server: check connection is available in SQLKiller

[Defined2014]

What problem does this PR solve?

Issue Number: close #57531

Problem Summary:

What changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  As TiDB can't close connection immediately after killing client process #57531 metioned, run select sleep(300) and kill the mysql-client. Then the sql can't found by show processlist.
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[codecov]

Codecov Report

Attention: Patch coverage is 94.82759% with 3 lines in your changes missing coverage. Please review.

Project coverage is 73.6238%. Comparing base (62d4284) to head (4f6c85b).
Report is 8 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #60685        +/-   ##
================================================
+ Coverage   73.1298%   73.6238%   +0.4940%     
================================================
  Files          1722       1722                
  Lines        476908     479248      +2340     
================================================
+ Hits         348762     352841      +4079     
+ Misses       106738     104885      -1853     
- Partials      21408      21522       +114     

Flag	Coverage Δ
integration	42.5672% <59.6491%> (?)
unit	72.3806% <94.8275%> (+0.0297%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.7804% <ø> (ø)
parser	∅ <ø> (∅)
br	47.4433% <ø> (+0.0164%)	⬆️

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[Defined2014]

/retest

[tiprow]

@Defined2014: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

Details

In response to this:

/retest

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[Defined2014]

/retest

[tiprow]

@Defined2014: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

Details

In response to this:

/retest

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[YangKeao]

LGTM

[xhebox]

LGTM otherwise

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox, YangKeao

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [YangKeao,xhebox]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-05-07 02:17:29.130053346 +0000 UTC m=+407896.312956592: ☑️ agreed by YangKeao.
• 2025-05-07 03:07:50.382192946 +0000 UTC m=+410917.565096205: ☑️ agreed by xhebox.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #68201.
But this PR has conflicts, please resolve them!