1.背景概述
客户业务发生死锁的报错，根据业务程序日志及业务流程，发现造成死锁的原因是：事务1 delete + insert ，事务2 delete + insert 2个事务交替执行导致的死锁；由于GAP锁阻塞了插入意向锁，并且当delete的数据存在时死锁不会发生，当delete的数据不存在时，会发生死锁。
2.问题复现
本次测试基于 GreatSQL-8.0.32-24，隔离级别为 RR
2.1 创建测试表
greatsql> create database test;
greatsql> create table test(c1 int unique key, c2 int, c3 int, c4 int);
greatsql> insert into test values (1,1,1,1),(3,3,3,3),(5,5,5,5),(9,9,9,9);
greatsql> select * from test;
+------+------+------+------+
| c1 | c2 | c3 | c4 |
+------+------+------+------+
| 1 | 1 | 1 | 1 |
| 3 | 3 | 3 | 3 |
| 5 | 5 | 5 | 5 |
| 9 | 9 | 9 | 9 |
+------+------+------+------+
4 rows in set (0.01 sec)
2.2 事务执行顺序
按以下事务执行顺序，如果要删除的数据存在，则不会发生死锁；如果要删除的数据不存，并且要删除的数据在同一个GAP锁的区间内则会发生死锁；
| 时间 | 事务1 | 事务2 |
|---|---|---|
| T1 | BEGIN; | BEGIN; |
| T2 | delete from test where c1=?; | |
| T3 | delete from test where c1=?; | |
| T4 | insert into test value(?,?,?,?); | |
| T5 | insert into test value(?,?,?,?); |
2.3 当delete的数据存在时
事务1：delete
greatsql> begin;
Query OK, 0 rows affected (0.00 sec)
greatsql> delete from test where c1=3;
Query OK, 1 row affected (0.00 sec)
查看锁信息：
greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
| THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
| 59 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 59 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 3, 0x000000000201 |
| 59 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000201 |
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
3 rows in set (0.00 sec)
此时事务1给 3, 0x000000000201 这条数据加了 记录锁 X,REC_NOT_GAP
事务2：delete
greatsql> begin;
Query OK, 0 rows affected (0.00 sec)
greatsql> delete from test where c1=5;
Query OK, 1 row affected (0.00 sec)
查看锁信息：
greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
| THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
| 57 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 57 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 5, 0x000000000202 |
| 57 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000202 |
| 59 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 59 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 3, 0x000000000201 |
| 59 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000201 |
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
6 rows in set (0.00 sec)
此时事务2给 5, 0x000000000202 这条数据加了 记录锁 X,REC_NOT_GAP
事务1：insert
greatsql> insert into test value(3,3,3,3);
Query OK, 1 row affected (0.00 sec)
查看锁信息：
greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
| THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
| 57 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 57 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 5, 0x000000000202 |
| 57 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000202 |
| 59 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 59 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 3, 0x000000000201 |
| 59 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000201 |
| 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 3, 0x000000000201 |
| 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 5, 0x000000000202 |
| 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 3, 0x000000000206 |
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
9 rows in set (0.00 sec)
此时事务1给 被delete删除的数据 3, 0x000000000201 ，插入的数据 3, 0x000000000202，以及相邻的下一条数据 5, 0x000000000202 加了 间隙锁 S,GAP
事务2：insert
greatsql> insert into test value(5,5,5,5);
Query OK, 1 row affected (0.01 sec)
查看锁信息：
greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
| THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
| 57 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 57 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 5, 0x000000000202 |
| 57 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000202 |
| 57 | test | test | c1 | RECORD | S,GAP | GRANTED | 5, 0x000000000202 |
| 57 | test | test | c1 | RECORD | S,GAP | GRANTED | 9, 0x000000000203 |
| 57 | test | test | c1 | RECORD | S,GAP | GRANTED | 5, 0x000000000207 |
| 59 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 59 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 3, 0x000000000201 |
