决定服务器锁锁定数据库的锁方案。
Locking scheme that determines which server locks which database.
如果锁文件存在，应该在重新启动服务器之前删除它。
If the lock file is present, delete it before restarting the server.
粗略一看，这好像使用了两次服务器循环，和前面的合作锁代码一样。
At first glance, this seems to take the same two server round-trips as our earlier cooperative locking code.
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