Subject: Re: bug#62731: 29.0.60; diff-apply-hunk doesn't work for creating new files

From: Dmitry Gutov <[email-masked]>
Date: Wed, 16 Oct 2024 22:37:40 +0300

Hi Juri,

On 15/10/2024 19:13, Juri Linkov wrote:
>>> All right, the attached seems to support both creation and deletion,
>>> including applying hunks in reverse direction.
>>> Things got trickier but not by a lot.
>>
>> Now pushed to master, seems useful enough. Let's see if some unforeseen
>> problems are reported.
> 
> This change broke diff of files:
> 
> @@ -1957,7 +1970,7 @@ diff-find-source-location
>                                   diff-context-mid-hunk-header-re nil t)
>   			 (error "Can't find the hunk separator"))
>   		       (match-string 1)))))
> -	   (file (or (diff-find-file-name other noprompt)
> +	   (file (or (diff-find-file-name (xor other reverse) noprompt)
>                        (error "Can't find the file")))
>   	   (revision (and other diff-vc-backend
>                             (if reverse (nth 1 diff-vc-revisions)
> 
> So after 'dired-backup-diff', typing 'C-c C-c' visits wrong file:
> visits the backup when point is on the file line, and vice versa:
> visits the file when point is on backup file line.

Thanks for reporting.

Do you think we can fix it this way?

diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index cfa90d380ad..374df3ee2cb 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -2196,7 +2196,7 @@ diff-goto-source
    ;; This is a convenient detail when using smerge-diff.
    (if event (posn-set-point (event-end event)))
    (let ((buffer (when event (current-buffer)))
-        (reverse (not (save-excursion (beginning-of-line) (looking-at 
"[-<]")))))
+        (reverse (not (save-excursion (beginning-of-line) (looking-at 
"[+<]")))))
      (pcase-let ((`(,buf ,_line-offset ,pos ,src ,_dst ,_switched)
                   (diff-find-source-location other-file reverse)))
        (pop-to-buffer buf)

Otherwise, I don't quite understand the intent behind that line. But it 
might have been masking the problem which I (hopefully) fixed in the 
hunk you quoted/