Fix panic when a signal is delivered before the server is instantiated (

elastic#580)

Introduce a flag and a mutex in the beater so that we can't stop the server before assigning it (ie. read before write)
fixes elastic#514

beater/beater.go

@@ -18,6 +18,7 @@ type beater struct {
 	mutex   sync.Mutex // guards server and stopped
 	server  *http.Server
 	stopped bool
+	logger *logp.Logger
 }
 
 // Creates beater
@@ -41,6 +42,7 @@ func New(b *beat.Beat, ucfg *common.Config) (beat.Beater, error) {
 	bt := &beater{
 		config:  beaterConfig,
 		stopped: false,
+		logger: logp.NewLogger("beater"),
 	}
 	return bt, nil
 }
@@ -56,7 +58,8 @@ func (bt *beater) Run(b *beat.Beat) error {
 
 	lis, err := net.Listen("tcp", bt.config.Host)
 	if err != nil {
-		logp.Err("failed to listen: %s", err)
+		bt.logger.Errorf("failed to listen: %s", err.Error())
+
 		return err
 	}
 	go notifyListening(bt.config, pub.Send)
@@ -72,15 +75,15 @@ func (bt *beater) Run(b *beat.Beat) error {
 
 	err = run(bt.server, lis, bt.config)
 	if err == http.ErrServerClosed {
-		logp.Info("Listener stopped: %s", err.Error())
+		bt.logger.Infof("Listener stopped: %s", err.Error())
 		return nil
 	}
 	return err
 }
 
 // Graceful shutdown
 func (bt *beater) Stop() {
-	logp.Info("stopping apm-server...")
+	bt.logger.Info("stopping apm-server...")
 	bt.mutex.Lock()
 	if bt.server != nil {
 		stop(bt.server, bt.config.ShutdownTimeout)