Subject: Re: [PATCH 5/9] pull: ensure --rebase overrides ability to ff

From: Junio C Hamano <[email-masked]>
Date: Tue, 20 Jul 2021 16:35:00 -0700

"Elijah Newren via GitGitGadget" <[email-masked]> writes:

> From: Elijah Newren <[email-masked]>
>
> Now that the handling of fast-forward-only in combination with rebases
> has been moved before the merge-vs-rebase logic, we have an unnecessary
> special fast-forward case left within the rebase logic.

It is correct to say that we could call run_rebase() and it will do
the right thing, even when can_ff is true, in this codepath.

But I am not sure if you want to do this as a part of this series.

The code in question comes from 33b842a1 (pull: fast-forward "pull
--rebase=true", 2016-06-29).  It was meant as a mere optimization
to avoid calling run_rebase() when we know we have nothing to replay
on top of their head after we fast-forward, and doing a pure
fast-forward is easier by calling run_merge().

In other words, the code this patch touches should not have anything
to do with the rebase-vs-fast-forward logic.

Now, if a benchmarking tells us that there is no difference between
calling an empty run_rebase() and run_merge(), I'd be perfectly fine
with this change, with the justification that we are "discarding an
earlier optimization that has become irrelevant".  But that would be
totally outside the theme of this topic, I would think.

Thanks.

> @@ -1070,13 +1070,7 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
>  		    submodule_touches_in_range(the_repository, &upstream, &curr_head))
>  			die(_("cannot rebase with locally recorded submodule modifications"));
>  
> -		if (can_ff) {
> -			/* we can fast-forward this without invoking rebase */
> -			opt_ff = "--ff-only";
> -			ret = run_merge();
> -		} else {
> -			ret = run_rebase(&newbase, &upstream);
> -		}
> +		ret = run_rebase(&newbase, &upstream);
>  
>  		if (!ret && (recurse_submodules == RECURSE_SUBMODULES_ON ||
>  			     recurse_submodules == RECURSE_SUBMODULES_ON_DEMAND))
> diff --git a/t/t5520-pull.sh b/t/t5520-pull.sh
> index e2c0c510222..4b50488141f 100755
> --- a/t/t5520-pull.sh
> +++ b/t/t5520-pull.sh
> @@ -295,7 +295,7 @@ test_expect_success '--rebase (merge) fast forward' '
>  	# The above only validates the result.  Did we actually bypass rebase?
>  	git reflog -1 >reflog.actual &&
>  	sed "s/^[0-9a-f][0-9a-f]*/OBJID/" reflog.actual >reflog.fuzzy &&
> -	echo "OBJID HEAD@{0}: pull --rebase . ff: Fast-forward" >reflog.expected &&
> +	echo "OBJID HEAD@{0}: pull --rebase . ff (finish): returning to refs/heads/to-rebase" >reflog.expected &&
>  	test_cmp reflog.expected reflog.fuzzy
>  '
>  
> @@ -307,8 +307,8 @@ test_expect_success '--rebase (am) fast forward' '
>  
>  	# The above only validates the result.  Did we actually bypass rebase?
>  	git reflog -1 >reflog.actual &&
> -	sed "s/^[0-9a-f][0-9a-f]*/OBJID/" reflog.actual >reflog.fuzzy &&
> -	echo "OBJID HEAD@{0}: pull --rebase . ff: Fast-forward" >reflog.expected &&
> +	sed -e "s/^[0-9a-f][0-9a-f]*/OBJID/" -e "s/[0-9a-f][0-9a-f]*$/OBJID/" reflog.actual >reflog.fuzzy &&
> +	echo "OBJID HEAD@{0}: rebase finished: refs/heads/to-rebase onto OBJID" >reflog.expected &&
>  	test_cmp reflog.expected reflog.fuzzy
>  '