Fix race condition in logging

`logp.FileRotator` had an issue when `WriteLines` is called concurrently

CHANGELOG.asciidoc

@@ -31,6 +31,7 @@ https://github.com/elastic/beats/compare/v6.0.0-alpha1...master[Check the HEAD d
 *Affecting all Beats*
 
 - Don't stop with error loading the ES template if the ES output is not enabled. {pull}4436[4436]
+- Fix race condition in internal logging rotator. {pull}4519[4519]
 
 *Filebeat*
 

libbeat/logp/file_rotator.go

@@ -6,6 +6,7 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"sync"
 )
 
 const RotatorMaxFiles = 1024
@@ -21,6 +22,7 @@ type FileRotator struct {
 
 	current     *os.File
 	currentSize uint64
+	currentLock sync.RWMutex
 }
 
 func (rotator *FileRotator) CreateDirectory() error {
@@ -73,16 +75,26 @@ func (rotator *FileRotator) WriteLine(line []byte) error {
 	}
 
 	line = append(line, '\n')
+
+	rotator.currentLock.RLock()
 	_, err := rotator.current.Write(line)
+	rotator.currentLock.RUnlock()
+
 	if err != nil {
 		return err
 	}
+
+	rotator.currentLock.Lock()
 	rotator.currentSize += uint64(len(line))
+	rotator.currentLock.Unlock()
 
 	return nil
 }
 
 func (rotator *FileRotator) shouldRotate() bool {
+	rotator.currentLock.RLock()
+	defer rotator.currentLock.RUnlock()
+
 	if rotator.current == nil {
 		return true
 	}
@@ -112,6 +124,8 @@ func (rotator *FileRotator) FileExists(fileNo int) bool {
 }
 
 func (rotator *FileRotator) Rotate() error {
+	rotator.currentLock.Lock()
+	defer rotator.currentLock.Unlock()
 
 	if rotator.current != nil {
 		if err := rotator.current.Close(); err != nil {

libbeat/logp/file_rotator_test.go

@@ -174,3 +174,32 @@ func TestConfigSane(t *testing.T) {
 	}
 	assert.NotNil(t, rotator.CheckIfConfigSane())
 }
+
+func TestRaceConditions(t *testing.T) {
+	// Make sure concurrent `WriteLine` calls don't end up in race conditions around `rotator.current`
+	if testing.Verbose() {
+		LogInit(LOG_DEBUG, "", false, true, []string{"rotator"})
+	}
+
+	dir, err := ioutil.TempDir("", "test_rotator_")
+	if err != nil {
+		t.Errorf("Error: %s", err.Error())
+		return
+	}
+
+	Debug("rotator", "Directory: %s", dir)
+
+	rotateeverybytes := uint64(10)
+	keepfiles := 20
+
+	rotator := FileRotator{
+		Path:             dir,
+		Name:             "testbeat",
+		RotateEveryBytes: &rotateeverybytes,
+		KeepFiles:        &keepfiles,
+	}
+
+	for i := 0; i < 1000; i++ {
+		go rotator.WriteLine([]byte(string(i)))
+	}
+}