sessionctx: fix data-race bug when alloc task id (#28022)

[ti-srebot]

cherry-pick #28022 to release-5.2
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/28283

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git push git@github.com:ti-srebot/tidb.git pr/28283:release-5.2-9a7f572d0ab1

---

What problem does this PR solve?

Issue Number: close #27952

Problem Summary:
The task id allocator will produce same task id in some cases. When there is a join that cannot be pushed, the build table and probe table will invoke "open" function simultanously. That will cause data race without mutex protection

What is changed and how it works?

Proposal: xxx

the fix is simple,. add lock guard for task id allocator

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

sessionctx: fix data-race bug when alloc task id

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• fzhedu
• wjhuang2016

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[ti-srebot]

/run-all-tests

[ti-srebot]

@hanfei1991 you're already a collaborator in bot's repo.

[fzhedu]

/LGTM

[zhouqiang-cl]

/merge

[ti-chi-bot]

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

Commit hash: 80d749b

[ti-chi-bot]

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