Add snapshot and diff service

Remove rootfs service in place of snapshot service. Adds
diff service for extracting and creating diffs. Diff
creation is not yet implemented. This service allows
pulling or creating images without needing root access to
mount. Additionally in the future this will allow containerd
to ensure extractions happen safely in a chroot if needed.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

cmd/dist/common.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"context"
 	"crypto/tls"
+	"encoding/json"
 	"fmt"
 	"net"
 	"net/http"
@@ -18,8 +19,10 @@ import (
 	"github.com/containerd/containerd/images"
 	"github.com/containerd/containerd/remotes"
 	"github.com/containerd/containerd/remotes/docker"
+	"github.com/containerd/containerd/rootfs"
 	contentservice "github.com/containerd/containerd/services/content"
 	imagesservice "github.com/containerd/containerd/services/images"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
 	"github.com/urfave/cli"
 	"google.golang.org/grpc"
@@ -151,3 +154,36 @@ func passwordPrompt() (string, error) {
 	}
 	return string(line), nil
 }
+
+func getImageLayers(ctx context.Context, image images.Image, cs content.Store) ([]rootfs.Layer, error) {
+	p, err := content.ReadBlob(ctx, cs, image.Target.Digest)
+	if err != nil {
+		return nil, errors.Wrapf(err, "failed to read manifest blob")
+	}
+
+	var manifest ocispec.Manifest
+	if err := json.Unmarshal(p, &manifest); err != nil {
+		return nil, errors.Wrap(err, "failed to unmarshal manifest")
+	}
+
+	diffIDs, err := image.RootFS(ctx, cs)
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to resolve rootfs")
+	}
+
+	if len(diffIDs) != len(manifest.Layers) {
+		return nil, errors.Errorf("mismatched image rootfs and manifest layers")
+	}
+
+	layers := make([]rootfs.Layer, len(diffIDs))
+	for i := range diffIDs {
+		layers[i].Diff = ocispec.Descriptor{
+			// TODO: derive media type from compressed type
+			MediaType: ocispec.MediaTypeImageLayer,
+			Digest:    diffIDs[i],
+		}
+		layers[i].Blob = manifest.Layers[i]
+	}
+
+	return layers, nil
+}