BR: Fix stuck when many missing-peer regions in cluster (#28497)

[ti-srebot]

cherry-pick #28497 to release-5.2
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/28680

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

git push [email protected]:ti-srebot/tidb.git pr/28680:release-5.2-6782628bc181

---

What problem does this PR solve?

Issue Number: close #27534

Problem Summary:
Even BR would skip when store count less than max-replica, the replica requirement via placement rules is still possible to make the restore stuck.

What is changed and how it works?

What's Changed:
"Parallel" all scatter request... (No many goroutines spawned, see the code)

How it Works:
So it wouldn't be stuck.

Check List

Tests

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

Release note

Fixed a bug that caused BR get stuck when many missing-peer regions in cluster.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• kennytm

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[ti-srebot]

/run-all-tests

[ti-srebot]

@YuJuncen please accept the invitation then you can push to the cherry-pick pull requests. Comment with /cherry-pick-invite if the invitation is expired.
https://github.com/ti-srebot/tidb/invitations

[YuJuncen]

/cherry-pick-invite

[ti-srebot]

@YuJuncen please accept the invitation then you can push to the cherry-pick pull requests. Comment with /cherry-pick-invite if the invitation is expired.
https://github.com/ti-srebot/tidb/invitations

[YuJuncen]

[2021-10-11T07:07:51.451Z] ./tools/check/check-errdoc.sh
[2021-10-11T07:08:01.473Z] Generate successfully, output:
[2021-10-11T07:08:01.473Z] Non-normalized error: insert statement not found
[2021-10-11T07:08:01.473Z] Duplicated error code: types:1292
[2021-10-11T07:08:01.473Z] Non-normalized error: can not add because sorted
[2021-10-11T07:08:01.473Z] Non-normalized error: Pattern not match
[2021-10-11T07:08:01.473Z] Non-normalized error: label constraint should be in format '{+|-}key=value'
[2021-10-11T07:08:01.473Z] Non-normalized error: unsupported label constraint
[2021-10-11T07:08:01.473Z] Non-normalized error: conflicting label constraints
[2021-10-11T07:08:01.473Z] Non-normalized error: label constraints in map syntax have invalid replicas
[2021-10-11T07:08:01.473Z] Non-normalized error: invalid label constraints format
[2021-10-11T07:08:01.473Z] Non-normalized error: label constraints with invalid REPLICAS
[2021-10-11T07:08:01.473Z] Non-normalized error: invalid bundle ID
[2021-10-11T07:08:01.473Z] Non-normalized error: invalid bundle ID format
[2021-10-11T07:08:01.473Z] Non-normalized error: REPLICAS must be 1 if ROLE=leader
[2021-10-11T07:08:01.473Z] Non-normalized error: the ROLE field is not specified
[2021-10-11T07:08:01.473Z] Non-normalized error: no rule of such role to drop
[2021-10-11T07:08:01.473Z] Non-normalized error: retryable: lock not found
[2021-10-11T07:08:01.473Z] Non-normalized error: retryable: already rollback
[2021-10-11T07:08:01.473Z] Non-normalized error: retryable: replaced by another transaction

🤔

[YuJuncen]

/run-check_dev 🤔

[glorv]

/merge

[ti-chi-bot]

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

Commit hash: 02e7d0e

[zhouqiang-cl]

/merge

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