Disable stderr logging unless logfile is selected

main.go

@@ -137,7 +137,7 @@ func main() {
 	toneTestOutput := flag.String("tone-out", "incoming.pcm", "file to save incoming audio to in tone-test mode")
 	tcpOnly := flag.Bool("tcp", false, "disable UDP, force audio through TCP tunnel")
 	logLevel := flag.String("log", "warn", "log level: debug, info, warn, error")
-	logFile := flag.String("logfile", "", "write logs to this file (in addition to stderr)")
+	logFile := flag.String("logfile", "", "write logs to this file (logging is disabled when omitted)")
 
 	flag.Parse()
 
@@ -155,17 +155,17 @@ func main() {
 	default:
 		level = barnlog.LevelWarn
 	}
-	// Default: log to stderr
+	// Logging is opt-in. Select /dev/stderr explicitly when terminal logging is
-	var writer io.Writer = os.Stderr
+	// desired; otherwise library diagnostics must not corrupt terminal output.
+	barnlog.SetLogger(nil)
 	if *logFile != "" {
 		f, err := os.OpenFile(*logFile, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0644)
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "cannot open log file %s: %v\n", *logFile, err)
 		} else {
-			writer = io.MultiWriter(os.Stderr, f)
+			barnlog.SetLogger(barnlog.NewWriterLogger(f, level))
 		}
 	}
-	barnlog.SetLogger(barnlog.NewWriterLogger(writer, level))
 
 	if *profile == true {
 		go func() {