executor,sessionctx: add correctness for system variables (#12311)

[xiaoronglv]

What problem does this PR solve?

For MySQL, thread_pool_size is the number of thread groups, which determine how many queries can execute simultaneously.

This commit is to add constraints for global variable thread_pool_size for TiDB, which is a subtask of epic Complete correctness for system.

What is changed and how it works?

Add restriction for global variable thread_pool_size, of which

• the value must be equal or greater than 1
• the value must be equal or less than 64

Check List

Tests

• Unit test

Code changes

• Has exported variable/fields change

Side effects

• Possible performance regression
• Increased code complexity

[xiaoronglv]

Hi @zz-jason

This commit is going to cherry-pick a commit from master to release-3.0.

Please help to review my code.

Thanks,

Ryan

[zz-jason]

@xiaoronglv Sure! Thank you for the help! BTW, it's better to copy the original PR description to the cherry-picked PR.

[zz-jason]

LGTM

[qw4990]

LGTM

[sre-bot]

Sorry @qw4990, you don't have permission to trigger auto merge event on this branch.

[xiaoronglv]

Hi @zz-jason

It seems @qw4990 doesn't have the permission to merge this PR. Could you please help to merge it?

Thanks.

[zz-jason]

/merge

[sre-bot]

/run-all-tests