前提：
以下分析基于标准的配置选项：
tx_isolation = REPEATABLE-READ
innodb_locks_unsafe_for_binlog = OFF
lock->type_mode用来表示锁的类型，实际上lock->type_mode包含了几乎所有锁的模式信息，例如锁类型判断是X锁还是S锁 lock->type_mode &LOCK_TYPE_MASK
| LOCK_MODE_MASK |
0xFUL |
用于表示锁模式掩码 |
| LOCK_TYPE_MASK |
0xF0UL |
用于表示锁类型，LOCK_TABLE或者LOCK_REC |
| LOCK_WAIT |
256 |
表示需要锁等待，还没有获得锁，只是在等待队列中等待 |
| LOCK_ORDINARY |
0 |
普通的next-key锁，锁记录，并锁记录前面的gap，这样可以防止幻读。 假设索引包括10，11，13，20，则next-key锁为： (negative infinity, 10], (10, 11], (11, 13], (13, 20], (20, positive infinity) 我们经常在innodb_locks表中看到的supremum pseudo-record就是锁住了最大值往后的gap. |
| LOCK_GAP |
512 |
只持有记录前的gap锁，例如，在一个gap上的x锁无法修改bit被设置的记录 在从索引记录链上移除记录时会加该类型的锁。 |
| LOCK_REC_NOT_GAP |
1024 |
也就是普通记录锁，只锁住记录，因此不会阻塞向该记录之前的gap中插入记录。 |
| LOCK_INSERT_INTENTION |
2048 |
插入意图锁,目的是让插入索引记录时等待，直到在gap上没有其他冲突的锁 记住，即使获得了等待的锁，也依然会持有插入意图锁 |
不同锁模式
enum lock_mode {
LOCK_IS = 0, /* intention shared */
LOCK_IX, /* intention exclusive */
LOCK_S, /* shared */
LOCK_X, /* exclusive */
LOCK_AUTO_INC, /* locks the auto-inc counter of a table
in an exclusive mode */
LOCK_NONE, /* this is used elsewhere to note consistent read */
LOCK_NUM = LOCK_NONE/* number of lock modes */
};
当使用唯一键或主键来检索数据时，gap锁是没有必要的（但如果查找的where条件中只包含了唯一索引的部分列时，则需要gap锁）
INSERT INTENTION锁在INSERT数据之前，它也是一种GAP锁，当多个事务同时插入记录到同一个索引gap时，如果不是GAP中的同一个位置，则无需等待。例如索引中有两个记录4和7，两个并发事务同时插入5和6，都会各自在获取记录排他锁之前先锁住INSERT INTENTION锁，但互相并不阻塞。
1.创建测试表
drop table section;
CREATE TABLE `section` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`title` varchar(255) NOT NULL,
`tree_left` int(11) DEFAULT NULL,
`tree_right` int(11) DEFAULT NULL,
`tree_level` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `tree_left` (`tree_left`) ,
KEY `tree_right` (`tree_right`)
) ENGINE=InnoDB;
INSERT INTO `section` VALUES (‘1′, ‘root’, ‘1’, ’14’, ‘0’);
INSERT INTO `section` VALUES (‘4′, ‘left tree’, ‘8’, ’13’, ‘1’);
INSERT INTO `section` VALUES (’10’, ‘left tree3′, ’11’, ’11’, ‘3’);
INSERT INTO `section` VALUES (’11’, ‘right Tree’, ‘2’, ‘7’, ‘1’);
INSERT INTO `section` VALUES (’16’, ‘right Tree 2′, ‘3’, ‘6’, ‘2’);
INSERT INTO `section` VALUES (’27’, ‘right Tree 3′, ‘4’, ‘5’, ‘3’);
2.场景测试，目的是找出关键函数
断点：
lock_rec_create
sel_set_rec_lock
lock0lock.c:1050
a.根据主键值删除记录
delete from section where id =8;
这里会根据主键扫描，堆栈如下：
ha_innobase::index_read
->row_search_for_mysql
|–>sel_set_rec_lock (第4280行)
–>二级索引lock_sec_rec_read_check_and_lock
–>聚集索引lock_clust_rec_read_check_and_lock
->lock_rec_lock
|–>lock_rec_lock_fast->lock_rec_lock_fast->lock_rec_create
|–>lock_rec_lock_slow
传递参数type_mode = 1027 = 3+1024 = LOCK_X +LOCK_REC_NOT_GAP
lock->type_mode = (type_mode & ~LOCK_TYPE_MASK) | LOCK_REC = 1059 = LOCK_X | LOCK_REC_NOT_GAP | LOCK_REC
不过如果是根据范围(或者where条件不是确定主键时)来删除记录，则在主键记录上加LOCK_X锁(锁模式值为3)
b.根据二级索引值删除记录
delete from section where tree_left = 8;
首先根据二级索引查找记录，对二级索引记录加锁
ha_innobase::index_read->row_search_for_mysql->sel_set_rec_lock->lock_sec_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_fast->lock_rec_create
type_mode=3 = LOCK_X
lock->type_mode = (type_mode & ~LOCK_TYPE_MASK) | LOCK_REC = 35 = LOCK_REC | LOCK_X
第二次，根据二级索引记录回查聚集索引
row_sel_get_clust_rec_for_mysql->lock_clust_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_fast->lock_rec_create
type_mode=1027
lock->type_mode = LOCK_X | LOCK_REC_NOT_GAP | LOCK_REC
获得主键记录后，就可以执行更新了(row_update_for_mysql)
第三次，扫描下一条记录，看看是否满足条件。
ha_innobase::general_fetch->row_search_for_mysql->sel_set_rec_lock->lock_sec_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_slow->lock_rec_create
type_mode=515
