varaible: fix the new session can't see the instance-level variable store-limit

[AilinKid]

Signed-off-by: ailinkid 314806019@qq.com

What problem does this PR solve?

Issue Number: close #26648

What is changed and how it works?

What's Changed: Thanks to Morgan's new framework of variable, the instance variable is more elegant to implement

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

1: start a cluster with 3 tidb nodes
2: set the instance storelimit in node1, node1 can see it while node2 & node3 can't see it
3: set the global storelimit in node1, all the nodes can see it.

Release note

varaible: fix the new session can't see the instance-level variable store-limit

[ti-chi-bot]

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• morgo
• tangenta

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[morgo]

This switch for if you change a global var, and you want to run code on other TiDB servers. I just wanted to confirm this is the case here?

(I had thought we would not need to add items to this switch, but that's turned out to be wrong. We may need to change SetGlobal() so that it fires on all TiDB servers).

[AilinKid]

Yes we need this. the case is that:

statement	Node1 instance	Node2 instance
set global	yes(apply local instance)	yes(propagated to instance2)
set session	yes(apply local instance)	no

I investigate for a while, maybe this code is actually what I want.

[AilinKid]

manual test:

1: start a cluster with 3 tidb nodes
2: set the instance storelimit in node1, node1 can see it while node2 & node3 can't see it
3: set the global storelimit in node1, all the nodes can see it.

[AilinKid]

/run-all-tests

[morgo]

LGTM

[AilinKid]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 4ccb061

[ti-chi-bot]

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