vfs: [bugfix] Update dir modification time

A subtle bug where dir modification time is not updated when the dir already exists
in the cache. It is only noticeable when some clients use dir modification time to
invalidate cache.

vfs/dir.go

@@ -694,9 +694,10 @@ func (d *Dir) _readDirFromEntries(entries fs.DirEntries, dirTree dirtree.DirTree
 			if node == nil || !node.IsDir() {
 				node = newDir(d.vfs, d.f, d, item)
 			}
+			dir := node.(*Dir)
+			dir.mu.Lock()
+			dir.modTime = item.ModTime(context.TODO())
 			if dirTree != nil {
-				dir := node.(*Dir)
-				dir.mu.Lock()
 				err = dir._readDirFromDirTree(dirTree, when)
 				if err != nil {
 					dir.read = time.Time{}
@@ -704,10 +705,10 @@ func (d *Dir) _readDirFromEntries(entries fs.DirEntries, dirTree dirtree.DirTree
 					dir.read = when
 					dir.cleanupTimer.Reset(d.vfs.Opt.DirCacheTime * 2)
 				}
-				dir.mu.Unlock()
-				if err != nil {
-					return err
-				}
+			}
+			dir.mu.Unlock()
+			if err != nil {
+				return err
 			}
 		default:
 			err = fmt.Errorf("unknown type %T", item)