Re: [PATCH v3 5/7] git-rebase.txt: document behavioral inconsistencies between modes

From: Elijah Newren <newren@gmail.com>
Date: Jun 21, 2018, 21:25 UTC
Message-ID: <CABPp-BH5GYed+BBMadY869qGVpP7DDcD3HQtJSed_RPDVFJ-vA@mail.gmail.com>
In-Reply-To: <xmqqfu1fswdh.fsf@gitster-ct.c.googlers.com>

Hi Junio,

On Thu, Jun 21, 2018 at 1:44 PM, Junio C Hamano <gitster@pobox.com> wrote:

Show 8 quoted lines

> Elijah Newren <newren@gmail.com> writes:
>
>> +BEHAVIORAL INCONSISTENCIES
>> +--------------------------
>> +
>> +  * --no-ff vs. --force-rebase
>
> Do we want to `--quote` these?

Ah, good point.

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>> + * empty commits:
>> +
>> +    am-based rebase will drop any "empty" commits, whether the
>> +    commit started empty (had no changes relative to its parent to
>> +    start with) or ended empty (all changes were already applied
>> +    upstream in other commits).
>> +
>> +    merge-based rebase does the same.
>> +
>> +    interactive-based rebase will by default drop commits that
>> +    started empty and halt if it hits a commit that ended up empty.
>
> I think the description is accurate.
>
> WRT a change that ends up being empty (as opposed to a change that
> is empty from the beginning), I'd think that the current behaviour
> is desireable one.  "am" based rebase is solely to transplant an
> existing history and want to stop much less than "interactive" one
> whose purpose is to polish a series before making it publishable,
> and asking for confirmation ("this has become empty--do you want to
> drop it?") is more appropriate from the workflow point of view; so
> it may deserve s/inconsistencies/differences/.

Are you arguing the default for an explicit interactive rebase, or also an implied one?

I would argue that implied interactive rebases also solely have the purpose of "transplanting an existing history", as you say. Thus, if the user passes --rebase-merges (or --strategy or --strategy-options after git-rebase--merge becomes part of git-rebase--interactive), then the default should be the same as for am-based rebase, i.e. just silently drop the empty commits.

Show 5 quoted lines

>> +    The --keep-empty option exists for interactive rebases to allow
>> +    it to keep commits that started empty.
>
> On the other hand, lack of --keep-empty on the "am" side is probably
> a bug that wants to be fixed.

I personally don't like --keep-empty; it's too granular.  I'd prefer a
  --empty={drop,halt,keep}
flag, and have it apply to both patches that were empty when the
rebase started as well as patches that became empty after application
on the new base.  I think that'd be useful for both am-based and
interactive-based rebases.

Then we could also make an explicit --interactive flag imply --empty=halt, if so desired, and otherwise make --empty=drop the default.

I'm interested in implementing that, if other folks think it's not crazy.

Thoughts?

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>> +  * empty commit messages:
>> +
>> +    am-based rebase will silently apply commits with empty commit
>> +    messages.
>> +
>> +    merge-based and interactive-based rebases will by default halt
>> +    on any such commits.  The --allow-empty-message option exists to
>> +    allow interactive-based rebases to apply such commits without
>> +    halting.
>
> Ditto for desirable difference coming from workflow point of view.

I can see the argument for having a difference for rebases if the interactive rebase was explicit, though I disagree with the argument. See https://public-inbox.org/git/CABPp-BHrcUHX_zHxpojV5=sxJ1=NoDg9uhxv+NH5BsHsQYavPQ@mail.gmail.com/ for my views on this.

However, I'm curious again -- is your argument only for interactive_rebase=explicit or also for interactive_rebase=implied?

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>> +  * directory rename detection:
>> +
>> +    merge-based and interactive-based rebases work fine with
>> +    directory rename detection.  am-based rebases sometimes do not.
>
> I quite do not get what the big deal is with "directory rename"; it
> is merely a degree of magic heuristics employed while detecting file
> renames.  It is natural to expect that the less information you make
> available to the machinery, the less amount of heuristics gets
> exercised to make "magic" happen.  Is the internal implementation
> detail described below (omitted) all that interesting to general
> readers of "git rebase --help", I wonder?  I would understand it if
> this were under Documentation/technical/, though.

You are right that the last two paragraphs are probably much too technical and belong elsewhere; I can move them. However, I am not sure if you were requesting just those paragraphs be moved, or if you wanted the whole "directory rename detection" section removed from the manpage. Could you clarify? Also, if the latter...How would users know how much "information is made available to the underlying machinery", as you put it? Or, at a higher level, what is the right way to communicate to them how they can obtain a behavior they might want (directory rename detection), if it's not mentioned somewhere in the manpage? Is there an alternate place to put this in the document?

Thanks, Elijah