Prefer X-Registry-Supports-Signatures to lookaside

In general, a single server should never have both, so the order mostly
doesn't matter.  The one place where the difference is important is
the ability of lookside to have a global default; if lookaside is preferred,
such a global default makes X-R-S-S impossible to use.

The disadvantage of this is that simply upgrading a registry to a server
version which supports X-R-S-S breaks any previously configured lookaside
for the server.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

docker/docker_image_dest.go

@@ -71,10 +71,10 @@ func (d *dockerImageDestination) SupportsSignatures() error {
 		return err
 	}
 	switch {
-	case d.c.signatureBase != nil:
-		return nil
 	case d.c.supportsSignatures:
 		return nil
+	case d.c.signatureBase != nil:
+		return nil
 	default:
 		return errors.Errorf("X-Registry-Supports-Signatures extension not supported, and lookaside is not configured")
 	}
@@ -277,10 +277,10 @@ func (d *dockerImageDestination) PutSignatures(signatures [][]byte) error {
 		return err
 	}
 	switch {
-	case d.c.signatureBase != nil:
-		return d.putSignaturesToLookaside(signatures)
 	case d.c.supportsSignatures:
 		return d.putSignaturesToAPIExtension(signatures)
+	case d.c.signatureBase != nil:
+		return d.putSignaturesToLookaside(signatures)
 	default:
 		return errors.Errorf("X-Registry-Supports-Signatures extension not supported, and lookaside is not configured")
 	}

docker/docker_image_src.go

@@ -189,10 +189,10 @@ func (s *dockerImageSource) GetSignatures(ctx context.Context, instanceDigest *d
 		return nil, err
 	}
 	switch {
-	case s.c.signatureBase != nil:
-		return s.getSignaturesFromLookaside(ctx, instanceDigest)
 	case s.c.supportsSignatures:
 		return s.getSignaturesFromAPIExtension(ctx, instanceDigest)
+	case s.c.signatureBase != nil:
+		return s.getSignaturesFromLookaside(ctx, instanceDigest)
 	default:
 		return [][]byte{}, nil
 	}