GNU bug report logs - #5352
diff-jump-to-old-file inverts hunk application as well

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Michael Orlitzky <michael <at> orlitzky.com>
To: bug-gnu-emacs <at> gnu.org
Subject: diff-jump-to-old-file inverts hunk application as well
Date: Sun, 10 Jan 2010 23:39:20 -0500

The custom diff-jump-to-old-file causes a reversal of the traditional
diff/patch behavior present in Emacs <= 22. From diff-mode.el, it seems
sort of intentional that this is the case; nevertheless, the new
behavior does not strike me as useful. From diff-mode.el:

(defun diff-apply-hunk (&optional reverse)
  "Apply the current hunk to the source file and go to the next.
  By default, the new source file is patched, but if the variable
  `diff-jump-to-old-file' is non-nil, then the old source file is
  patched instead (some commands, such as `diff-goto-source' can change
  the value of this variable when given an appropriate prefix argument).

Why should the default be to patch the new file, which is by definition
already patched? Take for example,

  gantu ~ $ cat new.txt
  Line 1
  Line 2
  Line 3
  Line 4
  gantu ~ $ cat source.txt
  Line 1
  Line 2
  Line 4
  gantu ~ $ diff -c source.txt new.txt > test.patch
  gantu ~ $ emacs -nw test.patch

(The diff -c accomplishes the same thing as M-x diff would within
Emacs). In this case, when the patch is opened, it is expected that M-x
diff-apply-hunk will patch the source (old) file to the new. That is, it
should add "Line 3" to the third line of the source file. This is how
Emacs <= 22 worked.

With v23, the new behavior is that M-x diff-apply-hunk will attempt to
apply the hunk to new.txt which, of course, already contains that line.
Even if one performs the diff within Emacs and explicitly chooses the
source/new files, the default behavior is to attempt to re-patch the new
file.

Message #8 received at 5352 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Michael Orlitzky <michael <at> orlitzky.com>
Cc: 5352 <at> debbugs.gnu.org
Subject: Re: bug#5352: diff-jump-to-old-file inverts hunk application as well
Date: Mon, 09 May 2022 16:32:11 +0200

Michael Orlitzky <michael <at> orlitzky.com> writes:

> The custom diff-jump-to-old-file causes a reversal of the traditional
> diff/patch behavior present in Emacs <= 22. From diff-mode.el, it seems
> sort of intentional that this is the case; nevertheless, the new
> behavior does not strike me as useful. From diff-mode.el:
>
> (defun diff-apply-hunk (&optional reverse)
>   "Apply the current hunk to the source file and go to the next.
>   By default, the new source file is patched, but if the variable
>   `diff-jump-to-old-file' is non-nil, then the old source file is
>   patched instead (some commands, such as `diff-goto-source' can change
>   the value of this variable when given an appropriate prefix argument).
>
> Why should the default be to patch the new file, which is by definition
> already patched? Take for example,

(I'm going through old bug reports that unfortunately weren't resolved
at the time.)

I think the logic here is when you have a diff that's been partially
applied, so that the "new" file isn't literally the one you've already
applied the patch to.

In that case, you usually want to apply the hunk to the new file, and
not the old one.

In any case, the current behaviour has been in place for a decade, so I
think it's too late to change it now (because that would break things
for people that's used to the direction it works in now).

So I'm closing this bug report.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

Message #13 received at 5352 <at> debbugs.gnu.org (full text, mbox):

From: Michael Orlitzky <michael <at> orlitzky.com>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: 5352 <at> debbugs.gnu.org
Subject: Re: bug#5352: diff-jump-to-old-file inverts hunk application as well
Date: Mon, 09 May 2022 12:18:04 -0400

On Mon, 2022-05-09 at 16:32 +0200, Lars Ingebrigtsen wrote:
> 
> So I'm closing this bug report.
> 

This seems to be working like I expect it to now, so maybe the issue
was elsewhere and has been fixed in the meantime.

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