tests/pessimistictest: fix DATA RACE for tests

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #34504

Problem Summary:

What is changed and how it works?

Fix data race...
transaction.PrewriteMaxBackoff = 20000 should be after dom.Close(),
because before dom.Close(), the background goroutines are still reading variable transaction.PrewriteMaxBackoff

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Verify no DATA RACE after this change:

cd tests/pessimistictest/
go test -race ./...

• 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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei
• tisonkun

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[tisonkun]

Thanks! I must reverse the order when reasoning :(

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/9c20932084f5a32a4597ae3ea59c6f532b0b5e6d

[hawkingrei]

BTW, This problem also shows that this parameter is not designed correctly, there should be an atomic type to avoid. It has been improved in the tikv/client-go#488.

[hawkingrei]

BTW, This problem also shows that this parameter is not designed correctly, there should be an atomic type to avoid. It has been improved in the tikv/client-go#488.

Further, a dynamic configuration change should not be a normal type of change.

[hawkingrei]

/merge

[ti-chi-bot]

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

Commit hash: da989cc

[sre-bot]

TiDB MergeCI notify

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci/integration-cdc-test	🟢 all 34 tests passed	32 min	Existing passed
idc-jenkins-ci-tidb/integration-common-test	🟢 all 11 tests passed	15 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 12 tests passed	7 min 56 sec	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	7 min 20 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	7 min 2 sec	Existing passed
idc-jenkins-ci-tidb/integration-ddl-test	🟢 all 6 tests passed	5 min 58 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	4 min 37 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	3 min 53 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	3 min 41 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed