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

From: Jeff King <peff@peff.net>
To: Jacob Keller <jacob.keller@gmail.com>
Cc: Junio C Hamano <gitster@pobox.com>,
	Jacob Keller <jacob.e.keller@intel.com>,
	Git mailing list <git@vger.kernel.org>
Subject: Re: [RFC 3/3] refspec: add support for negative refspecs
Date: Tue, 18 Aug 2020 13:41:16 -0400	[thread overview]
Message-ID: <20200818174116.GA2473110@coredump.intra.peff.net> (raw)
In-Reply-To: <CA+P7+xpcm51cLPDDW+F1J-XZ2VvwNDWjnZqm54f3DKXxDfBF5Q@mail.gmail.com>

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.

> > > @@ -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".

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.

-Peff