vtgate does not report autocommit system variable and SERVER_STATUS_IN_TRANS flag correctly

[aquarapid]

Logging these together, since they're both session related, although they may be addressed separately. Both these issues are problematic for more advanced MySQL drivers (e.g. the MySQL JDBC driver), since they can use these signals to determine things like: whether a commit statement can be elided because the session is already in autocommit mode.

autocommit:

• vtgate does not reflect the status of the autocommit system variable properly when it's been changed, e.g.:

mysql> select @@autocommit;
+--------------+
| @@autocommit |
+--------------+
|            1 |
+--------------+
1 row in set (0.00 sec)
mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)
mysql> select @@autocommit;
+--------------+
| @@autocommit |
+--------------+
|            1 |
+--------------+
1 row in set (0.00 sec)

However, it does recognize that a transaction has started, since it will reject a follow-up statement if the transaction timeout (default 30 secs in OLTP mode) has expired.

SERVER_STATUS_IN_TRANS and SERVER_STATUS_AUTOCOMMIT flags:

• Even when in a transaction, a result will be returned to the client via vtgate without the SERVER_STATUS_IN_TRANS flag set . This can be seen in a network dump (screenshot attached) as the "In transaction" field.
• The SERVER_STATUS_AUTOCOMMIT flag is also always just returned as 0, regardless of the status of autocommit.
  

[aquarapid]

Fixed by #6551