mysql: handle db name correctly

[sougou]

Fixes #5206

The conn schemaname var can get set through a new connection
or a ComInitDB packet. However, a use statement bypasses this
mechanism and gets handled at the vtgate level. This causes
problems because schemaName in the connection disagrees with
TargetString in vtgate.

To fix this, the new scheme calls a new handler function: ComInitDB
every time schemaName is initialized or changed in the connection.
This way, the TargetString remains the authoritative source
for the current db name.

I couldn't write a test for this specific behavior change because
the mysql go client does not implement support for ComInitDB.
But I've verified the things generally work as expected.

Signed-off-by: Sugu Sougoumarane ssougou@gmail.com

[morgo]

I managed to manually verify this as fixed, by using local example. Here is the previous behavior:

morgo@morgox1:~/vitess/src/vitess.io/vitess$ mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 27
Server version: 5.5.10-Vitess (Ubuntu)

Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> show databases;
+-----------+
| Databases |
+-----------+
| commerce  |
| customer  |
+-----------+
2 rows in set (0.00 sec)

mysql> use commerce;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show tables;
+-----------------------+
| Tables_in_vt_commerce |
+-----------------------+
| customer_seq          |
| order_seq             |
| product               |
+-----------------------+
3 rows in set (0.01 sec)

mysql> use customer;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show tables;
+-----------------------+
| Tables_in_vt_commerce |
+-----------------------+
| customer_seq          |
| order_seq             |
| product               |
+-----------------------+
3 rows in set (0.00 sec)

mysql> use customer;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show tables;
+-----------------------+
| Tables_in_vt_commerce |
+-----------------------+
| customer_seq          |
| order_seq             |
| product               |
+-----------------------+
3 rows in set (0.00 sec)

mysql> use commerce;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show tables;
+-----------------------+
| Tables_in_vt_commerce |
+-----------------------+
| customer_seq          |
| order_seq             |
| product               |
+-----------------------+
3 rows in set (0.01 sec)

Here is the new behavior:

morgo@morgox1:~/vitess/src/vitess.io/vitess$ mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 29
Server version: 5.5.10-Vitess (Ubuntu)

Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> show databases;
+-----------+
| Databases |
+-----------+
| commerce  |
| customer  |
+-----------+
2 rows in set (0.01 sec)

mysql> use commerce;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show tables;
+-----------------------+
| Tables_in_vt_commerce |
+-----------------------+
| customer_seq          |
| order_seq             |
| product               |
+-----------------------+
3 rows in set (0.00 sec)

mysql> use customer;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show tables;
+-----------------------+
| Tables_in_vt_customer |
+-----------------------+
| corder                |
| customer              |
+-----------------------+
2 rows in set (0.00 sec)

mysql> use customer;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show tables;
+-----------------------+
| Tables_in_vt_customer |
+-----------------------+
| corder                |
| customer              |
+-----------------------+
2 rows in set (0.00 sec)

mysql> use commerce;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show tables;
+-----------------------+
| Tables_in_vt_commerce |
+-----------------------+
| customer_seq          |
| order_seq             |
| product               |
+-----------------------+
3 rows in set (0.01 sec)

[morgo]

@sougou LGTM except the resharding_rbr test is failing. Did you want to bump the CI?

[deepthi]

LGTM