Added documentation for unused bytes at the end of Protocol::ColumnDefinition41

[daniyaalk]

Two unused, undocumented extra bytes in each ColumnDefinition Packet

Here is a packet capture of the Column definitions from a select * from db; query where the default mysql schema is selected. I have added a space to pad between the packet header and body.

01000001 16
27000002 03646566056d7973716c02646202646204486f737404486f73740cff00fc030000fe0340000000
23000003 03646566056d7973716c0264620264620244620244620cff0000010000fe8340000000
27000004 03646566056d7973716c026462026462045573657204557365720cff0080000000fe8b40000000
35000005 03646566056d7973716c0264620264620b53656c6563745f707269760b53656c6563745f707269760cff0004000000fe0101000000
35000006 03646566056d7973716c0264620264620b496e736572745f707269760b496e736572745f707269760cff0004000000fe0101000000
35000007 03646566056d7973716c0264620264620b5570646174655f707269760b5570646174655f707269760cff0004000000fe0101000000
35000008 03646566056d7973716c0264620264620b44656c6574655f707269760b44656c6574655f707269760cff0004000000fe0101000000
35000009 03646566056d7973716c0264620264620b4372656174655f707269760b4372656174655f707269760cff0004000000fe0101000000
3100000a 03646566056d7973716c0264620264620944726f705f707269760944726f705f707269760cff0004000000fe0101000000
3300000b 03646566056d7973716c0264620264620a4772616e745f707269760a4772616e745f707269760cff0004000000fe0101000000
3d00000c 03646566056d7973716c0264620264620f5265666572656e6365735f707269760f5265666572656e6365735f707269760cff0004000000fe0101000000
3300000d 03646566056d7973716c0264620264620a496e6465785f707269760a496e6465785f707269760cff0004000000fe0101000000
3300000e 03646566056d7973716c0264620264620a416c7465725f707269760a416c7465725f707269760cff0004000000fe0101000000
4900000f 03646566056d7973716c026462026462154372656174655f746d705f7461626c655f70726976154372656174655f746d705f7461626c655f707269760cff0004000000fe0101000000
3f000010 03646566056d7973716c026462026462104c6f636b5f7461626c65735f70726976104c6f636b5f7461626c65735f707269760cff0004000000fe0101000000
3f000011 03646566056d7973716c026462026462104372656174655f766965775f70726976104372656174655f766965775f707269760cff0004000000fe0101000000
3b000012 03646566056d7973716c0264620264620e53686f775f766965775f707269760e53686f775f766965775f707269760cff0004000000fe0101000000
45000013 03646566056d7973716c026462026462134372656174655f726f7574696e655f70726976134372656174655f726f7574696e655f707269760cff0004000000fe0101000000
43000014 03646566056d7973716c02646202646212416c7465725f726f7574696e655f7072697612416c7465725f726f7574696e655f707269760cff0004000000fe0101000000
37000015 03646566056d7973716c0264620264620c457865637574655f707269760c457865637574655f707269760cff0004000000fe0101000000
33000016 03646566056d7973716c0264620264620a4576656e745f707269760a4576656e745f707269760cff0004000000fe0101000000
37000017 03646566056d7973716c0264620264620c547269676765725f707269760c547269676765725f707269760cff0004000000fe0101000000
.......................<ResultSet Row Packets Truncated>.......................

As described by https://dev.mysql.com/doc/dev/mysql-server/latest/page_protocol_com_query_response_text_resultset.html, the first packet contains the count of columns.
Whereas the following packets shall contain ColumnDefinitions. Parsing the ColumnDefinition packets as documented in https://dev.mysql.com/doc/dev/mysql-server/latest/page_protocol_com_query_response_text_resultset_column_definition.html,

We get:

Hex	Name	Type	Value
03646566	catalog	string	def
056d7973716c	schema	string	mysql
026462	table	string	db
026462	org_table	string	db
04486f7374	name	string	Host
04486f7374	org_name	string	Host
0c	length	int	0c
ff00	charset	int	ff00
fc030000	col_length	int	fc030000
fe	type	int	fe
0340	flags	int	0340
00	decimals	int	00
0000	??????????????????????????????	-> Two bytes, undocumented.

Looking into the source code, these fields are reserved for future use.

    pos[9] = (char)field->decimals;
    pos[10] = 0;  // For the future
    pos[11] = 0;  // For the future
    pos += 12;

Mysql CLI dump for reference:

➜  ~ mysql -V                   
mysql  Ver 8.0.40-0ubuntu0.24.10.1 for Linux on x86_64 ((Ubuntu))
~ mysql -uroot -pasd -h127.0.0.1 -P3307 --ssl-mode=DISABLED
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 66
Server version: 8.0.32 MySQL Community Server - GPL

Copyright (c) 2000, 2024, Oracle and/or its affiliates.

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> SELECT VERSION();
+-----------+
| VERSION() |
+-----------+
| 8.0.32    |
+-----------+
1 row in set (0.00 sec)

mysql> use mysql;
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> select * from db;
+-----------+--------------------+---------------+-------------+-------------+-------------+-------------+-------------+-----------+------------+-----------------+------------+------------+-----------------------+------------------+------------------+----------------+---------------------+--------------------+--------------+------------+--------------+
| Host      | Db                 | User          | Select_priv | Insert_priv | Update_priv | Delete_priv | Create_priv | Drop_priv | Grant_priv | References_priv | Index_priv | Alter_priv | Create_tmp_table_priv | Lock_tables_priv | Create_view_priv | Show_view_priv | Create_routine_priv | Alter_routine_priv | Execute_priv | Event_priv | Trigger_priv |
+-----------+--------------------+---------------+-------------+-------------+-------------+-------------+-------------+-----------+------------+-----------------+------------+------------+-----------------------+------------------+------------------+----------------+---------------------+--------------------+--------------+------------+--------------+
| localhost | performance_schema | mysql.session | Y           | N           | N           | N           | N           | N         | N          | N               | N          | N          | N                     | N                | N                | N              | N                   | N                  | N            | N          | N            |
| localhost | sys                | mysql.sys     | N           | N           | N           | N           | N           | N         | N          | N               | N          | N          | N                     | N                | N                | N              | N                   | N                  | N            | N          | Y            |
+-----------+--------------------+---------------+-------------+-------------+-------------+-------------+-------------+-----------+------------+-----------------+------------+------------+-----------------------+------------------+------------------+----------------+---------------------+--------------------+--------------+------------+--------------+
2 rows in set (0.00 sec)

mysql> 

[mysql-oca-bot]

Hi, thank you for your contribution. Please confirm this code is submitted under the terms of the OCA (Oracle's Contribution Agreement) you have previously signed by cutting and pasting the following text as a comment:
"I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it."
Thanks

[daniyaalk]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

[mysql-oca-bot]

Hi, thank you for your contribution. Your code has been assigned to an internal queue. Please follow
bug http://bugs.mysql.com/bug.php?id=117346 for updates.
Thanks