bisync: fix race when CaptureOutput is used concurrently #8815

Before this change CaptureOutput could trip the race detector when
used concurrently. In particular if go routines using the logging are
outlasting the return from `fun()`.

This fixes the problem with a mutex.

cmd/bisync/bilib/output.go

@@ -4,15 +4,19 @@ package bilib
 import (
 	"bytes"
 	"log/slog"
+	"sync"
 
 	"github.com/rclone/rclone/fs/log"
 )
 
 // CaptureOutput runs a function capturing its output at log level INFO.
 func CaptureOutput(fun func()) []byte {
+	var mu sync.Mutex
 	buf := &bytes.Buffer{}
 	oldLevel := log.Handler.SetLevel(slog.LevelInfo)
 	log.Handler.SetOutput(func(level slog.Level, text string) {
+		mu.Lock()
+		defer mu.Unlock()
 		buf.WriteString(text)
 	})
 	defer func() {
@@ -20,5 +24,7 @@ func CaptureOutput(fun func()) []byte {
 		log.Handler.SetLevel(oldLevel)
 	}()
 	fun()
+	mu.Lock()
+	defer mu.Unlock()
 	return buf.Bytes()
 }