Allow swarm init with --availability=drain

This fix adds a new flag `--availability` to `swarm join`.

Related documentation has been updated.

An integration test has been added.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

command/swarm/init.go

@@ -20,6 +20,7 @@ type initOptions struct {
 	// Not a NodeAddrOption because it has no default port.
 	advertiseAddr   string
 	forceNewCluster bool
+	availability    string
 }
 
 func newInitCommand(dockerCli command.Cli) *cobra.Command {
@@ -41,6 +42,7 @@ func newInitCommand(dockerCli command.Cli) *cobra.Command {
 	flags.StringVar(&opts.advertiseAddr, flagAdvertiseAddr, "", "Advertised address (format: <ip|interface>[:port])")
 	flags.BoolVar(&opts.forceNewCluster, "force-new-cluster", false, "Force create a new cluster from current state")
 	flags.BoolVar(&opts.autolock, flagAutolock, false, "Enable manager autolocking (requiring an unlock key to start a stopped manager)")
+	flags.StringVar(&opts.availability, flagAvailability, "active", "Availability of the node (active/pause/drain)")
 	addSwarmFlags(flags, &opts.swarmOptions)
 	return cmd
 }
@@ -56,6 +58,15 @@ func runInit(dockerCli command.Cli, flags *pflag.FlagSet, opts initOptions) erro
 		Spec:             opts.swarmOptions.ToSpec(flags),
 		AutoLockManagers: opts.swarmOptions.autolock,
 	}
+	if flags.Changed(flagAvailability) {
+		availability := swarm.NodeAvailability(strings.ToLower(opts.availability))
+		switch availability {
+		case swarm.NodeAvailabilityActive, swarm.NodeAvailabilityPause, swarm.NodeAvailabilityDrain:
+			req.Availability = availability
+		default:
+			return fmt.Errorf("invalid availability %q, only active, pause and drain are supported", opts.availability)
+		}
+	}
 
 	nodeID, err := client.SwarmInit(ctx, req)
 	if err != nil {