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Re: [RFC] Rebasing merges: a jorney to the ultimate solution(RoadClear)

On Thu, Mar 8, 2018 at 3:20 AM, Phillip Wood <phillip.wood@xxxxxxxxxxxx> wrote:
> On 07/03/18 07:26, Johannes Schindelin wrote:
>> Hi Buga,
>> On Tue, 6 Mar 2018, Igor Djordjevic wrote:
>>> On 06/03/2018 19:12, Johannes Schindelin wrote:
>>>>>> And I guess being consistent is pretty important, too - if you add new
>>>>>> content during merge rebase, it should always show up in the merge,
>>>>>> period.
>>>>> Yes, that should make it easy for the user to know what to expect from
>>>>> rebase.
>>>> [...]
>>>> It will be slightly inconsistent. But in a defendable way, I think.
>>> I like where this discussion is heading, and here`s what I thought
>>> about it :)
>>> [...]
>>> Here`s a twist - not letting `merge` trying to be too smart by
>>> figuring out whether passed arguments correspond to rewritten
>>> versions of the original merge parents (which would be too
>>> restrictive, too, I`m afraid), but just be explicit about it, instead!
>> That's the missing piece, I think.
>>> So, it could be something like:
>>>      merge -C deadbee 123abc:cafecafe 234bcd:bedbedbed
>> I like where this is heading, too, but I do not think that we can do this
>> on a per-MERGE_HEAD basis. The vast majority of merge commits, in
>> practice, have two parents. So the `merge` command would actually only
>> have one revision to merge (because HEAD is the implicit first parent). So
>> that is easy.
>> But as soon as you go octopus, you can either perform an octopus merge, or
>> rebase the original merge commit. You cannot really mix and match here.
>> Unless we reimplement the octopus merge (which works quite a bit
>> differently from the "rebase merge commit" strategy, even if it is
>> incremental, too), which has its own challenges: if there are merge
>> conflicts before merging the last MERGE_HEAD, the octopus merge will exit
>> with status 2, telling you "Should not be doing an octopus.". While we
>> will want to keep merge conflict markers and continue with the "rebase the
>> original merge commit" strategy.
>> And it would slam the door shut for adding support for *other* merge
>> strategies to perform a more-than-two-parents merge.
>> Also, I do not think that it makes a whole lot of sense in practice to let
>> users edit what will be used for "original parent". If the user wants to
>> do complicated stuff, they can already do that, via `exec`. The `merge`
>> command really should be about facilitating common workflows, guiding the
>> user to what is sane.
>> Currently my favorite idea is to introduce a new flag: -R (for "rebase the
>> original merge commit"). It would look like this:
>>       merge -R -C <original-merge> <merge-head> # <oneline>
>> This flag would of course trigger the consistency check (does the number
>> of parents of the original merge commit agree with the parameter list? Was
>> an original merge commit specified to begin with?), and it would not fall
>> back to the recursive merge, but error out if that check failed.
>> Side note: I wonder whether we really need to perform the additional check
>> that ensures that the <merge-head> refers to the rewritten version of the
>> original merge commit's parent.
>> Second side note: if we can fast-forward, currently we prefer that, and I
>> think we should keep that behavior with -R, too.
> I think that would be a good idea to avoid unpleasant surprises.
>> If the user wants to force a new merge, they simply remove that -R flag.
>> What do you think?
> I did wonder about using 'pick <original-merge>' for rebasing merges and
> keeping 'merge ...' for recreating them but I'm not sure if that is a
> good idea. It has the advantage that the user cannot specify the wrong
> parents for the merge to be rebased as 'git rebase' would work out if
> the parents have been rebased, but maybe it's a bit magical to use pick
> for merge commits. Also there isn't such a simple way for the user to go
> from 'rabase this merge' to 'recreate this merge' as they'd have to
> write the whole merge line themselves (though I guess something like
> emacs' git-rebase.el would be able to help with that)
> Best Wishes
> Phillip

Since the ultimate commit hashes of newly rebased commits would be
unknown at the time of writing the todo file, I'm not sure how this
would work to specify the parents?

>> Ciao,
>> Dscho