server: fix data race in the mockProxyProtocolProxy

[hawkingrei]

What problem does this PR solve?

Issue Number: close #44609

Problem Summary:

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

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

[Ehco1996]

lgtm

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Ehco1996, xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-16 01:37:54.771806431 +0000 UTC m=+308871.186410497: ☑️ agreed by Ehco1996.
• 2023-06-16 06:52:52.327397251 +0000 UTC m=+327768.742001329: ☑️ agreed by xhebox.

[tiprow]

@hawkingrei: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
tiprow_fast_test	261e849	link	true	/test tiprow_fast_test

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[ti-chi-bot]

@hawkingrei: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	261e849	link	true	/test unit-test
pull-br-integration-test	261e849	link	true	/test pull-br-integration-test

Full PR test history. Your PR dashboard.

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[ti-chi-bot]

@hawkingrei: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-br-integration-test	261e849	link	true	/test pull-br-integration-test

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[ti-chi-bot]

PR needs rebase.

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