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From: Sergey Organov <sorganov@gmail.com>
To: Igor Djordjevic <igor.d.djordjevic@gmail.com>
Cc: Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	Phillip Wood <phillip.wood@dunelm.org.uk>,
	Jacob Keller <jacob.keller@gmail.com>,
	Git Mailing List <git@vger.kernel.org>,
	Johannes Sixt <j6t@kdbg.org>, Junio C Hamano <gitster@pobox.com>
Subject: Re: [RFC] Rebasing merges: a jorney to the ultimate solution(RoadClear)
Date: Tue, 20 Mar 2018 17:43:04 +0300
Message-ID: <87r2oevmdj.fsf@javad.com> (raw)
In-Reply-To: <34e8d563-a035-b09e-e959-748f2b4f4b99@gmail.com>

Igor Djordjevic <igor.d.djordjevic@gmail.com> writes:

> Hi Sergey,
>
> On 19/03/2018 06:44, Sergey Organov wrote:
>> 
>> > > > > > Second side note: if we can fast-forward, currently we prefer
>> > > > > > that, and I think we should keep that behavior with -R, too.
>> > > > >
>> > > > > I agree.
>> > > >
>> > > > I'm admittedly somewhat lost in the discussion, but are you
>> > > > talking fast-forward on _rebasing_ existing merge? Where would it
>> > > > go in any of the suggested algorithms of rebasing and why?
>> > > >
>> > > > I readily see how it can break merges. E.g., any "git merge
>> > > > --ff-only --no-ff" merge will magically disappear. So, even if
>> > > > somehow supported, fast-forward should not be performed by default
>> > > > during _rebasing_ of a merge.
>> > >
>> > > Hmm, now that you brought this up, I can only agree, of course.
>> > >
>> > > What I had in my mind was more similar to "no-rebase-cousins", like 
>> > > if we can get away without actually rebasing the merge but still 
>> > > using the original one, do it. But I guess that`s not what Johannes 
>> > > originally asked about.
>> > >
>> > > This is another definitive difference between rebasing (`pick`?) and 
>> > > recreating (`merge`) a merge commit - in the case where we`re rebasing, 
>> > > of course it doesn`t make sense to drop commit this time (due to 
>> > > fast-forward). This does make sense in recreating the merge (only).
>> >
>> > Eh, I might take this back. I think my original interpretation (and 
>> > agreement) to fast-forwarding is correct.
>> >
>> > But the confusion here comes from `--no-ff` as used for merging, as 
>> > opposed to `--no-ff` as used for rebasing. I _think_ Johannes meant 
>> > the latter one.
>> >
>> > In rebasing, `--no-ff` means that even if a commit inside todo list 
>> > isn`t to be changed, do not reuse it but create a new one. Here`s 
>> > excerpt from the docs[1]:
>> >
>> >   --no-ff
>> >     With --interactive, cherry-pick all rebased commits instead of 
>> >     fast-forwarding over the unchanged ones. This ensures that the 
>> >     entire history of the rebased branch is composed of new commits.
>> >
>> >     Without --interactive, this is a synonym for --force-rebase.
>> >
>> >
>> > So fast-forwarding in case of rebasing (merge commits as well) is 
>> > something you would want by default, as it wouldn`t drop/lose 
>> > anything, but merely reuse existing commit (if unchanged), instead of 
>> > cherry-picking (rebasing) it into a new (merge) commit anyway.
>> 
>> This sounds like breakage. E.g., it seems to be breaking every "-x ours"
>> merge out there.
>
> Either you are not understanding how rebase fast-forward works, or 
> I`m missing what you are pointing to... Mind explaining how can 
> something that`s left unchanged suddenly become a breakage?

It was misunderstanding on my side indeed, sorry.

>
>> Fast-forwarding existing merge, one way or another, still seems to be
>> wrong idea to me, as merge commit is not only about content change, but
>> also about joint point at particular place in the DAG.
>
> Not sure what this has to do with rebase fast-forwarding, either - 
> nothing changes for fast-forwarded (merge or non-merge) commit in 
> question, both content, joint point and everything else stays exactly 
> the same. If anything changed, then it can`t/won`t be fast-forwarded, 
> being unchanged is a prerequisite.
>
> Let me elaborate a bit. Here`s a starting diagram:

[... detailed explanation skipped for brevity ...]

