Re: diff-index --cc no longer permitted, gitk is now broken (slightly)

From: Sergey Organov <sorganov@gmail.com>
Date: Sep 8, 2021, 19:04 UTC
Message-ID: <87pmtiopm9.fsf@osv.gnss.ru>
In-Reply-To: <cbd0d173-ef17-576b-ab7a-465d42c82265@kdbg.org>

Johannes Sixt <j6t@kdbg.org> writes:

Show 17 quoted lines

> Am 08.09.21 um 15:43 schrieb Sergey Organov:
>> Besides, nobody yet told us why gitk uses --cc option in invocation of
>> 'diff-index' in the first place. Does it actually *rely* on particular
>> undocumented behavior of "diff-index --cc", or is it just a copy-paste
>> *leftover*?
>
> No, it is not a left-over. The thing is,
>
> - there is one point in the code where gitk adds options -p -C --cc (and
> more) to the command line (around line 8034),
>
> - and there is a totally different point in the code where it is decided
> whether diff-index, diff-tree, or diff-files is invoked (proc diffcmd
> around line 7871).
>
> IOW, Gitk expects that these option combinations can always be passed to
> all three commands.

I see, but the problem here is that while diff-files and diff-tree both accept --cc according to their documentation, diff-index does not. This means that, strictly speaking, gitk makes a mistake treating all 3 commands universally with respect to command-line arguments when it uses --cc.

Show 5 quoted lines

>
> Gitk does not want to look at a commit and then decide which incarnation
> of the command it wants to use (--cc vs. -p) depending on whether it is
> a merge commit or not. This decision is delegated to command that is
> invoked.

The problem is not in the kind of commit, the problem is in the command being invoked. diff-index doesn't support --cc according to its documentation, and thus gitk relies on undocumented behavior of diff-index. It might well be the case that it just happened to be "working", thus nobody cared.

> Therefore, silent fall-back from --cc to -p in case of non-merge
> commits or non-conflicted index is absolutely necessary.

I didn't change semantics of --cc, so this thing was not broken at all. I just disabled the --cc option in diff-index command, to match the documentation.

As a side note, in fact Git does no "silent fall-back from --cc to -p in case of non-merge commits", even though the behavior could be indeed seen like this. Instead, --cc implies -p, and, as --cc does not otherwise affect treating of non-merge commits, only -p is left active for them. Once again, this has not been recently changed, so does not need to be fixed.

Thanks, -- Sergey Organov