Re: [PATCH 7/7] xdiff: make diff3 the default conflictStyle

[Felipe Contreras <felipe.contreras@gmail.com>]

Elijah Newren wrote:
> On Fri, Jun 11, 2021 at 2:05 PM Felipe Contreras
> <felipe.contreras@gmail.com> wrote:
> >
> > Elijah Newren wrote:
> > > On Fri, Jun 11, 2021 at 10:57 AM Felipe Contreras <felipe.contreras@gmail.com> wrote:
> >
> > > > Or just use the base of the virtual merge:
> > > >
> > > >   <<<<<<< HEAD
> > > >   D
> > > >   ||||||| merged common ancestors
> > > >   1
> > > >   =======
> > > >   C
> > > >   >>>>>>> C
> > >
> > > I think that implementing this choice would look like this (again, not
> > > compiled or tested and I'm not familiar with xdiff so take it with a
> > > big grain of salt):
> > >
> > >
> > > diff --git a/ll-merge.c b/ll-merge.c
> > > index 095a4d820e..dbc7f76951 100644
> > > --- a/ll-merge.c
> > > +++ b/ll-merge.c
> > > @@ -130,6 +130,8 @@ static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
> > >       memset(&xmp, 0, sizeof(xmp));
> > >       xmp.level = XDL_MERGE_ZEALOUS;
> > >       xmp.favor = opts->variant;
> > > +     if (git_xmerge_style >= 0 && opts->virtual_ancestor)
> > > +             xmp.favor = XDL_MERGE_FAVOR_BASE;
> >
> > The only time git_xmerge_style isn't >= 0 is when no merge style has
> > been configured by the user.
> 
> Yep, probably should have just been
> 
> +     if (opts->virtual_ancestor)
> +             xmp.favor = XDL_MERGE_FAVOR_BASE;
> 
> Though the difference doesn't matter a lot.  Since
> merge.conflictStyle=merge (which is the current default) doesn't
> display the contents from the merge base in a three-way content merge,
> setting xmp.favor to XDL_MERGE_FAVOR_BASE vs. leaving it as 0 for the
> recursive/intermediate merges won't generally end up affecting the end
> result.  It'd only matter for diff3 and zdiff3 users.

OK, so:

  if (git_xmerge_style > 0 && opts->virtual_ancestor)

> Going on a slight tangent, I think there's actually a related bug
> here.  We probably should not honor XDL_MERGE_FAVOR_{OURS,THEIRS} when
> opts->virtual_ancestor is true; that's just asking for trouble.  I
> think it'd paradoxically result in reversing the desired behavior
> (e.g. users would see what they'd consider XDL_MERGE_FAVOR_THEIRS
> behavior when they asked for XDL_MERGE_FAVOR_OURS) in some cases as a
> result.

Maybe write a test-case and find out?

> > In fact, I don't see why any style should change that desired behavior.
> > If you said there's issues with the "merge" style too, perhaps the above
> > will help for those cases too.
> >
> > > > We don't have to use diff3 all the way.
> > >
> > > Right, thus my mention in the other email to consider adding a
> > > XDL_MERGE_FAVOR_BASE -- which you then also mention here in your third
> > > option, and which I've now given at least a partial patch for.  Not
> > > sure if it's a crazy idea or a great idea, since I don't do very many
> > > criss-cross merges myself.
> >
> > I thought you meant as a separate configurable flag, not something done
> > by default.
> >
> > Now that I understand what you meant I think it could be a great idea.
> 
> If someone that does lots of criss-cross merges can comment on the
> idea, and agree that it's worth a shot, I can try to turn it into real
> patches.

I would flip the conditional: if nobody that does lots of criss-crosses
objects...

If we waited for a tiny minority of users to speak up before doing
something we migth wait forever.

> (I might even try to investigate the zdiff3 stuff too, which sounds
> like something I've wanted many times...but I'd really rather
> concentrate on merge-ort until its upstreaming is finished.)

Well, I just re-sent the patch:

https://lore.kernel.org/git/20210613143155.836591-1-felipe.contreras@gmail.com/

Cheers.

-- 
Felipe Contreras