MySQL：RR分析死锁一列

编程语言 2019-03-14 16:55:04 阅读次数: 0

水平有限 有误请指出
版本：Percona MySQL 5.7.22
对于锁的学习我做了一些输出详细参考如下：
https://github.com/gaopengcarl/percona-server-locks-detail-5.7.22.git
其中有readme

本文也是一个朋友问我死锁问题。@越前

一、问题提出

如下构造方式，问为什么RC模式下不会死锁，RR模式下死锁。

drop table tt;

CREATE TABLE `tt` (
  `id` int(11) NOT NULL,
  `c1` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `c1` (`c1`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert into tt values(1,1);
session 1 session 2
begin;
select * from tt where c1=1 for update;
update tt set id=2 where c1=1;
begin;select * from tt where c1=1 for update;堵塞
select * from tt where c1=1 for update;
死锁回滚

二、分析方式

首先分析session 1 第一句：

select * from tt where c1=1 for update;
执行后的加锁行为

  • RR
---TRANSACTION 231106, ACTIVE 9 sec
3 lock struct(s), heap size 1160, 2 row lock(s)
MySQL thread id 11, OS thread handle 140737153623808, query id 303 localhost root
TABLE LOCK table `test`.`tt` trx id 231106 lock mode IX
RECORD LOCKS space id 127 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231106 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000001; asc     ;;

RECORD LOCKS space id 127 page no 3 n bits 72 index PRIMARY of table `test`.`tt` trx id 231106 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 6; hex 0000000386c0; asc       ;;
 2: len 7; hex aa0000003f0110; asc     ?  ;;
 3: len 4; hex 80000001; asc     ;;
  • RC
---TRANSACTION 231105, ACTIVE 7 sec
3 lock struct(s), heap size 1160, 2 row lock(s)
MySQL thread id 11, OS thread handle 140737153623808, query id 295 localhost root
TABLE LOCK table `test`.`tt` trx id 231105 lock mode IX
RECORD LOCKS space id 127 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231105 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000001; asc     ;;

RECORD LOCKS space id 127 page no 3 n bits 72 index PRIMARY of table `test`.`tt` trx id 231105 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 6; hex 0000000386c0; asc       ;;
 2: len 7; hex aa0000003f0110; asc     ?  ;;
 3: len 4; hex 80000001; asc     ;;

我们可以看到因为 c1是主键因此加锁方式不管怎么样都是LOCK_X|LOCK_REC_NOT_GAP，主键上也是同样的。就是锁住了二级唯一索引和主键的相关记录。

然后分析session 1 第二句：

update tt set id=2 where c1=1;
执行后的加锁行为

这一句比较重要，在二级唯一索引c1上会做一个删除和插入操作，也就是会将原来的1,1记录标记为del flag，同时插入2,1这条记录，这会引起一个锁的继承操作(lock_rec_inherit_to_gap_if_gap_lock调用会出现GAP LOCK)，但是之前还会涉及到唯一性检查因此还涉及到LOCK_S锁和next key lock的存在(对于二级索引做唯一性检查始终是next key lock)。这里的del flag也是形成死锁的重要原因。(对于二级索引的update操作总是先删除然后插入记录，主键则会进行判断是否可以容下新的记录)

  • RR
---TRANSACTION 231106, ACTIVE 1626 sec
5 lock struct(s), heap size 1160, 5 row lock(s), undo log entries 2
MySQL thread id 11, OS thread handle 140737153623808, query id 305 localhost root
TABLE LOCK table `test`.`tt` trx id 231106 lock mode IX
RECORD LOCKS space id 127 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231106 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000001; asc     ;;

RECORD LOCKS space id 127 page no 3 n bits 72 index PRIMARY of table `test`.`tt` trx id 231106 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 4; hex 80000001; asc     ;;
 1: len 6; hex 0000000386c2; asc       ;;
 2: len 7; hex 2c000000410dca; asc ,   A  ;;
 3: len 4; hex 80000001; asc     ;;

RECORD LOCKS space id 127 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231106 lock mode S(LOCK_S) locks gap and rec(LOCK_ORDINARY[next_key_lock])
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000001; asc     ;;

RECORD LOCKS space id 127 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231106 lock mode S(LOCK_S) locks gap before rec(LOCK_GAP)
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000002; asc     ;;
  • RC
5 lock struct(s), heap size 1160, 5 row lock(s), undo log entries 2
MySQL thread id 11, OS thread handle 140737153623808, query id 316 localhost root
TABLE LOCK table `test`.`tt` trx id 231123 lock mode IX
RECORD LOCKS space id 128 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231123 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000001; asc     ;;

