Re: [PATCH 7/7] apply --allow-overlap: fix a corner case

From: Junio C Hamano <gitster@pobox.com>
Date: Dec 6, 2019, 13:45 UTC
Message-ID: <xmqqr21hy3cf.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <15c4cce5a027d56c7ddbe5523cf0f3beabd06ed7.1575637705.git.gitgitgadget@gmail.com>

"Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com> writes:

Show 5 quoted lines

> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>
> Yes, yes, this is supposed to be only a band-aid option for `git add -p`
> not Doing The Right Thing. But as long as we carry the `--allow-overlap`
> option, we might just as well get it right.

It probably depends on the definition of "right", where it may not even exist (otherwise it wouldn't be a band-aid but be a real feature) ;-)

> This fixes the case where one hunk inserts a line before the first line,
> and is followed by a hunk whose context overlaps with the first one's
> and which appends a line at the end.

The in-code comment makes me wonder if we need to further loosen the check in the other direction, though. What makes begin more special than end? Can a hunk be seen that pretends to extend to the end but no longer does because there was an overlapping hunk that has been wiggled in?

Show 27 quoted lines

>
> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
>  apply.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/apply.c b/apply.c
> index f8a046a6a5..720a631eaa 100644
> --- a/apply.c
> +++ b/apply.c
> @@ -2661,6 +2661,16 @@ static int find_pos(struct apply_state *state,
>  	unsigned long backwards, forwards, current;
>  	int backwards_lno, forwards_lno, current_lno;
>  
> +	/*
> +	 * When running with --allow-overlap, it is possible that a hunk is
> +	 * seen that pretends to start at the beginning (but no longer does),
> +	 * and that *still* needs to match the end. So trust `match_end` more
> +	 * than `match_beginning`.
> +	 */
> +	if (state->allow_overlap && match_beginning && match_end &&
> +	    img->nr - preimage->nr != 0)
> +		match_beginning = 0;
> +
>  	/*
>  	 * If match_beginning or match_end is specified, there is no
>  	 * point starting from a wrong line that will never match and