code: func (d *FSDirectory) onIndexOutputClosed(name string) {
  	d.staleFilesLock.Lock()
  	defer d.staleFilesLock.Unlock()
  	d.staleFiles[name] = true
  }
docs: /*
  Sub classes should call this method on closing an open IndexOuput,
  reporting the name of the file that was closed. FSDirectory needs
  this information to take care of syncing stale files.
  */