support using multiple snapshotters simultaneously

e.g. dist pull --snapshotter btrfs ...; ctr run --snapshotter btrfs ...
(empty string defaults for overlayfs)

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/dist/common.go

@@ -44,6 +44,13 @@ var registryFlags = []cli.Flag{
 	},
 }
 
+var snapshotterFlags = []cli.Flag{
+	cli.StringFlag{
+		Name:  "snapshotter",
+		Usage: "Snapshotter name. Empty value stands for the daemon default value.",
+	},
+}
+
 func getClient(context *cli.Context) (*containerd.Client, error) {
 	address := context.GlobalString("address")
 	//timeout := context.GlobalDuration("connect-timeout")

cmd/dist/pull.go

@@ -20,7 +20,7 @@ command. As part of this process, we do the following:
 2. Prepare the snapshot filesystem with the pulled resources.
 3. Register metadata for the image.
 `,
-	Flags: registryFlags,
+	Flags: append(registryFlags, snapshotterFlags...),
 	Action: func(clicontext *cli.Context) error {
 		var (
 			ref = clicontext.Args().First()
@@ -38,7 +38,7 @@ command. As part of this process, we do the following:
 
 		// TODO: Show unpack status
 		fmt.Printf("unpacking %s...", img.Target().Digest)
-		err = img.Unpack(ctx)
+		err = img.Unpack(ctx, clicontext.String("snapshotter"))
 		fmt.Println("done")
 		return err
 	},

cmd/dist/rootfs.go

@@ -21,7 +21,7 @@ var rootfsUnpackCommand = cli.Command{
 	Name:      "unpack",
 	Usage:     "unpack applies layers from a manifest to a snapshot",
 	ArgsUsage: "[flags] <digest>",
-	Flags:     []cli.Flag{},
+	Flags:     snapshotterFlags,
 	Action: func(clicontext *cli.Context) error {
 		ctx, cancel := appContext(clicontext)
 		defer cancel()
@@ -49,7 +49,7 @@ var rootfsUnpackCommand = cli.Command{
 		for _, image := range images {
 			if image.Target().Digest == dgst {
 				fmt.Printf("unpacking %s (%s)...", dgst, image.Target().MediaType)
-				if err := image.Unpack(ctx); err != nil {
+				if err := image.Unpack(ctx, clicontext.String("snapshotter")); err != nil {
 					fmt.Println()
 					return err
 				}