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。