Re: [RFC v2] Rebasing merges: a jorney to the ultimate solution (Road Clear)

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Mar 8, 2018, 07:01 UTC
Message-ID: <nycvar.QRO.7.76.6.1803080746460.20700@ZVAVAG-6OXH6DA.rhebcr.pbec.zvpebfbsg.pbz>
In-Reply-To: <87vae7ykys.fsf@javad.com>

Hi Sergey,

On Wed, 7 Mar 2018, Sergey Organov wrote:

Show 24 quoted lines

> Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:
> >
> > On Wed, 7 Mar 2018, Sergey Organov wrote:
> >
> >> Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:
> >> 
> >> > On Tue, 6 Mar 2018, Sergey Organov wrote:
> >> >
> >> >> This is v2 of my "Rebasing merges" proposal.
> >> >
> >> > Didn't we settle on Phillip's "perform successive three-way merges
> >> > between the original merge commit and the new tips with the old
> >> > tips as base" strategy?
> >> 
> >> It seems you did, dunno exactly why.
> >
> > That is not true. You make it sound like I was the only one who liked
> > this, and not Phillip and Buga, too.
> >
> > Are you interested in the best solution, or in your solution :-)
> 
> I'm interested in any that works, and only you say that those suggested
> by Phillip is somehow superior. I still believe it's mine that superior,
> even if slightly.

That is misrepresenting what happened.

First, you came up with a strategy. I pointed out shortcomings that implied that we cannot use it unchanged. Then, Buga fixed your strategy by using additional steps (making the process more complicated than before, still without a simple-enough explanation for my liking, fixing the shortcomings). Then, Phillip presented a super-simple strategy and Buga confirmed that it also fixes the shortcomings I pointed out.

I am very excited that we finally found something that works *and* is easy to reason about.

Let's focus on that strategy rather than going back to the strategy which has known flaws and only an unsatisfyingly complex explanation.

Show 9 quoted lines

> >> The main problem with this decision is that we still don't see how
> >> and when to stop for user amendment using this method. OTOH, the
> >> original has this issue carefully discussed.
> >
> > Why would we want to stop, unless there are merge conflicts?
> 
> There is somewhat lengthy discussion about it that you probably missed.
> Not to repeat it, just see how 'rerere' works when it fires during
> rebase, even with no conflicts.

I did not miss that discussion. My question was a follow-up: Why would we want to stop, unless there are merge conflicts?

Show 15 quoted lines

> >> > It has the following advantages:
> >> >
> >> > - it is *very simple* to describe
> >> 
> >> The original is as simple if not simpler:
> >> 
> >> "rebase sides of the merge commit and then three-way merge them back
> >> using original merge commit as base"
> >
> > And that is also wrong, as I had proved already! Only Buga's addition made
> > it robust against dropping/modifying commits, and that addition also makes
> > it more complicated.
> 
> No. Get your facts straight. The [RFC v2] already fixed that original
> mistake. Could you please finally read it?

I do not see Buga's additions, and it is still a lengthy document that is hard to understand.

Phillip's alternative, in contrast, fit in at most two 80x25 pages and was intuitive (at least after seeing that the tree resulting from a merge is identical to the tree resulting from a rebase, once all merge conflicts are handled appropriately).

> > And it still has no satisfactory simple explanation why it works.
> 
> It has. It's there in the [RFC v2]. You seem to be the only one who
> doesn't get it. I suppose you just didn't bother to read.

I tried to read it, and got lost in all those figures that really do not do anything to make this strategy obvious to me.

Granted, I now know *how* it works.

I gave up understanding *why* it is supposed to work.

With Phillip's mail, it only took me 5 minutes to get a rudimentary understanding why it works.

Show 8 quoted lines

> >> No problems with octopuses, and no virtual merge bases of recursive
> >> merges to reason about.
> >
> > But those are no problems for Phillip's strategy, either!
> 
> I thought it was you who started to discuss virtual merge bases and
> related problems, as well as how it's difficult to support octopus
> merges, but it's fine with me if there are none of these problems.

Yes, I started explaining virtual merge bases, and how they are used in the recursive merge. Because I was asked how the recursive merge works, and I happen to know how it works very intimately.

> > So your point is...?
> 
> Still the same -- use what's better, the [RFC v2].

I strongly disagree that your approach is superior. It is more complex, and still has no simple answer to the question "why is this supposed to do what I want it to do?"

It does a lot of criss-crossing, and when I see that, I immediately suspect that it would lose information e.g. when commits were amended during the rebase. There are just way too many paths for obsolete changes to creep in again.

Show 8 quoted lines

> >> > - it is *very easy* to reason about, once it is pointed out that
> >> > rebases and merges result in the same trees.
> >> 
> >> The original is as easy to reason about, if not easier, especially as
> >> recursive merge strategy is not being used there in new ways.
> >
> > So do it. I still have to hear a single-sentence, clear and obvious
> > explanation why it works.

Please.

Show 6 quoted lines

> > And please do not describe why your original version works, because it
> > does not work.
> 
> Original [RFC] didn't work because of rather simple mistake that I've
> already admitted and fixed. [RFC v2] has got the fix. Read [RFC v2] and
> get your facts straight.

I don't care how many mistakes you made, and where the original ideas had to be enhanced. I am only interested in the outcome.

We already have a nice, simple outcome, and to be quite honest: I find this discussion here a bit pointless. Why would I abandon a simple strategy that obviously works for a complex strategy where it still is not obvious under what circumstances it works, and why?

> > Describe why the one amended with Buga's hack works.
> 
> It doesn't matter as these hacks are not needed anymore.

So maybe you want to also describe the "interdiff", and not expect anybody to read two lengthy documents and figure out where the changes are.

Show 24 quoted lines

> >> I honestly don't see any advantages of Phillip's method over the
> >> original, except personal preferences. At the same time, I have no
> >> objection of using it either, provided consistency check problem is
> >> solved there as well.
> >
> > Okay, let me reiterate then, because I do not want this point to be
> > missed:
> >
> > Phillip's method is essentially merging the new tips into the original
> > merge, pretending that the new tips were not rebased but merged into
> > upstream.
> >
> > So it exploits the duality of the rebase and merge operation, which both
> > result in identical trees (potentially after resolving merge
> > conflicts).
> >
> > I cannot think of any such interpretation for your proposal augmented by
> > Buga's fix-ups. And I haven't heard any such interpretation from your
> > side, either.
> 
> No fix-ups or augmentations are needed. It was a mistake that has been
> fixed in [RFC v2]. You've missed essential part of the discussion.
> 
> Read the [RFC v2], please:

Okay, I am done here. If all my questions and all my concerns are answered with the suggestion to spend an hour pouring over a lengthy description of your strategy that is so much more complex than my preferred alternative, asking me to figure out from that long document what the answers to my questions are, then I respectfully decline. I do have other things to care about, too, and I will *not* spend an hour trying to figure out answers only because you refuse to give them directly.

Ciao, Johannes