Manifest and layer soft deletion.

Implement the delete API by implementing soft delete for layers
and blobs by removing link files and updating the blob descriptor
cache.  Deletion is configurable - if it is disabled API calls
will return an unsupported error.

We invalidate the blob descriptor cache by changing the linkedBlobStore's
blobStatter to a blobDescriptorService and naming it blobAccessController.

Delete() is added throughout the relevant API to support this functionality.

Signed-off-by: Richard Scothern <richard.scothern@gmail.com>

docs/client/repository.go

@@ -354,7 +354,7 @@ func (ms *manifests) Delete(dgst digest.Digest) error {
 	defer resp.Body.Close()
 
 	switch resp.StatusCode {
-	case http.StatusOK:
+	case http.StatusAccepted:
 		return nil
 	default:
 		return handleErrorResponse(resp)
@@ -366,7 +366,8 @@ type blobs struct {
 	ub     *v2.URLBuilder
 	client *http.Client
 
-	statter distribution.BlobStatter
+	statter distribution.BlobDescriptorService
+	distribution.BlobDeleter
 }
 
 func sanitizeLocation(location, source string) (string, error) {
@@ -484,6 +485,10 @@ func (bs *blobs) Resume(ctx context.Context, id string) (distribution.BlobWriter
 	panic("not implemented")
 }
 
+func (bs *blobs) Delete(ctx context.Context, dgst digest.Digest) error {
+	return bs.statter.Clear(ctx, dgst)
+}
+
 type blobStatter struct {
 	name   string
 	ub     *v2.URLBuilder
@@ -535,3 +540,32 @@ func buildCatalogValues(maxEntries int, last string) url.Values {
 
 	return values
 }
+
+func (bs *blobStatter) Clear(ctx context.Context, dgst digest.Digest) error {
+	blobURL, err := bs.ub.BuildBlobURL(bs.name, dgst)
+	if err != nil {
+		return err
+	}
+
+	req, err := http.NewRequest("DELETE", blobURL, nil)
+	if err != nil {
+		return err
+	}
+
+	resp, err := bs.client.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	switch resp.StatusCode {
+	case http.StatusAccepted:
+		return nil
+	default:
+		return handleErrorResponse(resp)
+	}
+}
+
+func (bs *blobStatter) SetDescriptor(ctx context.Context, dgst digest.Digest, desc distribution.Descriptor) error {
+	return nil
+}