fix: handle nested artifact manifests in local blob access method

Added support to synthesize artifact blobs for nested OCI image manifests or indexes in cases where fewer references are provided. Updated error handling and blob synthesis logic accordingly.

Signed-off-by: Jakob Möller <jakob.moeller@sap.com>

Author: jakobmoellerdev

api/ocm/extensions/repositories/genericocireg/accessmethod_localblob.go

@@ -2,6 +2,7 @@ package genericocireg
 
 import (
 	"bytes"
+	"fmt"
 	"io"
 	"os"
 	"strings"
@@ -10,9 +11,9 @@ import (
 	"github.com/mandelsoft/goutils/errors"
 	"github.com/mandelsoft/goutils/finalizer"
 	"github.com/opencontainers/go-digest"
-
 	"ocm.software/ocm/api/oci"
 	"ocm.software/ocm/api/oci/artdesc"
+	"ocm.software/ocm/api/oci/extensions/repositories/artifactset"
 	"ocm.software/ocm/api/ocm/cpi/accspeccpi"
 	"ocm.software/ocm/api/ocm/extensions/accessmethods/localblob"
 	"ocm.software/ocm/api/utils/blobaccess/blobaccess"
@@ -99,8 +100,15 @@ func (m *localBlobAccessMethod) getBlob() (blobaccess.DataAccess, error) {
 		err  error
 	)
 	if len(refs) < 2 {
-		_, data, err = m.namespace.GetBlobData(digest.Digest(m.spec.LocalReference))
-		if err != nil {
+		if m.spec.MediaType == artdesc.MediaTypeImageIndex || m.spec.MediaType == artdesc.MediaTypeImageManifest {
+			// if we have a nested manifest or index, we can use the blob synthesis utility here to download
+			// the entire artifact set.
+			artblob, err := artifactset.SynthesizeArtifactBlob(m.namespace, m.spec.LocalReference)
+			if err != nil {
+				return nil, fmt.Errorf("failed to synthesize artifact blob: %w", err)
+			}
+			data = artblob
+		} else if _, data, err = m.namespace.GetBlobData(digest.Digest(m.spec.LocalReference)); err != nil {
 			return nil, err
 		}
 	} else {
@@ -126,7 +134,7 @@ func (m *localBlobAccessMethod) MimeType() string {
 	return m.spec.MediaType
 }
 
-////////////////////////////////////////////////////////////////////////////////
+// //////////////////////////////////////////////////////////////////////////////
 
 type composedBlock struct {
 	m    *localBlobAccessMethod