class ConnectionPool

def checkin(force: false)
  if ::Thread.current[@key]
    if ::Thread.current[@key_count] == 1 || force
      if ::Thread.current[@discard_key]
        begin
          @available.decrement_created
          ::Thread.current[@discard_key].call(::Thread.current[@key])
        rescue
          nil
        ensure
          ::Thread.current[@discard_key] = nil
        end
      else
        @available.push(::Thread.current[@key])
      end
      ::Thread.current[@key] = nil
      ::Thread.current[@key_count] = nil
    else
      ::Thread.current[@key_count] -= 1
    end
  elsif !force
    raise ConnectionPool::Error, "no connections are checked out"
  end
  nil
end