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

From: Igor Djordjevic <igor.d.djordjevic@gmail.com>
Date: Mar 6, 2018, 19:43 UTC
Message-ID: <754e2735-1288-9a8d-c8bd-ab39cf733812@gmail.com>
In-Reply-To: <nycvar.QRO.7.76.6.1803061812090.20700@ZVAVAG-6OXH6DA.rhebcr.pbec.zvpebfbsg.pbz>

On 06/03/2018 19:12, Johannes Schindelin wrote:

Show 47 quoted lines

> 
> > > 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.
> 
> Indeed. We have seen time and time again that consistent behavior is the
> only thing that lets us adhere to the Law of Least Surprise.
> 
> And here lies the rub: do we really want to let `merge -C <commit>` behave
> completely differently than `merge`? Granted, in one case we provide a
> template merge commit, in the other case, we do not. And the idea is
> already to behave differently, although that difference only extends to
> the commit message so far.
> 
> But given the benefit (i.e. that the strategy to transform the original
> merge commit into the new merge commit), I am willing to run that risk,
> especially since I foresee only few users wanting to create new merge
> commits from scratch using the `merge` todo command.
> 
> Of course, even then we need to be careful: the user might have
> *changed* or *moved* the original `merge` command. For example, if the
> merge command read:
> 
> 	merge -C deadbee cafecafe bedbedbed
> 
> and the user switched the order of the merged branches into
> 
> 	merge -C deadbee bedbedbed cafecafe
> 
> we would have to detect the changed order of the arguments so that we
> could still find the original branch tips.
> 
> But the user might also have changed the branch(es) to merge completely,
> in which case we might not even be able to find original branch tips.
> 
> My preferred solution would be to let the `merge` command figure out
> whether the passed arguments correspond to the rewritten versions of the
> original merge parents. And only in that case would we use the fancy
> strategy, in all other cases we would fall back to performing a regular
> recursive (or octopus) merge.
> 
> How does that sound?
> 
> 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 :)

First, starting from non-interactive rebase, I guess we may now agree that _rebasing_ merges is an actually expected behavior, not recreating them (thus keeping manual conflict resolutions and amendments, not losing them).

Now, interactive rebase is a totally different story, we already said user can change pretty much about everything, making merge _recreation_ to be a more sane choice, but let`s leave this other extreme for a brief moment.

In the least interesting situation, though, user could just review and close todo list, without changing anything - and in that case it would be important, consistency wise, to behave exactly like in case of non-interactive rebase, meaning still rebasing merges, not recreating them.

Ok, so that still aligns with what`s written so far - we need to be able to rebase merges interactively, too (not just recreate them), to stay consistent in less complex interactive rebases.

But, what if user really wants to _recreate_ merges, for whatever reason? Come on, this is interactive rebase we`re talking about, why being restrictive? :)

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!

So, it could be something like:

	merge -C deadbee 123abc:cafecafe 234bcd:bedbedbed

The format is still something to think about, but the point is rather simple - explicitly map old and new merge parents, showing this inside todo list by default.

This makes it much easier for later processing (and correct, no need to guess which one goes where), but also gives more power to the user, being able to decide which merge parents get "rebased", and which ones should go into the merge just like "new".

So if a user gets an interactive todo list like that and just closes it, we still have exact situation like non-interactive rebase (and no guessing on implementation side).

But, user might still decide to introduce new merge parents into the mix, even, where we could then just be merging those (as there is no old merge parent to actually rebase from):

	merge -C deadbee 123abc:cafecafe 234bcd:bedbedbed new-branch
	
Here, "new-branch" is something new, introduced inside interactive 
rebase, and it will be just merged into the other two (which are 
still being rebased).

Also, another example - if original merge parent "123abc" was merged from the other side using `-s ours` strategy, that means all the content this branch originally had will still be missing from the rebased merge (expect for what`s been cherry-picked elsewhere).

But, I would argue it`s quite legit to want to revise that decision, and let that content in this time. To make that happen, one would just remove "123abc:" from the todo list:

	merge -C deadbee cafecafe 234bcd:bedbedbed new-branch

..., meaning that only "bedbedbed" should be rebased according original merge parent "234bcd", where both "cafecafe" and "new-branch" should be just merged in, no previous context existing (no rebase).

In the end, user might even decide to swap old/new parent mapping, and that should be possible, too (might be pretty strange, though, causing conflicts, but we shouldn`t judge).

Or, one could map old merge parent to some totally new merge parent, like "new-branch" in that example above, all this being a fair game.

As one might suspect, to recreate merge from scratch instead, just drop all the old merge parents mappings:

	merge -C deadbee cafecafe bedbedbed

There, in my opinion, something like this provides the most consistent user experience (we always behave the same), while adding additional possibilities on top (getting to actually decide which merge parents get rebased, and which just merged), plus avoids all guess work (old and new merge parents, to be used for merge rebasing, are explicitly mapped).

What do you think? (unrelated to the parent mapping format itself, which could/should probably be made better, if possible)

Regards, Buga