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>

client.go

@@ -3,6 +3,7 @@ package containerd
 import (
 	"context"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"log"
 	"net/http"
@@ -25,6 +26,7 @@ import (
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/images"
 	"github.com/containerd/containerd/plugin"
+	"github.com/containerd/containerd/reference"
 	"github.com/containerd/containerd/remotes"
 	"github.com/containerd/containerd/remotes/docker"
 	"github.com/containerd/containerd/remotes/docker/schema1"
@@ -552,3 +554,120 @@ func (c *Client) Version(ctx context.Context) (Version, error) {
 		Revision: response.Revision,
 	}, nil
 }
+
+type imageFormat string
+
+const (
+	ociImageFormat imageFormat = "oci"
+)
+
+type importOpts struct {
+	format    imageFormat
+	refObject string
+}
+
+type ImportOpt func(c *importOpts) error
+
+func WithOCIImportFormat() ImportOpt {
+	return func(c *importOpts) error {
+		if c.format != "" {
+			return errors.New("format already set")
+		}
+		c.format = ociImageFormat
+		return nil
+	}
+}
+
+// WithRefObject specifies the ref object to import.
+// If refObject is empty, it is copied from the ref argument of Import().
+func WithRefObject(refObject string) ImportOpt {
+	return func(c *importOpts) error {
+		c.refObject = refObject
+		return nil
+	}
+}
+
+func resolveImportOpt(ref string, opts ...ImportOpt) (importOpts, error) {
+	var iopts importOpts
+	for _, o := range opts {
+		if err := o(&iopts); err != nil {
+			return iopts, err
+		}
+	}
+	// use OCI as the default format
+	if iopts.format == "" {
+		iopts.format = ociImageFormat
+	}
+	// if refObject is not explicitly specified, use the one specified in ref
+	if iopts.refObject == "" {
+		refSpec, err := reference.Parse(ref)
+		if err != nil {
+			return iopts, err
+		}
+		iopts.refObject = refSpec.Object
+	}
+	return iopts, nil
+}
+
+// Import imports an image from a Tar stream using reader.
+// OCI format is assumed by default.
+//
+// Note that unreferenced blobs are imported to the content store as well.
+func (c *Client) Import(ctx context.Context, ref string, reader io.Reader, opts ...ImportOpt) (Image, error) {
+	iopts, err := resolveImportOpt(ref, opts...)
+	if err != nil {
+		return nil, err
+	}
+	switch iopts.format {
+	case ociImageFormat:
+		return c.importFromOCITar(ctx, ref, reader, iopts)
+	default:
+		return nil, errors.Errorf("unsupported format: %s", iopts.format)
+	}
+}
+
+type exportOpts struct {
+	format imageFormat
+}
+
+type ExportOpt func(c *exportOpts) error
+
+func WithOCIExportFormat() ExportOpt {
+	return func(c *exportOpts) error {
+		if c.format != "" {
+			return errors.New("format already set")
+		}
+		c.format = ociImageFormat
+		return nil
+	}
+}
+
+// TODO: add WithMediaTypeTranslation that transforms media types according to the format.
+// e.g. application/vnd.docker.image.rootfs.diff.tar.gzip
+//      -> application/vnd.oci.image.layer.v1.tar+gzip
+
+// Export exports an image to a Tar stream.
+// OCI format is used by default.
+// It is up to caller to put "org.opencontainers.image.ref.name" annotation to desc.
+func (c *Client) Export(ctx context.Context, desc ocispec.Descriptor, opts ...ExportOpt) (io.ReadCloser, error) {
+	var eopts exportOpts
+	for _, o := range opts {
+		if err := o(&eopts); err != nil {
+			return nil, err
+		}
+	}
+	// use OCI as the default format
+	if eopts.format == "" {
+		eopts.format = ociImageFormat
+	}
+	pr, pw := io.Pipe()
+	switch eopts.format {
+	case ociImageFormat:
+		go func() {
+			pw.CloseWithError(c.exportToOCITar(ctx, desc, pw, eopts))
+		}()
+	default:
+		return nil, errors.Errorf("unsupported format: %s", eopts.format)
+	}
+	return pr, nil
+}