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

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Junio,

On Fri, 6 Dec 2019, Junio C Hamano wrote:

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

Indeed. My hope is that we can get rid of it once the scripted
`git-add--interactive.perl` is removed in favor of the built-in add -i/-p.
This is a long way off, of course.

> > 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?

The beginning is more special than the end because it is associated with
the line number 1. The end line number is flexible already.

There is another difference: after splitting hunks, the first hunk is
applied first, and may render the line numbers of succeeding hunks
incorrect. The same is not true for the last hunk: it cannot render the
preceding hunks' line numbers incorrect, as it has not been applied yet.

I think that's why `--allow-overlap` works with this patch when adding
lines both to the beginning and to the end after splitting a single hunk.

Ciao,
Dscho

> > 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
>