lock->type_mode = 547 = LOCK_GAP | LOCK_X | LOCK_REC
从show engine innodb status里的信息来看，也符合
TABLE LOCK table `test`.`section` trx id E1885D33 lock mode IX
RECORD LOCKS space id 109 page no 4 n bits 80 index `tree_left` of table `test`.`section` trx id E1885D33 lock_mode X
RECORD LOCKS space id 109 page no 3 n bits 80 index `PRIMARY` of table `test`.`section` trx id E1885D33 lock_mode X locks rec but not gap
RECORD LOCKS space id 109 page no 4 n bits 80 index `tree_left` of table `test`.`section` trx id E1885D33 lock_mode X locks gap before rec
c.二级索引上存在重复值
测试表：
drop table t1;
create table t1 (a int primary key , b int ,key(b));
insert into t1 values (1,10),(2,20),(3,20),(4,30);
delete from t1 where b = 20;
lock_sec_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_fast->lock_rec_create
heap_no=3
type_mode=3,lock->type_mode=35
row_sel_get_clust_rec_for_mysql->lock_clust_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_fast->lock_rec_create
type_mode=1027
lock->type_mode = 1059
row_update_for_mysql
继续查下一条数据。
sel_set_rec_lock->lock_sec_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_fast
type_mode=3，
row_sel_get_clust_rec_for_mysql->lock_clust_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_fast
mode=1027
row_update_for_mysql
row_search_for_mysql
lock_sec_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_slow->lock_rec_create
type_mode=547
show engine innodb status的结果：
4 lock struct(s), heap size 1248, 5 row lock(s), undo log entries 2
MySQL thread id 389, OS thread handle 0x7fa35e53c700, query id 1545 localhost root
TABLE LOCK table `test`.`t1` trx id E1885D8E lock mode IX
RECORD LOCKS space id 118 page no 4 n bits 72 index `b` of table `test`.`t1` trx id E1885D8E lock_mode X
RECORD LOCKS space id 118 page no 3 n bits 72 index `PRIMARY` of table `test`.`t1` trx id E1885D8E lock_mode X locks rec but not gap
RECORD LOCKS space id 118 page no 4 n bits 72 index `b` of table `test`.`t1` trx id E1885D8E lock_mode X locks gap before rec
因此，如果事务不提交的话，试图插入tree_left值为[4,11)范围的值都会被阻塞掉。
c.二级索引上存在重复值
测试表：
drop table t1;
create table t1 (a int primary key , b int ,key(b));
insert into t1 values (1,10),(2,20),(3,20),(4,30);
delete from t1 where b = 20;
lock_sec_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_fast->lock_rec_create
heap_no=3
type_mode=3,lock->type_mode=35
row_sel_get_clust_rec_for_mysql->lock_clust_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_fast->lock_rec_create
type_mode=1027
lock->type_mode = 1059
row_update_for_mysql
继续查下一条数据。
sel_set_rec_lock->lock_sec_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_fast
type_mode=3，
row_sel_get_clust_rec_for_mysql->lock_clust_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_fast
mode=1027
row_update_for_mysql
row_search_for_mysql
lock_sec_rec_read_check_and_lock->lock_rec_lock->lock_rec_lock_slow->lock_rec_create
type_mode=547
show engine innodb status的结果：
4 lock struct(s), heap size 1248, 5 row lock(s), undo log entries 2
MySQL thread id 389, OS thread handle 0x7fa35e53c700, query id 1545 localhost root
TABLE LOCK table `test`.`t1` trx id E1885D8E lock mode IX
RECORD LOCKS space id 118 page no 4 n bits 72 index `b` of table `test`.`t1` trx id E1885D8E lock_mode X
RECORD LOCKS space id 118 page no 3 n bits 72 index `PRIMARY` of table `test`.`t1` trx id E1885D8E lock_mode X locks rec but not gap
RECORD LOCKS space id 118 page no 4 n bits 72 index `b` of table `test`.`t1` trx id E1885D8E lock_mode X locks gap before rec
这里只是通过几个gdb挖掘出几个关键的函数，下回将开始深入分析这些函数的执行流程。
