executor: fix slow log push down bug (#68765)

[ti-chi-bot]

This is an automated cherry-pick of #68765

What problem does this PR solve?

Issue Number: close #68758

bug 1: if the new line symbol is at the ending of internal chunk, the file scan will be finsihed too early because empty string will break the loop

bug 2: slow log has some time drift because record time and write time is different. We should tolerate it

Problem Summary:

What changed and how does it work?

Check List

Tests

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

  • I checked and no code files have been changed.

checked with real slow log files collected from user.

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

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Improved accuracy of slow query log filtering with time-range boundary handling.
  • Enhanced reverse-scan functionality for slow query logs, particularly with large entries.

• Tests

  • Added comprehensive test coverage for slow-query reverse-scan edge cases and time boundary conditions.

[ti-chi-bot]

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: d4318ccc-00f4-406c-8130-e698d874ec54

📥 Commits

Reviewing files that changed from the base of the PR and between af202b4 and c7bfca9.

📒 Files selected for processing (2)

• pkg/executor/slow_query.go
• pkg/executor/slow_query_test.go

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📝 Walkthrough

Walkthrough

This PR fixes a slow-log query regression by introducing internal time-tolerance boundaries during reverse-scan and file-discovery operations. The tolerance widens the search windows to prevent boundary-adjacent logs from being omitted, addressing the issue where queries returned fewer rows than expected.

Changes

Slow Query Time Tolerance Implementation

Layer / File(s)	Summary
Time Tolerance Constant and Helper Functions pkg/executor/slow_query.go	Defines slowLogTimeRangeInternalTolerance constant and introduces slowLogMayOverlapTimeRangeWithTolerance and slowLogTimeWithTolerance helpers that compute tolerance-adjusted time boundaries for overlap detection. Extracts line-splitting into splitSlowLogLines helper to normalize \r\n line endings and trim trailing empty lines.
Reverse-Scan and File-Discovery Updates pkg/executor/slow_query.go	Updates reverse-scan initialization to widen minStartTime using tolerance, refactors file discovery (getAllFiles) to apply tolerance-adjusted comparisons when filtering candidate files, and modifies file-end time overlap checks and compressed-log pruning to use tolerance-widened boundaries instead of strict comparisons.
Test Helpers, Imports, and Extended Coverage pkg/executor/slow_query_test.go	Normalizes pkg/meta/model import path, renames TestSplitbyColon to TestSplitByColon, adds getTimeColIdxFromRetrieverOutput helper to locate the "Time" column in retriever output, and extends TestSlowQueryRetrieverReversedScanWithLimit with a cross-chunk boundary case. Introduces new TestSlowQueryRetrieverReversedScanWithTimeJitter test validating forward and reverse scan behavior under millisecond jitter around time-range boundaries.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested labels

size/M, cherry-pick-approved, approved, lgtm, type/cherry-pick-for-release-nextgen-20251011

Suggested reviewers

• gengliqi
• wshwsh12
• hawkingrei

Poem

🐰 A tolerance for time's gentle sway,
Where logs at boundaries won't slip away.
Wider windows catch each line with care,
Reverse scans now skip no logs out there! 🎯

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 8.33% 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
Title check	✅ Passed	The title clearly identifies the main change: fixing a slow log push down bug in the executor package, which aligns with the primary objectives of the PR.
Description check	✅ Passed	The description identifies the linked issue (#68758), explains two specific bugs being fixed, lists completed tests, and follows the template structure despite being an automated cherry-pick.
Linked Issues check	✅ Passed	The code changes directly address both bugs mentioned in issue #68758: introducing tolerance for time drift in slow log scanning [#68758] and fixing early termination when newlines fall at chunk boundaries [#68758].
Out of Scope Changes check	✅ Passed	All changes are scoped to slow query execution logic: tolerance-adjusted time range comparisons, helper functions for overlap checking, newline handling refactoring, and corresponding test updates with no unrelated modifications.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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