> Does this settle your concerns, or I`m missing something?

Yes, it does, thank you! Leaving as many leading commits as possible
unchanged during rebase is what fast-forward mean in this case then, and
it's pretty OK with me.

>> As for fast-forwarding re-merge, explicitly requested, I'm not sure. On
>> one hand, it's inline with the default "git merge" behavior, on the
>> other hand, it still feels wrong, somehow.
>
> Regarding fast-forwarding in context of merging, in case where we are 
> recreating merges (not rebasing them), following existing `git merge` 
> logic might make sense, where I would expect rebasing todo list `merge` 
> command to pick-up tricks from `git merge` as needed, like learning 
> to accept `--no-ff` option, for example, thus not fast-forwarding 
> merges (on request) even when possible.
>
> Though, I do agree that in case you want to recreate an existing merge 
> (instead of just rebasing it), `merge` command fast-forwarding might 
> probably not be what you want for the most of the time, but I`m afraid 
> having rebase todo list `merge` command default behavior different than 
> `git merge` default one (in regards to fast-forwarding) would be 
> confusing... or not?
>
> From what I could grasp so far, usually Git commands` default 
> behavior is (explained to be) chosen per "most common use case", so 
> might be non fast-forwarding would be fine as default for rebase todo 
> list `merge` command, even though different than `git merge`
> itself...?

As far as I can tell, fast-forward as default for "git merge" has been
chosen to avoid excessive unintended merges in typical workflows, and
therefore this decision is not actually applicable to rebasing, I think.
I'm inclined to disable merges to fast-forward during history rebasing,
at least by default.

As far as I can tell, current "--preserve-merges" does fast-forward and
breaks "git merge --ff-only --no-ff" merges, among other things it
breaks, and that was my primary concern here. Didn't check
what "--recreate-merges" does though.

