Re: Unreliable 'git rebase --onto'

>
> On Wed, Jan 08, 2020 at 10:43:49PM +0100, Eugeniu Rosca wrote:
> > Hello Git community,
> >
> > Below is a simple reproduction scenario for what looks to be a bug (?)
> > in 'git rebase --onto' (v2.25.0-rc1-19-g042ed3e048af).
> >
> > I would appreciate your confirmation of the misbehavior.
> > If the behavior is correct/expected, I would appreciate some feedback
> > how to avoid it in future, since it occurs with the default parameters.
> >
> > 1. git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> >
> > 2. ### Cherry pick an upstream commit, to contrast the results with
> >    'git rebase --onto':
> >    $ git checkout -b v4.18-cherry-pick v4.18
> >    $ git cherry-pick 463fa44eec2fef50
> >    Auto-merging drivers/input/touchscreen/atmel_mxt_ts.c
> >    warning: inexact rename detection was skipped due to too many files.
> >    warning: you may want to set your merge.renamelimit variable to at least 7216 and retry the command.

> >    [v4.18-cherry-pick bd142b45bf3a] Input: atmel_mxt_ts - disable IRQ across suspend
> >     Author: Evan Green <evgreen@chromium.org>
> >     Date: Wed Oct 2 14:00:21 2019 -0700
> >     1 file changed, 4 insertions(+)
> >
> > 3. ### In spite of the warning, the result matches the original commit:
> >    $ vimdiff <(git show 463fa44eec2fef50) <(git show v4.18-cherry-pick)
> >
> > 4. ### Now, backport the same commit via 'git rebase --onto'
> >    $ git rebase --onto v4.18 463fa44eec2fef50~ 463fa44eec2fef50
> >    First, rewinding head to replay your work on top of it...
> >    Applying: Input: atmel_mxt_ts - disable IRQ across suspend
> >
> > 5. ### The result is different:
> >    $ git branch v4.18-rebase-onto
> >    $ git diff v4.18-cherry-pick v4.18-rebase-onto
> >
> > diff --git a/drivers/input/touchscreen/atmel_mxt_ts.c b/drivers/input/touchscreen/atmel_mxt_ts.c
> > index b45958e89cc5..2345b587662b 100644
> > --- a/drivers/input/touchscreen/atmel_mxt_ts.c
> > +++ b/drivers/input/touchscreen/atmel_mxt_ts.c
> > @@ -3139,8 +3139,6 @@ static int __maybe_unused mxt_suspend(struct device *dev)
> >
> >       mutex_unlock(&input_dev->mutex);
> >
> > -     disable_irq(data->irq);
> > -
> >       return 0;
> >  }
> >
> > @@ -3162,6 +3160,8 @@ static int __maybe_unused mxt_resume(struct device *dev)
> >
> >       mutex_unlock(&input_dev->mutex);
> >
> > +     disable_irq(data->irq);
> > +
> >       return 0;
> >  }
> >
> >
> > In a nutshell, purely from user's perspective:
> >  - I get a warning from 'git cherry pick', with perfect results
> >  - I get no warning from 'git rebase --onto', with wrong results
> >
> > Does git still behave expectedly? TIA!
>
> This is a known issue with the 'am' backend of 'git rebase'.
>
> The good news is that work is already well under way to change the
> default backend from 'am' to 'merge', which will solve this issue.
> From the log message of aa523de170 (rebase: change the default backend
> from "am" to "merge", 2019-12-24):
>
>   The am-backend drops information and thus limits what we can do:
>   [...]
>     * reduction in context from only having a few lines beyond those
>       changed means that when context lines are non-unique we can apply
>       patches incorrectly.[2]
>   [...]
>   [2] https://lore.kernel.org/git/CABPp-BGiu2nVMQY_t-rnFR5GQUz_ipyEE8oDocKeO+>
>
> Alas, there is unexpected bad news: with that commit the runtime of
> your 'git rebase --onto' command goes from <1sec to over 50secs.
> Cc-ing Elijah, author of that patch...