Re: [PATCH] pull: abort if --ff-only is given and fast-forwarding is impossible

From: Elijah Newren <newren@gmail.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: "Alex Henrie" <alexhenrie24@gmail.com>,
	"Phillip Wood" <phillip.wood123@gmail.com>,
	"Git Mailing List" <git@vger.kernel.org>,
	"Ævar Arnfjörð" <avarab@gmail.com>,
	"Felipe Contreras" <felipe.contreras@gmail.com>
Subject: Re: [PATCH] pull: abort if --ff-only is given and fast-forwarding is impossible
Date: Mon, 12 Jul 2021 13:51:18 -0700	[thread overview]
Message-ID: <CABPp-BH+LPbfdgixvSEGpLxwCHHSK99PFmE3q04jPZjLaqr5Xg@mail.gmail.com> (raw)
In-Reply-To: <xmqq8s2b489p.fsf@gitster.g>

On Mon, Jul 12, 2021 at 12:55 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> Alex Henrie <alexhenrie24@gmail.com> writes:
>
> > Sorry, I misspoke. I was thinking of the case where fast-forwarding is
> > impossible.
>
> When we cannot fast-forward (i.e. we have our own development that
> is not in the tip of their history),
>
>  --ff-only would cause the operation fail
>  --ff would become no-op (as it merely allows fast-forwarding)
>  --no-ff would become no-op (as it merely forbids fast-forwarding)
>
> and the latter two case, we'd either merge or rebase (with possibly
> specified mode like --preserve-merges).  I thought the current
> documentation is already fairly clear on this point?

git pull's --no-ff is documented to "create a merge commit in all
cases", and thus as worded, seems incompatible with rebasing to me.
Treating --no-ff as a no-op when we cannot fast-forward (i.e. allowing
rebasing to happen) could be seen as a backwards incompatible change
at this point.

Having --ff be compatible with rebasing works because the end result
will be the same as described in the existing documentation.

> > If fast-forwarding is possible, --ff-only already effectively
> > implies --no-rebase, and we might want to make that explicit in
> > the documentation.
>
> When we fast-forward (i.e. their history is descendant from ours,
> and the user did not give --no-ff), it does not matter if it is done
> using the merge backend, the rebase backend, or by the "git pull"
> wrapper. The end user does not care.  The end result is that the tip
> of the branch now points at the tip of the history we pulled from
> the other side and that is all what matters.
>
> So, from that point of view, I do not think we want to say rebase or
> merge or anything else for such a case in the documentation.

All three of --ff, --no-ff, and --ff-only come from
Documentation/merge-options.txt and are shared between git-merge and
git-pull.  The description of each of those items mentions "merges" or
"merging" at least once in every sentence.