test: stabilize flaky TestMainBackupLoop

[flaky-claw]

What problem does this PR solve?

Issue Number: close #68203

Problem Summary:
Flaky test TestMainBackupLoop in br/pkg/backup intermittently fails, so this PR stabilizes that path.

What changed and how does it work?

Root Cause

Random mock response assignment could leave a live store with no responses, causing the mock sender and RunLoop to wait until timeout.

Fix

Deterministic round-robin assignment keeps both mock stores participating in every subcase and removes the invalid empty-live-store setup.

Verification

Spec:

• target: br/pkg/backup :: TestMainBackupLoop
• strategy: tidb.issue_scoped.v2
• plan mode: BASELINE_ONLY
• requirements: required case must execute; no skip; repeat count = 1
• execution surface: GO_TEST_WITH_TAGS
• build tags: intest, deadlock
• baseline gates: required_flaky_gate, build_safety_gate, intent_guard_gate
• feedback surface source: baseline_only

Observed result:

• status: passed
• required case executed: yes
• submission decision: ALLOWED
• note: Required flaky case executed during validation.
  Required flaky case was not skipped.
  target_flaky passed.
  build passed.

Gate checklist:

• target_flaky: PASS
• package_raw: SKIPPED
• package_failpoint: SKIPPED
• build: PASS

Commands:

• go test -json -tags=intest,deadlock ./br/pkg/backup -run '^TestMainBackupLoop$' -count=1
• make build

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.

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

Fixes #68203

Summary by CodeRabbit

• Tests
  • Improved test stability by using deterministic round-robin mapping for backup responses instead of random selection.

[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: 749b915d-9f55-43d1-91e2-d32c5053b184

📥 Commits

Reviewing files that changed from the base of the PR and between 6bb13b7 and fe61277.

📒 Files selected for processing (1)

• br/pkg/backup/client_test.go

---

📝 Walkthrough

Walkthrough

In TestMainBackupLoop, a new storeIDForSplit(i) helper is introduced that uses modulo indexing (stores[i%len(stores)].GetId()) to deterministically assign store IDs. This helper replaces the previous random store selection across all five test case scenarios when populating mockBackupResponses.

Changes

Fix flaky TestMainBackupLoop with deterministic store selection

Layer / File(s)	Summary
storeIDForSplit helper and all test case updates br/pkg/backup/client_test.go	Adds storeIDForSplit(i) that picks stores[i%len(stores)].GetId(), then applies it in all five split-loop blocks (normal run, canceled, store drops never returns, store drops comes back, store drops while watched) replacing the prior random store selection.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Suggested labels

size/S, release-note-none, approved, lgtm

Suggested reviewers

• GMHDBJD
• YuJuncen
• Leavrth

Poem

🐇 No more random hops for me,
Round-robin stores, deterministically!
Flaky tests shall flap no more,
Modulo maps each range to store.
Steady builds, the warren cheers! 🎉

🚥 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
Title check	✅ Passed	The title 'test: stabilize flaky TestMainBackupLoop' accurately describes the main change—stabilizing a flaky test—and is concise and clear.
Description check	✅ Passed	The description includes all required template sections: problem statement with issue link, root cause analysis, fix explanation, verification details, test checklist with unit test selected, and release notes.
Linked Issues check	✅ Passed	The PR successfully addresses the objective of stabilizing the flaky TestMainBackupLoop test (#68203) by implementing deterministic round-robin mock response assignment to eliminate timeout issues.
Out of Scope Changes check	✅ Passed	All changes are scoped to the test file (br/pkg/backup/client_test.go) and directly address the flaky test stabilization objective with no out-of-scope modifications.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}