RECORD LOCKS space id 128 page no 3 n bits 72 index PRIMARY of table `test`.`tt` trx id 231123 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 32
 0: len 4; hex 80000001; asc     ;;
 1: len 6; hex 0000000386d3; asc       ;;
 2: len 7; hex 370000003206e2; asc 7   2  ;;
 3: len 4; hex 80000001; asc     ;;

RECORD LOCKS space id 128 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231123 lock mode S(LOCK_S) locks gap and rec(LOCK_ORDINARY[next_key_lock])
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000001; asc     ;;

RECORD LOCKS space id 128 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231123 lock mode S(LOCK_S) locks gap before rec(LOCK_GAP)
Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000002; asc     ;;

到这里RR和RC加锁并没有什么不同，因为都是唯一值，同时锁继承也都是二级索引上的都是LOCK_S|LOCK_ORDINARY[next_key_lock]，但是下面就会出现不同了。

然后分析session 2的第一句：

select * from tt where c1=1 for update;

实际上这个时候存在2条c1=1的记录只有1,1标记为删除了，1,2没有提交，都是需要访问的。
然后堵塞行为为：

  • RR

LOCK WAIT 2 lock struct(s), heap size 1160, 1 row lock(s)
MySQL thread id 10, OS thread handle 140737153824512, query id 350 localhost root statistics
select * from tt where c1=1 for update
------- TRX HAS BEEN WAITING 11 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 129 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231146 lock_mode X(LOCK_X) locks gap and rec(LOCK_ORDINARY[next_key_lock]) waiting(LOCK_WAIT)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000001; asc     ;;
  • RC
LOCK WAIT 2 lock struct(s), heap size 1160, 1 row lock(s)
MySQL thread id 10, OS thread handle 140737153824512, query id 325 localhost root statistics
select * from tt where c1=1 for update
------- TRX HAS BEEN WAITING 9 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 128 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231128 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP) waiting(LOCK_WAIT)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000001; asc     ;;

我们这里可以看到对于RR模式下唯一键c1的1,1已经删除了。我做了debug发现这里会在函数中row_search_mvcc加锁前做判断如下：

if (!set_also_gap_locks
            || srv_locks_unsafe_for_binlog
            || trx->isolation_level <= TRX_ISO_READ_COMMITTED
            || (unique_search && !rec_get_deleted_flag(rec, comp))
            || dict_index_is_spatial(index)) {

            goto no_gap_lock;
        } else {
            lock_type = LOCK_ORDINARY;
        }

我们抛开其他来分析这两句

  • trx->isolation_level <= TRX_ISO_READ_COMMITTED
    如果是RC模式则直接上LOCK_REC_NOT_GAP及只锁住记录本身
  • (unique_search && !rec_get_deleted_flag(rec, comp))
    如果不是RC，如果是二级唯一索引并且没有被标记为del flag则标记为LOCK_REC_NOT_GAP。但是如果标记为del flag则标记为LOCK_ORDINARY就是next key lock。

分析session 1的最后一个语句也就是产生死锁的语句：

select * from tt where c1=1 for update;

如上这个语句会访问1,1标记为删除了，1,2没有提交 的两个记录。这个时候就出现了不同。

  • RC
    只需要唯一索引 1,1上 LOCK_REC_NOT_GAP|LOCK_X 及记录索引，这个锁在本事物的第一句语句上已经获得了，因此直接通过了，不需要做检测。
  • RR

需要在唯一索引 1,1上 LOCK_ORDINARY|LOCK_X 也就是就是next key lock。这个锁在本事物中并没有获取过，因此需要重新的检测所的兼容性，最终加入了等待列表。

我们来看一下函数lock_rec_lock_slow，我做debug的时候明显看到了不同的逻辑：

if (lock_rec_has_expl(mode, block, heap_no, trx)) {
 
        /* The trx already has a strong enough lock on rec: do 1,1 key lock RR NEX KEY LOCK  stronager
        nothing */
        lock_rec_print(mode,block,heap_no,index,thr_get_trx(thr));
        err = DB_SUCCESS;

    } else {

        const lock_t* wait_for = lock_rec_other_has_conflicting(
            mode, block, heap_no, trx,index);

        if (wait_for != NULL) {

            /* If another transaction has a non-gap conflicting
            request in the queue, as this transaction does not
            have a lock strong enough already granted on the
            record, we may have to wait. */

