Re: How to debug a "git merge"?

[Jeff King <peff@peff.net>]

On Thu, Mar 15, 2018 at 10:51:27AM +0100, Lars Schneider wrote:

> > Maybe a stupid question, but: did you make sure that the merge does
> > indeed pick the wrong version of the file? The other option is that
> > somebody mistakenly did a "checkout --ours" or similar while resolving
> > the conflict.
> 
> No stupid question at all. That's exactly what they did and I did not
> realize it! Thank you!

Oh good. :) I have run across this same situation before myself, which
is why I thought to ask about it.

> Next time I won't stumble over this. I wonder if this is a common enough
> problem to do something about it? For instance what if `git log` (or just
> `git show`) has an option `--verify-merges` or `--reenact-merges` or 
> something? This option would perform a "default recursive merge" and 
> show the diff between the actual merge and the default merge?
> 
> In the most common case there is no diff. If there are merge conflicts
> then we would just show the conflicting files. If there is no merge
> conflict for a file *but* a difference then we would show it. I think
> this would have helped me to realize this kind of problem earlier.
> 
> Would that option make sense to you?

Yes, it's absolutely a good idea, and a frequent wish-list item. The
problem is that it's tricky to implement. The only working patches I
know of were Thomas Rast's "--remerge-diff" from 2014:

  https://public-inbox.org/git/cover.1409860234.git.tr@thomasrast.ch/

The tricky thing is that you have to actually re-run the diff, and we
don't know what options were used for the original diff (e.g., what
strategy). And also, merge-recursive really wants to have a valid
working tree (though I think maybe that has gotten better over the
years).

But even with those warts, I still found it useful in many cases.

-Peff