Issue warning on unsupported CDI hook

To allow for CDI hooks to be added gradually we provide a generic no-op hook
for unrecognised subcommands. This will log a warning instead of erroring out.

An unsupported hook could be the result of a CDI specification referring to a
new hook that is not yet supported by an older NVIDIA Container Toolkit
version or a hook that has been removed in newer version.

Signed-off-by: Evan Lezar <elezar@nvidia.com>

cmd/nvidia-ctk/hook/hook.go

@@ -27,7 +27,7 @@ type hookCommand struct {
 	logger logger.Interface
 }
 
-// NewCommand constructs a hook command with the specified logger
+// NewCommand constructs CLI subcommand for handling CDI hooks.
 func NewCommand(logger logger.Interface) *cli.Command {
 	c := hookCommand{
 		logger: logger,
@@ -37,10 +37,21 @@ func NewCommand(logger logger.Interface) *cli.Command {
 
 // build
 func (m hookCommand) build() *cli.Command {
-	// Create the 'hook' command
+	// Create the 'hook' subcommand
 	hook := cli.Command{
 		Name:  "hook",
 		Usage: "A collection of hooks that may be injected into an OCI spec",
+		// We set the default action for the `hook` subcommand to issue a
+		// warning and exit with no error.
+		// This means that if an unsupported hook is run, a container will not fail
+		// to launch. An unsupported hook could be the result of a CDI specification
+		// referring to a new hook that is not yet supported by an older NVIDIA
+		// Container Toolkit version or a hook that has been removed in newer
+		// version.
+		Action: func(ctx *cli.Context) error {
+			commands.IssueUnsupportedHookWarning(m.logger, ctx)
+			return nil
+		},
 	}
 
 	hook.Subcommands = commands.New(m.logger)