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From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
To: Denton Liu <liu.denton@gmail.com>
Cc: Git Mailing List <git@vger.kernel.org>, Jeff King <peff@peff.net>
Subject: Re: [PATCH 0/3] rebase: learn --keep-base
Date: Wed, 27 Mar 2019 16:39:50 +0100
Message-ID: <871s2sba2h.fsf@evledraar.gmail.com> (raw)
In-Reply-To: <20190326213031.GA21504@dev-l>


On Tue, Mar 26 2019, Denton Liu wrote:

> Hi Ævar,
>
> On Tue, Mar 26, 2019 at 09:35:48PM +0100, Ævar Arnfjörð Bjarmason wrote:
>>
>> On Tue, Mar 26 2019, Denton Liu wrote:
>>
>> > Hi Ævar,
>> >
>> > On Tue, Mar 26, 2019 at 03:35:34PM +0100, Ævar Arnfjörð Bjarmason wrote:
>> >>
>> >> On Sat, Mar 23 2019, Denton Liu wrote:
>> >>
>> >> > This series teaches rebase the --keep-base option.
>> >> >
>> >> > 'git rebase --keep-base <upstream>' is equivalent to
>> >> > 'git rebase --onto <upstream>... <upstream>' or
>> >> > 'git rebase --onto $(git merge-base <upstream> HEAD) <upstream>' .
>> >> >
>> >> > This seems to be a common case that people (including myself!) run into; I was
>> >> > able to find these StackOverflow posts about this use case:
>> >> >
>> >> > * https://stackoverflow.com/questions/53234798/can-i-rebase-on-a-branchs-fork-point-without-explicitly-specifying-the-parent
>> >> > * https://stackoverflow.com/questions/41529128/how-do-you-rebase-only-changes-between-two-branches-into-another-branch
>> >> > * https://stackoverflow.com/a/4207357
>> >>
>> >> Like with another series of yours I think this would be best squashed
>> >> into one patch.
>> >
>> > Will do.
>> >
>> >>
>> >> Maybe I've misunderstood this but isn't this like --fork-point except
>> >> with just plain "git merge-base" instead of "git merge-base
>> >> --fork-point", but then again 2/3 shows multiple base aren't supported,
>> >> but merge-base supports that.
>> >>
>> >
>> > --fork-point gets used to determine the _set of_ commits which are to be
>> > rebased, whereas --keep-base (and --onto) determine the base where that
>> > set of commits will be spliced. As a result, these two options cover
>> > orthogonal use-cases.
>>
>> Right. After playing with this a bit more though --fork-point is mostly
>> there, it it does find the same fork point, as evidenced all your tests
>> (that aren't asserting incompatibility with other options) passing with
>> this:
>>
>>     diff --git a/t/t3416-rebase-onto-threedots.sh b/t/t3416-rebase-onto-threedots.sh
>>     index 9c2548423b..ab2d50e69a 100755
>>     --- a/t/t3416-rebase-onto-threedots.sh
>>     +++ b/t/t3416-rebase-onto-threedots.sh
>>     @@ -116,7 +116,7 @@ test_expect_success 'rebase --keep-base master from topic' '
>>             git checkout topic &&
>>             git reset --hard G &&
>>
>>     -       git rebase --keep-base master &&
>>     +       git rebase $(git merge-base --fork-point master HEAD) &&
>>             git rev-parse C >base.expect &&
>>             git merge-base master HEAD >base.actual &&
>>             test_cmp base.expect base.actual &&
>>     @@ -140,7 +140,7 @@ test_expect_success 'rebase -i --keep-base master from topic' '
>>             git reset --hard G &&
>>
>>             set_fake_editor &&
>>     -       EXPECT_COUNT=2 git rebase -i --keep-base master &&
>>     +       EXPECT_COUNT=2 git rebase -i $(git merge-base --fork-point master HEAD) &&
>>             git rev-parse C >base.expect &&
>>             git merge-base master HEAD >base.actual &&
>>             test_cmp base.expect base.actual &&
>>
>> I've poked at some of this recently in
>> https://public-inbox.org/git/20190221214059.9195-3-avarab@gmail.com/ as
>> noted in the feedback there (I haven't gotten around to v2 yet) it's
>> entirely possible that I haven't understood this at all :)
>>
>> But it seems to me that this patch/implementation conflates two
>> unrelated things.
>>
>> Once is that we use --fork-point to mean that we're going to find the
>> divergence point with "merge-base --fork-point". This gets you halfway
>> to where you want to be, i.e. AFAICT the --keep-base and --fork-point
>> will always find the same commit for "git rebase" and "git rebase
>> --keep-base". See the "options.restrict_revision = get_fork_point(...)"
>> part of the code.
>
> I don't think this is true. The code that --keep-base uses to find the
> merge base is get_oid_mb, see the relevant snippet
>
> 	if (strstr(options.onto_name, "...")) {
> 		if (get_oid_mb(options.onto_name, &merge_base) < 0)
>
> whereas the --fork-point code uses get_fork_point, as you mentioned
> above. As a result, they don't necessarily refer to the same commit in
> the case where upstream is rewound.
>
>>
>> The other, which you want to disable, is that --fork-point *also* says
>> "OK, once we've found the divergence point, let's then rebase it on the
>> latest upstream. Or in the example above the "master" part of "git
>> merge-base --fork-point master HEAD".
>
> Correct, I guess in essence this is what I'm doing.
>
>>
>> Shouldn't --keep-base just be implemented in terms of skipping *that*
>> part, i.e. we find the fork point using the upstream info, but then
>> don't rebase *on* upstream.
>>
>> The reason the distinction matters is because with your patch these two
>> act differently:
>>
>>     git rebase --keep-base
>>     git rebase $(git merge-base @{u} HEAD)
>>
>> The latter will skip work ("Current branch master is up to date"), but
>> --keep-base will always re-rebase things. There's some cases where
>> --fork-point does that, which I was trying to address with my linked WIP
>> patch above.
>
> I believe this is desired behaviour. Suppose we have this (modified)
> graph from the git-merge-base docs, where B3 was formerly part of
> origin/master but it was then rewound:
>
>            ---o---o---B2--o---o---o---B (origin/master)
>                    \
>                     B3
>                      \
>                       Derived (local master)
>
> If we run "git rebase --keep-base", we'll get the following graph:
>
>            ---o---o---B2--o---o---o---B (origin/master)
>                    \
>                     Derived (local master)
>
> which I believe is the desired behaviour (we're abandoning B3 since
> upstream abandoned it).
>
> I hope I'm understanding you correctly. Please let me know if I've
> misinterpreted anything you've said or if anything I've said is unclear.

