Fix signature handling with GC.

If a schema 1 manifest is uploaded with the `disablesignaturestore` option set
to true, then no signatures will exist.  Handle this case.

If a schema 1 manifest is pushed, deleted, garbage collected and pushed again, the
repository will contain signature links from the first version, but the blobs will
not exist.  Disable the signature store in the garbage-collect command so
signatures are not fetched.

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

registry/storage/manifeststore.go

@@ -12,6 +12,7 @@ import (
 	"github.com/docker/distribution/manifest/manifestlist"
 	"github.com/docker/distribution/manifest/schema1"
 	"github.com/docker/distribution/manifest/schema2"
+	"github.com/docker/distribution/registry/storage/driver"
 )
 
 // A ManifestHandler gets and puts manifests of a particular type.
@@ -161,13 +162,20 @@ func (ms *manifestStore) GetSignatures(ctx context.Context, manifestDigest diges
 		return nil, err
 	}
 
+	var digests []digest.Digest
 	alg := string(digest.SHA256)
 	signaturePaths, err := ms.blobStore.driver.List(ctx, path.Join(signaturesPath, alg))
-	if err != nil {
+
+	switch err.(type) {
+	case nil:
+		break
+	case driver.PathNotFoundError:
+		// Manifest may have been pushed with signature store disabled
+		return digests, nil
+	default:
 		return nil, err
 	}
 
-	var digests []digest.Digest
 	for _, sigPath := range signaturePaths {
 		sigdigest, err := digest.ParseDigest(alg + ":" + path.Base(sigPath))
 		if err != nil {