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

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Sep 23, 2016, 21:13 UTC
Message-ID: <alpine.DEB.2.20.1609232305590.129229@virtualbox>
In-Reply-To: <1mu0zrn.cs3kf31kttpjkM%lists@haller-berlin.de>

Hi Stefan,

On Fri, 23 Sep 2016, Stefan Haller wrote:

Show 47 quoted lines

> Stefan Beller <sbeller@google.com> wrote:
> 
> > On Thu, Sep 22, 2016 at 12:48 PM, Kevin Daudt <me@ikke.info> wrote:
> > > 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.
> 
> Thanks, but all of this still talks about the issues in very generic
> terms ("most attempts to rearrange patches"). I'm interested in more
> details as to exactly what kind of attempts do or don't work. In
> particular, I'm interested in fixup/squash commands (without reordering
> anything else), or dropping (non-merge) commits.
> 
> I could of course experiment with these and try to find out myself, but
> I was hoping someone would just know the answer off the top of their
> head, saving me some time.

The fundamental problem here is the underlying design of bolting on the "recreate a merge" functionality onto the "pick" command.

That is, if you try to rebase non-linear commit history, it will still generate a linear list of "pick <commit-name>" lines, as if it were linear, except that it will include the merge commits, too.

It then will try to guess what you want to do by recording which commit was rewritten as which commit. And when it encounters a "pick" with a merge commit, it will try to merge the *rewritten* commit.

In other words, the design does not allow for changing the tip of any merged branch. Not reordering, not dropping.

And I do not think that there is a way to fix that design. That is why I came up with the Git garden shears (see the link I sent elsewhere in this thread).

Ciao, Johannes