crane pull: support pulling index to OCI Layout (#1215)

* crane pull: support pulling index to OCI Layout

Prior to this, we'd also resolve an index to an image when using crane
pull. For OCI Layouts, we can actually pull the whole index, so when
--format=oci and --platform is unset, just do that.

* Address feedback

* Add platform test

Author: jonjohnsonjr

Diff for: .github/workflows/e2e.yaml

@@ -63,3 +63,21 @@ jobs:
         ./app/crane pull --format=oci $img $layout
         ./app/crane push --image-refs=foo.images $layout $dst
         diff <(./app/crane manifest $img) <(./app/crane manifest $(cat foo.images))
+
+        # Make sure we can roundtrip an index (distroless).
+        distroless=$(mktemp -d)
+        remote="gcr.io/distroless/static"
+        local="localhost:1338/distroless:static"
+
+        ./app/crane pull --format=oci $remote $distroless
+        ./app/crane push $distroless $local
+        diff <(./app/crane manifest $remote) <(./app/crane manifest $local)
+
+        # And that it works for a single platform (pulling from what we just pushed).
+        distroless=$(mktemp -d)
+        remote="$local"
+        local="localhost:1338/distroless/platform:static"
+
+        ./app/crane pull --platform=linux/arm64 --format=oci $remote $distroless
+        ./app/crane push $distroless $local
+        diff <(./app/crane manifest --platform linux/arm64 $remote) <(./app/crane manifest $local)

Diff for: cmd/crane/cmd/pull.go

@@ -18,8 +18,11 @@ import (
 	"fmt"
 
 	"github.com/google/go-containerregistry/pkg/crane"
+	"github.com/google/go-containerregistry/pkg/name"
 	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/cache"
+	"github.com/google/go-containerregistry/pkg/v1/layout"
+	"github.com/google/go-containerregistry/pkg/v1/remote"
 	"github.com/spf13/cobra"
 )
 
@@ -33,16 +36,38 @@ func NewCmdPull(options *[]crane.Option) *cobra.Command {
 		Args:  cobra.MinimumNArgs(2),
 		RunE: func(_ *cobra.Command, args []string) error {
 			imageMap := map[string]v1.Image{}
+			indexMap := map[string]v1.ImageIndex{}
 			srcList, path := args[:len(args)-1], args[len(args)-1]
 			for _, src := range srcList {
-				img, err := crane.Pull(src, *options...)
+				o := crane.GetOptions(*options...)
+				ref, err := name.ParseReference(src, o.Name...)
 				if err != nil {
-					return fmt.Errorf("pulling %s: %w", src, err)
+					return fmt.Errorf("parsing reference %q: %w", src, err)
+				}
+
+				rmt, err := remote.Get(ref, o.Remote...)
+				if err != nil {
+					return err
+				}
+
+				// If we're writing an index to a layout and --platform hasn't been set,
+				// pull the entire index, not just a child image.
+				if format == "oci" && rmt.MediaType.IsIndex() && o.Platform == nil {
+					idx, err := rmt.ImageIndex()
+					if err != nil {
+						return err
+					}
+					indexMap[src] = idx
+					continue
+				}
+
+				img, err := rmt.Image()
+				if err != nil {
+					return err
 				}
 				if cachePath != "" {
 					img = cache.Image(img, cache.NewFilesystemCache(cachePath))
 				}
-
 				imageMap[src] = img
 			}
 
@@ -59,6 +84,20 @@ func NewCmdPull(options *[]crane.Option) *cobra.Command {
 				if err := crane.MultiSaveOCI(imageMap, path); err != nil {
 					return fmt.Errorf("saving oci image layout %s: %w", path, err)
 				}
+
+				// crane.MultiSaveOCI doesn't support index, so just append these at the end.
+				p, err := layout.FromPath(path)
+				if err != nil {
+					return err
+				}
+				for ref, idx := range indexMap {
+					anns := map[string]string{
+						"dev.ggcr.image.name": ref,
+					}
+					if err := p.AppendIndex(idx, layout.WithAnnotations(anns)); err != nil {
+						return err
+					}
+				}
 			default:
 				return fmt.Errorf("unexpected --format: %q (valid values are: tarball, legacy, and oci)", format)
 			}

Diff for: pkg/crane/pull.go

@@ -133,8 +133,11 @@ func MultiSaveOCI(imgMap map[string]v1.Image, path string) error {
 			return err
 		}
 	}
-	for _, img := range imgMap {
-		if err = p.AppendImage(img); err != nil {
+	for ref, img := range imgMap {
+		anns := map[string]string{
+			"dev.ggcr.image.name": ref,
+		}
+		if err = p.AppendImage(img, layout.WithAnnotations(anns)); err != nil {
 			return err
 		}
 	}