改进MySQL的table_cache(2)

如果这是该query打开的第一个表：设置thd->version = refresh_version，这样，当我们打开剩余表的过程中，如果version发生了变化，则需要back off，关闭所有已经打开的并重新打开表

目前refresh_version只会被FLUSH TABLES命令改变

 if (thd->handler_tables)        

    mysql_ha_flush(thd);   //刷新（关闭并标记为re-open）所有需要reopen的表

查询table cache的过程：

 for (table= (TABLE*) hash_first(&open_cache, (uchar*) key, key_length,                  //基于同一个key来查找hash表

                                  &state);

       table && table->in_use ;

       table= (TABLE*) hash_next(&open_cache, (uchar*) key, key_length,

                                 &state))

{

**********************************

flush tables marked for flush.

 Normally, table->s->version contains the value of

      refresh_version from the moment when this table was

      (re-)opened and added to the cache.

      If since then we did (or just started) FLUSH TABLES

      statement, refresh_version has been increased.

      For "name-locked" TABLE instances, table->s->version is set

      to 0 (see lock_table_name for details).

      In case there is a pending FLUSH TABLES or a name lock, we

      need to back off and re-start opening tables.

      If we do not back off now, we may dead lock in case of lock

      order mismatch with some other thread:

      c1: name lock t1; -- sort of exclusive lock

      c2: open t2;      -- sort of shared lock

      c1: name lock t2; -- blocks

      c2: open t1; -- blocks

*********************************

             if (table->needs_reopen_or_name_lock())  //Is this instance of the table should be reopen or represents a name-lock?