code: func (s *session) markFileNum(num int64) {
  	nextFileNum := num + 1
  	for {
  		old, x := s.stNextFileNum, nextFileNum
  		if old > x {
  			x = old
  		}
  		if atomic.CompareAndSwapInt64(&s.stNextFileNum, old, x) {
  			break
  		}
  	}
  }
docs: // Mark file number as used.