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MYSQL5.6super_read_only和EventScheduler冲突导致启动失败该怎么办

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关于MYSQL 5.6 super_read_only和Event Scheduler冲突导致启动失败

版本percona-server-5.6.22-72.0，这个版本肯定有这个问题
这个问题出现在线上我们将库设置为super_read_only后启动报错，整个MYSQLD CRASH掉
2017-04-23 01:15:46 22577 [ERROR] Event Scheduler: Failed to open table mysql.event
2017-04-23 01:15:46 22577 [ERROR] Event Scheduler: Error while loading from disk.
2017-04-23 01:15:46 22577 [Note] Event Scheduler: Purging the queue. 0 events
2017-04-23 01:15:46 22577 [ERROR] Aborting
在5.6.25以及其他5.7的版本上并没有这个问题。网上说可能是BUG但是没有说出具体原因，也有
帖子说和super_read_only 有关，确实关闭super_read_only就不会再出问题，
但是为了找到问题，我想挖一挖代码如下：
首先要找到报错源头，首先找到Event Scheduler: Error while loading from disk.的位置如下：
这段代码出现在
events.cc的Events::init(my_bool opt_noacl_or_bootstrap) 方法下
 if (event_queue->init_queue(thd) || load_events_from_db(thd) ||
      (opt_event_scheduler == EVENTS_ON && scheduler->start(&err_no)))
  {
    sql_print_error("Event Scheduler: Error while loading from disk."); --这里
    res= TRUE; /* fatal error: request unireg_abort */
    goto end;
  }
这里3个条件都可能引起这个错误：
1、初始化队列失败，函数接口event_queue->init_queue
2、加载event数据失败，函数接口load_events_from_db
3、参数event_scheduler设置是否为ON，并且scheduler->start调用失败


而错误 Event Scheduler: Failed to open table mysql.event 正是load_events_from_db(thd)这个
方法报出来的，查看其部分代码
/*
    NOTE: even if we run in read-only mode, we should be able to lock the
    mysql.event table for writing. In order to achieve this, we should call
    mysql_lock_tables() under the super user.


    Same goes for transaction access mode.
    Temporarily reset it to read-write.
  */--这里源码也有不算清楚的解释


  saved_master_access= thd->security_ctx->master_access;
  thd->security_ctx->master_access |= SUPER_ACL;
  bool save_tx_read_only= thd->tx_read_only;
  thd->tx_read_only= false;


  ret= db_repository->open_event_table(thd, TL_WRITE, &table); --这里的返回值进行判断


  thd->tx_read_only= save_tx_read_only;
  thd->security_ctx->master_access= saved_master_access;


  if (ret)
  {
    sql_print_error("Event Scheduler: Failed to open table mysql.event"); ---这里
    DBUG_RETURN(TRUE);
  }
我们可以看到他的是通过调用db_repository->open_event_table(thd, TL_WRITE, &table)来
接收其返回值如果为true则报错。接下来看open_event_table
bool
Event_db_repository::open_event_table(THD *thd, enum thr_lock_type lock_type,
                                      TABLE **table)
{
  TABLE_LIST tables;
  DBUG_ENTER("Event_db_repository::open_event_table");


  tables.init_one_table("mysql", 5, "event", 5, "event", lock_type);


  if (open_and_lock_tables(thd, &tables, FALSE, MYSQL_LOCK_IGNORE_TIMEOUT))
    DBUG_RETURN(TRUE);


  *table= tables.table;
  tables.table->use_all_columns();


  if (table_intact.check(*table, &event_table_def))
  {
    close_thread_tables(thd);
    my_error(ER_EVENT_OPEN_TABLE_FAILED, MYF(0));
    DBUG_RETURN(TRUE);
  }


  DBUG_RETURN(FALSE);
}
大概这个函数会判断
1、event表是否可以lock
2、event表是否损坏


最终会调用lock_tables-->mysql_lock_tables，如果mysql_lock_locks返回一个
NULL指针则报错如下：


    DEBUG_SYNC(thd, "before_lock_tables_takes_lock");


    if (! (thd->lock= mysql_lock_tables(thd, start, (uint) (ptr - start), flags))) --如果mysql_lock_tables返回一个NULL给thd->lock，产生异常
      DBUG_RETURN(TRUE);


    DEBUG_SYNC(thd, "after_lock_tables_takes_lock");


接下来mysql_lock_tables 又调用lock_tables_check函数进行table
lock的检查，如果lock_tables_check函数返回一个大于0的值则异常
那么调用流程清晰了
Events::init-->Events::load_events_from_db-->open_event_table
-->open_and_lock_tables -->lock_tables-->mysql_lock_tables-->
lock_tables_check


