containerd: allow containers without an image

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/ctr/list.go

@@ -48,9 +48,14 @@ var listCommand = cli.Command{
 		w := tabwriter.NewWriter(os.Stdout, 10, 1, 3, ' ', 0)
 		fmt.Fprintln(w, "ID\tIMAGE\tPID\tSTATUS")
 		for _, c := range containers {
-			image, err := c.Image(ctx)
-			if err != nil {
-				return err
+			var imageName string
+			if image, err := c.Image(ctx); err != nil {
+				if err != containerd.ErrNoImage {
+					return err
+				}
+				imageName = "-"
+			} else {
+				imageName = image.Name()
 			}
 			var (
 				status string
@@ -73,7 +78,7 @@ var listCommand = cli.Command{
 			}
 			if _, err := fmt.Fprintf(w, "%s\t%s\t%d\t%s\n",
 				c.ID(),
-				image.Name(),
+				imageName,
 				pid,
 				status,
 			); err != nil {

container.go

@@ -3,7 +3,6 @@ package containerd
 import (
 	"context"
 	"encoding/json"
-	"fmt"
 	"path/filepath"
 	"sync"
 
@@ -17,7 +16,10 @@ import (
 	"github.com/pkg/errors"
 )
 
-var ErrNoRunningTask = errors.New("no running task")
+var (
+	ErrNoImage       = errors.New("container does not have an image")
+	ErrNoRunningTask = errors.New("no running task")
+)
 
 type Container interface {
 	ID() string
@@ -96,7 +98,7 @@ func (c *container) Task(ctx context.Context, attach IOAttach) (Task, error) {
 // Image returns the image that the container is based on
 func (c *container) Image(ctx context.Context) (Image, error) {
 	if c.c.Image == "" {
-		return nil, fmt.Errorf("container is not based on an image")
+		return nil, ErrNoImage
 	}
 	i, err := c.client.ImageService().Get(ctx, c.c.Image)
 	if err != nil {