            RecLock    rec_lock(thr, index, block, heap_no, mode);

            err = rec_lock.add_to_waitq(wait_for);

        }

三、总结

最终RR下形成了环路

  • session1 首先获得唯一索引上的 1,1记录的 LOCK_REC_NOT_GAP|LOCK_X
  • 然后session 1做update 获得唯一索引上的 1,1记录的 LOCK_ORDINARY(next key lock)|LOCK_S
  • 然后session 2需要获取唯一索引上的 1,1记录的 LOCK_ORDINARY(next key lock)|LOCK_X 发生等待
  • 然后session 1需要获取唯一索引上的 1,1记录的 LOCK_ORDINARY(next key lock)|LOCK_X 加入等待队列进行等待

死锁发生因此发生，而RC模式下最后两部需要获取的都是 LOCK_REC_NOT_GAP|LOCK_X，虽然session 2处于等待但是session因为已经获得相同级别的锁不会在进行检测加锁等待，而直接通过了。

下面是死锁的记录：


*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 117 page no 4 n bits 72 index c1 of table `t1`.`tt` trx id 230530 lock_mode X(LOCK_X) locks gap and rec(LOCK_ORDINARY[next_key_lock]) waiting(LOCK_WAIT)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000001; asc     ;;

*** (2) TRANSACTION:
TRANSACTION 230525, ACTIVE 68 sec starting index read
mysql tables in use 1, locked 1
6 lock struct(s), heap size 1160, 6 row lock(s), undo log entries 2
MySQL thread id 6, OS thread handle 140737153423104, query id 156 localhost root statistics
select * from tt where c1=1 for update
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 117 page no 4 n bits 72 index c1 of table `t1`.`tt` trx id 230525 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000001; asc     ;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 117 page no 4 n bits 72 index c1 of table `t1`.`tt` trx id 230525 lock_mode X(LOCK_X) locks gap and rec(LOCK_ORDINARY[next_key_lock]) waiting(LOCK_WAIT)
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
 0: len 4; hex 80000001; asc     ;;
 1: len 4; hex 80000001; asc     ;;