最终我们分析掉这个版本的BUG由于lock_tables_check函数检查返回异常
而导致这个错误。
函数调用栈如下:
#0  mysql_lock_tables (thd=0x1c0b5e0, tables=0x1b62ca0, count=1, flags=2048) at /home/percona-server-5.6.22-72.0/sql/lock.cc:296
#1  0x00000000007910c9 in lock_tables (thd=0x1c0b5e0, tables=0x7fffffffdae0, count=1, flags=2048) at /home/percona-server-5.6.22-72.0/sql/sql_base.cc:6125
#2  0x000000000079086f in open_and_lock_tables (thd=0x1c0b5e0, tables=0x7fffffffdae0, derived=false, flags=2048, prelocking_strategy=0x7fffffffda90)
    at /home/percona-server-5.6.22-72.0/sql/sql_base.cc:5889
#3  0x0000000000781ed2 in open_and_lock_tables (thd=0x1c0b5e0, tables=0x7fffffffdae0, derived=false, flags=2048)
    at /home/percona-server-5.6.22-72.0/sql/sql_base.h:477
#4  0x0000000000a26d20 in Event_db_repository::check_system_tables (thd=0x1c0b5e0) at /home/percona-server-5.6.22-72.0/sql/event_db_repository.cc:1202
#5  0x00000000008ff5fb in Events::init (opt_noacl_or_bootstrap=0 '\000') at /home/percona-server-5.6.22-72.0/sql/events.cc:858
#6  0x000000000063e21d in mysqld_main (argc=83, argv=0x18f4c58) at /home/percona-server-5.6.22-72.0/sql/mysqld.cc:5784
#7  0x0000000000632634 in main (argc=11, argv=0x7fffffffe398) at /home/percona-server-5.6.22-72.0/sql/main.cc:25

跟踪lock_tables_check函数发现如下


198         if (!(flags & MYSQL_LOCK_IGNORE_GLOBAL_READ_ONLY) && !t->s->tmp_table)
(gdb) n
200           if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE &&
(gdb) n
204                      opt_super_readonly ? "--read-only (super)" : "--read-only");
(gdb) n
205             DBUG_RETURN(1); 
上面由于这段代码：
200           if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE &&
201               enforce_ro && opt_readonly && !thd->slave_thread)
202           {
203             my_error(ER_OPTION_PREVENTS_STATEMENT, MYF(0),
204                      opt_super_readonly ? "--read-only (super)" : "--read-only");
205             DBUG_RETURN(1);
206           }

if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE && enforce_ro && opt_readonly && !thd->slave_thread)
条件满足返回了 DBUG_RETURN(1); 造成整个报错
而这里
enforce_ro 起到了关键作用，也是问题的根源，相信和这里有关
  if (!opt_super_readonly)
    enforce_ro= !(thd->security_ctx->master_access & SUPER_ACL);

(gdb) p  t->alias
$12 = 0x1c5b4d0 "event"
(gdb) list
200           if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE &&
201               enforce_ro && opt_readonly && !thd->slave_thread)
202           {
203             my_error(ER_OPTION_PREVENTS_STATEMENT, MYF(0),
204                      opt_super_readonly ? "--read-only (super)" : "--read-only");
205             DBUG_RETURN(1);
206           }
207         }
208       }
209
(gdb) p enforce_ro
$13 = true
(gdb) p  t->alias
$15 = 0x1c5b4d0 "event"
(gdb) p thd->security_ctx->master_access
$16 = 32768

我们研究下 
lock_tables_check(THD *thd, TABLE **tables, uint count, uint flags)
{
  uint system_count= 0, i= 0;
  bool enforce_ro= true;
  /*
    Identifies if the executed sql command can updated either a log
    or rpl info table.
  */
  bool log_table_write_query= false;

  DBUG_ENTER("lock_tables_check");

  if (!opt_super_readonly)
    enforce_ro= !(thd->security_ctx->master_access & SUPER_ACL);
 后面省略

这句问题的根源，默认enforce_ro= true; 
如果opt_super_readonly开启则
enforce_ro= !(thd->security_ctx->master_access & SUPER_ACL);不运行
那么enforce_ro= true

如果opt_super_readonly不开启则
enforce_ro= !(thd->security_ctx->master_access & SUPER_ACL);运行
SUPER_ACL是一个宏#define SUPER_ACL (1L << 15)
当做4字节INT的话,及
1000 0000 0000 0000
而thd->security_ctx->master_access是32768及
1000 0000 0000 0000
显然他们按位与得到是1 及ture，然后!true
所以enforce_ro= false;
如果为flase则
200           if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE &&
201               enforce_ro && opt_readonly && !thd->slave_thread)
202           {
203             my_error(ER_OPTION_PREVENTS_STATEMENT, MYF(0),
204                      opt_super_readonly ? "--read-only (super)" : "--read-only");
205             DBUG_RETURN(1);
206           }
不会执行，则不会再次报错

