Re: Limitiations of git rebase --preserve-merges --interactive

From: Stefan Beller <sbeller@google.com>
Date: Sep 22, 2016, 20:54 UTC
Message-ID: <CAGZ79kYoFH0dbQwvL_2JOtd8aybH8MKCtzy+p_88+QykVTGUtg@mail.gmail.com>
In-Reply-To: <20160922194848.GB6641@ikke.info>

On Thu, Sep 22, 2016 at 12:48 PM, Kevin Daudt <me@ikke.info> wrote:

Show 20 quoted lines

> On Thu, Sep 22, 2016 at 07:33:11PM +0000, Anatoly Borodin wrote:
>> Hi Stefan,
>>
>> this section was added to the manual in the commit
>> cddb42d2c58a9de9b2b5ef68817778e7afaace3e by "Jonathan Nieder"
>> <jrnieder@gmail.com> 6 years ago. Maybe he remembers better?
>>
>
> Just to make it clear, this section explicitly talks about 'bugs' with
> preserve-merges and interactive rebase.  Without the --preserve-merges
> option, those operations works as expected.
>
> The reason, as that section explains, is that it's not possible to store
> the merge structure in the flat todo list. I assume this means git
> internally remembers where the merge commit was, and then restores it
> while rebasing.
>
> Changing the order, or dropping commits might then give unexpected
> results.
>

The commit message may help as well:

    rebase -i -p: document shortcomings

    The rebase --preserve-merges facility presents a list of commits
    in its instruction sheet and uses a separate table to keep
    track of their parents.  Unfortunately, in practice this means
    that with -p after most attempts to rearrange patches, some
    commits have the "wrong" parent and the resulting history is
    rarely what the caller expected.

    Yes, it would be nice to fix that.  But first, add a warning to the
    manual to help the uninitiated understand what is going on.