Re: [PATCH 1/3] diff: have submodule_format logic avoid additional diff headers

[Elijah Newren <newren@gmail.com>]

On Wed, Aug 31, 2022 at 3:20 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,
> > created in create_filepairs_for_header_only_notifications().  These are
> > represented by inserting additional pairs in diff_queued_diff which
> > always have a mode of 0 and a null_oid.  When these were added, one
> > code path was noted to assume that at least one of the diff_filespecs
> > in the pair were valid, and that codepath was corrected.
> >
> > The submodule_format handling is another codepath with the same issue;
> > it would operate on these additional headers and attempt to display them
> > as submodule changes.  Prevent that by explicitly checking for both
> > modes being 0.
>
> It may make sense to give a concrete name for the condition these
> new codepaths check, which presumably exists in the part that was
> touched in 95433eeed9 when "that codepath was corrected".  I think
> you want to treat a diffpair with at least one side with non-zero
> mode as a "real" thing (as opposed to the phony "additional headers"
> hack), so perhaps
>
>         int diff_filepair_is_phoney(struct diff_filespec *one,
>                                     struct diff_filespec *two)
>         {
>                 return !DIFF_FILE_VALID(one) && !DIFF_FILE_VALID(two);
>         }
>
> or something like that.  The use of the FILE_VALID macro here is
> very much deliberate, and tries to match the more recent hack after
> this hunk that says:
>
>         if (!DIFF_FILE_VALID(one) && !DIFF_FILE_VALID(two)) {
>                 /*
>                  * We should only reach this point for pairs from
>                  * create_filepairs_for_header_only_notifications().  For
>                  * these, we should avoid the "/dev/null" special casing
>                  * above, meaning we avoid showing such pairs as either
>                  * "new file" or "deleted file" below.
>                  */
>                 lbl[0] = a_one;
>                 lbl[1] = b_two;
>         }
>
> We shouldn't expect readers to understand (one->mode || two->mode)
> is about the same hack as the other one.

I like that; I'll make that change.