所以解决这个问题或者说BUG，就是设置如果opt_super_readonly不开启，
就是不要设置super_read_only权限。

在5.7.14版本中，我粗略查看lock_tables_check代码，有改动。5.7.14没遇到这个 问题

  1. if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE &&

  2.         check_readonly(thd, true))

  3.           DBUG_RETURN(1);


可以看到 enforce_ro已经不再作为判断的标准


而5.6.22这个版本确实有这个问题，但是这个问题不是每个版本都有。如果遇到可以参考。

附带5.6.22，5.7.14代码：
5.6.22

点击(此处)折叠或打开

  1. static int

  2. lock_tables_check(THD *thd, TABLE **tables, uint count, uint flags)

  3. {

  4.   uint system_count= 0, i= 0;

  5.   bool enforce_ro= true;

  6.   /*

  7.     Identifies if the executed sql command can updated either a log

  8.     or rpl info table.

  9.   */

  10.   bool log_table_write_query= false;

  12.   DBUG_ENTER("lock_tables_check");

  14.   if (!opt_super_readonly)

  15.     enforce_ro= !(thd->security_ctx->master_access & SUPER_ACL);

  16.   log_table_write_query=

  17.      is_log_table_write_query(thd->lex->sql_command);

  19.   for (i=0 ; i

  20.   {

  21.     TABLE *t= tables[i];

  23.     /* Protect against 'fake' partially initialized TABLE_SHARE */

  24.     DBUG_ASSERT(t->s->table_category != TABLE_UNKNOWN_CATEGORY);

  26.     /*

  27.       Table I/O to performance schema tables is performed

  28.       only internally by the server implementation.

  29.       When a user is requesting a lock, the following

  30.       constraints are enforced:

  31.     */

  32.     if (t->s->table_category == TABLE_CATEGORY_LOG &&

  33.         (flags & MYSQL_LOCK_LOG_TABLE) == 0 &&

  34.         !log_table_write_query)

  35.     {

  36.       /*

  37.         A user should not be able to prevent writes,

  38.         or hold any type of lock in a session,

  39.         since this would be a DOS attack.

  40.       */

  41.       if (t->reginfo.lock_type >= TL_READ_NO_INSERT ||

  42.           thd->lex->sql_command == SQLCOM_LOCK_TABLES)

  43.       {

  44.         my_error(ER_CANT_LOCK_LOG_TABLE, MYF(0));

  45.         DBUG_RETURN(1);

  46.       }

  47.     }

  49.     if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE)

  50.     {

  51.       if (t->s->table_category == TABLE_CATEGORY_SYSTEM)

  52.         system_count++;

  54.       if (t->db_stat & HA_READ_ONLY)

  55.       {

  56.         my_error(ER_OPEN_AS_READONLY, MYF(0), t->alias);

  57.         DBUG_RETURN(1);

  58.       }

  59.     }

  61.     /*

  62.       If we are going to lock a non-temporary table we must own metadata

  63.       lock of appropriate type on it (I.e. for table to be locked for

  64.       write we must own metadata lock of MDL_SHARED_WRITE or stronger

  65.       type. For table to be locked for read we must own metadata lock

  66.       of MDL_SHARED_READ or stronger type).

  67.       The only exception are HANDLER statements which are allowed to

  68.       lock table for read while having only MDL_SHARED lock on it.

  69.     */

  70.     DBUG_ASSERT(t->s->tmp_table ||

  71.                 thd->mdl_context.is_lock_owner(MDL_key::TABLE,

  72.                                  t->s->db.str, t->s->table_name.str,

  73.                                  t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE ?

  74.                                  MDL_SHARED_WRITE : MDL_SHARED_READ) ||

  75.                 (t->open_by_handler &&

  76.                  thd->mdl_context.is_lock_owner(MDL_key::TABLE,

  77.                                   t->s->db.str, t->s->table_name.str,

  78.                                   MDL_SHARED)));

  80.     /*

  81.       Prevent modifications to base tables if READ_ONLY is activated.

  82.       In any case, read only does not apply to temporary tables.

  83.     */

  84.     if (!(flags & MYSQL_LOCK_IGNORE_GLOBAL_READ_ONLY) && !t->s->tmp_table)

  85.     {

  86.       if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE &&

  87.           enforce_ro && opt_readonly && !thd->slave_thread)

  88.       {

  89.         my_error(ER_OPTION_PREVENTS_STATEMENT, MYF(0),

  90.                  opt_super_readonly ? "--read-only (super)" : "--read-only");

  91.         DBUG_RETURN(1);

  92.       }

  93.     }

  94.   }

  96.   /*

  97.     Locking of system tables is restricted:

  98.     locking a mix of system and non-system tables in the same lock

  99.     is prohibited, to prevent contention.

