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

[Phillip Wood <phillip.wood123@gmail.com>]

Hi Alex


On 11/07/2021 02:26, Alex Henrie wrote:
> The warning about pulling without specifying how to reconcile divergent
> branches says that after setting pull.rebase to true, --ff-only can
> still be passed on the command line to require a fast-forward. Make that
> actually work.

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

I don't think enforcing fast-forward only for rebases makes sense unless 
it is given on the command line. If the user gives `--rebase` 
`--ff-only` on the command line then we should either error out or take 
the last one in which case `pull --rebase --ff-only` would fast-forward 
only but `pull --ff-only --rebase` would rebase. We should also decide 
what to do when the user has pull.ff set to something other than only 
and also has pull.rebase to something other than false set - I'd guess 
we'd want to rebase unless there is a merge option on the command line 
but I haven't thought about those cases.

Best Wishes

Phillip

> Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
> ---
>   advice.c                     |  5 +++++
>   advice.h                     |  1 +
>   builtin/merge.c              |  2 +-
>   builtin/pull.c               | 11 ++++++++---
>   t/t7601-merge-pull-config.sh | 24 ++++++++++++++++++++++++
>   5 files changed, 39 insertions(+), 4 deletions(-)
> 
> diff --git a/advice.c b/advice.c
> index 0b9c89c48a..337e8f342b 100644
> --- a/advice.c
> +++ b/advice.c
> @@ -286,6 +286,11 @@ void NORETURN die_conclude_merge(void)
>   	die(_("Exiting because of unfinished merge."));
>   }
>   
> +void NORETURN die_ff_impossible(void)
> +{
> +	die(_("Not possible to fast-forward, aborting."));
> +}
> +
>   void advise_on_updating_sparse_paths(struct string_list *pathspec_list)
>   {
>   	struct string_list_item *item;
> diff --git a/advice.h b/advice.h
> index bd26c385d0..1624043838 100644
> --- a/advice.h
> +++ b/advice.h
> @@ -95,6 +95,7 @@ void advise_if_enabled(enum advice_type type, const char *advice, ...);
>   int error_resolve_conflict(const char *me);
>   void NORETURN die_resolve_conflict(const char *me);
>   void NORETURN die_conclude_merge(void);
> +void NORETURN die_ff_impossible(void);
>   void advise_on_updating_sparse_paths(struct string_list *pathspec_list);
>   void detach_advice(const char *new_name);
>   
> diff --git a/builtin/merge.c b/builtin/merge.c
> index a8a843b1f5..aa920ac524 100644
> --- a/builtin/merge.c
> +++ b/builtin/merge.c
> @@ -1620,7 +1620,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
>   	}
>   
>   	if (fast_forward == FF_ONLY)
> -		die(_("Not possible to fast-forward, aborting."));
> +		die_ff_impossible();
>   
>   	if (autostash)
>   		create_autostash(the_repository,
> diff --git a/builtin/pull.c b/builtin/pull.c
> index 3e13f81084..d979660482 100644
> --- a/builtin/pull.c
> +++ b/builtin/pull.c
> @@ -1046,9 +1046,14 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
>   
>   	can_ff = get_can_ff(&orig_head, &merge_heads.oid[0]);
>   
> -	if (rebase_unspecified && !opt_ff && !can_ff) {
> -		if (opt_verbosity >= 0)
> -			show_advice_pull_non_ff();
> +	if (!can_ff) {
> +		if (opt_ff) {
> +			if (!strcmp(opt_ff, "--ff-only"))
> +				die_ff_impossible();
> +		} else {
> +			if (rebase_unspecified && opt_verbosity >= 0)
> +				show_advice_pull_non_ff();
> +		}
>   	}
>   
>   	if (opt_rebase) {
> diff --git a/t/t7601-merge-pull-config.sh b/t/t7601-merge-pull-config.sh
> index 52e8ccc933..b5a09a60f9 100755
> --- a/t/t7601-merge-pull-config.sh
> +++ b/t/t7601-merge-pull-config.sh
> @@ -183,6 +183,30 @@ test_expect_success 'pull prevents non-fast-forward with "only" in pull.ff' '
>   	test_must_fail git pull . c3
>   '
>   
> +test_expect_success 'pull prevents non-fast-forward with pull.ff=only and pull.rebase=true' '
> +	git reset --hard c1 &&
> +	test_config pull.ff only &&
> +	test_config pull.rebase true &&
> +	test_must_fail git pull . c3
> +'
> +
> +test_expect_success 'pull prevents non-fast-forward with pull.ff=only and pull.rebase=false' '
> +	git reset --hard c1 &&
> +	test_config pull.ff only &&
> +	test_config pull.rebase false &&
> +	test_must_fail git pull . c3
> +'
> +
> +test_expect_success 'pull prevents non-fast-forward with --rebase --ff-only' '
> +	git reset --hard c1 &&
> +	test_must_fail git pull --rebase --ff-only . c3
> +'
> +
> +test_expect_success 'pull prevents non-fast-forward with --no-rebase --ff-only' '
> +	git reset --hard c1 &&
> +	test_must_fail git pull --no-rebase --ff-only . c3
> +'
> +
>   test_expect_success 'merge c1 with c2 (ours in pull.twohead)' '
>   	git reset --hard c1 &&
>   	git config pull.twohead ours &&
>