From: Elijah Newren <newren@gmail.com>
To: Eugeniu Rosca <roscaeugeniu@gmail.com>
Cc: "Eugeniu Rosca" <erosca@de.adit-jv.com>,
"SZEDER Gábor" <szeder.dev@gmail.com>,
"Junio C Hamano" <gitster@pobox.com>, "Jeff King" <peff@peff.net>,
"Ævar Arnfjörð" <avarab@gmail.com>,
"Git Mailing List" <git@vger.kernel.org>
Subject: Re: Unreliable 'git rebase --onto'
Date: Thu, 9 Jan 2020 18:35:27 -0800 [thread overview]
Message-ID: <CABPp-BHvJHpSJT7sdFwfNcPn_sOXwJi3=o14qjZS3M8Rzcxe2A@mail.gmail.com> (raw)
In-Reply-To: <20200110000603.GA19040@erosca>
Hi Eugeniu,
On Thu, Jan 9, 2020 at 4:06 PM Eugeniu Rosca <roscaeugeniu@gmail.com> wrote:
>
> Hi Elijah,
>
> On Thu, Jan 09, 2020 at 10:05:52AM -0800, Elijah Newren wrote:
> > On Thu, Jan 9, 2020 at 2:53 AM Eugeniu Rosca <erosca@de.adit-jv.com> wrote:
> > > Some years ago I was hit by 'git merge' producing slightly different
> > > results compared to 'git rebase --onto' and 'git cherry-pick A..B'
> > > (maybe I can come up with a reproduction scenario for that too).
> >
> > If you can, I'd be interested to see it and take a look. I'd normally
> > assume it was just some case where A..B included "evil" merge commits
> > (merge commits that made additional changes not part of the actual
> > merging) since rebasing or cherry-picking such a range would exclude
> > the merge commits and thus drop those changes -- but you identified a
> > real bug with the default rebase backend so I'm interested to see if
> > you happen to have more bugs I should know about.
>
> Here is a _simplified_ scenario to get a totally unexpected result from
> 'git merge' (initially reproduced years ago, but still happening on
> 2.25.0.rc2):
>
> ## Preparation
> 0. git --version
> git version 2.25.0.rc2
> 1. git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> 2. git remote add linux-stable https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
> 3. git fetch linux-stable
>
> # Reproduction
> 4. git checkout f7a8e38f07a1
> 5. git merge --no-edit e18da11fc0f959
> ## Merge v4.4.3 commit
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e18da11fc0f959
> which is a linux-stable backport of vanilla v4.5-rc1 commit
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f7a8e38f07a1
> the latter being checked out at step 4.
>
> 6. git show HEAD
> ## Inspect the _automatic_ conflict resolution performed by git in
> drivers/mtd/nand/nand_base.c. Git decided to integrate e18da11fc0f959
> alongside f7a8e38f07a1, while essentially they are the same commit.
> We end up with two times commit f7a8e38f07a1.
>
> What do you think about that?
Ooh, interesting case; thanks for sending it along. I think this is
the same as https://lore.kernel.org/git/20190816184051.GB13894@sigill.intra.peff.net/
, which struck git itself not that long back. It didn't do any actual
harm, though, it was just surprising. I'm not familiar with the xdiff
part of the codebase, so I don't know if this is a heuristic thing, or
something more along the lines of the diff3 issues mentioned at
https://www.cis.upenn.edu/~bcpierce/papers/diff3-short.pdf. I read up
on this area a little bit a few months ago and I'd like to dig more at
the diff3 stuff in general, but it may be a little while. If you see
more issues like this, though, I'm definitely interested in saving and
cataloging them for when I get back to this.
Elijah
prev parent reply other threads:[~2020-01-10 2:35 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-08 21:43 Unreliable 'git rebase --onto' Eugeniu Rosca
2020-01-08 22:35 ` SZEDER Gábor
2020-01-09 0:55 ` Elijah Newren
2020-01-09 15:03 ` SZEDER Gábor
2020-01-09 17:53 ` Elijah Newren
2020-01-21 19:18 ` [PATCH v1] rebase -i: stop checking out the tip of the branch to rebase Alban Gruin
2020-01-21 20:07 ` Elijah Newren
2020-01-22 20:24 ` Junio C Hamano
2020-01-22 20:47 ` Junio C Hamano
2020-01-24 14:45 ` Alban Gruin
2020-01-24 14:45 ` [PATCH v2] " Alban Gruin
2020-01-24 14:55 ` Alban Gruin
2020-01-24 18:12 ` Junio C Hamano
2020-01-24 15:05 ` [PATCH v3] " Alban Gruin
2020-01-24 18:30 ` Junio C Hamano
2020-02-05 14:31 ` Johannes Schindelin
2020-01-24 17:11 ` [PATCH v2] " Andrei Rybak
2020-01-09 11:13 ` Unreliable 'git rebase --onto' Eugeniu Rosca
[not found] ` <CABPp-BHsyMOz+hi7EYoAnAWfzms7FRfwqCoarnu8H+vyDoN6SQ@mail.gmail.com>
2020-01-09 10:53 ` Eugeniu Rosca
2020-01-09 18:05 ` Elijah Newren
2020-01-10 0:06 ` Eugeniu Rosca
2020-01-10 2:35 ` Elijah Newren [this message]
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