br: handle region leader miss (#52822)

[ti-chi-bot]

This is an automated cherry-pick of #52822

What problem does this PR solve?

Issue Number: close #51124 close #50501

Problem Summary:
the leader of region, scanned from PD, is nil

What changed and how does it work?

check regions' leader after scan from PD.

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

[Leavrth]

/test pull-br-integration-test

[tiprow]

@Leavrth: No presubmit jobs available for pingcap/[email protected]

In response to this:

/test pull-br-integration-test

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lance6716, YuJuncen

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

The pull request process is described here

Needs approval from an approver in each of these files:

• br/OWNERS [YuJuncen]
• br/pkg/lightning/OWNERS [lance6716]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-22 07:33:32.199194248 +0000 UTC m=+2243365.956329822: ☑️ agreed by lance6716.
• 2024-05-22 09:13:55.497336271 +0000 UTC m=+2249389.254471843: ☑️ agreed by YuJuncen.

[Leavrth]

/retest

[Leavrth]

/retest

[codecov]

Codecov Report

Attention: Patch coverage is 36.36364% with 14 lines in your changes are missing coverage. Please review.

Please upload report for BASE (release-7.5@e406d57). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #52855   +/-   ##
================================================
  Coverage               ?   71.9948%           
================================================
  Files                  ?       1411           
  Lines                  ?     410253           
  Branches               ?          0           
================================================
  Hits                   ?     295361           
  Misses                 ?      95027           
  Partials               ?      19865           

Flag	Coverage Δ
unit	71.9948% <36.3636%> (?)

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

Components	Coverage Δ
dumpling	53.9913% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.4049% <0.0000%> (?)