Little refinements on DAG code

[fuzhe1989]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

What is changed and how it works?

Proposal: xxx

What's Changed:

No logical changes.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch:

Check List

Tests

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

Side effects

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• SeaRise

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[fuzhe1989]

/run-all-tests

[sre-bot]

Coverage detail: https://ci-internal.pingcap.net/job/tics_ghpr_unit_test/118/cobertura/
(Coverage detail url is limited office network access)

lines: 35.5% (43335 out of 122148)
branches: 5.4% (65917 out of 1219975)

[fuzhe1989]

/run-all-tests

[sre-bot]

Coverage detail: https://ci-internal.pingcap.net/job/tics_ghpr_unit_test/119/cobertura/
(Coverage detail url is limited office network access)

lines: 35.5% (43328 out of 122148)
branches: 5.4% (65884 out of 1219975)

[SeaRise]

/merge

[ti-chi-bot]

@SeaRise: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: e158f65

[sre-bot]

Coverage detail: https://ci-internal.pingcap.net/job/tics_ghpr_unit_test/127/cobertura/
(Coverage detail url is limited office network access)

lines: 35.5% (43332 out of 122148)
branches: 5.4% (65903 out of 1219969)