| 59 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000201 |
| 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 3, 0x000000000201 |
| 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 5, 0x000000000202 |
| 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 3, 0x000000000206 |
+-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+
12 rows in set (0.00 sec)
此时事务2给 被delete删除的数据 5, 0x000000000202 ，插入的数据 5, 0x000000000207，以及相邻的下一条数据 9, 0x000000000203 加了 间隙锁 S,GAP
由于GAP锁之间是相互兼容的，所以没有发生锁等待及死锁，此时事务1，事务2都执行完成，可以正常提交。
2.4 当delete的数据不存在时
事务1：delete
greatsql> begin;
Query OK, 0 rows affected (0.01 sec)
greatsql> delete from test where c1=6;
Query OK, 0 rows affected (0.00 sec)
查看锁信息：
greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+
| THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+
| 61 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 61 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 |
+-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+
2 rows in set (0.00 sec)
此时事务1给 9, 0x000000000203 这条数据加了 间隙锁 X,GAP
事务2：delete
greatsql> begin;
Query OK, 0 rows affected (0.00 sec)
greatsql> delete from test where c1=7;
Query OK, 0 rows affected (0.00 sec)
查看锁信息：
greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+
| THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+
| 62 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 62 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 |
| 61 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 61 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 |
+-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+
4 rows in set (0.00 sec)
此时事务2给 9, 0x000000000203 这条数据加了 间隙锁 X,GAP 间隙锁可以相互兼容，因此没有报错
事务1：insert
greatsql> insert into test value(6,6,6,6);
---hang住，处于锁等待
查看锁信息：
greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+-----------+---------------+-------------+------------+-----------+------------------------+-------------+-------------------+
| THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------+---------------+-------------+------------+-----------+------------------------+-------------+-------------------+
| 62 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 62 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 |
| 61 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 61 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 |
| 61 | test | test | c1 | RECORD | X,GAP,INSERT_INTENTION | WAITING | 9, 0x000000000203 |
+-----------+---------------+-------------+------------+-----------+------------------------+-------------+-------------------+
5 rows in set (0.00 sec)
此时事务1，要申请给 9, 0x000000000203 这条数据加 间隙锁，插入意向锁 X,GAP,INSERT_INTENTION；由于事务2已经加了 间隙锁 X,GAP 间隙锁与插入意向锁并不兼容，因此事务1的insert处于锁等待状态
事务2：insert
greatsql> insert into test value(7,7,7,7);
ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction
查看锁信息：
greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks;
+-----------+---------------+-------------+-----------------+-----------+------------------------+-------------+------------------------+
| THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA |
+-----------+---------------+-------------+-----------------+-----------+------------------------+-------------+------------------------+
| 61 | test | test | NULL | TABLE | IX | GRANTED | NULL |
| 61 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 |
| 61 | test | test | c1 | RECORD | X,GAP | GRANTED | 6, 0x000000000213 |
| 61 | test | test | GEN_CLUST_INDEX | RECORD | X | GRANTED | supremum pseudo-record |
| 61 | test | test | GEN_CLUST_INDEX | RECORD | X,GAP | GRANTED | 0x000000000213 |
| 61 | test | test | c1 | RECORD | X,GAP,INSERT_INTENTION | GRANTED | 9, 0x000000000203 |
+-----------+---------------+-------------+-----------------+-----------+------------------------+-------------+------------------------+
6 rows in set (0.01 sec)
事务2回滚，只有事务1的加锁信息。
由于此时事务2，要申请给 9, 0x000000000203 这条数据加 间隙锁，插入意向锁由于事务1已经加了 间隙锁 X,GAP 间隙锁与插入意向锁并不兼容，因此事务2的insert处于锁等待状态，2个事务相互等待锁导致死锁，此时事务2回滚。
3.总结
此次死锁的发生主要是GAP 锁 和 插入意向锁的冲突，建议让业务修改一下逻辑，先判断数据是否存在 select 一下，存在的话 delete 后在 insert ； 不存在的话直接 insert 不用delete了。
Enjoy GreatSQL 😃
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