Group export metadata descriptors by platform

exporter/containerimage/export.go

@@ -234,7 +234,14 @@ func (e *imageExporterInstance) Export(ctx context.Context, src *exporter.Source
 	if err != nil {
 		return nil, nil, err
 	}
-	desc := descriptors[0]
+	var desc *ocispecs.Descriptor
+	if len(descriptors) != 1 {
+		desc = descriptors["index"][0]
+	} else {
+		for _, v := range descriptors {
+			desc = v[0]
+		}
+	}
 	defer func() {
 		if err == nil {
 			descref = NewDescriptorReference(*desc, done)
@@ -360,7 +367,7 @@ func (e *imageExporterInstance) Export(ctx context.Context, src *exporter.Source
 	}
 	resp[exptypes.ExporterImageDescriptorKey] = base64.StdEncoding.EncodeToString(dtdesc)
 
-	dtdesclist, err := json.Marshal(map[string]any{"list": descriptors})
+	dtdesclist, err := json.Marshal(descriptors)
 	if err != nil {
 		return nil, nil, err
 	}

exporter/containerimage/writer.go

@@ -61,7 +61,11 @@ type ImageWriter struct {
 	opt WriterOpt
 }
 
-func (ic *ImageWriter) Commit(ctx context.Context, inp *exporter.Source, sessionID string, inlineCache exptypes.InlineCache, opts *ImageCommitOpts) ([]*ocispecs.Descriptor, error) {
+func platformStringify(p *ocispecs.Platform) string {
+	return fmt.Sprintf("%s/%s", p.OS, p.Architecture)
+}
+
+func (ic *ImageWriter) Commit(ctx context.Context, inp *exporter.Source, sessionID string, inlineCache exptypes.InlineCache, opts *ImageCommitOpts) (map[string][]*ocispecs.Descriptor, error) {
 	if _, ok := inp.Metadata[exptypes.ExporterPlatformsKey]; len(inp.Refs) > 0 && !ok {
 		return nil, errors.Errorf("unable to export multiple refs, missing platforms mapping")
 	}
@@ -180,9 +184,11 @@ func (ic *ImageWriter) Commit(ctx context.Context, inp *exporter.Source, session
 		}
 		mfstDesc.Annotations[exptypes.ExporterConfigDigestKey] = configDesc.Digest.String()
 
-		return []*ocispecs.Descriptor{
-			mfstDesc,
-			configDesc,
+		return map[string][]*ocispecs.Descriptor{
+			platformStringify(mfstDesc.Platform): {
+				mfstDesc,
+				configDesc,
+			},
 		}, nil
 	}
 
@@ -228,7 +234,7 @@ func (ic *ImageWriter) Commit(ctx context.Context, inp *exporter.Source, session
 
 	labels := map[string]string{}
 
-	var descriptors []*ocispecs.Descriptor
+	var descriptors = make(map[string][]*ocispecs.Descriptor)
 	var attestationManifests []ocispecs.Descriptor
 
 	for i, p := range ps.Platforms {
@@ -272,7 +278,7 @@ func (ic *ImageWriter) Commit(ctx context.Context, inp *exporter.Source, session
 		dp := p.Platform
 		mfstDesc.Platform = &dp
 		idx.Manifests = append(idx.Manifests, *mfstDesc)
-		descriptors = append(descriptors, mfstDesc, configDesc)
+		descriptors[platformStringify(&dp)] = append(descriptors[platformStringify(&dp)], mfstDesc, configDesc)
 
 		labels[fmt.Sprintf("containerd.io/gc.ref.content.%d", i)] = mfstDesc.Digest.String()
 
@@ -328,7 +334,7 @@ func (ic *ImageWriter) Commit(ctx context.Context, inp *exporter.Source, session
 
 	for i, mfst := range attestationManifests {
 		idx.Manifests = append(idx.Manifests, mfst)
-		descriptors = append(descriptors, &mfst)
+		descriptors[platformStringify(mfst.Platform)] = append(descriptors[platformStringify(mfst.Platform)], &mfst)
 		labels[fmt.Sprintf("containerd.io/gc.ref.content.%d", len(ps.Platforms)+i)] = mfst.Digest.String()
 	}
 
@@ -350,7 +356,7 @@ func (ic *ImageWriter) Commit(ctx context.Context, inp *exporter.Source, session
 		return nil, idxDone(errors.Wrapf(err, "error writing manifest list blob %s", idxDigest))
 	}
 	idxDone(nil)
-	descriptors = append([]*ocispecs.Descriptor{&idxDesc}, descriptors...)
+	descriptors["index"] = []*ocispecs.Descriptor{&idxDesc}
 
 	return descriptors, nil
 }

exporter/oci/export.go

@@ -163,7 +163,14 @@ func (e *imageExporterInstance) Export(ctx context.Context, src *exporter.Source
 	if err != nil {
 		return nil, nil, err
 	}
-	desc := descriptors[0]
+	var desc *ocispecs.Descriptor
+	if len(descriptors) != 1 {
+		desc = descriptors["index"][0]
+	} else {
+		for _, v := range descriptors {
+			desc = v[0]
+		}
+	}
 	defer func() {
 		if err == nil {
 			descref = containerimage.NewDescriptorReference(*desc, done)
@@ -195,7 +202,7 @@ func (e *imageExporterInstance) Export(ctx context.Context, src *exporter.Source
 	}
 	resp[exptypes.ExporterImageDescriptorKey] = base64.StdEncoding.EncodeToString(dtdesc)
 
-	dtdesclist, err := json.Marshal(map[string]any{"list": descriptors})
+	dtdesclist, err := json.Marshal(descriptors)
 	if err != nil {
 		return nil, nil, err
 	}