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>

export.go

@@ -0,0 +1,65 @@
+package containerd
+
+import (
+	"archive/tar"
+	"context"
+	"io"
+
+	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/images"
+	"github.com/containerd/containerd/oci"
+	ocispecs "github.com/opencontainers/image-spec/specs-go"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+func (c *Client) exportToOCITar(ctx context.Context, desc ocispec.Descriptor, writer io.Writer, eopts exportOpts) error {
+	tw := tar.NewWriter(writer)
+	img := oci.Tar(tw)
+
+	// For tar, we defer creating index until end of the function.
+	if err := oci.Init(img, oci.InitOpts{SkipCreateIndex: true}); err != nil {
+		return err
+	}
+	cs := c.ContentStore()
+	handlers := images.Handlers(
+		images.ChildrenHandler(cs),
+		exportHandler(cs, img),
+	)
+	// For tar, we need to use Walk instead of Dispatch for ensuring sequential write
+	if err := images.Walk(ctx, handlers, desc); err != nil {
+		return err
+	}
+	// For tar, we don't use oci.PutManifestDescriptorToIndex() which allows appending desc to existing index.json
+	// but requires img to support random read access so as to read index.json.
+	return oci.WriteIndex(img,
+		ocispec.Index{
+			Versioned: ocispecs.Versioned{
+				SchemaVersion: 2,
+			},
+			Manifests: []ocispec.Descriptor{desc},
+		},
+	)
+}
+
+func exportHandler(cs content.Store, img oci.ImageDriver) images.HandlerFunc {
+	return func(ctx context.Context, desc ocispec.Descriptor) ([]ocispec.Descriptor, error) {
+		r, err := cs.Reader(ctx, desc.Digest)
+		if err != nil {
+			return nil, err
+		}
+		w, err := oci.NewBlobWriter(img, desc.Digest.Algorithm())
+		if err != nil {
+			return nil, err
+		}
+		if _, err = io.Copy(w, r); err != nil {
+			return nil, err
+		}
+		if err = w.Commit(desc.Size, desc.Digest); err != nil {
+			return nil, err
+		}
+		if err = w.Close(); err != nil {
+			return nil, err
+		}
+		return nil, nil
+	}
+}