code: func (ms *FileMsgStore) getMsgIndex(slice *fileSlice, seq uint64) (*msgIndex, error) {
  	bm := ms.bufferedMsgs[seq]
  	if bm != nil {
  		return bm.index, nil
  	}
  	return ms.readMsgIndex(slice, seq)
  }
docs: // getMsgIndex returns a msgIndex object for message with sequence `seq`,
  // or nil if message is not found (or no longer valid: expired, removed
  // due to limits, etc).
  // This call first checks that the record is not present in
  // ms.bufferedMsgs since it is possible that message and index are not
  // yet stored on disk.