Re: [PATCH v5 3/3] rebase: add a config option for --rebase-merges

[Phillip Wood <phillip.wood123@gmail.com>]

Hi Alex

On 12/03/2023 20:57, Alex Henrie wrote:
> On Tue, Mar 7, 2023 at 9:23 AM Phillip Wood <phillip.wood123@gmail.com> wrote:
> 
>> On 04/03/2023 23:24, Alex Henrie wrote:
>>> On Thu, Mar 2, 2023 at 2:37 AM Phillip Wood <phillip.wood123@gmail.com> wrote:
>>>
>>>> On 25/02/2023 18:03, Alex Henrie wrote:
>>>
>>>>> +rebase.merges::
>>>>> +     Whether and how to set the `--rebase-merges` option by default. Can
>>>>> +     be `rebase-cousins`, `no-rebase-cousins`, or a boolean. Setting to
>>>>> +     true is equivalent to `--rebase-merges` without an argument, setting to
>>>>> +     `rebase-cousins` or `no-rebase-cousins` is equivalent to
>>>>> +     `--rebase-merges` with that value as its argument, and setting to false
>>>>> +     is equivalent to `--no-rebase-merges`. Passing `--rebase-merges` on the
>>>>> +     command line without an argument overrides a `rebase.merges=false`
>>>>> +     configuration but does not override other values of `rebase.merge`.
>>>>
>>>> I'm still not clear why the commandline doesn't override the config in
>>>> all cases as is our usual practice. After all if the user has set
>>>> rebase.merges then they don't need to pass --rebase-merges unless they
>>>> want to override the config.
>>>
>>> Given the current push to turn rebase-merges on by default, it seems
>>> likely that rebase-cousins will also be turned on by default at some
>>> point after that.
>>
>> It is good to try and future proof things but this seems rather
>> hypothetical. I don't really see how the choice of whether
>> --rebase-merges is turned on by default is related to the choice of
>> whether or not to rebase cousins by default. It is worth noting that the
>> default was changed to from rebase-cousins to no-rebase-cousins early in
>> the development of --rebase-merges[1] as it was felt to be less surprising.
> 
>> [1]
>> https://lore.kernel.org/git/nycvar.QRO.7.76.6.1801292251240.35@ZVAVAG-6OXH6DA.rhebcr.pbec.zvpebfbsg.pbz/
> 
> Thank you for sharing that link. Even though I got the tests right, I
> got confused and started thinking that rebase-cousins was a more
> thorough version of rebase-merges. In fact, they do opposite things:
> rebase-merges tries to preserve the graph and rebase-cousins
> intentionally restructures the graph. In my opinion using the word
> "rebase" in the names of both options was another unfortunate UI
> decision, but I understand the difference now.
> 
>>> There will be a warning about the default changing,
>>> and we'll want to let users suppress that warning by setting
>>> rebase.rebaseMerges=rebase-cousins. It would then be very confusing if
>>> a --rebase-merges from some old alias continued to mean
>>> --rebase-merges=no-rebase-cousins when the user expects it to start
>>> behaving as though the default has already changed.
>>
>> But aren't you breaking those aliases now when
>> rebase.rebaseMerges=rebase-cousins? That's what I'm objecting to. It
>> seems like we're breaking things now to avoid a hypothetical future
>> change breaking them which does not seem like the right trade off to me.
>>
>> It also does not fit with the way other optional arguments interact with
>> their associated config setting. For example "git branch/checkout/switch
>> --track" and branch.autoSetupMerge. If the optional argument to --track
>> is omitted it defaults to "direct" irrespective of the config.
 >
> What I really don't want is to paint ourselves into a corner.

Nor do I but we already seem to be in some kind of corner as what you're 
proposing breaks the status quo and the existing convention that command 
line options override config settings.

> You're
> right that it's unlikely that the default will ever change from
> no-rebase-cousins to rebase-cousins; I was mistaken. However, Glen
> thinks that in the future we might have some kind of
> rebase-evil-merges mode as well, and that that might become the
> default. If we don't let the rebase.rebaseMerges config value control
> the default behavior of --rebase-merges without an argument on the
> command line, we would have to introduce a separate config option for
> the transition, which would be ugly.

I'm optimistic that Elijah's work on rebasing evil merges will allow us 
to improve --rebase-merges. I expect that we'll enable it by default 
once it is merged. I do not think that we should add another argument 
for --rebase-merges to disable it though as that would be orthogonal to 
"rebase-cousins" and "no-rebase-cousins" which control what commits get 
rebased not how merges are rebased. If users want to disable the better 
rebasing of merges we'll need to add a --remerge option or something 
like that.

> More voices would be helpful here. Does anyone else have an opinion on
> how likely it is that the default rebase-merges mode will change in
> the future? Or on whether rebase.rebaseMerges should be allowed to
> affect --rebase-merges in order to facilitate such a change?

Getting other's views would indeed be helpful

Best Wishes

Phillip


>>>>> +test_expect_success '--rebase-merges overrides rebase.merges=false' '
>>>>> +     test_config rebase.merges false &&
>>>>> +     git checkout -b override-config-merges-false E &&
>>>>> +     before="$(git rev-parse --verify HEAD)" &&
>>>>> +     test_tick &&
>>>>> +     git rebase --rebase-merges C &&
>>>>> +     test_cmp_rev HEAD $before
>>>>
>>>> This test passes if the rebase does nothing, maybe pass --force and
>>>> check the graph?
>>>
>>> The rebase is supposed to do nothing here.
>>
>> It's not doing nothing though - it is rebasing the branch, it just
>> happens that everything fast-forwards so HEAD ends up unchanged. My
>> point is that this test should verify the branch has been rebased. Maybe
>> you could check the reflog message for HEAD@{0} is
>>
>>          rebase (finish): returning to refs/heads/override-config-merges-false
>>
>>> Checking that the commit
>>> hash is the same is just a quick way to check that the entire graph is
>>> the same. What more would be checked by checking the graph instead of
>>> the hash?
>>
>> By using --force and checking the graph you check that the rebase
>> actually happened.
> 
> I got the impression that people like that not checking the graph
> itself (or the reflog) makes the tests more concise, but I don't care
> much either way. For what it's worth, the way I did it matches the
> existing tests in the file. If you can find at least one other person
> who thinks that it ought to change for this patch series to be
> accepted, and no one else objects, I'll change it.
> 
>> Thanks for working on this
> 
> You're welcome; hopefully we can get the remaining details ironed out quickly.
> 
> -Alex