Add diff service implementation

Add snapshot subcommand to ctr for creating diffs of RW layers.

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

rootfs/diff.go

@@ -0,0 +1,52 @@
+package rootfs
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/containerd/containerd"
+	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/snapshot"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+type MountDiffer interface {
+	DiffMounts(ctx context.Context, lower, upper []containerd.Mount, media, ref string) (ocispec.Descriptor, error)
+}
+
+type DiffOptions struct {
+	MountDiffer
+	content.Store
+	snapshot.Snapshotter
+}
+
+func Diff(ctx context.Context, snapshotID, contentRef string, sn snapshot.Snapshotter, md MountDiffer) (ocispec.Descriptor, error) {
+	info, err := sn.Stat(ctx, snapshotID)
+	if err != nil {
+		return ocispec.Descriptor{}, err
+	}
+
+	lowerKey := fmt.Sprintf("%s-parent-view", info.Parent)
+	lower, err := sn.View(ctx, lowerKey, info.Parent)
+	if err != nil {
+		return ocispec.Descriptor{}, err
+	}
+	defer sn.Remove(ctx, lowerKey)
+
+	var upper []containerd.Mount
+	if info.Kind == snapshot.KindActive {
+		upper, err = sn.Mounts(ctx, snapshotID)
+		if err != nil {
+			return ocispec.Descriptor{}, err
+		}
+	} else {
+		upperKey := fmt.Sprintf("%s-view", snapshotID)
+		upper, err = sn.View(ctx, upperKey, snapshotID)
+		if err != nil {
+			return ocispec.Descriptor{}, err
+		}
+		defer sn.Remove(ctx, lowerKey)
+	}
+
+	return md.DiffMounts(ctx, lower, upper, ocispec.MediaTypeImageLayer, contentRef)
+}