export: Copy distribution source labels to manifest annotations

Persist manifest/indexes distribution source labels as annotations in
the index.json. This could allow the importer to fetch the missing blobs
from the external repository.

These can't really be persisted directly in blob descriptors because
that would alter the digests.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

images/archive/exporter.go

@@ -24,11 +24,14 @@ import (
 	"io"
 	"path"
 	"sort"
+	"strings"
 
 	"github.com/containerd/containerd/v2/content"
 	"github.com/containerd/containerd/v2/errdefs"
 	"github.com/containerd/containerd/v2/images"
+	"github.com/containerd/containerd/v2/labels"
 	"github.com/containerd/containerd/v2/platforms"
+	"github.com/containerd/log"
 	digest "github.com/opencontainers/go-digest"
 	ocispecs "github.com/opencontainers/image-spec/specs-go"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
@@ -191,6 +194,23 @@ func addNameAnnotation(name string, base map[string]string) map[string]string {
 	return annotations
 }
 
+func copySourceLabels(ctx context.Context, infoProvider content.InfoProvider, desc ocispec.Descriptor) (ocispec.Descriptor, error) {
+	info, err := infoProvider.Info(ctx, desc.Digest)
+	if err != nil {
+		return desc, err
+	}
+	for k, v := range info.Labels {
+		if strings.HasPrefix(k, labels.LabelDistributionSource) {
+			if desc.Annotations == nil {
+				desc.Annotations = map[string]string{k: v}
+			} else {
+				desc.Annotations[k] = v
+			}
+		}
+	}
+	return desc, nil
+}
+
 // ContentProvider provides both content and info about content
 type ContentProvider interface {
 	content.Provider
@@ -208,13 +228,22 @@ func Export(ctx context.Context, store ContentProvider, writer io.Writer, opts .
 
 	records := []tarRecord{
 		ociLayoutFile(""),
-		ociIndexRecord(eo.manifests),
+	}
+
+	manifests := make([]ocispec.Descriptor, 0, len(eo.manifests))
+	for _, desc := range eo.manifests {
+		d, err := copySourceLabels(ctx, store, desc)
+		if err != nil {
+			log.G(ctx).WithError(err).WithField("desc", desc).Warn("failed to copy distribution.source labels")
+			continue
+		}
+		manifests = append(manifests, d)
 	}
 
 	algorithms := map[string]struct{}{}
 	dManifests := map[digest.Digest]*exportManifest{}
 	resolvedIndex := map[digest.Digest]digest.Digest{}
-	for _, desc := range eo.manifests {
+	for _, desc := range manifests {
 		if images.IsManifestType(desc.MediaType) {
 			mt, ok := dManifests[desc.Digest]
 			if !ok {
@@ -304,6 +333,8 @@ func Export(ctx context.Context, store ContentProvider, writer io.Writer, opts .
 		}
 	}
 
+	records = append(records, ociIndexRecord(manifests))
+
 	if !eo.skipDockerManifest && len(dManifests) > 0 {
 		tr, err := manifestsRecord(ctx, store, dManifests)
 		if err != nil {