br: improve error handling in PaginateScanRegion function

[okJiang]

What problem does this PR solve?

Issue Number: close #64257

Problem Summary:

The scan region here is a batch. BR will retry this batch scan region.

• Expectation: If an error is reported in every retry, the whole batch scan region should be retreated to the leader handle.
• Now, when an error retry is encountered, the first scan region in the loop will be retreated, but when err is changed to nil in the first retreated scan region, the retreated will be canceled. The first scan region will be retreated, but the first time the scan region is retreated, err is changed to nil, so the retreated region is canceled, which is not as expected.

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.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.9027%. Comparing base (749c973) to head (efdd992).
⚠️ Report is 24 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #64259        +/-   ##
================================================
+ Coverage   72.7381%   74.9027%   +2.1645%     
================================================
  Files          1859       1884        +25     
  Lines        503905     519506     +15601     
================================================
+ Hits         366531     389124     +22593     
+ Misses       115136     106659      -8477     
- Partials      22238      23723      +1485     

Flag	Coverage Δ
integration	48.8456% <100.0000%> (?)
unit	72.6339% <100.0000%> (+0.3263%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.8700% <ø> (ø)
parser	∅ <ø> (∅)
br	63.3997% <100.0000%> (+17.0143%)	⬆️

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[okJiang]

/retest

[okJiang]

/retest

[okJiang]

/retest

[3pointer]

Suggested change

	return lastRegions, retryErr
	return lastRegions, retryErr

should better use last error, or the error stack will be very large.

[okJiang]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: 3pointer, rleungx

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• br/OWNERS [3pointer]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-11-07 06:55:24.153330368 +0000 UTC m=+425973.596360247: ☑️ agreed by 3pointer.
• 2025-11-07 08:18:22.49041611 +0000 UTC m=+430951.933445989: ☑️ agreed by rleungx.

[ti-chi-bot]

@okJiang: 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
idc-jenkins-ci-tidb/unit-test	efdd992	link	unknown	/test unit-test

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