raft: add fast path to raftLog.match()

[hakuuww]

This PR addresses the TODO optimizations to add a fast path to raftLog.match() to avoid doing a linear scan to find a fork point on "our" raftLog against the appending LeadSlice in the common-case.

l is "our" log, s is the log append LeadSlice from MsgApp.

By raft invariants, we have a guarantee that our (follower) log l is a prefix of the l.accTerm leader's log.
So if the accTerm leader sends us log appends (indicated by the LeadSlice s.term field, and checked by l.accTerm == s.term),

we may skip all the matchTerm checks for the entries in the appending LeadSlice sas they will all succeed meaning there are no log inconsistencies between the log append LeadSlice s and our log l.

It is also possible that we have not yet appended the first log entry from the s.term leader so l.accTerm == s.term would evaluate to false. In this case, we can still use the Raft log matching property to do a fast-path check:
Check the term match at min(s.lastIndex(), l.lastIndex()) entry of s. If the term matches, there is no mismatch between the two logs.

In either case, if there is no mismatch, return min(s.lastIndex(), l.lastIndex()). All entryIDs up to and including this index are guaranteed to match.

Note that there is still one remaining TODO in raftLog.match() untouched.
However with the termCache in place the potential penalty cost of doing a linear scan (l.matchTerm(id)) for all id within LeadSlice s.entries.id) is no longer substantial, as it is very unlikely that the term check falls back to a storage call.
Therefore the TODO may be updated to reflect the current state.

Epic: None
Release note: None

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[cockroach-teamcity]

This change is 

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

Thank you for updating your pull request.

Before a member of our team reviews your PR, I have some potential action items for you:

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[hakuuww]

@hakuuww made 1 comment.
Reviewable status: complete! 0 of 0 LGTMs obtained.

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pkg/raft/log.go line 194 at r2 (raw file):

//
// All the entries up to the returned index are already present in the log, and
// do not need to be rewritten. The caller can safely fast-forward the appending

"appended" indicates entries in s LeadSlice are already appended to the caller's raftLog.
However, this is not true at this point in time, as the LeadSlice is still not appended yet.

Also it won't be "appended" if match returns 0, false.

[hakuuww]

@hakuuww made 1 comment and resolved 1 discussion.
Reviewable status: complete! 0 of 0 LGTMs obtained.

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pkg/raft/log.go line 194 at r2 (raw file):

Previously, hakuuww (Anthony Xu) wrote…

"appended" indicates entries in s LeadSlice are already appended to the caller's raftLog.
However, this is not true at this point in time, as the LeadSlice is still not appended yet.

Also it won't be "appended" if match returns 0, false.

Appending makes a bit more sense as we are in the process of attempting to append the LeadSlice to raftLog.