copr: Fix bug that mpp node availability detect does not work in some corner cases (#28201)

[ti-srebot]

cherry-pick #28201 to release-5.1
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/28288

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git push git@github.com:ti-srebot/tidb.git pr/28288:release-5.1-624f7cab3b4e

---

What problem does this PR solve?

Issue Number: close pingcap/tiflash#3118

Problem Summary:
As described in the issue.

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:
The root cause is we detect the TiFlash node's availability in balanceBatchCopTask, but here is a shortcut in balanceBatchCopTask that if there is only one batchCopTask, it will return directly without balancing, which means the availability check is also skipped.

tidb/store/copr/batch_coprocessor.go

Lines 107 to 111 in 3c9c01e

	func balanceBatchCopTask(ctx context.Context, kvStore *kvStore, originalTasks []*batchCopTask, mppStoreLastFailTime map[string]time.Time, ttl time.Duration) []*batchCopTask {
	if len(originalTasks) <= 1 {
	return originalTasks
	}
	isMPP := mppStoreLastFailTime != nil

This pr fix this by not trigger the shortcut if the query is mpp query.

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 bug that mpp node availability detect does not work in some corner cases

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• fzhedu
• windtalker

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[ti-srebot]

/run-all-tests

[ti-srebot]

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

[windtalker]

LGTM

[fzhedu]

LGTM

[fzhedu]

/merge

[ti-chi-bot]

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

Commit hash: f1962c4

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