Join the logging of the message and the newline into one Write

Fixes #646

CHANGELOG.asciidoc

@@ -26,6 +26,8 @@ https://github.com/elastic/beats/compare/1.0.0...master[Check the HEAD diff]
 ==== Bugfixes
 
 *Affecting all Beats*
+- Fix logging issue with file based output where newlines could be misplaced
+  during concurrent logging {pull}650[650]
 
 *Packetbeat*
 - Fix setting direction to out and use its value to decide when dropping events if ignore_outgoing is enabled {pull}557[557]

libbeat/logp/file_rotator.go

@@ -66,15 +66,13 @@ func (rotator *FileRotator) WriteLine(line []byte) error {
 			return err
 		}
 	}
+
+	line = append(line, '\n')
 	_, err := rotator.current.Write(line)
 	if err != nil {
 		return err
 	}
-	_, err = rotator.current.Write([]byte("\n"))
-	if err != nil {
-		return err
-	}
-	rotator.current_size += uint64(len(line) + 1)
+	rotator.current_size += uint64(len(line))
 
 	return nil
 }