swarm: improve cli output on node promote/demote for unchanged role

As of now promoting (or demoting) a node that has its role
left unchanged will always print a successful message.

This PR fixes the issue by matching the behavior on swarmkit's
swarmctl command and printing a message when desired role is
the current role of the node.

As a result this also avoids calling update when it is not
necessary.

Signed-off-by: Alexandre Beslic <alexandre.beslic@gmail.com>

api/client/node/demote.go

@@ -22,6 +22,10 @@ func newDemoteCommand(dockerCli *client.DockerCli) *cobra.Command {
 
 func runDemote(dockerCli *client.DockerCli, nodes []string) error {
 	demote := func(node *swarm.Node) error {
+		if node.Spec.Role == swarm.NodeRoleWorker {
+			fmt.Fprintf(dockerCli.Out(), "Node %s is already a worker.\n", node.ID)
+			return errNoRoleChange
+		}
 		node.Spec.Role = swarm.NodeRoleWorker
 		return nil
 	}

api/client/node/promote.go

@@ -22,6 +22,10 @@ func newPromoteCommand(dockerCli *client.DockerCli) *cobra.Command {
 
 func runPromote(dockerCli *client.DockerCli, nodes []string) error {
 	promote := func(node *swarm.Node) error {
+		if node.Spec.Role == swarm.NodeRoleManager {
+			fmt.Fprintf(dockerCli.Out(), "Node %s is already a manager.\n", node.ID)
+			return errNoRoleChange
+		}
 		node.Spec.Role = swarm.NodeRoleManager
 		return nil
 	}

api/client/node/update.go

@@ -1,6 +1,7 @@
 package node
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/docker/docker/api/client"
@@ -13,6 +14,10 @@ import (
 	"golang.org/x/net/context"
 )
 
+var (
+	errNoRoleChange = errors.New("role was already set to the requested value")
+)
+
 func newUpdateCommand(dockerCli *client.DockerCli) *cobra.Command {
 	nodeOpts := newNodeOptions()
 
@@ -53,6 +58,9 @@ func updateNodes(dockerCli *client.DockerCli, nodes []string, mergeNode func(nod
 
 		err = mergeNode(&node)
 		if err != nil {
+			if err == errNoRoleChange {
+				continue
+			}
 			return err
 		}
 		err = client.NodeUpdate(ctx, node.ID, node.Version, node.Spec)