code: func (w *IndexWriter) checkpointNoSIS() (err error) {
  	w.Lock() // synchronized
  	defer w.Unlock()
  	w.changeCount++
  	return w.deleter.checkpoint(w.segmentInfos, false)
  }
docs: /*
  Checkpoints with IndexFileDeleter, so it's aware of new files, and
  increments changeCount, so on close/commit we will write a new
  segments file, but does NOT bump segmentInfos.version.
  */