test: fix flaky DisaggReadSnapshot — skip background tasks during write+mergeDeltaAll (#10898)

[ti-chi-bot]

This is an automated cherry-pick of #10898

What problem does this PR solve?

Issue Number: close #10897

Problem Summary:
DMStoreForSegmentReadTaskTest.DisaggReadSnapshot is flaky — the initial write(4096) schedules a background flush task
whose completion handler (placeDeltaIndex) and follow-up background merge delta both acquire is_updating on
DeltaValueSpace. When mergeDeltaAll() runs before these background tasks release the lock, it fails silently,
leaving 5 persisted column files instead of the expected 4.

What is changed and how it works?

Enable skip_check_segment_update failpoint before the initial write(4096) + mergeDeltaAll() block in the test,
with a scoped guard that disables it on exit. This prevents checkSegmentUpdate from scheduling any background
flush/merge tasks during the setup phase, eliminating the race entirely.

This is consistent with other tests in the same file (fetchPagesNoTinyNoInMem, fetchPagesTinyNoInMem,
fetchPagesTinyInMem) which already enable skip_check_segment_update via disableFlushCache() before their
write+mergeDeltaAll blocks.

Check List

Tests

• Unit test

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

None

Summary by CodeRabbit

• Tests
  • Updated the disaggregated read snapshot test setup to bypass segment update checks during data preparation, then restore the checks afterward.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

1. It must be LGTMed and approved by the reviewers firstly.
2. For pull requests to TiDB-x branches, it must have no failed tests.
3. AFTER it has lgtm and approved labels, please wait for the cherry-pick merging approval from triage owners.

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[coderabbitai]

📝 Walkthrough

Walkthrough

The DisaggReadSnapshot test temporarily enables skip_check_segment_update during stable-data writes and delta merging. An RAII scope guard disables the failpoint after setup.

Changes

DisaggReadSnapshot test setup

Layer / File(s)	Summary
Failpoint control during test setup dbms/src/Storages/DeltaMerge/tests/gtest_segment_read_task.cpp	The test enables skip_check_segment_update before stable-data operations and disables it through a scope guard afterward.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Poem

A rabbit guards the segment gate,
Sets the failpoint, pauses fate.
Writes and merges proceed in line,
Then RAII makes the state benign.
Hop, hop—no flaky sign!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the flaky DisaggReadSnapshot test and the fix to skip background tasks during setup.
Description check	✅ Passed	The description follows the template, explains the race and fix, records issue and test details, and addresses side effects and release notes.
Linked Issues check	✅ Passed	The change addresses issue #10897 by preventing background flush and merge tasks from racing with mergeDeltaAll during test setup.
Out of Scope Changes check	✅ Passed	The seven added lines are limited to the affected unit test and directly support the linked issue objective.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

🧹 Nitpick comments (1)

dbms/src/Storages/DeltaMerge/tests/gtest_segment_read_task.cpp (1)

656-657: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Use camelCase for the scope guard variable.

Rename fp_guard to fpGuard. The new local variable uses snake_case.

As per coding guidelines, “Method and variable names should use camelCase (e.g., readBlock, totalBytes).”

Proposed rename

-        auto fp_guard
+        auto fpGuard

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@dbms/src/Storages/DeltaMerge/tests/gtest_segment_read_task.cpp` around lines
656 - 657, Rename the local scope guard variable in the segment read test from
fp_guard to fpGuard, leaving its fail-point disabling behavior unchanged.

Source: Coding guidelines

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@dbms/src/Storages/DeltaMerge/tests/gtest_segment_read_task.cpp`:
- Around line 656-657: Rename the local scope guard variable in the segment read
test from fp_guard to fpGuard, leaving its fail-point disabling behavior
unchanged.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: e6dcf79e-e5e6-451a-ba0c-5df416a2686b

📥 Commits

Reviewing files that changed from the base of the PR and between 76536b2 and 379723b.

📒 Files selected for processing (1)

• dbms/src/Storages/DeltaMerge/tests/gtest_segment_read_task.cpp

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JaySon-Huang, JinheLin

The full list of commands accepted by this bot can be found here.

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Details

Needs approval from an approver in each of these files:

• OWNERS [JaySon-Huang,JinheLin]

Approvers can indicate their approval by writing /approve in a comment
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-08-11 05:15:30.128926166 +0000 UTC m=+3109916.165021222: ☑️ agreed by JaySon-Huang.
• 2026-08-11 07:24:22.030277884 +0000 UTC m=+3117648.066372939: ☑️ agreed by JinheLin.

[JaySon-Huang]

/test pull-integration-test

[ti-chi-bot]

@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-integration-test	379723b	link	true	/test pull-integration-test

Full PR test history. Your PR dashboard.

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