Add internal/imagesource/impl.NoSignatures, use it to reduce boilerplate

Should not change behavior.

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

docker/internal/tarfile/src.go

@@ -26,6 +26,7 @@ import (
 // Source is a partial implementation of types.ImageSource for reading from tarPath.
 type Source struct {
 	impl.PropertyMethodsInitialize
+	impl.NoSignatures
 	impl.DoesNotAffectLayerInfosForCopy
 	stubs.NoGetBlobAtInitialize
 
@@ -314,14 +315,3 @@ func (s *Source) GetBlob(ctx context.Context, info types.BlobInfo, cache types.B
 
 	return nil, 0, fmt.Errorf("Unknown blob %s", info.Digest)
 }
-
-// GetSignatures returns the image's signatures.  It may use a remote (= slow) service.
-// This source implementation does not support manifest lists, so the passed-in instanceDigest should always be nil,
-// as there can be no secondary manifests.
-func (s *Source) GetSignatures(ctx context.Context, instanceDigest *digest.Digest) ([][]byte, error) {
-	if instanceDigest != nil {
-		// How did we even get here? GetManifest(ctx, nil) has returned a manifest.DockerV2Schema2MediaType.
-		return nil, errors.New(`Manifest lists are not supported by "docker-daemon:"`)
-	}
-	return [][]byte{}, nil
-}

internal/imagesource/impl/signatures.go

@@ -0,0 +1,18 @@
+package impl
+
+import (
+	"context"
+
+	"github.com/opencontainers/go-digest"
+)
+
+// NoSignatures implements GetSignatures() that returns nothing.
+type NoSignatures struct{}
+
+// GetSignatures returns the image's signatures.  It may use a remote (= slow) service.
+// If instanceDigest is not nil, it contains a digest of the specific manifest instance to retrieve signatures for
+// (when the primary manifest is a manifest list); this never happens if the primary manifest is not a manifest list
+// (e.g. if the source never returns manifest lists).
+func (stub NoSignatures) GetSignatures(ctx context.Context, instanceDigest *digest.Digest) ([][]byte, error) {
+	return nil, nil
+}

oci/layout/oci_src.go

@@ -23,6 +23,7 @@ import (
 
 type ociImageSource struct {
 	impl.PropertyMethodsInitialize
+	impl.NoSignatures
 	impl.DoesNotAffectLayerInfosForCopy
 	stubs.NoGetBlobAtInitialize
 
@@ -150,14 +151,6 @@ func (s *ociImageSource) GetBlob(ctx context.Context, info types.BlobInfo, cache
 	return r, fi.Size(), nil
 }
 
-// GetSignatures returns the image's signatures.  It may use a remote (= slow) service.
-// If instanceDigest is not nil, it contains a digest of the specific manifest instance to retrieve signatures for
-// (when the primary manifest is a manifest list); this never happens if the primary manifest is not a manifest list
-// (e.g. if the source never returns manifest lists).
-func (s *ociImageSource) GetSignatures(ctx context.Context, instanceDigest *digest.Digest) ([][]byte, error) {
-	return [][]byte{}, nil
-}
-
 // getExternalBlob returns the reader of the first available blob URL from urls, which must not be empty.
 // This function can return nil reader when no url is supported by this function. In this case, the caller
 // should fallback to fetch the non-external blob (i.e. pull from the registry).

sif/src.go

@@ -23,6 +23,7 @@ import (
 
 type sifImageSource struct {
 	impl.PropertyMethodsInitialize
+	impl.NoSignatures
 	impl.DoesNotAffectLayerInfosForCopy
 	stubs.NoGetBlobAtInitialize
 
@@ -198,14 +199,3 @@ func (s *sifImageSource) GetManifest(ctx context.Context, instanceDigest *digest
 	}
 	return s.manifest, imgspecv1.MediaTypeImageManifest, nil
 }
-
-// GetSignatures returns the image's signatures.  It may use a remote (= slow) service.
-// If instanceDigest is not nil, it contains a digest of the specific manifest instance to retrieve signatures for
-// (when the primary manifest is a manifest list); this never happens if the primary manifest is not a manifest list
-// (e.g. if the source never returns manifest lists).
-func (s *sifImageSource) GetSignatures(ctx context.Context, instanceDigest *digest.Digest) ([][]byte, error) {
-	if instanceDigest != nil {
-		return nil, errors.New("manifest lists are not supported by the sif transport")
-	}
-	return nil, nil
-}

tarball/tarball_src.go

@@ -22,6 +22,7 @@ import (
 
 type tarballImageSource struct {
 	impl.PropertyMethodsInitialize
+	impl.NoSignatures
 	impl.DoesNotAffectLayerInfosForCopy
 	stubs.NoGetBlobAtInitialize
 
@@ -252,16 +253,6 @@ func (is *tarballImageSource) GetManifest(ctx context.Context, instanceDigest *d
 	return is.manifest, imgspecv1.MediaTypeImageManifest, nil
 }
 
-// GetSignatures returns the image's signatures.  It may use a remote (= slow) service.
-// This source implementation does not support manifest lists, so the passed-in instanceDigest should always be nil,
-// as there can be no secondary manifests.
-func (*tarballImageSource) GetSignatures(ctx context.Context, instanceDigest *digest.Digest) ([][]byte, error) {
-	if instanceDigest != nil {
-		return nil, fmt.Errorf("manifest lists are not supported by the %q transport", transportName)
-	}
-	return nil, nil
-}
-
 func (is *tarballImageSource) Reference() types.ImageReference {
 	return &is.reference
 }