[PATCH 1 of 6 V2] obsstore: add a 'cachekey' method

[Yuya Nishihara]

On Thu, 1 Jun 2017 15:28:19 -0700, Jun Wu wrote:
> Excerpts from Sean Farley's message of 2017-06-01 13:27:44 -0700:
> > > Both series are complementary and useful. Obscache is very efficient for 
> > > its purpose and the other one improve other aspect of evolution related 
> > > computation. We already have many rev indexed caches so nothing really 
> > > new here.
> > 
> > This was my thinking as well. Though, I'm not trying to muddy the waters
> > here.
> 
> There are 4 revsets: obsolete, unstable, bumped, and divergent. My patch
> speeds up all of them significantly while obscache only speeds up the first
> one.
> 
> I admit that obscache is slightly faster than indexing on the "obsolete"
> revset. But that perf difference would be like just 20ms for hg-committed. I
> think that 20ms does not justify the complexity of a complete
> (huge-repo-friendly) implementation obscache and the coupling with revision
> number in design. And I do think eventually we want the related revset to be
> lazy so there won't be noticeable perf difference.

I haven't read these patches carefully, but I like Jun's radixlink idea which
seemed clever. If the perf win is just a few tens of milliseconds, I prefer not
having two separate cache layers that could complicate things in future.