Cherry pick of #11330 on release-3.4

[jingyih]

Cherry pick of #11330 on release-3.4.

#11330: mvcc/kvstore:when the number key-value is greater than one

Modified ./test to automatically detect branch when finding merge base. (Thanks to @jpbetz)

[jingyih]

Technically this is a performance improvement, not a bug fix. Given that we do not have a plan to release etcd 3.5 soon, I would like to see how people feel about backporting this performance improvement to etcd 3.4 (so that everybody can benefit from it early).

Basically, when cluster is large, the compaction of the in-memory index tree could take a long time to finish (hundreds of milliseconds). While server applies the compaction log entry, other requests (including linearizable read requests) might be blocked waiting on it.

[jingyih]

cc @gyuho @xiang90 @spzala @jpbetz @WIZARD-CXY

[gyuho]

can fix CI? otherwise, lgtm

[jpbetz]

It's my fault that fmt is failing, but I think it can be fixed by switching from master to release-3.4 here: https://github.com/etcd-io/etcd/blob/master/test#L584. We should improve that test code automatically detect the correct branch, e.g.

git log --oneline "$(git merge-base HEAD $(git rev-parse --abbrev-ref --symbolic-full-name @{u}))"...HEAD

Would someone be willing to apply this to the release branches and master?

@jingyih You can apply to /test on this PR?

[WIZARD-CXY]

lgtm

[jingyih]

@jingyih You can apply to /test on this PR?

Done

[jingyih]

shellcheck is failing, which I cannot reproduce locally. Let me try to debug and fix it here.

[jpbetz]

Latest test failure:

test:584:43: warning: Quote this to prevent word splitting. [SC2046]
test:584:94: warning: This { is literal. Check expression (missing ;/
?) or quote it. [SC1083]
test:584:96: warning: This } is literal. Check expression (missing ;/
?) or quote it. [SC1083]

We might need to do something like:

gitBranch=$(git rev-parse --abbrev-ref --symbolic-full-name "@{u}")
gitBaseHash=$(git merge-base HEAD "${gitBranch}")
git log --oneline "${gitBaseHash}"...HEAD | while read -r l; do

Fair warning: I haven't run this. It needs to be tested with shellcheck.

[wojtek-t]

/cc

[gyuho]

I will cut a new release today.