insert事务产生duplicate key error引发的死锁分析

先看程序报错：

2017-06-12 21:18:40.856 [ForkJoinPool.commonPool-worker-12] ERROR com.jd.gms.maindata.accurate.service.impl.FixServiceImpl[51] - FixServiceImpl.fixSkuAttribute error java.lang.RuntimeException: org.springframework.dao.DeadlockLoserDataAccessException: ### Error updating database. Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLTransactionRollbackException: Deadlock found when trying to get lock; try restarting transaction ### The error may involve SkuAttribute.draftInsert-Inline ### The error occurred while setting parameters ### SQL: insert into sku_1 ( sku_id, prop_type, com_att_id, com_att_val_id, val, source, category_id, version, created, modified ) values ( ?, ?, ?, ?, ?, ?, ?, ?, now(), now() ) ON DUPLICATE KEY UPDATE sku_id = ?, prop_type = ?, com_att_id = ?, com_att_val_id = ?, val = ?, source = ?, category_id = ?, version = ?, modified = now() ### Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLTransactionRollbackException: Deadlock found when trying to get lock;

从报错信息可以看出有死锁，insert SQL导致，研发有些不理解，insert操作怎么会导致死锁？

我们都知道， insert into values存在X row record lock。

但后面的需要更加注意，对于存在auto_increment列，存在X lock在auto_increment列的index，特殊的表级AUTO-INC lock，如果insert产生duplicate-key错误，则在duplicate index record设置S lock，如果多个session插入同一行则可以产生deadlock。

官方文档对于insert 加锁说明：

Insert sets an exclusive lock on the inserted row. This lock is an index-record lock, not a next-key lock (that is, there is no gap lock) and does not prevent other sessions from inserting into the gap before the inserted row.Prior to inserting the row, a type of gap lock called an insertion intention gap lock is set. This lock signals the intent to insert in such a way that multiple transactions inserting into the same index gap need not wait for each other if they are not inserting at the same position within the gap.If a duplicate-key error occurs, a shared lock on the duplicate index record is set. This use of a shared lock can result in deadlock should there be multiple sessions trying to insert the same row if another session already has an exclusive lock.

大概意思就是：insert会对插入成功的行加上排它锁，这个排它锁是个记录锁，而非next-key锁（也不是gap锁了），不会阻止其他并发的事务往这条记录之前插入记录。在插入之前，会先在插入记录所在的间隙加上一个插入意向gap锁，并发的事务可以对同一个gap加意向gap锁。如果insert 的事务出现了duplicate-key error ，事务会对duplicate index record加共享锁。这个共享锁在并发的情况下是会产生死锁的，比如有两个并发的insert都对要对同一条记录加共享锁，而此时这条记录又被其他事务加上了排它锁，排它锁的事务提交或者回滚后，两个并发的insert操作是会发生死锁。

先看下加锁的表结构，存在id主键和sku_id,prop_type,com_att_id,source的唯一索引：

CREATE TABLE `sku_1` (
      `id` bigint(20) NOT NULL  AUTO_INCREMENT COMMENT '自增',
      `sku_id` bigint(20) NOT NULL,
      `prop_type` tinyint(4) DEFAULT  NULL,
      `com_att_id` int(11) DEFAULT  NULL,
      `com_att_val_id` varchar(150)  DEFAULT NULL,
      `val` varchar(200) DEFAULT  NULL,
      `source` int(11) DEFAULT  NULL,
      `category_id` int(11) DEFAULT  NULL,
      `version` varchar(50) DEFAULT  NULL,
      `created` datetime DEFAULT  NULL,
      `modified` datetime DEFAULT  NULL,
      PRIMARY KEY (`id`),
      UNIQUE KEY `unique`  (`sku_id`,`prop_type`,`com_att_id`,`source`)
    ) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;

演示下insert 的事务出现了duplicate key error的情况：

步骤	事务1	事务2	事务3
1	mysql> begin; Query OK, 0 rows affected (0.00 sec)	mysql> begin; Query OK, 0 rows affected (0.00 sec)	mysql> begin; Query OK, 0 rows affected (0.00 sec)
2	mysql> insert into sku_1 (sku_id,prop_type,com_att_id,com_att_val_id,val,source,category_id,version,created,modified) values (9807755,4,3495,'8339','L',58,12004,'10000',now(),now()); Query OK, 1 row affected (0.04 sec)
3		mysql> insert into sku_1 (sku_id,prop_type,com_att_id,com_att_val_id,val,source,category_id,version,created,modified) values (9807755,4,3495,'8339','L',58,12004,'10000',now(),now()); 锁等待	mysql> insert into sku_1 (sku_id,prop_type,com_att_id,com_att_val_id,val,source,category_id,version,created,modified) values (9807755,4,3495,'8339','L',58,12004,'10000',now(),now()); 锁等待
4	mysql> rollback; Query OK, 0 rows affected (0.00 sec)
5		mysql> insert into sku_1 (sku_id,prop_type,com_att_id,com_att_val_id,val,source,category_id,version,created,modified) values (9807755,4,3495,'8339','L',58,12004,'10000',now(),now()); Query OK, 1 row affected (37.17 sec)	mysql> insert into sku_1 (sku_id,prop_type,com_att_id,com_att_val_id,val,source,category_id,version,created,modified) values (9807755,4,3495,'8339','L',58,12004,'10000',now(),now()); ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction 死锁发生了