Re: Can't squash merge with merge.ff set to false

From: Bryan Turner <bturner@atlassian.com>
Date: Jan 5, 2018, 20:54 UTC
Message-ID: <CAGyf7-E-7KUCAM8SeG-2powp2XN+H=16VG-6XBmj1wUT9tjVcw@mail.gmail.com>
In-Reply-To: <CAHd499A_ANzFA8HSQLJWakzzWanbmVGnur=um=kquYHu0aCBcQ@mail.gmail.com>

On Fri, Jan 5, 2018 at 12:35 PM, Robert Dailey <rcdailey.lists@gmail.com> wrote:

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> On Fri, Jan 5, 2018 at 2:26 PM, Paul Smith <paul@mad-scientist.net> wrote:
>> On Fri, 2018-01-05 at 12:12 -0800, Bryan Turner wrote:
>>> On Fri, Jan 5, 2018 at 11:59 AM, Robert Dailey <rcdailey.lists@gmail.com> wrote:
>>>
>>> As for why the two aren't allowed together, my assumption would be
>>> because if you're only squashing a single commit "--squash" and that
>>> commit is fast-forward from the target, a new commit is not created
>>> and instead the target branch is fast-forwarded. With "--no-ff", it's
>>> questionable what "--squash" should do in that case. Fast-forward
>>> anyway? Rewrite the commit simply to get new committer details and
>>> SHA-1?
>>
>> If it only failed when you were squash-merging a single commit that was
>> also fast-forwardable, I guess that would be one thing.  But even if I
>> have multiple commits and I want to squash-merge them, which clearly is
>> a separate operation giving different results, I get this error.

I think there's a reasonable argument that having the failure be consistent is easier to reason about, and therefore provides a "better" user experience (to some definition of "better" which all people may not share in common).

If the failure was delayed until "git merge --squash" decided it wanted to fast-forward, the failure might seem more arbitrary.

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>>
>> I don't think Git should try to be clever here (if that's what it's
>> doing--I always assumed it was just a missing configuration case in the
>> error check).  If I asked for a squash-merge then Git should give me a
>> squash merge.
>>
>> So in answer to your question, --squash should give me a squash merge
>> and the setting of --ff / --no-ff should be completely ignored, as it's
>> irrelevant.
>>
>> My $0.02.
>
> Seems like --ff works, but is also misleading since in my case (more
> than one commit) I'm not doing a ff merge and there's no possibility
> of it.

"--ff" doesn't say "git merge" _must_ fast-forward ("--ff-only"); it says that it _can_. At a general level with "--squash", that seems to be exactly correct. A "--squash" merge can create a new commit, or it can fast-forward an existing commit if the situation allows. Based on that, passing "--ff" doesn't seem misleading to me.

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> I think your idea of the 2 being distinctly separate makes
> sense. Basically, --squash takes precedence and if the mechanism to
> implement squash in certain scenarios (such as single commit) is
> fast-forward merge, then that decision is made for the user and is no
> longer something they can control.

The two _aren't_ distinctly separate, though. "git merge --squash --ff-only" has very different semantics to "git merge --squash --ff", in that it will only create a new squashed commit (or fast-forward a single commit) if the incoming commit(s) are fast-forward from the target. So there _is_ a setting for the fast-forward mode (given "--ff", "--ff-only", and "--no-ff" are a tri-state switch, and therefore comprise a single setting) that does impact squashing.