store/tikv: make snapshot thread safe (#17593)

[tiancaiamao]

What problem does this PR solve?

Cherry pick #17593 to release-4.0

Problem Summary:

A correct fix need to cherry pick a series of commits:

#16056
#17555
#17593
#17678
#17911

That maybe too much for 4.0

What is changed and how it works?

make snapshot thread safe

Check List

Tests

• Unit test

Release note

• No release note

[tiancaiamao]

PTAL @coocood @lysu

[ti-srebot]

@coocood,Thanks for you review.

[coocood]

@tiancaiamao
Just move code from Next to Open doesn't completely fix the bug.
The PointGet executor maybe built many times during execution. for example, in IndexJoin.

[tiancaiamao]

@tiancaiamao
Just move code from Next to Open doesn't completely fix the bug.
The PointGet executor maybe built many times during execution. for example, in IndexJoin.

Move code from Next to Open so the snapshot is not captured everytime and reduce conflict (the transaction is modified and snapshot may be nil)

This code is not safe, as txn maybe set to nil after Valid() and before GetSnapshot.

if txn.Valid() {
     snapshot = txn.GetSnapshot()
}

I override GetSnapshot to provide the snapshot.
Not in a atomic way, but it should work , as a pointer is in a machine word size (assumption on hardware), either nil or a kv.Snapshot is OK.

[tiancaiamao]

Link issue #16817

[tiancaiamao]

/run-all-tests

[tiancaiamao]

github is unavailable

[coocood]

LGTM

[ti-srebot]

@coocood, Thanks for your review, however we are sorry that your vote won't be count. You already give a LGTM to this PR

[coocood]

@bobotu PTAL

[bobotu]

Not in a atomic way, but it should work , as a pointer is in a machine word size (assumption on hardware), either nil or a kv.Snapshot is OK.

This may have a problem with ARMv7?

In particular, the model was originally non-multicopy-atomic: writes could become visible to some other threads before becoming visible to all

Anyway, it isn't a blocker for this PR.

LGTM

[ti-srebot]

@bobotu, Thanks for your review, however we are sorry that your vote won't be count. You are not a reviewer or committer or co-leader or leader for the related sigs:execution(slack).

[coocood]

LGTM

[ti-srebot]

@coocood, Thanks for your review, however we are sorry that your vote won't be count. You already give a LGTM to this PR

[ti-srebot]

@SunRunAway,Thanks for you review.

[jebter]

/merge

[jebter]

/merge

[zhouqiang-cl]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@tiancaiamao merge failed.

[jebter]

/run-common-test

[jebter]

/run-check_dev