When we have 2 or more subnets in cluster.yaml, loop through all the …

…subnets to automatically find and chooose an appropriate subnet for the specified controllerIP

ref 3rd comment in coreos#439 (comment)

multi-node/aws/pkg/config/config.go

@@ -180,8 +180,9 @@ type StackTemplateOptions struct {
 
 type stackConfig struct {
 	*Config
-	UserDataWorker     string
-	UserDataController string
+	UserDataWorker        string
+	UserDataController    string
+	ControllerSubnetIndex int
 }
 
 func execute(filename string, data interface{}, compress bool) (string, error) {
@@ -227,6 +228,25 @@ func (c Cluster) stackConfig(opts StackTemplateOptions, compressUserData bool) (
 
 	stackConfig.Config.TLSConfig = compactAssets
 
+	controllerIPAddr := net.ParseIP(stackConfig.ControllerIP)
+	if controllerIPAddr == nil {
+		return nil, fmt.Errorf("invalid controllerIP: %s", stackConfig.ControllerIP)
+	}
+	controllerSubnetFound := false
+	lastSubnetIndex := len(stackConfig.Subnets) - 1
+	for i, subnet := range stackConfig.Subnets {
+		_, instanceCIDR, err := net.ParseCIDR(subnet.InstanceCIDR)
+		if err != nil {
+			return nil, fmt.Errorf("invalid instanceCIDR: %v", err)
+		}
+		if instanceCIDR.Contains(controllerIPAddr) {
+			stackConfig.ControllerSubnetIndex = i
+			controllerSubnetFound = true
+		} else if !controllerSubnetFound && i == lastSubnetIndex {
+			return nil, fmt.Errorf("Fail-fast occurred possibly because of a bug: ControllerSubnetIndex couldn't be determined for subnets (%v) and controllerIP (%v)", stackConfig.Subnets, stackConfig.ControllerIP)
+		}
+	}
+
 	if stackConfig.UserDataWorker, err = execute(opts.WorkerTmplFile, stackConfig.Config, compressUserData); err != nil {
 		return nil, fmt.Errorf("failed to render worker cloud config: %v", err)
 	}

multi-node/aws/pkg/config/templates/stack-template.json

@@ -261,7 +261,7 @@
             ],
             "PrivateIpAddress": "{{.ControllerIP}}",
             "SubnetId": {
-              "Ref": "Subnet0"
+              "Ref": "Subnet{{.ControllerSubnetIndex}}"
             }
           }
         ],