Rollback after error can clobber updates

[aquarapid]

This seems to be just another form of #6264 , but happens with default settings.

This occurs in current master (b9bb5d0), but has been reported against 6.0 as well.

Scenario:

• Setup by using examples/local/101_initial_cluster.sh
• Connect to vtgate, and run through the following testcase.
• Create table with unique key and insert a value:

$ mysql -A -u root -P 15306 -h 127.0.0.1 commerce
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 5
Server version: 5.7.9-Vitess MySQL Community Server - GPL
.
.
mysql> create table t1 (c1 bigint primary key, c2 bigint, unique key (c2));
Query OK, 0 rows affected (0.12 sec)

mysql> insert into t1 (c1, c2) values (1, 1);
Query OK, 1 row affected (0.03 sec)

mysql> select * from t1;
+----+------+
| c1 | c2   |
+----+------+
|  1 |    1 |
+----+------+
1 row in set (0.01 sec)

• Verify autocommit is on (by default):

mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
1 row in set (0.01 sec)

• Insert a value with duplicate unique key:

mysql> insert into t1 (c1, c2) values (2, 1);
ERROR 1062 (23000): vtgate: http://localhost:15001/: execInsertUnsharded: target: commerce.0.master, used tablet: zone1-100 (localhost): vttablet: rpc error: code = AlreadyExists desc = Duplicate entry '1' for key 't1.c2' (errno 1062) (sqlstate 23000) (CallerID: userData1): Sql: "insert into t1(c1, c2) values (:vtg1, :vtg2)", BindVars: {vtg1: "type:INT64 value:\"2\" "vtg2: "type:INT64 value:\"1\" "}

• Verify table is unchanged:

mysql> select * from t1;
+----+------+
| c1 | c2   |
+----+------+
|  1 |    1 |
+----+------+
1 row in set (0.00 sec)

• Update existing row, verifying it has changed:

mysql> update t1 set c2 = 2 where c1 = 1;
Query OK, 1 row affected (0.01 sec)

mysql> select * from t1;
+----+------+
| c1 | c2   |
+----+------+
|  1 |    2 |
+----+------+
1 row in set (0.00 sec)

• Now, wait 30 seconds, and check table again:

mysql> select sleep(30);
+-----------+
| sleep(30) |
+-----------+
|         0 |
+-----------+
1 row in set (30.00 sec)
mysql> select * from t1;
ERROR 1317 (HY000): vtgate: http://localhost:15001/: target: commerce.0.master, used tablet: zone1-100 (localhost): vttablet: rpc error: code = Aborted desc = transaction 1591663223087763322: ended at 2020-06-08 17:47:59.896 PDT (exceeded timeout: 30s) (CallerID: userData1)

• That didn't work, check again:

mysql> select * from t1;
+----+------+
| c1 | c2   |
+----+------+
|  1 |    1 |
+----+------+
1 row in set (0.01 sec)

Table has reverted to previous state.