ctr: Add images tag subcommand

Adds a subcommand for tagging images managed by containerd.

Usage:

  ctr images tag docker.io/library/alpine:latest docker.io/library/alpine:new

Comes with a --force flag to allow you to gracefully overwrite image references
that already exist.

Signed-off-by: Eli Uriegas <eli.uriegas@docker.com>

cmd/ctr/commands/images/images.go

@@ -46,6 +46,7 @@ var Command = cli.Command{
 		pullCommand,
 		pushCommand,
 		removeCommand,
+		tagCommand,
 		setLabelsCommand,
 	},
 }

cmd/ctr/commands/images/tag.go

@@ -0,0 +1,88 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package images
+
+import (
+	"fmt"
+
+	"github.com/containerd/containerd/cmd/ctr/commands"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/urfave/cli"
+)
+
+var tagCommand = cli.Command{
+	Name:        "tag",
+	Usage:       "tag an image",
+	ArgsUsage:   "[flags] <source_ref> <target_ref> [<target_ref>, ...]",
+	Description: `Tag an image for use in containerd.`,
+	Flags: []cli.Flag{
+		cli.BoolFlag{
+			Name:  "force",
+			Usage: "force target_ref to be created, regardless if it already exists",
+		},
+	},
+	Action: func(context *cli.Context) error {
+		var (
+			ref = context.Args().First()
+		)
+		if ref == "" {
+			return fmt.Errorf("please provide an image reference to tag from")
+		}
+		if context.NArg() <= 1 {
+			return fmt.Errorf("please provide an image reference to tag to")
+		}
+
+		client, ctx, cancel, err := commands.NewClient(context)
+		if err != nil {
+			return err
+		}
+		defer cancel()
+
+		ctx, done, err := client.WithLease(ctx)
+		if err != nil {
+			return err
+		}
+		defer done(ctx)
+
+		imageService := client.ImageService()
+		image, err := imageService.Get(ctx, ref)
+		if err != nil {
+			return err
+		}
+		// Support multiple references for one command run
+		for _, targetRef := range context.Args()[1:] {
+			image.Name = targetRef
+			// Attempt to create the image first
+			if _, err = imageService.Create(ctx, image); err != nil {
+				// If user has specified force and the image already exists then
+				// delete the original image and attempt to create the new one
+				if errdefs.IsAlreadyExists(err) && context.Bool("force") {
+					if err = imageService.Delete(ctx, targetRef); err != nil {
+						return err
+					}
+					if _, err = imageService.Create(ctx, image); err != nil {
+						return err
+					}
+				} else {
+					return err
+				}
+			}
+			fmt.Println(targetRef)
+		}
+		return nil
+	},
+}