eventservice: keep transaction cursor inside scan window

[asddongmen]

What problem does this PR solve?

Issue Number: ref #6022

An interrupted EventService scan can publish a transaction cursor (commitTs=C, startTs=S). If the adaptive scan window later shrinks below C, the existing code skips the now-empty range even though a nonzero TxnStartTs also prevents signal resolved-ts, so the dispatcher must wait for the window to reach C again.

This is a narrow liveness corner case. It is not intended to attribute the previously observed low-CPU periodic throughput issue to this path.

What is changed and how it works?

Treat either a row position or transaction start-ts as a resume cursor. When scan-window capping moves the end below the cursor commit-ts, normalize the effective range to [C,C].

For TxnStartTs=S, EventStore scans from (C,S+1) to the end of commit-ts C, so only later transactions sharing C are resumed. Ordinary empty ranges without a cursor, DDL/syncpoint handling, and large-transaction behavior are unchanged.

A focused unit test covers the transaction-cursor case after the scan window shrinks behind C.

Check List

Tests

• Unit test

Questions

Will it cause performance regression or break compatibility?

No compatibility change. An extra scan is possible only for a valid resume cursor that the old path skipped; the scan is required to make progress.

Do you need to update user documentation, design documentation or monitoring documentation?

No.

Release note

Fix a corner case where a shrinking scan window can stall a dispatcher with a transaction resume cursor.

Summary by CodeRabbit

• Bug Fixes
  • Improved scan-range resume handling for transaction-start and row-position cursors.
  • Preserved valid scan requests when adaptive window limits shrink the range.
  • Ensured collapsed ranges retain matching start and end positions instead of being discarded.

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[asddongmen]

/run-check-issue-triage-complete

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 107eee2a-da15-4a85-a4cc-7d56e7a5e82b

📥 Commits

Reviewing files that changed from the base of the PR and between b813e76 and f06b1e9.

📒 Files selected for processing (2)

• pkg/eventservice/event_broker.go
• pkg/eventservice/event_broker_test.go

Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review.

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📝 Walkthrough

Walkthrough

The event broker now preserves transaction-start and row-position cursors when adaptive window capping shrinks a scan range. The regression test verifies a scan request with matching commit timestamps and the original transaction cursor.

Changes

Scan-range resume handling

Layer / File(s)	Summary
Resume-range correction and regression coverage pkg/eventservice/event_broker.go, pkg/eventservice/event_broker_test.go	The broker recognizes both supported resume cursor types and restores a shrunken range to the transaction commit timestamp. The test verifies the scan request, transaction cursor, and empty row position.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to f06b1

This localized fix keeps transaction resume cursors within the scan window to prevent a narrow dispatcher stall; no actionable merge-blocking risk remains after normal checks and review.

Suggested labels: lgtm, approved

Suggested reviewers: lidezhu

Poem

A rabbit checked the scan with care,
And kept both cursors safely there.
The window shrank, the range held true,
Commit times matched as they should do.
“Hop approved!” the rabbit said.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description covers the issue, implementation, tests, compatibility, documentation, and release note requirements.
Title check	✅ Passed	The title clearly and concisely describes the main change to preserve a transaction cursor within the scan window.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

Warning

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🔧 golangci-lint (2.12.2)

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[asddongmen]

/test all