Yeah. I'm still confused, but mainly because I haven't allocated enough
brainpower to try to understand it :)

So yeah, I can believe it's subtly different, would be great to have a
v2 whose docs/tests cover those subtleties, right now (as seen in my
discussion upthread) the tests that are there can identically use the
fork point.

I also wonder if we can holistically think about this UI / how we can
expose different things. E.g. for the times I've needed this and have
manually dug up the fork point I haven't wanted to handle the case of
upstream rewinding, just re-rebase-i on some old base, while still
having upstream tracking info, and for rebase to exit early if there's
nothing to do (similar to if I feed it the fork point as a rev).

>>
>> Whereas the thing you actually want to work is:
>>
>>     git rebase -i --keep-base
>>     git rebase -i $(git merge-base @{u} HEAD)
>>
>> I.e. to have both of those allow you to re-arrange/fixup whatever and
>> still rebase on the same divergence point with @{u}, and won't run
>> rebase when there's no work to do unless you give it --force-rebase.
>>
>> > reason that --onto already disallows multiple bases. If we have multiple
>> > bases, how do we determine which one is the "true base" to use? It makes
>> > more sense to error out and let the user manually specify it.
>>
>> Ah, makes sense.
>>
>> >> I'd find something like the "DISCUSSION ON FORK-POINT MODE" in
>> >> git-merge-base helpful with examples of what we'd pick in the various
>> >> scenarios, and also if whatever commit this picks was something you
>> >> could have "git merge-base" spew out, so you could get what rebase would
>> >> do here from other tooling (which maybe is possible, but I'm confused by
>> >> the "no multiple bases"...).
>> >
>> > If I'm understanding you correctly then yes, this could be done with
>> > other tooling. See the 0/3 for equivalent commands.
>> >
>> > Perhaps I should update the rebase documentation to mention that
>> > --fork-point and --keep-base are orthogonal because it's unclear for
>> > you, it's probably unclear for other users as well.
>> >
>> > Thanks,
>> >
>> > Denton

