feat: change foreign layer error message at repository.Fetch() (#377)

Signed-off-by: wangxiaoxuan273 <wangxiaoxuan119@gmail.com>

Author: wangxiaoxuan273

copy.go

@@ -26,6 +26,7 @@ import (
 	"oras.land/oras-go/v2/content"
 	"oras.land/oras-go/v2/errdef"
 	"oras.land/oras-go/v2/internal/cas"
+	"oras.land/oras-go/v2/internal/descriptor"
 	"oras.land/oras-go/v2/internal/platform"
 	"oras.land/oras-go/v2/internal/registryutil"
 	"oras.land/oras-go/v2/internal/status"
@@ -258,6 +259,9 @@ func copyGraph(ctx context.Context, src content.ReadOnlyStorage, dst content.Sto
 func doCopyNode(ctx context.Context, src content.ReadOnlyStorage, dst content.Storage, desc ocispec.Descriptor) error {
 	rc, err := src.Fetch(ctx, desc)
 	if err != nil {
+		if descriptor.IsForeignLayer(desc) && errors.Is(err, errdef.ErrNotFound) {
+			return fmt.Errorf("the artifact with foreign layer %s is not supported: %w", desc.Digest, err)
+		}
 		return err
 	}
 	defer rc.Close()

internal/descriptor/descriptor.go

@@ -18,6 +18,7 @@ package descriptor
 import (
 	"github.com/opencontainers/go-digest"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"oras.land/oras-go/v2/internal/docker"
 )
 
 // Descriptor contains the minimun information to describe the disposition of
@@ -45,3 +46,16 @@ func FromOCI(desc ocispec.Descriptor) Descriptor {
 		Size:      desc.Size,
 	}
 }
+
+// IsForeignLayer checks if a descriptor describes a foreign layer.
+func IsForeignLayer(desc ocispec.Descriptor) bool {
+	switch desc.MediaType {
+	case ocispec.MediaTypeImageLayerNonDistributable,
+		ocispec.MediaTypeImageLayerNonDistributableGzip,
+		ocispec.MediaTypeImageLayerNonDistributableZstd,
+		docker.MediaTypeForeignLayer:
+		return true
+	default:
+		return false
+	}
+}

internal/docker/mediatype.go

@@ -20,4 +20,5 @@ const (
 	MediaTypeConfig       = "application/vnd.docker.container.image.v1+json"
 	MediaTypeManifestList = "application/vnd.docker.distribution.manifest.list.v2+json"
 	MediaTypeManifest     = "application/vnd.docker.distribution.manifest.v2+json"
+	MediaTypeForeignLayer = "application/vnd.docker.image.rootfs.foreign.diff.tar.gzip"
 )