pkg/manifests.List: add SetSubject()/Subject()

Add methods for reading and writing the index-level "subject" field.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

common/internal/deepcopy.go

@@ -0,0 +1,29 @@
+package internal
+
+import (
+	v1 "github.com/opencontainers/image-spec/specs-go/v1"
+	"golang.org/x/exp/maps"
+	"golang.org/x/exp/slices"
+)
+
+// DeepCopyDescriptor copies a Descriptor, deeply copying its contents
+func DeepCopyDescriptor(original *v1.Descriptor) *v1.Descriptor {
+	tmp := *original
+	if original.URLs != nil {
+		tmp.URLs = slices.Clone(original.URLs)
+	}
+	if original.Annotations != nil {
+		tmp.Annotations = maps.Clone(original.Annotations)
+	}
+	if original.Data != nil {
+		tmp.Data = slices.Clone(original.Data)
+	}
+	if original.Platform != nil {
+		tmpPlatform := *original.Platform
+		if original.Platform.OSFeatures != nil {
+			tmpPlatform.OSFeatures = slices.Clone(original.Platform.OSFeatures)
+		}
+		tmp.Platform = &tmpPlatform
+	}
+	return &tmp
+}

common/pkg/manifests/manifests.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"os"
 
+	"github.com/containers/common/internal"
 	"github.com/containers/image/v5/manifest"
 	digest "github.com/opencontainers/go-digest"
 	imgspec "github.com/opencontainers/image-spec/specs-go"
@@ -39,6 +40,8 @@ type List interface {
 	MediaType(instanceDigest digest.Digest) (string, error)
 	SetArtifactType(instanceDigest digest.Digest, artifactType string) error
 	ArtifactType(instanceDigest digest.Digest) (string, error)
+	SetSubject(subject *v1.Descriptor) error
+	Subject() (*v1.Descriptor, error)
 	Serialize(mimeType string) ([]byte, error)
 	Instances() []digest.Digest
 	OCIv1() *v1.Index
@@ -488,6 +491,26 @@ func (l *list) ArtifactType(instanceDigest digest.Digest) (string, error) {
 	return oci.ArtifactType, nil
 }
 
+// SetSubject sets the image index's subject.
+// The field is specific to the OCI image index format, and is not present in Docker manifest lists.
+func (l *list) SetSubject(subject *v1.Descriptor) error {
+	if subject != nil {
+		subject = internal.DeepCopyDescriptor(subject)
+	}
+	l.oci.Subject = subject
+	return nil
+}
+
+// Subject retrieves the subject which might have been set on the image index.
+// The field is specific to the OCI image index format, and is not present in Docker manifest lists.
+func (l *list) Subject() (*v1.Descriptor, error) {
+	s := l.oci.Subject
+	if s != nil {
+		s = internal.DeepCopyDescriptor(s)
+	}
+	return s, nil
+}
+
 // FromBlob builds a list from an encoded manifest list or image index.
 func FromBlob(manifestBytes []byte) (List, error) {
 	manifestType := manifest.GuessMIMEType(manifestBytes)

common/pkg/manifests/manifests_test.go

@@ -462,3 +462,43 @@ func TestPlatform(t *testing.T) {
 	assert.NoError(t, list.SetOSFeatures(instanceDigest, []string{}))
 	assert.Nil(t, list.OCIv1().Manifests[0].Platform, "expected platform to be nil")
 }
+
+func TestSubject(t *testing.T) {
+	bytes, err := os.ReadFile(ociFixture)
+	if err != nil {
+		t.Fatalf("error loading blob: %v", err)
+	}
+	list, err := FromBlob(bytes)
+	if err != nil {
+		t.Fatalf("error parsing blob: %v", err)
+	}
+	for _, wrote := range []*v1.Descriptor{
+		nil,
+		{},
+		{
+			MediaType: v1.MediaTypeImageManifest,
+			Digest:    expectedInstance,
+			Size:      1234,
+		},
+	} {
+		err := list.SetSubject(wrote)
+		if err != nil {
+			t.Fatalf("error setting subject: %v", err)
+		}
+		b, err := list.Serialize("")
+		if err != nil {
+			t.Fatalf("error serializing list: %v", err)
+		}
+		list, err = FromBlob(b)
+		if err != nil {
+			t.Fatalf("error parsing list: %v", err)
+		}
+		read, err := list.Subject()
+		if err != nil {
+			t.Fatalf("error retrieving subject: %v", err)
+		}
+		if !reflect.DeepEqual(read, wrote) {
+			t.Fatalf("expected subject %v, got %v", wrote, read)
+		}
+	}
+}