  reply index

Thread overview: 123+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-23 15:25 Denton Liu
2019-03-23 15:25 ` [PATCH 1/3] rebase: teach rebase --keep-base Denton Liu
2019-03-24  3:46   ` Eric Sunshine
2019-03-24 13:20   ` Junio C Hamano
2019-03-25  0:06     ` Denton Liu
2019-03-25  5:41       ` Denton Liu
2019-04-01 10:45         ` Junio C Hamano
2019-03-24 13:37   ` Junio C Hamano
2019-03-25  5:47   ` Denton Liu
2019-03-25 18:50   ` Johannes Schindelin
2019-03-25 19:29     ` Denton Liu
2019-03-26 13:27       ` Johannes Schindelin
2019-03-23 15:25 ` [PATCH 2/3] t3416: test " Denton Liu
2019-03-23 15:25 ` [PATCH 3/3] git-rebase.txt: document --keep-base option Denton Liu
2019-03-24 13:15 ` [PATCH 0/3] rebase: learn --keep-base Junio C Hamano
2019-03-25  0:04   ` Denton Liu
2019-04-01 10:45     ` Junio C Hamano
2019-03-26 14:35 ` Ævar Arnfjörð Bjarmason
2019-03-26 17:50   ` Denton Liu
2019-03-26 20:35     ` Ævar Arnfjörð Bjarmason
2019-03-26 21:30       ` Denton Liu
2019-03-27 15:39         ` Ævar Arnfjörð Bjarmason [this message]
2019-03-28 22:17 ` [PATCH v2] rebase: teach rebase --keep-base Denton Liu
2019-03-28 23:13   ` Ævar Arnfjörð Bjarmason
2019-03-29 15:47   ` Johannes Schindelin
2019-03-29 17:52     ` Denton Liu
2019-04-01 10:46     ` Junio C Hamano
2019-04-01 20:51   ` [PATCH v3 0/4] " Denton Liu
2019-04-01 20:51     ` [PATCH v3 1/4] t3431: add rebase --fork-point tests Denton Liu
2019-04-04 20:28       ` Denton Liu
2019-04-05 11:15       ` SZEDER Gábor
2019-04-08  4:38         ` Junio C Hamano
2019-04-05 14:55       ` Johannes Schindelin
2019-04-05 17:25         ` Denton Liu
2019-04-05 17:51           ` Johannes Sixt
2019-04-05 18:51             ` Johannes Schindelin
2019-04-05 20:19               ` Johannes Schindelin
2019-04-05 21:10                 ` SZEDER Gábor
2019-04-01 20:51     ` [PATCH v3 2/4] t3432: test rebase fast-forward behavior Denton Liu
2019-04-01 20:52     ` [PATCH v3 3/4] rebase: fast-forward --onto in more cases Denton Liu
2019-04-02  1:25       ` Junio C Hamano
2019-04-02  1:48         ` Junio C Hamano
2019-04-02  4:44           ` Denton Liu
2019-04-01 20:52     ` [PATCH v3 4/4] rebase: teach rebase --keep-base Denton Liu
2019-04-05 21:39     ` [PATCH v4 0/4] " Denton Liu
2019-04-05 21:40       ` [PATCH v4 1/4] t3431: add rebase --fork-point tests Denton Liu
2019-04-05 21:40       ` [PATCH v4 2/4] t3432: test rebase fast-forward behavior Denton Liu
2019-04-05 21:40       ` [PATCH v4 3/4] rebase: fast-forward --onto in more cases Denton Liu
2019-04-05 21:40       ` [PATCH v4 4/4] rebase: teach rebase --keep-base Denton Liu
2019-04-15 22:29       ` [PATCH v5 0/5] " Denton Liu
2019-04-15 22:29         ` [PATCH v5 1/5] t3431: add rebase --fork-point tests Denton Liu
2019-04-15 22:29         ` [PATCH v5 2/5] t3432: test rebase fast-forward behavior Denton Liu
2019-04-15 22:29         ` [PATCH v5 3/5] rebase: fast-forward --onto in more cases Denton Liu
2019-04-16  6:26           ` Junio C Hamano
2019-04-16 13:59           ` Phillip Wood
