Re: [PATCH 2/3] diff: fix filtering of additional headers under --remerge-diff

[Elijah Newren <newren@gmail.com>]

On Wed, Aug 31, 2022 at 3:26 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> "Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com> writes:
>
> > From: Elijah Newren <newren@gmail.com>
> >
> > Commit 95433eeed9 ("diff: add ability to insert additional headers for
> > paths", 2022-02-02) introduced the possibility of additional headers.
> > Because there could be conflicts with no content differences (e.g. a
> > modify/delete conflict resolved in favor of taking the modified file
> > as-is), that commit also modified the diff_queue_is_empty() and
> > diff_flush_patch() logic to ensure these headers were included even if
> > there was no associated content diff.
>
> In the longer term, I think we may have to redo the way additional
> headers are inserted to the diff_queue.  All the diff code I am
> familiar with (read: written before this hack was introduced) trusts
> that diff_queue.nr is the number of paths that are returned by the
> diff frontend, and unless there is diffcore_break involved, there
> will be at most one diff_filepair that is about a path.
>
> Why do these need to be separate entries in the queue, not a new
> attribute added to an existing filepair?  Are we inserting pieces of
> information that are not about any paths that are involved in the
> diff?

They usually are added to an existing filepair, but as your last
question alludes to, there isn't always one that exists.

One example where this can happen is with a modify/delete conflict.
If someone got one of those and decided to keep the modified file
as-is when resolving the conflict, then there'd be no content diff to
show for that file when investigating the resulting merge commit with
--remerge-diff.  But since the point of --remerge-diff is to show
whatever work the user did to resolve conflicts, and that file was
conflicted, we want to show _something_ for that path.  Thus the
reason for creating a new "phoney" filepair just for attaching the
"remerge CONFLICT (modify/delete) ..." notice.