+	*Context
+
+	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 := err.(type) {
+		case storage.ErrUnknownLayer:
+			w.WriteHeader(http.StatusNotFound)
+			lh.Errors.Push(v2.ErrorCodeBlobUnknown, err.FSLayer)
+		default:
+			lh.Errors.Push(v2.ErrorCodeUnknown, err)
+		}
+		return
+	}
+	defer layer.Close()
+
+	http.ServeContent(w, r, layer.Digest().String(), layer.CreatedAt(), layer)
+}