2019-04-17  6:44             ` Denton Liu
2019-04-17 14:14               ` Phillip Wood
2019-04-19 17:08           ` Denton Liu
2019-04-15 22:29         ` [PATCH v5 4/5] rebase: fast-forward --fork-point " Denton Liu
2019-04-15 22:29         ` [PATCH v5 5/5] rebase: teach rebase --keep-base Denton Liu
2019-04-17 18:01       ` [PATCH v6 0/5] " Denton Liu
2019-04-17 18:01         ` [PATCH v6 1/6] t3431: add rebase --fork-point tests Denton Liu
2019-04-17 18:01         ` [PATCH v6 2/6] t3432: test rebase fast-forward behavior Denton Liu
2019-04-17 18:01         ` [PATCH v6 3/6] rebase: refactor can_fast_forward into goto tower Denton Liu
2019-04-17 18:01         ` [PATCH v6 4/6] rebase: fast-forward --onto in more cases Denton Liu
2019-04-17 19:59           ` Phillip Wood
2019-04-17 18:01         ` [PATCH v6 5/6] rebase: fast-forward --fork-point " Denton Liu
2019-04-17 18:01         ` [PATCH v6 6/6] rebase: teach rebase --keep-base Denton Liu
2019-04-21  8:11         ` [PATCH v7 0/6] rebase: learn --keep-base Denton Liu
2019-04-21  8:11           ` [PATCH v7 1/6] t3431: add rebase --fork-point tests Denton Liu
2019-04-23 23:12             ` Denton Liu
2019-04-21  8:11           ` [PATCH v7 2/6] t3432: test rebase fast-forward behavior Denton Liu
2019-04-21  8:11           ` [PATCH v7 3/6] rebase: refactor can_fast_forward into goto tower Denton Liu
2019-04-21  8:11           ` [PATCH v7 4/6] rebase: fast-forward --onto in more cases Denton Liu
2019-04-21  8:11           ` [PATCH v7 5/6] rebase: fast-forward --fork-point " Denton Liu
2019-04-21  8:11           ` [PATCH v7 6/6] rebase: teach rebase --keep-base Denton Liu
2019-05-08  0:12           ` [RFC WIP PATCH v8 00/13] learn --keep-base & more Ævar Arnfjörð Bjarmason
2019-05-08  3:57             ` Junio C Hamano
2019-07-19 19:14               ` Junio C Hamano
2019-07-19 21:01                 ` Denton Liu
2019-08-25  9:11             ` [PATCH v9 0/9] rebase: learn --keep-base and improvements on fast-forward behaviour Denton Liu
2019-08-25  9:11               ` [PATCH v9 1/9] t3431: add rebase --fork-point tests Denton Liu
2019-08-25  9:12               ` [PATCH v9 2/9] t3432: test rebase fast-forward behavior Denton Liu
2019-08-25  9:12               ` [PATCH v9 3/9] t3432: distinguish "noop-same" v.s. "work-same" in "same head" tests Denton Liu
2019-08-25  9:12               ` [PATCH v9 4/9] t3432: test for --no-ff's interaction with fast-forward Denton Liu
2019-08-25  9:12               ` [PATCH v9 5/9] rebase: refactor can_fast_forward into goto tower Denton Liu
2019-08-25 13:17                 ` Pratyush Yadav
2019-08-26 23:17                   ` Denton Liu
2019-08-25  9:12               ` [PATCH v9 6/9] rebase: fast-forward --onto in more cases Denton Liu
2019-08-25  9:12               ` [PATCH v9 7/9] rebase: fast-forward --fork-point " Denton Liu
2019-08-25  9:12               ` [PATCH v9 8/9] rebase tests: test linear branch topology Denton Liu
2019-08-25  9:12               ` [PATCH v9 9/9] rebase: teach rebase --keep-base Denton Liu
