Re: rebase preserve-merges: incorrect merge commits

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Jan 8, 2018, 13:56 UTC
Message-ID: <nycvar.QRO.7.76.6.1801081452290.31@ZVAVAG-6OXH6DA.rhebcr.pbec.zvpebfbsg.pbz>
In-Reply-To: <CAJs94Eas59UxZ+490AUSOuB37i+JBEvbCnGBMVpKi1G469OTbw@mail.gmail.com>

Hi Matwey,

On Mon, 8 Jan 2018, Matwey V. Kornilov wrote:

> I think that rebase preserve-merges algorithm needs further
> improvements. Probably, you already know it.

Yes. preserve-merges is a fundamentally flawed design.

Please have a look here:

	https://github.com/git/git/pull/447

Since we are in a feature freeze in preparation for v2.16.0, I will submit these patch series shortly after v2.16.0 is released.

> As far as I understand the root cause of this that when new merge
> commit is created by rebase it is done simply by git merge
> $new_parents without taking into account any actual state of the
> initial merge commit.

Indeed. preserve-merges does not allow commits to be reordered. (Actually, it *does* allow it, but then fails to handle it correctly.) We even have test cases that mark this as "known breakage".

But really, I do not think it is worth trying to fix the broken design. Better to go with the new recreate-merges. (I am biased, of course, because I invented recreate-merges. But then, I also invented preserve-merges, so ...)

Ciao, Johannes