mysqlのレプリケーションが取れなくなったエラーの対処法

以下のようなエラーが通知された。

Slave_SQL_Running: No

SLAVEのサーバに入り、mysqlの状態を確認。

mysql> SHOW SLAVE STATUS\G
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: ***
                  Master_User: repl
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: mysql-bin.000002
          Read_Master_Log_Pos: 218234266
               Relay_Log_File: mysqld-relay-bin.000007
                Relay_Log_Pos: 3668140
        Relay_Master_Log_File: mysql-bin.000002
             Slave_IO_Running: Yes
            Slave_SQL_Running: No
              Replicate_Do_DB:
          Replicate_Ignore_DB: test,system_db
           Replicate_Do_Table:
       Replicate_Ignore_Table:
      Replicate_Wild_Do_Table:
  Replicate_Wild_Ignore_Table:
                   Last_Errno: 1396
                   Last_Error: Error 'Operation DROP USER failed for 'readonly'@'localhost'' on query. Default database: ''. Query: 'drop user 'readonly'@'localhost''
                 Skip_Counter: 1
          Exec_Master_Log_Pos: 218231194
              Relay_Log_Space: 3746406
              Until_Condition: None
               Until_Log_File:
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File:
           Master_SSL_CA_Path:
              Master_SSL_Cert:
            Master_SSL_Cipher:
               Master_SSL_Key:
        Seconds_Behind_Master: NULL
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error:
               Last_SQL_Errno: 1396
               Last_SQL_Error: Error 'Operation DROP USER failed for 'readonly'@'localhost'' on query. Default database: ''. Query: 'drop user 'readonly'@'localhost''
  Replicate_Ignore_Server_Ids:
             Master_Server_Id: 1
1 row in set (0.00 sec)

readonlyというユーザのdropに失敗している模様。
SLAVEにしか必要ないユーザだったので、原因のクエリをスキップして、レプリケーションを起動させる。

mysql> SET GLOBAL SQL_SLAVE_SKIP_COUNTER = 1;
Query OK, 0 rows affected (0.00 sec)

mysql> START SLAVE SQL_THREAD;
Query OK, 0 rows affected (0.00 sec)

状態を確認。

mysql> SHOW SLAVE STATUS\G
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: ***
                  Master_User: repl
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: mysql-bin.000002
          Read_Master_Log_Pos: 218234609
               Relay_Log_File: mysqld-relay-bin.000007
                Relay_Log_Pos: 3671555
        Relay_Master_Log_File: mysql-bin.000002
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB:
          Replicate_Ignore_DB: test,system_db
           Replicate_Do_Table:
       Replicate_Ignore_Table:
      Replicate_Wild_Do_Table:
  Replicate_Wild_Ignore_Table:
                   Last_Errno: 0
                   Last_Error:
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 218234609
              Relay_Log_Space: 3746749
              Until_Condition: None
               Until_Log_File:
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File:
           Master_SSL_CA_Path:
              Master_SSL_Cert:
            Master_SSL_Cipher:
               Master_SSL_Key:
        Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error:
               Last_SQL_Errno: 0
               Last_SQL_Error:
  Replicate_Ignore_Server_Ids:
             Master_Server_Id: 1
1 row in set (0.00 sec)

問題なく動いた。