Variables should use status var interface

[morgo]

Feature Request

Is your feature request related to a problem? Please describe:

I found a bug in #9365

It looks like the variables are initialized before the config file is parsed. So if the port is non-default, it won't be reflected.

Describe the feature you'd like:

Instead of having configuration all in one place, I think a nicer design would be to have individual components 'register' sysvars. This would make it work closer to how status vars currently work.

The advantage would be that plugins could also add additional variables to the server.

(This would make it work similar to MySQL too..)

Describe alternatives you've considered:

An easy workaround would be to update the sysvars after the server starts.

Teachability, Documentation, Adoption, Migration Strategy:

It would take some initial work to move sysvars to the correct component. Initially, they could all be put under the server.

[morgo]

PTAL @tiancaiamao

Do you see any flaws in this strategy?

[tiancaiamao]

individual components 'register' sysvars sounds a good idea.
/cc @lysu

[lysu]

+1 plugin also needs a mechanism to "register sysvar" into hosted TiDB too, and now plugins use ugly way to do it. we should improve it.

[ghost]

@lysu do you mind if I take this over?

[lysu]

@lysu do you mind if I take this over?

:D surely maybe after finish #18717 then maybe have time to do this

[ghost]

I meant I would like to work on this if that is okay with you :)

I think it works best as multiple PRs:

1. Convert variables to using status var interface.
2. Refine the sysVar struct to hold min/max/type/validation (anonymous function), and move it out of ValidateSetSystemVar: https://github.com/pingcap/tidb/blob/master/sessionctx/variable/varsutil.go#L326-L327
3. Fix Support for typed system variables when running SELECT @@foreign_key_checks #15131
4. Add an information_schema.variables_info table (based on what is there in MySQL 8.0).
5. Start work on instance scoped variables. i.e. instance/node level variables #19332

[lysu]

I meant I would like to work on this if that is okay with you :)

I think it works best as multiple PRs:

1. Convert variables to using status var interface.
2. Refine the sysVar struct to hold min/max/type/validation (anonymous function), and move it out of ValidateSetSystemVar: https://github.com/pingcap/tidb/blob/master/sessionctx/variable/varsutil.go#L326-L327
3. Fix Support for typed system variables when running SELECT @@foreign_key_checks #15131
4. Add an information_schema.variables_info table (based on what is there in MySQL 8.0).
5. Start work on instance scoped variables. i.e. instance/node level variables #19332

👍 Yes, it's ok for me~ 😊