Re: feature suggestion: optimize common parts for checkout --conflict=diff3

[Jeff King <peff@peff.net>]

On Thu, Mar 07, 2013 at 01:50:46PM -0500, Jeff King wrote:

> On Thu, Mar 07, 2013 at 10:40:46AM -0800, Junio C Hamano wrote:
> 
> > Where we differ is if such information loss is a good thing to have.
> >
> > We could say "both sides added, identically" is auto-resolved when
> > you use the zealous option, and do so regardless of how the merge
> > conflicts are presented.  Then it becomes perfectly fine to eject
> > "A" and "E" out of the conflicted block and merge them to be part of
> > pre/post contexts.  The same goes for reducing "<C|=C>" to "C".  As
> > long as we clearly present the users what the option does and what
> > its implications are, it is not bad to have such an option, I think.
> 
> Exactly. I do think it has real-world uses (see the example script I
> posted yesterday), but it would never replace diff3. I'm going to try it
> out for a bit. As I mentioned yesterday, I see those sorts of
> cherry-pick-with-something-on-top conflicts when I am rebasing onto or
> merging my topics into what you have picked up from the same topic on
> the list.

I wanted to give an update on how this has been going. I've been running
with zdiff3 for almost a month. I keep my merge.conflictstyle set to
diff3, and when I see something that I think might benefit from the
"both sides added" zealousness, I do a "git checkout --conflict=zdiff3"
and examine the result.

I have seen it help, and always when rebasing patches that were accepted
upstream. For example, imagine I added a big block of text in one patch
(e.g., an entire test script). Then I added more tests in a follow-on
patch. Or I change some of the lines from expect_failure to
expect_success. You can see this in t1060 of the
jk/check-corrupt-objects-carefully topic (I didn't try, but you could
probably reproduce by just rebasing it on top of the current master).

When I rebase my version of the patches on your master with the new
content, the conflict for the first patch is useless in diff3. I see
that the base had nothing, upstream added a hundred lines, and my patch
added ninety lines. But it's hard to see which lines are missing or
modified because of the size of the conflict. It looks like:

       <<<<<<< ours
       #!/bin/sh
       test_description=whatever
       ...
          end of some test
       '
       test_done
       ||||||| base
       =======
       #!/bin/sh
       test_description=whatever
       ...
          end of another test
       '
       test_done
       >>>>>>> theirs

The interesting part is in the "...", which contains different lines in
each version, but it may be hundreds of lines long. Using zdiff3, I get:

       #!/bin/sh
       test_description=whatever
       ...
       <<<<<<< ours
       test_expect_success 'some_new_test' '
       ...
       ||||||| base
       =======
       >>>>>>> theirs
       '
       test_done

I can see that nothing was tweaked; I just didn't add any content there,
and upstream did. Contrast this with zealous "merge" conflicts, which
would look like:

      #!/bin/sh
      test_description=whatever
      ...
      <<<<<<< ours
      test_expect_success 'some_new_test' '
      ...
      =======
      >>>>>>> theirs
      '
      test_done

which similarly condenses, but is missing a piece of information: that
there was nothing in the base. I don't know whether the conflict is
there because my patch removed some content that got changed upstream,
or whether upstream added some content that I did not have in my patch.

So I think it is useful when rebasing on top of what upstream took,
specifically when:

  1. You have a series that updates the same hunk repeatedly (because
     from your perspective, you see only the tip of what upstream took).

  2. Upstream takes your patch but tweaks it (either as a fixup, to deal
     with a merge conflict, or whatever). You get to see the minimal
     tweak, not the fact that you have a giant hunk which differs from
     the upstream only by a few characters or a few lines.

So I do think zdiff3 is useful, and I plan to continue using it.

-Peff