Re: [RFC 3/3] refspec: add support for negative refspecs

[Jacob Keller <jacob.keller@gmail.com>]

On Tue, Aug 18, 2020 at 10:41 AM Jeff King <peff@peff.net> wrote:
>
> On Mon, Aug 17, 2020 at 05:04:00PM -0700, Jacob Keller wrote:
>
> > > > +     /* apply any negative refspecs now to prune the list of refs */
> > > > +     ref_map = apply_negative_refspecs(ref_map, rs);
> > > > +
> > > >       ref_map = ref_remove_duplicates(ref_map);
> > >
> > > How was the ordering here decided?  Should it result the same set if
> > > negative ones are excluded after duplicates are removed?
> >
> > Good question. This was what was done in peff's original patch. I need
> > to understand a bit more about what ref_remove_duplicates does to
> > really figure this out.
>
> The relevant commit is 2467a4fa03 (Remove duplicate ref matches in
> fetch, 2007-10-08), I think. We may end up with multiple refspecs
> requesting a particular ref. E.g.:
>
>   git fetch origin refs/heads/master refs/heads/*
>
> I don't think the order should matter. If we apply negative refspecs
> first, then we'd either remove both copies or leave both untouched (and
> if the latter, then de-dup to a single). If we apply negative refspecs
> after de-duping, then we'd either remove the single or leave it in
> place. But the result is the same either way.

I'm not sure this is quite true in the case where destinations are
supplied. Suppose this case:

git fetch refs/heads/*:refs/remotes/origin/*
refs/other/mybranch:refs/remotes/origin/mybranch

This would ofcourse error out due to de-duping where we determine that
both would fetch to the same place.. however if you also added a
negative refspec:

git fetch refs/heads/*:refs/remotes/origin/*
refs/other/mybranch:refs/remotes/origin/mybranch ^refs/heads/mybranch

then shouldn't this work? meaning we should de-dupe only after we
apply negative refspecs in this case?

>
> > > > @@ -1441,6 +1445,8 @@ int match_push_refs(struct ref *src, struct ref **dst,
> > > >               string_list_clear(&src_ref_index, 0);
> > > >       }
> > > >
> > > > +     *dst = apply_negative_refspecs(*dst, rs);
> > > > +
> > >
> > > The block of code whose tail is shown in the pre-context has
> > > prepared "delete these refs because we no longer have them" to the
> > > other side under MATCH_REFS_PRUNE but that was done based on the
> > > *dst list before we applied the negative refspec.  Is the ordering
> > > of these two correct, or should we filter the dst list with negative
> > > ones and use the resulting one in pruning operation?
> >
> > I think we need to swap the order here. I'll take a closer look.
>


> Hmm. I think the behavior we'd want is something like:
>
>   # make sure the other side has three refs
>   git branch prune/one HEAD
>   git branch prune/two HEAD
>   git branch prune/three HEAD
>   git push dst.git refs/heads/prune/*
>
>   # now drop two of ours, which are eligible for pruning
>   git branch -d prune/one
>   git branch -d prune/two
>
>   # push with pruning, omitting "two"
>   git push --prune dst.git refs/heads/prune/* ^refs/heads/prune/two
>
>   # we should leave "two" but still deleted "one"
>   test_write_lines one three >expect
>   git -C dst.git for-each-ref --format='%(refname:lstrip=3)' refs/heads/prune/ >actual
>   test_cmp expect actual
>
> I.e., the negative refspec shrinks the space we're considering pruning.
> And we'd probably want a similar test for "fetch --prune".
>
> I just tried that, though, and got an interesting result. The push
> actually complains:
>
>   $ git push --prune dst.git refs/heads/prune/* ^refs/heads/prune/two
>   error: src refspec refs/heads/prune/two does not match any
>   error: failed to push some refs to 'dst.git'
>
> For negative refspecs, would we want to loosen the "must-exist" check?
> Or really, is this getting into the "are we negative on the src or dst"
> thing you brought up earlier? Especially with --prune, what I really
> want to say is "do not touch the remote refs/heads/two".
>

Hmmm..

For regular push the negative refspec applies to the source. For prune
though we only provide a destination..

> We can get work around it by using a wildcard:
>
>   $ git push --prune dst.git refs/heads/prune/* ^refs/heads/prune/two*
>   To dst.git
>    - [deleted]         prune/one
>
> So it works as I'd expect already with your patch. But I do wonder if
> there are corner cases around the src/dst thing that might not behave
> sensibly.
>

Right, there's some interesting questions here still.

> -Peff

I'll be adding this as a test!

Thanks,
Jake