2019-08-25 22:59                 ` Philip Oakley
2019-08-26 19:37               ` [PATCH v9 0/9] rebase: learn --keep-base and improvements on fast-forward behaviour Junio C Hamano
2019-08-27  5:37               ` [PATCH v10 " Denton Liu
2019-08-27  5:37                 ` [PATCH v10 1/9] t3431: add rebase --fork-point tests Denton Liu
2019-08-27  5:37                 ` [PATCH v10 2/9] t3432: test rebase fast-forward behavior Denton Liu
2019-08-27  5:37                 ` [PATCH v10 3/9] t3432: distinguish "noop-same" v.s. "work-same" in "same head" tests Denton Liu
2019-08-27  5:37                 ` [PATCH v10 4/9] t3432: test for --no-ff's interaction with fast-forward Denton Liu
2019-08-27  8:11                   ` SZEDER Gábor
2019-08-27  5:37                 ` [PATCH v10 5/9] rebase: refactor can_fast_forward into goto tower Denton Liu
2019-08-27  5:37                 ` [PATCH v10 6/9] rebase: fast-forward --onto in more cases Denton Liu
2019-08-27  5:38                 ` [PATCH v10 7/9] rebase: fast-forward --fork-point " Denton Liu
2019-08-27  5:38                 ` [PATCH v10 8/9] rebase tests: test linear branch topology Denton Liu
2019-08-27  5:38                 ` [PATCH v10 9/9] rebase: teach rebase --keep-base Denton Liu
2019-05-08  0:12           ` [RFC WIP PATCH v8 01/13] t3431: add rebase --fork-point tests Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 02/13] t3432: test rebase fast-forward behavior Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 03/13] t3432: distinguish "noop-same" v.s. "work-same" in "same head" tests Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 04/13] t3432: test for --no-ff's interaction with fast-forward Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 05/13] rebase: refactor can_fast_forward into goto tower Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 06/13] rebase: fast-forward --onto in more cases Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 07/13] rebase: fast-forward --fork-point " Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 08/13] rebase: teach rebase --keep-base Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 09/13] rebase tests: test linear branch topology Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 10/13] rebase: don't rebase linear topology with --fork-point Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 11/13] rebase: eliminate side-effects from can_fast_forward() Ævar Arnfjörð Bjarmason
2019-05-17 21:16             ` Johannes Schindelin
2019-05-08  0:12           ` [RFC WIP PATCH v8 12/13] rebase: add a should_fast_forward() utility function Ævar Arnfjörð Bjarmason
2019-05-08  0:12           ` [RFC WIP PATCH v8 13/13] WIP: can_fast_forward() support for --preserve-merges and --rebase-merges Ævar Arnfjörð Bjarmason
2019-05-17 22:02             ` Johannes Schindelin
2019-04-06 19:44     ` [PATCH v3 0/4] rebase: teach rebase --keep-base Ævar Arnfjörð Bjarmason
2019-04-06 20:38       ` Denton Liu
2019-04-13 21:10         ` Ævar Arnfjörð Bjarmason

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