*** WE ROLL BACK TRANSACTION (1)

四、栈帧参考

最后留下几个栈帧以备分析使用

  • 锁继承
#0  lock_rec_set_nth_bit (lock=0x30b1230, i=3) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/include/lock0priv.ic:91
#1  0x0000000001a5d44a in RecLock::lock_alloc (trx=0x7fffd7803b10, index=0x7ffe7459ff80, mode=546, rec_id=..., size=9)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1484
#2  0x0000000001a5d826 in RecLock::create (this=0x7fffec0c0eb0, trx=0x7fffd7803b10, owns_trx_mutex=false, add_to_hash=true, prdt=0x0)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1537
#3  0x0000000001a5e60c in lock_rec_add_to_queue (type_mode=546, block=0x7fff9adb8150, heap_no=3, index=0x7ffe7459ff80, trx=0x7fffd7803b10, 
    caller_owns_trx_mutex=false) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1853
#4  0x0000000001a611ec in lock_rec_inherit_to_gap_if_gap_lock (block=0x7fff9adb8150, heir_heap_no=3, heap_no=1)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:2829
#5  0x0000000001a62ea3 in lock_update_insert (block=0x7fff9adb8150, rec=0x7fff9b9c408c "\200")
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:3659
#6  0x0000000001c53c25 in btr_cur_optimistic_insert (flags=0, cursor=0x7fffec0c23f0, offsets=0x7fffec0c24c8, heap=0x7fffec0c13e0, entry=0x7ffe7403bb70, 
    rec=0x7fffec0c24c0, big_rec=0x7fffec0c24b8, n_ext=0, thr=0x7ffe7403ba00, mtr=0x7fffec0c1bc0)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/btr/btr0cur.cc:3346
#7  0x0000000001b195fe in row_ins_sec_index_entry_low (flags=0, mode=2, index=0x7ffe7459ff80, offsets_heap=0x7ffe7403bf98, heap=0x7ffe7403c448, entry=0x7ffe7403bb70, 
    trx_id=0, thr=0x7ffe7403ba00, dup_chk_only=false) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0ins.cc:3166
#8  0x0000000001b1a15e in row_ins_sec_index_entry (index=0x7ffe7459ff80, entry=0x7ffe7403bb70, thr=0x7ffe7403ba00, dup_chk_only=false)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0ins.cc:3421
#9  0x0000000001b9e053 in row_upd_sec_index_entry (node=0x7ffe7403b6f8, thr=0x7ffe7403ba00)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0upd.cc:2337
#10 0x0000000001b9e1c3 in row_upd_sec_step (node=0x7ffe7403b6f8, thr=0x7ffe7403ba00)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0upd.cc:2364
  • RR加锁del flag记录
#0  lock_rec_set_nth_bit (lock=0x30b28b8, i=2) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/include/lock0priv.ic:91
#1  0x0000000001a5d44a in RecLock::lock_alloc (trx=0x7fffd7804080, index=0x7ffe74064ea0, mode=259, rec_id=..., size=9)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1484
#2  0x0000000001a5d826 in RecLock::create (this=0x7fffec0c1e00, trx=0x7fffd7804080, owns_trx_mutex=true, add_to_hash=true, prdt=0x0)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1537
#3  0x0000000001a5e1c4 in RecLock::add_to_waitq (this=0x7fffec0c1e00, wait_for=0x30b0e58, prdt=0x0)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1731
#4  0x0000000001a5ee37 in lock_rec_lock_slow (impl=0, mode=3, block=0x7fff4d027b20, heap_no=2, index=0x7ffe74064ea0, thr=0x7ffe7459ff60)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:2004
#5  0x0000000001a5f1ce in lock_rec_lock (impl=false, mode=3, block=0x7fff4d027b20, heap_no=2, index=0x7ffe74064ea0, thr=0x7ffe7459ff60)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:2082
#6  0x0000000001a69a02 in lock_sec_rec_read_check_and_lock (flags=0, block=0x7fff4d027b20, rec=0x7fff4da8c07e "\200", index=0x7ffe74064ea0, offsets=0x7fffec0c2690, 
    mode=LOCK_X, gap_mode=0, thr=0x7ffe7459ff60) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:6457
#7  0x0000000001b717f7 in sel_set_rec_lock (pcur=0x7ffe7459f6d0, rec=0x7fff4da8c07e "\200", index=0x7ffe74064ea0, offsets=0x7fffec0c2690, mode=3, type=0, 
    thr=0x7ffe7459ff60, mtr=0x7fffec0c2180) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0sel.cc:1270
#8  0x0000000001b7ab6a in row_search_mvcc (buf=0x7ffe7405b9c0 "\375\002", mode=PAGE_CUR_GE, prebuilt=0x7ffe7459f4b0, match_mode=1, direction=0)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0sel.cc:5591
  • RC加锁del flag记录
#0  lock_rec_lock_slow (impl=0, mode=1027, block=0x7fff3310cdf0, heap_no=2, index=0x7ffe74076d90, thr=0x7ffe7459fc20)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1962
#1  0x0000000001a5f1ce in lock_rec_lock (impl=false, mode=1027, block=0x7fff3310cdf0, heap_no=2, index=0x7ffe74076d90, thr=0x7ffe7459fc20)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:2082
#2  0x0000000001a69a02 in lock_sec_rec_read_check_and_lock (flags=0, block=0x7fff3310cdf0, rec=0x7fff33bdc07e "\200", index=0x7ffe74076d90, offsets=0x7fffec0c2690, 
    mode=LOCK_X, gap_mode=1024, thr=0x7ffe7459fc20) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:6457
#3  0x0000000001b717f7 in sel_set_rec_lock (pcur=0x7ffe7459f6d0, rec=0x7fff33bdc07e "\200", index=0x7ffe74076d90, offsets=0x7fffec0c2690, mode=3, type=1024, 
    thr=0x7ffe7459fc20, mtr=0x7fffec0c2180) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0sel.cc:1270
#4  0x0000000001b7ab6a in row_search_mvcc (buf=0x7ffe7403ae80 "\375\002", mode=PAGE_CUR_GE, prebuilt=0x7ffe7459f4b0, match_mode=1, direction=0)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0sel.cc:5591
#5  0x00000000019d5493 in ha_innobase::index_read (this=0x7ffe7403cda0, buf=0x7ffe7403ae80 "\375\002", key_ptr=0x7ffe74095600 "", key_len=5, 
    find_flag=HA_READ_KEY_EXACT) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/handler/ha_innodb.cc:9536
#6  0x0000000000f934aa in handler::index_read_map (this=0x7ffe7403cda0, buf=0x7ffe7403ae80 "\375\002", key=0x7ffe74095600 "", keypart_map=1, 
    find_flag=HA_READ_KEY_EXACT) at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/handler.h:2942

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