Push down condition check to TiKV for prewrite request

[zhangjinpeng87]

What problem does this PR solve?

Push down condition check to TiKV for Insert request.

What is changed and how it works?

Push down condition check to TiKV, and the batch_get for insert statement with unique constraint can be removed.

Check List

Tests

• Unit test

Code changes

• Has interface methods change

Related PRs

• Push down precondition to TiKV kvproto#343

• prewrite only when keys are not exist tikv/tikv#4085

These two PRs should be merged first.

[tiancaiamao]

I suggest handle prewriteMutation logic like this:

func prewriteMutation() {
    if mutation.Op == Op_Insert {
        Get() // Special handle for Op_insert, call the Get() operation
    }
    ....
}

This may result in worse performance, but the code is much simpler.
@zhangjinpeng1987

[tiancaiamao]

Why log error here? key already exists is a normal business logic.

[zhangjinpeng87]

I think it is better to log duplicated entry error in TiDB.

[winkyao]

I agree with tiancaiamao, if we log the duplicated error, there will be too many useless logs.

[zhangjinpeng87]

@disksing PTAL

[winkyao]

reset LGTM

[winkyao]

I agree with tiancaiamao, if we log the duplicated error, there will be too many useless logs.

[disksing]

LGTM

[zhangjinpeng87]

@winkyao PTAL again.

[zhangjinpeng87]

/run-all-tests

[tiancaiamao]

/run-integration-tests

[tiancaiamao]

LGTM