MDEV-31344: fast status variable lookup

[dmitry416]

Summary

• Added a hash table for status variables
• Now the variable is searched first in the hash table, and if it is not found, it is searched using the old path.

[CLAassistant]

All committers have signed the CLA.

[gkodinov]

Thank you for your work. This is a preliminary review.

Please make sure the buildbot servers compile and run tests successfully to completion.

Also, the whole diff needs work:

• the hash should not be filled in only at init time. It should also be filled in when status variables array version is increased (e.g. load a plugin etc).
• hash access should be protected under LOCK_all_status_vars.
• you need a test that covers the change as much as possible.
• to be able to test you might need some new status variable counters to observe the hash usage vs scans.

I would start with an exact design document of what the diff is trying to do. Then I'd add regression test to ensure that all the functional and non-functional requirements are tested. And then compare the implementation to the design spec and the tests and make sure it delivers.

[gkodinov]

Closing due to inactivity: lack of reaction to my review comments from 3 weeks ago. If you intend to keep working on this, please turn it back from draft to open.

[FooBarrior]

@gkodinov,

the hash should not be filled in only at init time. It should also be filled in when status variables array version is increased (e.g. load a plugin etc).

The thing is it's only practical to populate the hash from non-plugin vars: plugin vars are always array or func types, which are not supported by hash.

But we agreed with @dmitry416 that we'll do it in add_status_var, just for wholesomeness of the solution.