scatter: check pending peers before scattering (#31691)

[ti-srebot]

cherry-pick #31691 to release-5.4
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/32129

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/32129:release-5.4-15748a4996b1

---

Signed-off-by: Yu Juncen yu745514916@live.com

What problem does this PR solve?

Issue Number: close #31034

Problem Summary:
When scattering regions, BR sometimes meets "pending" regions and fail to scatter them then. (This behavior is defined by PD, we can found them in the DEBUG log.)

This PR checks the pending status before doing scatter.

Why those pending regions exist is still in investigating, for more details, see tikv/pd#4525 and #31034.

What is changed and how it works?

Check List

Tests

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

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

Fix a bug that caused region unbalanced after restoring.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3pointer
• fengou1

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[ti-srebot]

/run-all-tests

[ti-srebot]

@YuJuncen you're already a collaborator in bot's repo.

[3pointer]

LGTM

[3pointer]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 86ffa4c

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/95ab7c588144dc634b69115ad4f571b47cf8c773

[3pointer]

/rebuild

[ti-chi-bot]

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