From: Philip Oakley <philipoakley@iee.email>
Date: Fri, 29 Nov 2019 13:31:23 GMT
Subject: Re: git-rebase produces incorrect output
Message-ID: <eadb7c7e-5c8c-7e0b-2be0-ddc31d6ff7d4@iee.email>
In-Reply-To: <CAN_72e2h2avv-U9BVBYqXVKiC+5kHy-pjejyMSD3X22uRXE39g@mail.gmail.com>

On 29/11/2019 08:21, Pavel Roskin wrote:
> Hi!
>
> I've discovered an issue with "git rebase" producing a subtly
> incorrect file. In fact, that files even compiled but failed in unit
> tests! That's so scary that I'm going to stop using "git rebase" for
> now. Fortunately, "git rebase --merge" is working correctly, so I'll
> use it. Too bad there is no option to use "--merge" by default.
>
> The issue was observed in git 2.23 and reproduced in today's next
> branch (2.24.0.449.g4c06f74957) on up-to-date Fedora 31 x86_64.
>
> I've created a repository that demonstrates the issue:
> https://github.com/proski/git-rebase-demo
>
> The branch names should be self-explanatory. "master" is the base,
> "branch1" and "branch2" contain one change each. If "branch1" is
> rebased on top of "branch2", the result is incorrect, saved in the
> "rebase-bad" branch. If "git rebase -m" is used, the result is
> correct, saved in the "merge-good" branch.
>
> The files in "rebase-bad" and "merge-good" have exactly the same lines
> but in a different order. Yet the changes on branch1 and branch2
> affect non-overlapping parts of the file. There should be no doubt how
> the merged code should look like.
>
> I believe the change on branch2 shifts the lines, so that the first
> change from branch1 is applies to a place below the intended location,
> and then git goes back to an earlier line to apply the next hunk. I
> can imagine that it would do the right thing in case of swapped blocks
> of code. Yet I have a real life example where it does a very wrong
> thing.
>
> Indeed, "git diff origin/branch2 origin/rebase-bad" and "git diff
> origin/branch2 origin/merge-good" both produce diffs of 9957 bytes
> long, different only in the order of the hunks.
>
> Another interesting data point - "git rebase --interactive" is working
> correctly.
>
Which specific lines is this on?

Using the Github compare facility [1], I see multiple changes, some of 
which are probably just noise from the example.
  https://github.com/proski/git-rebase-demo/compare/merge-good...rebase-bad

Philip

[1] 
https://help.github.com/en/github/committing-changes-to-your-project/comparing-commits-across-time