bug#72556: 29.1.90; vc-diff does not undo hunk in end of source file

[Eli Zaretskii <eliz@gnu.org>]

> From: Juri Linkov <juri@linkov.net>
> Cc: Tomas Nordin <tomasn@posteo.net>,  Dmitry Gutov <dmitry@gutov.dev>,
>   72556@debbugs.gnu.org
> Date: Mon, 19 Aug 2024 19:49:05 +0300
> 
> >> To reproduce:
> >>
> >> $ mkdir apply
> >> $ cd apply
> >> $ echo "first line of code" > example.file
> >> $ git init
> >> $ git add .
> >> $ git commit -m 'init'
> >> $ echo "last line of code" >> example.file
> >> $ emacs -Q example.file
> >>
> >> C-x v =
> >> n
> >> C-c C-a
> >>
> >> The hunk is applied in the source buffer.
> >
> > Juri and Dmitry, any comments or suggestions?
> 
> I don't know why it fails at the end, so I tried to debug 'diff-apply-hunk',
> and found that the problem is in these lines of 'diff-find-source-location':
> 
>     ;; FIXME: Check for case where both OLD and NEW are found.
>     (pos (or (diff-find-text (car old))
> 
> When the hunk is at the beginning/end of the diff buffer,
> then it misses the top/bottom part of the context,
> so 'diff-find-text' wrongly matches it because it fails
> to see any changes in it due to insufficient context,
> and returns switched=nil.
> 
> A possible solution would be to add a condition to detect the case
> when the hunk is at the beginning/end of the diff buffer.

Thanks.  Would anyone like to submit a patch along these lines?