Add rootfs service

Add rootfs service to allow creation of rootfs from layer array

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

services/rootfs/preparer.go

@@ -0,0 +1,39 @@
+package rootfs
+
+import (
+	"context"
+
+	rootfsapi "github.com/docker/containerd/api/services/rootfs"
+	containerd_v1_types "github.com/docker/containerd/api/types/descriptor"
+	"github.com/docker/containerd/rootfs"
+	digest "github.com/opencontainers/go-digest"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+func NewPreparerFromClient(client rootfsapi.RootFSClient) rootfs.Preparer {
+	return remotePreparer{
+		client: client,
+	}
+}
+
+type remotePreparer struct {
+	client rootfsapi.RootFSClient
+}
+
+func (rp remotePreparer) Prepare(ctx context.Context, layers []ocispec.Descriptor) (digest.Digest, error) {
+	pr := rootfsapi.PrepareRequest{
+		Layers: make([]*containerd_v1_types.Descriptor, len(layers)),
+	}
+	for i, l := range layers {
+		pr.Layers[i] = &containerd_v1_types.Descriptor{
+			MediaType: l.MediaType,
+			Digest:    l.Digest,
+			MediaSize: l.Size,
+		}
+	}
+	resp, err := rp.client.Prepare(ctx, &pr)
+	if err != nil {
+		return "", nil
+	}
+	return resp.ChainID, nil
+}