-- Sergey

  reply index

Thread overview: 173+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-16 13:08 [RFC] Rebasing merges: a jorney to the ultimate solution (Road Clear) Sergey Organov
2018-02-18  4:16 ` Jacob Keller
2018-02-19  5:28   ` Sergey Organov
2018-02-19 23:44 ` Igor Djordjevic
2018-02-20 12:42   ` Sergey Organov
2018-02-27  0:07   ` Johannes Schindelin
2018-02-27  5:01     ` Sergey Organov
2018-02-27  5:30     ` Jacob Keller
2018-02-27 16:21       ` Johannes Schindelin
2018-02-27 18:55         ` Igor Djordjevic
2018-02-27 19:59           ` Igor Djordjevic
2018-02-27 23:27             ` Johannes Schindelin
2018-02-28  2:12               ` Igor Djordjevic
2018-02-28  4:35                 ` Igor Djordjevic
2018-02-28  6:14                   ` Sergey Organov
2018-02-28 20:53                     ` Igor Djordjevic
2018-02-28  5:44               ` Sergey Organov
2018-02-28 19:42                 ` Igor Djordjevic
2018-02-27 23:40             ` Igor Djordjevic
2018-02-28  0:10               ` Junio C Hamano
2018-02-28  2:35                 ` Igor Djordjevic
2018-02-28  5:27                 ` Sergey Organov
2018-02-28  0:36               ` Jacob Keller
2018-02-28  1:33                 ` Igor Djordjevic
2018-02-28  1:43                   ` Igor Djordjevic
2018-02-28  5:21                   ` Sergey Organov
2018-02-28 19:09                     ` Igor Djordjevic
2018-03-01  5:27                       ` Sergey Organov
2018-02-28  5:19               ` Sergey Organov
2018-02-28 20:25                 ` Igor Djordjevic
2018-02-28 22:17                   ` Igor Djordjevic
2018-03-01  5:19                     ` Sergey Organov
2018-03-01  5:39                   ` Sergey Organov
2018-03-02  1:16                     ` Igor Djordjevic
2018-03-02  5:40                       ` Sergey Organov
2018-03-02 17:45                         ` Igor Djordjevic
2018-03-02 11:17                       ` [RFC] Rebasing merges: a jorney to the ultimate solution(RoadClear) Phillip Wood
2018-03-02 12:36                         ` Phillip Wood
2018-03-02 16:02                           ` Jacob Keller
2018-03-02 23:33                             ` Igor Djordjevic
2018-03-06 10:36                               ` Phillip Wood
2018-03-06 18:12                                 ` Johannes Schindelin
2018-03-06 19:43                                   ` Igor Djordjevic
2018-03-07  7:26                                     ` Johannes Schindelin
2018-03-08 11:20                                       ` Phillip Wood
2018-03-08 12:16                                         ` Phillip Wood
2018-03-08 16:05                                           ` Igor Djordjevic
2018-03-11 12:00                                             ` Johannes Schindelin
2018-03-11 16:33                                               ` Igor Djordjevic
2018-03-12 10:37                                                 ` Johannes Schindelin
2018-03-12 12:56                                                   ` Sergey Organov
2018-03-13  0:01                                                     ` Igor Djordjevic
2018-03-26 12:03                                                       ` Johannes Schindelin
2018-03-27  5:08                                                         ` Sergey Organov
2018-03-27 13:35                                                           ` Johannes Schindelin
2018-04-02  6:07                                                             ` Sergey Organov
2018-03-12 23:54                                                   ` Igor Djordjevic
2018-03-13  6:25                                                     ` Sergey Organov
2018-03-08 15:56                                         ` Igor Djordjevic
2018-03-11 12:08                                           ` Johannes Schindelin
2018-03-11 17:34                                             ` Igor Djordjevic
2018-03-12 10:46                                               ` Johannes Schindelin
2018-03-13  0:16                                                 ` Igor Djordjevic
2018-03-26 13:07                                                   ` Johannes Schindelin
2018-03-27  5:51                                                     ` Sergey Organov
2018-03-27 13:49                                                       ` Johannes Schindelin
2018-03-28  5:57                                                         ` Sergey Organov
2018-03-30 13:41                                                           ` Johannes Schindelin
2018-03-30 16:36                                                             ` Sergey Organov
2018-03-28  5:57                                                         ` Sergey Organov
     [not found]                                                           ` <CA+P7+xoDQ2mzhxeZPFhaY+TaSoKkQm=5AtoduHH06-VggOJ2jg@mail.gmail.com>
2018-03-28 11:29                                                             ` Sergey Organov
     [not found]                                                               ` <CA+P7+xo19mHrWz9Fy-ifgCcVJM2xwzcLj7F2NvFe2LwGbaJiDQ@mail.gmail.com>
2018-03-29  5:53                                                                 ` Sergey Organov
2018-03-30 10:38                                                                   ` Johannes Schindelin
2018-03-30 12:36                                                                     ` Sergey Organov
2018-03-30 13:33                                                                       ` Johannes Schindelin
2018-03-30 15:13                                                                         ` Sergey Organov
2018-03-28 12:10                                                             ` Sergey Organov
2018-03-08 16:07                                         ` Jacob Keller
2018-03-11 12:11                                           ` Johannes Schindelin
2018-03-11 17:46                                             ` Igor Djordjevic
