variable: fix 34773 race error

[Yisaer]

Signed-off-by: yisaer disxiaofei@163.com

What problem does this PR solve?

Issue Number: close #34773

Problem Summary:

What is changed and how it works?

Check List

Tests

• Manual test (add detailed scripts or steps below)

I manually tested origin variable function which is worked as expected.

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
• qw4990

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[Yisaer]

/run-check-issue-triage-complete

[Yisaer]

/merge

[ti-chi-bot]

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

Commit hash: b34d2ea

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/b34d2ea2ba47c02668e5a0d21374d4563072e9eb

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! [3] CI still failing after this pr merged.
These failed integration tests don't seem to be introduced by the current PR.

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/integration-common-test	🔴 failed 3, success 8, total 11	15 min	Existing failure
idc-jenkins-ci-tidb/integration-compatibility-test	🔴 failed 1, success 0, total 1	9 min 51 sec	Existing failure
idc-jenkins-ci-tidb/integration-ddl-test	🔴 failed 4, success 2, total 6	5 min 50 sec	Existing failure
idc-jenkins-ci/integration-cdc-test	🟢 all 34 tests passed	27 min	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	22 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 12 tests passed	11 min	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	6 min 36 sec	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	6 min 4 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	5 min 28 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed