code: func (sb *sandbox) joinLeaveEnd() {
  	sb.Lock()
  	defer sb.Unlock()
  
  	if sb.joinLeaveDone != nil {
  		close(sb.joinLeaveDone)
  		sb.joinLeaveDone = nil
  	}
  }
docs: // joinLeaveEnd marks the end of this join/leave operation and
  // signals the same without race to other join and leave waiters