ctr: add ctr content fetch-blob

e.g., `ctr content fetch-blob docker.io/library/debian:latest sha256:43d28810c1b4c28a1be3bac8e0e40fcc472b2bfcfcda952544ed99cb874d2b1a`

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cmd/ctr/commands/content/content.go

@@ -29,6 +29,7 @@ import (
 	"github.com/containerd/containerd/content"
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/log"
+	"github.com/containerd/containerd/remotes"
 	units "github.com/docker/go-units"
 	digest "github.com/opencontainers/go-digest"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
@@ -47,6 +48,7 @@ var (
 			editCommand,
 			fetchCommand,
 			fetchObjectCommand,
+			fetchBlobCommand,
 			getCommand,
 			ingestCommand,
 			listCommand,
@@ -442,6 +444,60 @@ var (
 		},
 	}
 
+	fetchBlobCommand = cli.Command{
+		Name:        "fetch-blob",
+		Usage:       "retrieve blobs from a remote",
+		ArgsUsage:   "[flags] <remote> [<digest>, ...]",
+		Description: `Fetch blobs by digests from a remote.`,
+		Flags:       commands.RegistryFlags,
+		Action: func(context *cli.Context) error {
+			var (
+				ref     = context.Args().First()
+				digests = context.Args().Tail()
+			)
+			if len(digests) == 0 {
+				return errors.New("must specify digests")
+			}
+			ctx, cancel := commands.AppContext(context)
+			defer cancel()
+
+			resolver, err := commands.GetResolver(ctx, context)
+			if err != nil {
+				return err
+			}
+
+			ctx = log.WithLogger(ctx, log.G(ctx).WithField("ref", ref))
+
+			log.G(ctx).Debugf("resolving")
+			fetcher, err := resolver.Fetcher(ctx, ref)
+			if err != nil {
+				return err
+			}
+
+			fetcherByDigest, ok := fetcher.(remotes.FetcherByDigest)
+			if !ok {
+				return fmt.Errorf("fetcher %T does not implement remotes.FetcherByDigest", fetcher)
+			}
+
+			for _, f := range digests {
+				dgst, err := digest.Parse(f)
+				if err != nil {
+					return err
+				}
+				rc, _, err := fetcherByDigest.FetchByDigest(ctx, dgst)
+				if err != nil {
+					return err
+				}
+				_, err = io.Copy(os.Stdout, rc)
+				rc.Close()
+				if err != nil {
+					return err
+				}
+			}
+			return nil
+		},
+	}
+
 	pushObjectCommand = cli.Command{
 		Name:        "push-object",
 		Usage:       "push an object to a remote",