client: add Import() and Export() for importing/exporting image in OCI format

Export as a tar (Note: "-" can be used for stdout):

    $ ctr images export /tmp/oci-busybox.tar docker.io/library/busybox:latest

Import a tar (Note: "-" can be used for stdin):

    $ ctr images import foo/new:latest /tmp/oci-busybox.tar

Note: media types are not converted at the moment: e.g.
  application/vnd.docker.image.rootfs.diff.tar.gzip
  -> application/vnd.oci.image.layer.v1.tar+gzip

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

cmd/ctr/export.go

@@ -0,0 +1,106 @@
+package main
+
+import (
+	"io"
+	"os"
+
+	"github.com/containerd/containerd/reference"
+	digest "github.com/opencontainers/go-digest"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"github.com/pkg/errors"
+	"github.com/urfave/cli"
+)
+
+var imagesExportCommand = cli.Command{
+	Name:      "export",
+	Usage:     "export an image",
+	ArgsUsage: "[flags] <out> <image>",
+	Description: `Export an image to a tar stream
+`,
+	Flags: []cli.Flag{
+		cli.StringFlag{
+			Name:  "oci-ref-name",
+			Value: "",
+			Usage: "Override org.opencontainers.image.ref.name annotation",
+		},
+		cli.StringFlag{
+			Name:  "manifest",
+			Usage: "Digest of manifest",
+		},
+		cli.StringFlag{
+			Name:  "manifest-type",
+			Usage: "Media type of manifest digest",
+			Value: ocispec.MediaTypeImageManifest,
+		},
+	},
+	Action: func(clicontext *cli.Context) error {
+		var (
+			out   = clicontext.Args().First()
+			local = clicontext.Args().Get(1)
+			desc  ocispec.Descriptor
+		)
+
+		ctx, cancel := appContext(clicontext)
+		defer cancel()
+
+		client, err := newClient(clicontext)
+		if err != nil {
+			return err
+		}
+
+		if manifest := clicontext.String("manifest"); manifest != "" {
+			desc.Digest, err = digest.Parse(manifest)
+			if err != nil {
+				return errors.Wrap(err, "invalid manifest digest")
+			}
+			desc.MediaType = clicontext.String("manifest-type")
+		} else {
+			img, err := client.ImageService().Get(ctx, local)
+			if err != nil {
+				return errors.Wrap(err, "unable to resolve image to manifest")
+			}
+			desc = img.Target
+		}
+
+		if desc.Annotations == nil {
+			desc.Annotations = make(map[string]string)
+		}
+		if s, ok := desc.Annotations[ocispec.AnnotationRefName]; !ok || s == "" {
+			if ociRefName := determineOCIRefName(local); ociRefName != "" {
+				desc.Annotations[ocispec.AnnotationRefName] = ociRefName
+			}
+			if ociRefName := clicontext.String("oci-ref-name"); ociRefName != "" {
+				desc.Annotations[ocispec.AnnotationRefName] = ociRefName
+			}
+		}
+		var w io.WriteCloser
+		if out == "-" {
+			w = os.Stdout
+		} else {
+			w, err = os.Create(out)
+			if err != nil {
+				return nil
+			}
+		}
+		r, err := client.Export(ctx, desc)
+		if err != nil {
+			return err
+		}
+		if _, err := io.Copy(w, r); err != nil {
+			return err
+		}
+		if err := w.Close(); err != nil {
+			return err
+		}
+		return r.Close()
+	},
+}
+
+func determineOCIRefName(local string) string {
+	refspec, err := reference.Parse(local)
+	if err != nil {
+		return ""
+	}
+	tag, _ := reference.SplitObject(refspec.Object)
+	return tag
+}