# File lib/connection_pool.rb, line 97
  def checkin
    if ::Thread.current[@key]
      if ::Thread.current[@key_count] == 1
        @available.push(::Thread.current[@key])
        ::Thread.current[@key]= nil
      else
        ::Thread.current[@key_count]-= 1
      end
    else
      raise ConnectionPool::Error, 'no connections are checked out'
    end

    nil
  end