Re: combined diff does not detect binary files and ignores -diff attribute

From: Junio C Hamano <gitster@pobox.com>
Date: May 24, 2011, 04:46 UTC
Message-ID: <7v39k4aeos.fsf@alter.siamese.dyndns.org>
In-Reply-To: <20110523234131.GB10488@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 7 quoted lines

>> However, custom diff drivers (still) don't work. :-)
>
> Yeah, I didn't add any support for that. I'm not sure what it should do;
> custom diff drivers don't know how to handle combined diff, do they?
>
> If you write me a test case that explains what _should_ happen, I'll see
> what I can do. :)

I do not think it is sensible to expect anybody to come up with a sane semantics for combined diff to work with GIT_EXTERNAL_DIFF (and external diff driver that can be specified via the attributes mechanism) in any meaningful way.

The whole point of the external diff mechanism is that an external command can take _two_ files and represent the change between them in a way that is more suited for the need of the user than the patch form. The output from such an external command does not have any obligation to even follow the convention used by the patch output, namely:

  @@ from here to there things have changed @@
   this is common
  -this was the removed content
  +this is the new content

as the _whole_ point of the external diff mechanism is to give something that is _different_ from the patch form, in the hope that it is in a more appropriate form for whoever consumes the output.

On the other hand, combined diff is all about combining multiple patches show them side-by-side in a combined fashion. Without the above four kinds of cues, there is no way to even _align_ the change outputs from two parents, let alone _combining_ them.

Anybody interested can check "compare-cooking.perl" in the todo branch, which is used as an external diff driver to view the differences between "What's cooking" postings via these:

    [diff "whatscooking"]
            xfuncname = "^\\[(.*)\\]$"
            command = ./compare-cooking.perl

in the .git/config file, together with

    whats-cooking.txt diff=whatscooking

in the .gitattributes file. Running

    $ git log -p --ext-diff todo -- whats-cooking.txt

would give a sample output.

It is conceivable that we _could_ newly define a "combined external diff driver" that would take 3 or more files, and compute and show the combined result by itself, but that will certainly not go through the codepath you touched with the textconv patch. Calling out to such a new type of external diff driver would have to happen at the level where we have 1+N blob object names for a N-parent commit, namely, at the beginning of show_patch_diff(), bypassing the entire contents of that function and instead letting the new n-way external diff driver do everything.

I however highly doubt that such an interface would make sense. For example, what would be the desirable format to compare three versions of "What's cooking" postings, and how would the updated compare-cooking.perl script would look like?