common/kafka: rely on mechanism to enable or disable SASL

Instead of username. This should be the same, but the code is more
correct this way.

common/kafka/config.go

@@ -36,9 +36,9 @@ type Configuration struct {
 // SASLConfiguration defines SASL configuration.
 type SASLConfiguration struct {
 	// Username tells the SASL username
-	Username string `validate:"required_with=SASLAlgorithm"`
+	Username string `validate:"required_with=SASLMechanism"`
 	// Password tells the SASL password
-	Password string `validate:"required_with=SASLAlgorithm SASLUsername"`
+	Password string `validate:"required_with=SASLMechanism"`
 	// Mechanism tells the SASL algorithm
 	Mechanism SASLMechanism `validate:"required_with=SASLUsername"`
 	// OAuthTokenURL tells which URL to use to get an OAuthToken
@@ -111,7 +111,7 @@ func NewConfig(config Configuration) (*sarama.Config, error) {
 		kafkaConfig.Net.TLS.Config = tlsConfig
 	}
 	// SASL
-	if config.SASL.Username != "" {
+	if config.SASL.Mechanism != SASLNone {
 		kafkaConfig.Net.SASL.Enable = true
 		kafkaConfig.Net.SASL.User = config.SASL.Username
 		kafkaConfig.Net.SASL.Password = config.SASL.Password
@@ -137,6 +137,8 @@ func NewConfig(config Configuration) (*sarama.Config, error) {
 				tlsConfig,
 				config.SASL.Username, config.SASL.Password,
 				config.SASL.OAuthTokenURL)
+		default:
+			return nil, fmt.Errorf("unknown SASL mechanism: %s", config.SASL.Mechanism)
 		}
 	}
 	return kafkaConfig, nil

common/kafka/config_test.go

@@ -34,8 +34,9 @@ func TestKafkaNewConfig(t *testing.T) {
 					Enable: true,
 				},
 				SASL: SASLConfiguration{
-					Username: "hello",
-					Password: "password",
+					Username:  "hello",
+					Password:  "password",
+					Mechanism: SASLPlain,
 				},
 			},
 		}, {