2018-03-08 15:16                                       ` Igor Djordjevic
2018-03-08 16:21                                         ` Igor Djordjevic
2018-03-11 12:22                                           ` Johannes Schindelin
2018-03-14 14:24                                         ` Sergey Organov
2018-03-14 23:11                                           ` Igor Djordjevic
2018-03-15  6:00                                             ` Sergey Organov
2018-03-15 21:51                                               ` Igor Djordjevic
2018-03-17  2:08                                             ` Igor Djordjevic
2018-03-19  5:44                                               ` Sergey Organov
2018-03-19 21:35                                                 ` Igor Djordjevic
2018-03-20 14:43                                                   ` Sergey Organov [this message]
2018-03-26 13:47                                           ` Johannes Schindelin
2018-03-06 23:24                                   ` Junio C Hamano
2018-03-07  7:09                                     ` Johannes Schindelin
2018-03-07 18:20                                       ` Junio C Hamano
2018-03-08  7:03                                         ` Johannes Schindelin
2018-03-08  8:11                                           ` Junio C Hamano
2018-03-09 17:09                                             ` Johannes Schindelin
2018-03-02 16:00                         ` Jacob Keller
2018-03-02 18:14                           ` Igor Djordjevic
2018-03-03 17:29                             ` Igor Djordjevic
2018-03-05  5:35                               ` Sergey Organov
2018-03-02 11:31                     ` [RFC] Rebasing merges: a jorney to the ultimate solution (Road Clear) Phillip Wood
2018-03-03  0:29                       ` Igor Djordjevic
2018-03-05  5:00                         ` Sergey Organov
2018-03-06 10:52                           ` Phillip Wood
2018-03-06 16:56                             ` Junio C Hamano
2018-03-08  7:05                               ` Johannes Schindelin
2018-03-08  8:18                                 ` Junio C Hamano
2018-03-11 11:56                                   ` Johannes Schindelin
2018-03-13 18:24                                     ` Junio C Hamano
2018-03-26 13:17                                       ` Johannes Schindelin
2018-03-05 17:29                         ` Johannes Schindelin
2018-03-06 23:21                           ` Igor Djordjevic
2018-03-07  7:04                             ` Johannes Schindelin
2018-03-06 10:45                         ` Phillip Wood
2018-03-06 11:45                           ` Sergey Organov
2018-03-08 16:30                             ` Igor Djordjevic
2018-03-06 18:12                           ` Johannes Schindelin
2018-03-07  5:08                             ` Sergey Organov
2018-03-07  6:58                               ` Johannes Schindelin
2018-03-07 14:34                                 ` Sergey Organov
2018-03-08  6:45                                   ` Johannes Schindelin
2018-03-12 12:31                                     ` Sergey Organov
2018-03-26 11:37                                       ` Johannes Schindelin
2018-03-27  5:11                                         ` Sergey Organov
2018-03-27 12:55                                           ` Johannes Schindelin
2018-03-28  4:32                                             ` Sergey Organov
2018-03-08 11:08                             ` Phillip Wood
2018-03-05 17:52                       ` Johannes Schindelin
2018-03-13 16:10                       ` Sergey Organov
2018-03-14  1:12                         ` Igor Djordjevic
2018-03-14  7:21                           ` Sergey Organov
2018-03-15  0:09                             ` Igor Djordjevic
2018-03-15  7:52                               ` Sergey Organov
2018-03-15 23:08                                 ` Igor Djordjevic
2018-03-16  7:31                                   ` Sergey Organov
2018-03-17  3:04                                     ` Igor Djordjevic
2018-03-19  6:01                                       ` Sergey Organov
2018-03-26 14:11                                   ` Johannes Schindelin
2018-02-28  0:29         ` Jacob Keller
2018-02-27 11:57     ` Sergey Organov
2018-02-27 18:14       ` Junio C Hamano
2018-02-28  0:30         ` Jacob Keller
2018-02-28  5:54           ` Sergey Organov
2018-02-28  4:53         ` Sergey Organov
2018-03-06 13:26 ` [RFC v2] " Sergey Organov
2018-03-07  6:46   ` Johannes Schindelin
2018-03-07 13:27     ` Sergey Organov
2018-03-07 14:08       ` Johannes Schindelin
2018-03-07 15:16         ` Sergey Organov
2018-03-08  7:01           ` Johannes Schindelin
2018-03-12 12:42             ` Sergey Organov
2018-03-26 11:50               ` Johannes Schindelin
2018-03-08 19:58         ` Igor Djordjevic
2018-03-08 20:27           ` Igor Djordjevic
2018-03-08 22:05             ` Igor Djordjevic
2018-03-08 23:31               ` Igor Djordjevic
2018-03-11 15:47                 ` Johannes Schindelin
2018-03-11 20:53                   ` Igor Djordjevic
2018-03-12 10:20                     ` Johannes Schindelin
2018-03-12 13:49                       ` Sergey Organov
2018-03-26 12:44                         ` Johannes Schindelin
2018-03-27  5:32                           ` Sergey Organov
2018-03-13  0:29                       ` Igor Djordjevic
2018-03-26 13:58                         ` Johannes Schindelin
2018-03-12 13:07                     ` Sergey Organov
2018-03-11 15:40           ` Johannes Schindelin
2018-03-11 22:04             ` Igor Djordjevic
2018-03-12 12:05               ` Sergey Organov
2018-03-26 11:33                 ` Johannes Schindelin
2018-03-27  5:34                   ` Sergey Organov
2018-03-12 22:41               ` Igor Djordjevic

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