Use FieldLogger interface to represent logrus logger (#75)

The FieldLogger interface is satisfied by either *Logger or *Entry.
Accepting this interface in place of the concrete *Logger type allows
users to inject a logger with some fields already set. For example, the
application developer might want all logging from the library to have a
`subsystem=kcl` field.

Signed-off-by: Mike Pye <mail@mdpye.co.uk>

logger/logrus.go

@@ -34,12 +34,12 @@ type LogrusLogEntry struct {
 }
 
 type LogrusLogger struct {
-	logger *logrus.Logger
+	logger logrus.FieldLogger
 }
 
 // NewLogrusLogger adapts existing logrus logger to Logger interface.
 // The call is responsible for configuring logrus logger appropriately.
-func NewLogrusLogger(lLogger *logrus.Logger) Logger {
+func NewLogrusLogger(lLogger logrus.FieldLogger) Logger {
 	return &LogrusLogger{
 		logger: lLogger,
 	}

test/logger_test.go

@@ -79,10 +79,20 @@ func TestLogrusLoggerWithConfig(t *testing.T) {
 }
 
 func TestLogrusLogger(t *testing.T) {
-	// adapts to Logger interface
+	// adapts to Logger interface from *logrus.Logger
 	log := logger.NewLogrusLogger(logrus.StandardLogger())
 
 	contextLogger := log.WithFields(logger.Fields{"key1": "value1"})
 	contextLogger.Debugf("Starting with logrus")
 	contextLogger.Infof("Logrus is awesome")
 }
+
+func TestLogrusLoggerWithFieldsAtInit(t *testing.T) {
+	// adapts to Logger interface from *logrus.Entry
+	fieldLogger := logrus.StandardLogger().WithField("key0", "value0")
+	log := logger.NewLogrusLogger(fieldLogger)
+
+	contextLogger := log.WithFields(logger.Fields{"key1": "value1"})
+	contextLogger.Debugf("Starting with logrus")
+	contextLogger.Infof("Structured logging is awesome")
+}