[PATCH 1 of 2] templater: do not use index.partialmatch() directly to calculate shortest()

[Yuya Nishihara]

On Sun, 23 Oct 2016 12:52:34 -0700, Martin von Zweigbergk wrote:
> On Sat, Oct 22, 2016 at 11:35 PM, Yuya Nishihara <yuya at tcha.org> wrote:
> > # HG changeset patch
> > # User Yuya Nishihara <yuya at tcha.org>
> > # Date 1477199123 -32400
> > #      Sun Oct 23 14:05:23 2016 +0900
> > # Branch stable
> > # Node ID f180a39d749aeacb72936e629a372623b1f88b8c
> > # Parent  76c57e1fe79b0980b377b4f305635dea393d6315
> > templater: do not use index.partialmatch() directly to calculate shortest()
> >
> > cl.index.partialmatch() isn't a drop-in replacement for cl._partialmatch().
> > It has no knowledge about hidden revisions, and it can't accept a node shorter
> > than the length 4. Instead, let cl._partialmatch() use index.partialmatch()
> > if available.
> >
> > The test result is sampled with --pure.
> >
> > diff --git a/mercurial/templater.py b/mercurial/templater.py
> > --- a/mercurial/templater.py
> > +++ b/mercurial/templater.py
> > @@ -840,13 +840,8 @@ def shortest(context, mapping, args):
> >      cl = mapping['ctx']._repo.changelog
> >      def isvalid(test):
> >          try:
> > -            try:
> > -                cl.index.partialmatch(test)
> 
> Are there other callers of this method (index.partialmatch) or can we
> remove it now?

It's the low-level function behind cl._partialmatch(), so can't be removed.

> > -            except AttributeError:
> > -                # Pure mercurial doesn't support partialmatch on the index.
> > -                # Fallback to the slow way.
> > -                if cl._partialmatch(test) is None:
> > -                    return False
> > +            if cl._partialmatch(test) is None:
> > +                return False
> 
> How much slower is it? Correctness is almost always more important,
> but it would be good to know how much slower we're making it.

Here's some numbers. The performance characteristics is identical if no hidden
revision is involved. If there are hidden revisions, _partialmatch() can fall
back to O(len(repo)) path, which means shortest() becomes way slower if
len(shortest(node)) > minlength.

(with no hidden revision)
% hg log -R mozilla-central -r0:20000 -T '{node|shortest}\n' --time > /dev/null
(before) time: real 1.310 secs (user 1.290+0.000 sys 0.010+0.000)
(after)  time: real 1.540 secs (user 1.520+0.000 sys 0.020+0.000)

(with hidden revisions)
% hg log -R hg-committed -r0:20000 -T '{node|shortest}\n' --time > /dev/null
(before) time: real 1.530 secs (user 1.480+0.000 sys 0.040+0.000)
(after)  time: real 43.080 secs (user 43.060+0.000 sys 0.030+0.000)

If we want to choose the speed over the correctness, we can switch to use the
unfiltered changelog, which will give the same results on both pure and CPy
environments.