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Re: rebase preserve-merges: incorrect merge commits




Hi Matwey,

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

> 2018-01-08 19:32 GMT+03:00 Johannes Schindelin <Johannes.Schindelin@xxxxxx>:
> >
> > On Mon, 8 Jan 2018, Matwey V. Kornilov wrote:
> >
> >> 2018-01-08 17:42 GMT+03:00 Matwey V. Kornilov <matwey.kornilov@xxxxxxxxx>:
> >> > 2018-01-08 16:56 GMT+03:00 Johannes Schindelin <Johannes.Schindelin@xxxxxx>:
> >> >> 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 ...)
> >> >
> >> > Well. I just checked --recreate-merges=no-rebase-cousins from the PR
> >> > and found that it produces the same wrong result in my test example.
> >> > The topology is reproduced correctly, but merge-commit content is
> >> > broken.
> >> > I did git rebase --recreate-merges=no-rebase-cousins --onto abc-0.1 v0.1 abc-0.2
> >>
> >> Indeed, exactly as you still say in the documentation: "Merge conflict
> >> resolutions or manual amendments to merge commits are not preserved."
> >> My initial point is that they have to be preserved. Probably in
> >> recreate-merges, if preserve-merges is discontinued.
> >
> > Ah, but that is consistent with how non-merge-preserving rebase works: the
> > `pick` commands *also* do not record merge conflict resolution...
> >
> 
> I am sorry, didn't get it. When I do non-merge-preserving rebase
> --interactive there is no way to `pick' merge-commit at all.

Right, but you can `pick` commits and you can get merge conflicts. And you
need to resolve those merge conflicts and those merge conflict resolutions
are not preserved for future interactive rebases, unless you use `rerere`
(in which case it also extends to `pick`ing merge commits in
merge-preserving mode).

Ciao,
Johannes