store/copr: polish the tiflash-tikv fallback function.

[hanfei1991]

What problem does this PR solve?

Problem Summary:
fallback to tikv when batch cop or mpp meets io error

What is changed and how it works?

What's Changed:
Convert all kinds of io error in BatchCop and MPP to "TiFlashTimeout" error which can be identified by fallback function and retry in TiKV mode.
For now, mpp io error cannot be retried by itself. So even not a fatal error can be routed to tikv's retry. This problem can be tackled after we support mpp client's retry.

Related changes

• PR to update unistore: decrease the count ref when mpp request is done. ngaut/unistore#438 fix a leak in unistore after processing mpp request.

Check List

Tests

• Unit test for batch cop, mpp test.

Release note

• store/copr: polish the tiflash-tikv fallback function.

[hanfei1991]

/cc @winoros @xuyifangreeneyes

[winoros]

but both of us have no permission to approve, i'm asking others for help.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lysu
• winoros

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[winoros]

/merge

[ti-chi-bot]

@winoros: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: d9d51ed

[hanfei1991]

/assign @winoros

[winoros]

/merge

[ti-chi-bot]

@winoros: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[ti-chi-bot]

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

Commit hash: 8e259b0

[winoros]

/merge

[ti-chi-bot]

@winoros: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[ti-chi-bot]

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

Commit hash: 81243ff

[ti-chi-bot]

@hanfei1991: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.