Verify required checkpoint parameters are set

Without this change it was possible to run

  crictl checkpoint CTR-ID

without specifying '--export'. The export parameter is, however,
required as it tell the CRI implementation where to store the checkpoint
archive. This commit adds a check to ensure '--export' is set and not
empty.

Signed-off-by: Adrian Reber <[email protected]>

Author: adrianreber

Diff for: cmd/crictl/container.go

@@ -618,13 +618,19 @@ var checkpointContainerCommand = &cli.Command{
 		&cli.StringFlag{
 			Name:    "export",
 			Aliases: []string{"e"},
-			Usage:   "Specify the name of the archive used to export the checkpoint image.",
+			Usage:   "Specify the name of the tar archive (/path/to/checkpoint.tar) used to export the checkpoint image.",
 		},
 	},
 	Action: func(c *cli.Context) error {
 		if c.NArg() == 0 {
 			return fmt.Errorf("ID cannot be empty")
 		}
+		if c.String("export") == "" {
+			return fmt.Errorf(
+				"Cannot checkpoint a container without specifying the checkpoint destination. " +
+				"Use --export=/path/to/checkpoint.tar",
+			)
+		}
 		runtimeClient, err := getRuntimeService(c, 0)
 		if err != nil {
 			return err

Diff for: docs/crictl.md

@@ -369,6 +369,15 @@ CONTAINER           IMAGE               CREATED             STATE
 b25b4f26e3429       busybox:latest      14 seconds ago      Running             busybox             0                   158d7a6665ff3
 ```
 
+### Checkpoint a running container
+
+```sh
+$ crictl checkpoint --export=/path/to/checkpoint.tar 39fcdd7a4f1d4
+39fcdd7a4f1d4
+$ ls /path/to/checkpoint.tar
+/path/to/checkpoint.tar
+```
+
 ## More information
 
 * See the [Kubernetes.io Debugging Kubernetes nodes with crictl doc](https://kubernetes.io/docs/tasks/debug-application-cluster/crictl/)