Add Cluster Override

When working with multiple clusters it would be nice to temporarily
switch clusters by passing it as an option on the command line (without
changing the selected one) like `kubectl` handles "context" and `gcloud`
handles "project"

cmd/kaf/kaf.go

@@ -117,13 +117,15 @@ var (
 	protoFiles        []string
 	protoExclude      []string
 	verbose           bool
+	clusterOverride   string
 )
 
 func init() {
 	rootCmd.PersistentFlags().StringVar(&cfgFile, "config", "", "config file (default is $HOME/.kaf/config)")
 	rootCmd.PersistentFlags().StringSliceVarP(&brokersFlag, "brokers", "b", nil, "Comma separated list of broker ip:port pairs")
 	rootCmd.PersistentFlags().StringVar(&schemaRegistryURL, "schema-registry", "", "URL to a Confluent schema registry. Used for attempting to decode Avro-encoded messages")
 	rootCmd.PersistentFlags().BoolVarP(&verbose, "verbose", "v", false, "Whether to turn on sarama logging")
+	rootCmd.PersistentFlags().StringVarP(&clusterOverride, "cluster", "c", "", "set a temporary current cluster")
 	cobra.OnInitialize(onInit)
 }
 
@@ -144,6 +146,8 @@ func onInit() {
 		errorExit("Invalid config: %v", err)
 	}
 
+	config.ClusterOverride = clusterOverride
+
 	cluster := config.ActiveCluster()
 	if cluster != nil {
 		// Use active cluster from config

config.go

@@ -32,8 +32,9 @@ type Cluster struct {
 }
 
 type Config struct {
-	CurrentCluster string     `yaml:"current-cluster"`
-	Clusters       []*Cluster `yaml:"clusters"`
+	CurrentCluster  string `yaml:"current-cluster"`
+	ClusterOverride string
+	Clusters        []*Cluster `yaml:"clusters"`
 }
 
 func (c *Config) SetCurrentCluster(name string) error {
@@ -60,12 +61,21 @@ func (c *Config) SetCurrentCluster(name string) error {
 }
 
 func (c *Config) ActiveCluster() *Cluster {
-	if c == nil || c.CurrentCluster == "" {
+	if c == nil {
+		return nil
+	}
+
+	toSearch := c.ClusterOverride
+	if c.ClusterOverride == "" {
+		toSearch = c.CurrentCluster
+	}
+
+	if toSearch == "" {
 		return nil
 	}
 
 	for _, cluster := range c.Clusters {
-		if cluster.Name == c.CurrentCluster {
+		if cluster.Name == toSearch {
 			return cluster
 		}
 	}