api: Omit Cluster, Nodes, and Managers from swarm info when unavailable

Currently these fields are included in the response JSON with zero
values. It's better not to include them if the information is
unavailable (for example, on a worker node).

This turns Cluster into a pointer so that it can be left out.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

api/types/swarm/swarm.go

@@ -179,10 +179,10 @@ type Info struct {
 	Error            string
 
 	RemoteManagers []Peer
-	Nodes          int
-	Managers       int
+	Nodes          int `json:",omitempty"`
+	Managers       int `json:",omitempty"`
 
-	Cluster ClusterInfo
+	Cluster *ClusterInfo `json:",omitempty"`
 }
 
 // Peer represents a peer.

cli/command/system/info.go

@@ -97,7 +97,7 @@ func prettyPrintInfo(dockerCli *command.DockerCli, info types.Info) error {
 			fmt.Fprintf(dockerCli.Out(), " Error: %v\n", info.Swarm.Error)
 		}
 		fmt.Fprintf(dockerCli.Out(), " Is Manager: %v\n", info.Swarm.ControlAvailable)
-		if info.Swarm.ControlAvailable && info.Swarm.Error == "" && info.Swarm.LocalNodeState != swarm.LocalNodeStateError {
+		if info.Swarm.Cluster != nil && info.Swarm.ControlAvailable && info.Swarm.Error == "" && info.Swarm.LocalNodeState != swarm.LocalNodeStateError {
 			fmt.Fprintf(dockerCli.Out(), " ClusterID: %s\n", info.Swarm.Cluster.ID)
 			fmt.Fprintf(dockerCli.Out(), " Managers: %d\n", info.Swarm.Managers)
 			fmt.Fprintf(dockerCli.Out(), " Nodes: %d\n", info.Swarm.Nodes)

daemon/cluster/swarm.go

@@ -407,8 +407,7 @@ func (c *Cluster) Info() types.Info {
 			info.Error = err.Error()
 		}
 
-		// Strip JoinTokens
-		info.Cluster = swarm.ClusterInfo
+		info.Cluster = &swarm.ClusterInfo
 
 		if r, err := state.controlClient.ListNodes(ctx, &swarmapi.ListNodesRequest{}); err != nil {
 			info.Error = err.Error()