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

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

[Tomas Nordin]

Hello Emacs

I have got used to the diff-mode feature of undoing hunks. I have
noticed that if the diff hunk is an addition in the end of the source
file, I dont get the expected question if I want to undo the hunk.
Instead, the hunk is applied again.

My example is with a git repo, some edits made (and saved to disk) in a
file, and hitting C-x v = to view the diff. Move to one of the hunks.
One can now diff-apply-hunk to reverse the hunk. But if that hunk is the
last thing in the source file, the hunk is applied (again). This is not
what I expect.

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.

In GNU Emacs 29.1.90 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.38, cairo version 1.16.0) of 2023-11-18 built on fliptop2
Repository revision: d9e43f2197fa1d5ade1d483b15cc50c6d705b969
Repository branch: emacs-29
Windowing system distributor 'The X.Org Foundation', version 11.0.12101007
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bug#72556: 29.1.90; vc-diff does not undo hunk in end of source file

[Eli Zaretskii]

> From: Tomas Nordin <tomasn@posteo.net>
> Date: Sat, 10 Aug 2024 12:11:08 +0000
> 
> I have got used to the diff-mode feature of undoing hunks. I have
> noticed that if the diff hunk is an addition in the end of the source
> file, I dont get the expected question if I want to undo the hunk.
> Instead, the hunk is applied again.
> 
> My example is with a git repo, some edits made (and saved to disk) in a
> file, and hitting C-x v = to view the diff. Move to one of the hunks.
> One can now diff-apply-hunk to reverse the hunk. But if that hunk is the
> last thing in the source file, the hunk is applied (again). This is not
> what I expect.
> 
> 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?

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

[Juri Linkov]

>> 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.

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

[Eli Zaretskii]

> 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?

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

[Eli Zaretskii]

Ping! Would someone want to work on a patch, or should I close this
bug as wontfix?

> Cc: 72556@debbugs.gnu.org
> Date: Sat, 31 Aug 2024 11:09:36 +0300
> From: 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?
> 
> 
> 
> 

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

[Dmitry Gutov]

On 19/08/2024 19:49, Juri Linkov wrote:
> 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.

This might have drawbacks. Sometimes we want to split a hunk and then 
apply the top part. And when a hunk is split, the top part can't have 
context at the end.

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

[Dmitry Gutov]

Hi!

On 10/08/2024 15:11, Tomas Nordin wrote:
> I have
> noticed that if the diff hunk is an addition in the end of the source
> file, I dont get the expected question if I want to undo the hunk.
> Instead, the hunk is applied again.

No matter the resolution of this bug, just wanted to make sure you know 
that you can force the reverse direction by adding the prefix to the 
command.

So while 'C-c C-a' applies the hunk, 'C-u C-c C-a' reverses it (with a 
similar check that sees whether the hunk has been applied already).

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

[Tomas Nordin]

Dmitry Gutov <dmitry@gutov.dev> writes:

> Hi!
>
> On 10/08/2024 15:11, Tomas Nordin wrote:
>> I have
>> noticed that if the diff hunk is an addition in the end of the source
>> file, I dont get the expected question if I want to undo the hunk.
>> Instead, the hunk is applied again.
>
> No matter the resolution of this bug, just wanted to make sure you know 
> that you can force the reverse direction by adding the prefix to the 
> command.
>
> So while 'C-c C-a' applies the hunk, 'C-u C-c C-a' reverses it (with a 
> similar check that sees whether the hunk has been applied already).

I was loosely aware of that but never got it into my fingers. Instead I
was relying on that dwim behavior of diff-mode. But trying the prefix
variant out, it seems to be a solid way of "un-applying" a hunk.

Might not be worth the trouble to try and fix this problem if it is
complicated. There is a solid way to do what I want (which is maybe the
proper way), and my complaint about this is the only one I have seen. A
wont-fix is fine with me.