mount: adjust starting order

avoid possible nil wfs.Server

weed/command/mount_std.go

@@ -246,6 +246,7 @@ func RunMount(option *MountOptions, umask os.FileMode) bool {
 	glog.V(0).Infof("mounted %s%s to %v", *option.filer, mountRoot, dir)
 	server := fs.New(c, nil)
 	seaweedFileSystem.Server = server
+	seaweedFileSystem.StartBackgroundTasks()
 	err = server.Serve(seaweedFileSystem)
 
 	// check if the mount process has an error to report

weed/filesys/wfs.go

@@ -125,8 +125,6 @@ func NewSeaweedFileSystem(option *Option) *WFS {
 			glog.V(4).Infof("InvalidateEntry %s : %v", filePath, err)
 		}
 	})
-	startTime := time.Now()
-	go meta_cache.SubscribeMetaEvents(wfs.metaCache, wfs.signature, wfs, wfs.option.FilerMountRootPath, startTime.UnixNano())
 	grace.OnInterrupt(func() {
 		wfs.metaCache.Shutdown()
 	})
@@ -141,6 +139,11 @@ func NewSeaweedFileSystem(option *Option) *WFS {
 	return wfs
 }
 
+func (wfs *WFS) StartBackgroundTasks() {
+	startTime := time.Now()
+	go meta_cache.SubscribeMetaEvents(wfs.metaCache, wfs.signature, wfs, wfs.option.FilerMountRootPath, startTime.UnixNano())
+}
+
 func (wfs *WFS) Root() (fs.Node, error) {
 	return wfs.root, nil
 }