plugin: fix audit plugin will cause tidb panic (#23803)

[ti-srebot]

cherry-pick #23803 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/23819

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/23819:release-4.0-5f8ec4b904bd

---

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

What problem does this PR solve?

Issue Number: close #23786

What is changed and how it works?

What's Changed: fix the sessionVars.connectionInfo may be nil when exit a conn.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Manual test (add detailed scripts or steps below)

1: tiup playground nightly --tiflash 0  
2: substitute the tidb bin with the ours
3: open a leading conn1 and set the audit plugin as false ( admin plugins disable audit;)
5: open a test conn2
6: enable the audit plugin in the leading con1 ( admin plugins enable audit;)
7: exit the test conn2

before this pr: tidb panic

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x58 pc=0x2f5de34]
goroutine 809 [running]:
github.com/pingcap/tidb/server.(*Server).onConn.func4(0xc0006d3590, 0xc0006d3380, 0xc00155ddee)
/home/arenatlx/go/src/github.com/pingcap/tidb/server/server.go:486 +0xb4
github.com/pingcap/tidb/plugin.ForeachPlugin(0xc0011e6d01, 0xc00155df00, 0xc00141ba10, 0x0)
/home/arenatlx/go/src/github.com/pingcap/tidb/plugin/plugin.go:419 +0x137
github.com/pingcap/tidb/server.(*Server).onConn(0xc0012c2dd0, 0xc0011e6d00)
/home/arenatlx/go/src/github.com/pingcap/tidb/server/server.go:482 +0xaf9
created by github.com/pingcap/tidb/server.(*Server).Run
/home/arenatlx/go/src/github.com/pingcap/tidb/server/server.go:383 +0x8d5

this pr

HAPPY

Release note

• plugin: fix audit plugin will cause tidb panic

[ti-srebot]

/run-all-tests

[ti-srebot]

@AilinKid you're already a collaborator in bot's repo.

[AilinKid]

PTAL @djshow832 @lysu

[ti-chi-bot]

@wshwsh12: /lgtm is only allowed for the reviewers in list.

In response to this:

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[xhebox]

/lgtm

[xiongjiwei]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• xhebox
• xiongjiwei

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[xiongjiwei]

/merge

[ti-chi-bot]

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

Commit hash: 196ad55

[AilinKid]

/merge

[lysu]

/merge

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[AilinKid]

/merge

[AilinKid]

/merge