Add initial support for connection attributes.

[dveeden]

This sets attribute _client_name with the value "Go MySQL Driver"

Here processlist_id=58 is the connection from the Go MySQL Driver.

mysql> select * from performance_schema.session_connect_attrs; show processlist;
+----------------+-----------------+------------------+------------------+
| PROCESSLIST_ID | ATTR_NAME       | ATTR_VALUE       | ORDINAL_POSITION |
+----------------+-----------------+------------------+------------------+
|              3 | _os             | Linux            |                0 |
|              3 | _client_name    | libmysql         |                1 |
|              3 | _pid            | 3891             |                2 |
|              3 | _client_version | 5.7.7-rc         |                3 |
|              3 | _platform       | x86_64           |                4 |
|              3 | program_name    | Slave I/O Thread |                5 |
|              3 | channel_name    | NULL             |                6 |
|              6 | _os             | Linux            |                0 |
|              6 | _client_name    | libmysql         |                1 |
|              6 | _pid            | 3618             |                2 |
|              6 | _client_version | 5.7.7-rc         |                3 |
|              6 | _platform       | x86_64           |                4 |
|              6 | program_name    | Slave I/O Thread |                5 |
|              6 | channel_name    | testc1           |                6 |
|             58 | _client_name    | Go MySQL Driver  |                0 |
|             59 | _os             | Linux            |                0 |
|             59 | _client_name    | libmysql         |                1 |
|             59 | _pid            | 14429            |                2 |
|             59 | _client_version | 5.7.7-rc         |                3 |
|             59 | _platform       | x86_64           |                4 |
|             59 | program_name    | mysql            |                5 |
+----------------+-----------------+------------------+------------------+
21 rows in set (0.00 sec)

This could later be extended to:

• Send more information
  • _os
  • _client_version
  • _pid
  • _platform
  • program_name
• Send custom information
  • e.g. Key/Value pairs

The attributes send by the C library use _ and the application using the C library usually sets program_name to $0

[julienschmidt]

How is this useful?

[julienschmidt]

You forgot CLIENT_PLUGIN_AUTH_LENENC_CLIENT_DATA

[dveeden]

@julienschmidt this is not really useful as this only sends one key/value pair.
To make this more useful it should send more information by default (driver version, pid etc) and should allow applications to define custom attributes.

Some uses:

• Enrich audit logs by sending the end-user for which the application is working (like the X-Forwarded-For header in HTTP)
• Make it easier to identify which client application is using a connection (e.g. program_name=my_long_running_cron_job) and use this to whitelist/blacklist it from something which automatically kills long running queries.
• Have statistics on which end user, application, etc. uses the database.

[dveeden]

Added the missing CLIENT_PLUGIN_AUTH_LENENC_CLIENT_DATA

[dveeden]

Rebased as the client attributes were updated in #336