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>

services/diff/client.go

@@ -0,0 +1,56 @@
+package diff
+
+import (
+	"context"
+
+	"github.com/containerd/containerd"
+	diffapi "github.com/containerd/containerd/api/services/diff"
+	"github.com/containerd/containerd/api/types/descriptor"
+	mounttypes "github.com/containerd/containerd/api/types/mount"
+	"github.com/containerd/containerd/rootfs"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+// NewApplierFromClient returns a new Applier which communicates
+// over a GRPC connection.
+func NewApplierFromClient(client diffapi.DiffClient) rootfs.Applier {
+	return &remoteApplier{
+		client: client,
+	}
+}
+
+type remoteApplier struct {
+	client diffapi.DiffClient
+}
+
+func (r *remoteApplier) Apply(ctx context.Context, diff ocispec.Descriptor, mounts []containerd.Mount) (ocispec.Descriptor, error) {
+	req := &diffapi.ApplyRequest{
+		Diff:   fromDescriptor(diff),
+		Mounts: fromMounts(mounts),
+	}
+	resp, err := r.client.Apply(ctx, req)
+	if err != nil {
+		return ocispec.Descriptor{}, err
+	}
+	return toDescriptor(resp.Applied), nil
+}
+
+func fromDescriptor(d ocispec.Descriptor) *descriptor.Descriptor {
+	return &descriptor.Descriptor{
+		MediaType: d.MediaType,
+		Digest:    d.Digest,
+		Size_:     d.Size,
+	}
+}
+
+func fromMounts(mounts []containerd.Mount) []*mounttypes.Mount {
+	apiMounts := make([]*mounttypes.Mount, len(mounts))
+	for i, m := range mounts {
+		apiMounts[i] = &mounttypes.Mount{
+			Type:    m.Type,
+			Source:  m.Source,
+			Options: m.Options,
+		}
+	}
+	return apiMounts
+}