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

CHANGELOG.asciidoc

@@ -20,7 +20,8 @@ https://github.com/elastic/apm-server/compare/71df0d96445df35afe27f38bcf734a0828
 ==== Bug fixes
 - Updated systemd doc url {pull}354[354]
 - Updated readme doc urls {pull}356[356]
-- Use updated stack trace frame values for calculating error `grouping_keys` {pull}485[485] 
+- Use updated stack trace frame values for calculating error `grouping_keys` {pull}485[485]
+- Fix panic when a signal is delivered before the server is instantiated {pull}580[580]
 
 ==== Added
 - Include build time and revision in version information {pull}396[396]

beater/beater.go

@@ -6,15 +6,19 @@ import (
 	"net"
 	"net/http"
 	"regexp"
+	"sync"
 
 	"github.com/elastic/beats/libbeat/beat"
 	"github.com/elastic/beats/libbeat/common"
 	"github.com/elastic/beats/libbeat/logp"
 )
 
 type beater struct {
-	config *Config
-	server *http.Server
+	config  *Config
+	mutex   sync.Mutex // guards server and stopped
+	server  *http.Server
+	stopped bool
+	logger *logp.Logger
 }
 
 // Creates beater
@@ -36,14 +40,15 @@ func New(b *beat.Beat, ucfg *common.Config) (beat.Beater, error) {
 	}
 
 	bt := &beater{
-		config: beaterConfig,
+		config:  beaterConfig,
+		stopped: false,
+		logger: logp.NewLogger("beater"),
 	}
 	return bt, nil
 }
 
 func (bt *beater) Run(b *beat.Beat) error {
 	var err error
-	logger := logp.NewLogger("beater")
 
 	pub, err := newPublisher(b.Publisher, bt.config.ConcurrentRequests)
 	if err != nil {
@@ -53,24 +58,36 @@ func (bt *beater) Run(b *beat.Beat) error {
 
 	lis, err := net.Listen("tcp", bt.config.Host)
 	if err != nil {
-		logger.Errorf("failed to listen: %s", err.Error())
+		bt.logger.Errorf("failed to listen: %s", err.Error())
 
 		return err
 	}
 	go notifyListening(bt.config, pub.Send)
 
+	bt.mutex.Lock()
+	if bt.stopped {
+		defer bt.mutex.Unlock()
+		return nil
+	}
+
 	bt.server = newServer(bt.config, pub.Send)
+	bt.mutex.Unlock()
 
 	err = run(bt.server, lis, bt.config)
 	if err == http.ErrServerClosed {
-		logger.Infof("Listener stopped: %s", err.Error())
+		bt.logger.Infof("Listener stopped: %s", err.Error())
 		return nil
 	}
 	return err
 }
 
 // Graceful shutdown
 func (bt *beater) Stop() {
-	logp.NewLogger("beater").Info("stopping apm-server...")
-	stop(bt.server, bt.config.ShutdownTimeout)
+	bt.logger.Info("stopping apm-server...")
+	bt.mutex.Lock()
+	if bt.server != nil {
+		stop(bt.server, bt.config.ShutdownTimeout)
+	}
+	bt.stopped = true
+	bt.mutex.Unlock()
 }