session: fix multiple domain creation racing in domap (#36792)

[ti-srebot]

cherry-pick #36792 to release-6.1
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/36810

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

git push git@github.com:ti-srebot/tidb.git pr/36810:release-6.1-b1edc33fc1a9

---

Signed-off-by: xhe xw897002528@gmail.com

What problem does this PR solve?

Issue Number: close #36791

Problem Summary: As title, refer the issue. After investigation, rwmutex did not bring notable performance gain. Also no one will ever pass a nil store, verified by running all tests, so if len(dm.domains) == 1 && store == nil { removed.

xhe@M1PRO testkit % cat 1
creating domains:

// mutex
BenchmarkDomain-8 5 241125350 ns/op
// rwmutex
BenchmarkDomain-8 5 229305383 ns/op

one domain, creating sessions:

// mutex
BenchmarkDomain-8 128607 9408 ns/op
BenchmarkDomain-8 125948 9338 ns/op
// rwmutex
BenchmarkDomain-8 132254 9853 ns/op
BenchmarkDomain-8 107743 9331 ns/op

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.

Fix that incorrect TiDB states may appear on startup under very, very, very extreme cases 

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei
• wjhuang2016

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[ti-srebot]

/run-all-tests

[ti-srebot]

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

[xhebox]

mistake

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/dd3d00aba2a98e108ab7791e5a0ab3d5a2d33fd4

[xhebox]

/merge

[ti-chi-bot]

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

Commit hash: 0ba4274