  100.   */

  101.   if ((system_count > 0) && (system_count < count))

  102.   {

  103.     my_error(ER_WRONG_LOCK_OF_SYSTEM_TABLE, MYF(0));

  104.     DBUG_RETURN(1);

  105.   }

  107.   DBUG_RETURN(0);

  108. }





5.7.14

点击(此处)折叠或打开

  1. static int

  2. lock_tables_check(THD *thd, TABLE **tables, size_t count, uint flags)

  3. {

  4.   uint system_count= 0, i= 0;

  5.   /*

  6.     Identifies if the executed sql command can updated either a log

  7.     or rpl info table.

  8.   */

  9.   bool log_table_write_query= false;

  11.   DBUG_ENTER("lock_tables_check");

  13.   log_table_write_query=

  14.      is_log_table_write_query(thd->lex->sql_command);

  16.   for (i=0 ; i

  17.   {

  18.     TABLE *t= tables[i];

  20.     /* Protect against 'fake' partially initialized TABLE_SHARE */

  21.     DBUG_ASSERT(t->s->table_category != TABLE_UNKNOWN_CATEGORY);

  23.     /*

  24.       Table I/O to performance schema tables is performed

  25.       only internally by the server implementation.

  26.       When a user is requesting a lock, the following

  27.       constraints are enforced:

  28.     */

  29.     if (t->s->table_category == TABLE_CATEGORY_LOG &&

  30.         (flags & MYSQL_LOCK_LOG_TABLE) == 0 &&

  31.         !log_table_write_query)

  32.     {

  33.       /*

  34.         A user should not be able to prevent writes,

  35.         or hold any type of lock in a session,

  36.         since this would be a DOS attack.

  37.       */

  38.       if (t->reginfo.lock_type >= TL_READ_NO_INSERT ||

  39.           thd->lex->sql_command == SQLCOM_LOCK_TABLES)

  40.       {

  41.         my_error(ER_CANT_LOCK_LOG_TABLE, MYF(0));

  42.         DBUG_RETURN(1);

  43.       }

  44.     }

  46.     if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE)

  47.     {

  48.       if (t->s->table_category == TABLE_CATEGORY_SYSTEM)

  49.         system_count++;

  51.       if (t->db_stat & HA_READ_ONLY)

  52.       {

  53.         my_error(ER_OPEN_AS_READONLY, MYF(0), t->alias);

  54.         DBUG_RETURN(1);

  55.       }

  56.     }

  58.     /*

  59.       If we are going to lock a non-temporary table we must own metadata

  60.       lock of appropriate type on it (I.e. for table to be locked for

  61.       write we must own metadata lock of MDL_SHARED_WRITE or stronger

  62.       type. For table to be locked for read we must own metadata lock

  63.       of MDL_SHARED_READ or stronger type).

  64.     */

  65.     DBUG_ASSERT(t->s->tmp_table ||

  66.                 thd->mdl_context.owns_equal_or_stronger_lock(MDL_key::TABLE,

  67.                                    t->s->db.str, t->s->table_name.str,

  68.                                    t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE ?

  69.                                    MDL_SHARED_WRITE : MDL_SHARED_READ));

  71.     /*

  72.       Prevent modifications to base tables if READ_ONLY is activated.

  73.       In any case, read only does not apply to temporary tables.

  74.     */

  75.     if (!(flags & MYSQL_LOCK_IGNORE_GLOBAL_READ_ONLY) && !t->s->tmp_table)

  76.     {

  77.       if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE &&

  78.         check_readonly(thd, true))

  79.           DBUG_RETURN(1);

  80.     }

  81.   }

  83.   /*

  84.     Locking of system tables is restricted:

  85.     locking a mix of system and non-system tables in the same lock

  86.     is prohibited, to prevent contention.

  87.   */

  88.   if ((system_count > 0) && (system_count < count))

  89.   {

  90.     my_error(ER_WRONG_LOCK_OF_SYSTEM_TABLE, MYF(0));

  91.     DBUG_RETURN(1);

  92.   }

  94.   DBUG_RETURN(0);

  95. }

以上就是MYSQL 5.6 super_read_only和Event Scheduler冲突导致启动失败该怎么办，小编相信有部分知识点可能是我们日常工作会见到或用到的。希望你能通过这篇文章学到更多知识。更多详情敬请关注创新互联行业资讯频道。


标题名称：MYSQL5.6super_read_only和EventScheduler冲突导致启动失败该怎么办
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