crane: add --flatten for index append (#1566)

* crane: add --flatten for index append

While it's possible that folks might want to append an index to an
index, it's more likely that they would want to merge them. This flag
gives them an option to do either.

* Update cmd/crane/cmd/index.go

Co-authored-by: Jason Hall <jason@chainguard.dev>

* ./hack/update-codegen.sh

---------

Co-authored-by: Jason Hall <jason@chainguard.dev>

cmd/crane/cmd/index.go

@@ -112,7 +112,7 @@ func NewCmdIndexFilter(options *[]crane.Option) *cobra.Command {
 func NewCmdIndexAppend(options *[]crane.Option) *cobra.Command {
 	var baseRef, newTag string
 	var newManifests []string
-	var dockerEmptyBase bool
+	var dockerEmptyBase, flatten bool
 
 	cmd := &cobra.Command{
 		Use:   "append",
@@ -194,9 +194,40 @@ The platform for appended manifests is inferred from the config file or omitted
 					if err != nil {
 						return err
 					}
-					adds = append(adds, mutate.IndexAddendum{
-						Add: idx,
-					})
+					if flatten {
+						im, err := idx.IndexManifest()
+						if err != nil {
+							return err
+						}
+						for _, child := range im.Manifests {
+							switch {
+							case child.MediaType.IsImage():
+								img, err := idx.Image(child.Digest)
+								if err != nil {
+									return err
+								}
+								adds = append(adds, mutate.IndexAddendum{
+									Add:        img,
+									Descriptor: child,
+								})
+							case child.MediaType.IsIndex():
+								idx, err := idx.ImageIndex(child.Digest)
+								if err != nil {
+									return err
+								}
+								adds = append(adds, mutate.IndexAddendum{
+									Add:        idx,
+									Descriptor: child,
+								})
+							default:
+								return fmt.Errorf("unexpected child %q with media type %q", child.Digest, child.MediaType)
+							}
+						}
+					} else {
+						adds = append(adds, mutate.IndexAddendum{
+							Add: idx,
+						})
+					}
 				} else {
 					return fmt.Errorf("saw unexpected MediaType %q for %q", desc.MediaType, m)
 				}
@@ -229,6 +260,7 @@ The platform for appended manifests is inferred from the config file or omitted
 	cmd.Flags().StringVarP(&newTag, "tag", "t", "", "Tag to apply to resulting image")
 	cmd.Flags().StringSliceVarP(&newManifests, "manifest", "m", []string{}, "References to manifests to append to the base index")
 	cmd.Flags().BoolVar(&dockerEmptyBase, "docker-empty-base", false, "If true, empty base index will have Docker media types instead of OCI")
+	cmd.Flags().BoolVar(&flatten, "flatten", true, "If true, appending an index will append each of its children rather than the index itself")
 
 	return cmd
 }