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

From: Felipe Contreras <felipe.contreras@gmail.com>
To: Junio C Hamano <gitster@pobox.com>,
	Phillip Wood <phillip.wood123@gmail.com>
Cc: Alex Henrie <alexhenrie24@gmail.com>,
	git@vger.kernel.org, avarab@gmail.com,
	felipe.contreras@gmail.com, newren@gmail.com
Subject: Re: [PATCH] pull: abort if --ff-only is given and fast-forwarding is impossible
Date: Mon, 12 Jul 2021 12:30:30 -0500	[thread overview]
Message-ID: <60ec7c3636327_a45252081d@natae.notmuch> (raw)
In-Reply-To: <xmqqpmvn5ukj.fsf@gitster.g>

Junio C Hamano wrote:
> Phillip Wood <phillip.wood123@gmail.com> writes:
> 
> > Thanks for revising this patch, I like this approach much better. I do
> > however have some concerns about the interaction of pull.ff with the 
> > rebase config and command line options. I'd naively expect the
> > following behavior (where rebase can fast-forward if possible)
> >
> >   pull.ff  pull.rebase  commandline  action
> >    only     not false                rebase
> >    only     not false   --no-rebase  fast-forward only
> >     *       not false    --ff-only   fast-forward only
> >    only     not false    --ff        merge --ff
> >    only     not false    --no-ff     merge --no-ff
> >    only       false                  fast-forward only
> >    only       false      --rebase    rebase
> >    only       false      --ff        merge --ff
> >    only       false      --no-ff     merge --no-ff
> 
> Do you mean by "not false" something other than "true"?  Are you
> trying to capture what should happen when these configuration
> options are unspecified as well (and your "not false" is "either set
> to true or unspecified")?  I ask because the first row does not make
> any sense to me.  It seems to say
> 
>     "If pull.ff is set to 'only', pull.rebase is not set to 'false',
>     and the command line does not say anything, we will rebase".

No, pull.rebase can have values other than true, like "merges".

  git -c pull.ff=only -c pull.rebase=merges pull

This should rebase because pull.ff=only is meant only for --merge.

> I do agree with you that the command line options
> 
>     --ff-only
>     --ff (aka "allow ff")
>     --no-ff
> 
> should override pull.ff and

Yes.

>     --rebase
>     --no-rebase (aka "merge")
> 
> should override pull.rebase configuration settings

Yes.

> and also override pull.ff set to 'only'

No.

pull.ff=only is specifically set for when the user wants to do a merge,
*not* a rebase.

  git -c pull.ff=only pull # fast-forward merge
  git -c pull.ff=only pull --rebase # rebase
  git -c pull.ff=only pull --no-rebase # fast-forward merge

Whether the merge is implied becuse it's the default, or the user has
explicitely specified it with --no-rebase (should be --merge) does not
matter.

> (i.e. the user explicitly
> wants intregration of the two histories and at that point "I usually
> just follow along" which is "pull.ff=only" no longer applies).

Nowhere in the description of pull.ff does it say that --no-rebase turns
off pull.ff=only.

pull.ff::
	By default, Git does not create an extra merge commit when merging
	a commit that is a descendant of the current commit. Instead, the
	tip of the current branch is fast-forwarded. When set to `false`,
	this variable tells Git to create an extra merge commit in such
	a case (equivalent to giving the `--no-ff` option from the command
	line). When set to `only`, only such fast-forward merges are
	allowed (equivalent to giving the `--ff-only` option from the
	command line). This setting overrides `merge.ff` when pulling.

That would be a pretty convoluted semantic change.

And BTW, why would these four produce different results?

  git -c pull.ff=only -c pull.rebase=false pull
  git -c pull.ff=only pull --no-rebase
  git -c pull.rebase=false pull --ff-only
  git pull --ff-only --no-rebase

-- 
Felipe Contreras