Subject: Re: Unreliable 'git rebase --onto'

From: Eugeniu Rosca <[email-masked]>
Date: Fri, 10 Jan 2020 01:06:03 +0100

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 <[email-masked]> 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?

> Unfortunately, you should note that git-2.25 is going to have the same
> bug you reported; there are still some loose ends with my series to
> make -m the default, and the 2.25 release is expected within a few
> days, so my change of default won't happen until 2.26.  (That series
> would have needed to be completed several weeks ago for it to go into
> 2.25).

Thanks for this piece of information and for the time/effort spent!

-- 
Best Regards,
Eugeniu