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Subject: bug#80040: 30.2; diff-sanity-check-hunk false positive on "\ No newline at end, of file"
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When running `diff-sanity-check-hunk` on a diff hunk containing "\ No
newline at end of file" in the middle (like, for example, the shell
output of `diff -u <(printf 'old') <(printf 'new')`), the user is
prompted to "Try to auto-fix word wrap damage?".

If the user says yes to this prompt, the "\ No newline at end of file"
is simply moved up to the end of the previous line, i.e. treated as if
the previous line was accidentally wrapped.

This indirectly breaks e.g. `diff-apply-hunk`, which fails if the user
chooses to "fix" the damage, or aborts if they choose not to.

Here's a simple reproduction:

```elisp
(require 'diff-mode)

;; This works fine - newline at end of old content.
(with-temp-buffer
   ;; Diff as generated by e.g.:
   ;;
   ;;  diff -u <(printf 'old\n') <(printf 'new')
   ;;
   (insert "diff --git a/test.txt b/test.txt
--- a/test.txt
+++ b/test.txt
@@ -1 +1 @@
-old
+new
\\ No newline at end of file
")
   (diff-mode)
   (goto-char (point-min))
   (re-search-forward "^@@")
   (beginning-of-line)
   ;; No issue.
   (diff-sanity-check-hunk))

;; This throws a false-positive sanity check error.
(with-temp-buffer
   ;; Diff as generated by e.g.:
   ;;
   ;;  diff -u <(printf 'old') <(printf 'new')
   ;;
   (insert "diff --git a/test.txt b/test.txt
--- a/test.txt
+++ b/test.txt
@@ -1 +1 @@
-old
\\ No newline at end of file
+new
\\ No newline at end of file
")
   (diff-mode)
   (goto-char (point-min))
   (re-search-forward "^@@")
   (beginning-of-line)
   ;; Prompts to fix word-wrap damage.
   (diff-sanity-check-hunk))
```

Note that if `diff-sanity-check-hunk` is redefined as a no-op,
`diff-apply-hunk` does seem to work fine on patches of this form.

In GNU Emacs 30.2 (build 2, aarch64-apple-darwin25.0.0, NS
  appkit-2685.10 Version 26.0.1 (Build 25A362)) of 2025-11-12 built on
  shine.local
System Description:  macOS 26.0.1

Configured using:
  'configure --disable-dependency-tracking --disable-silent-rules
  --enable-locallisppath=/opt/homebrew/share/emacs/site-lisp
  --infodir=/opt/homebrew/Cellar/emacs-plus@30/30.2/share/info/emacs
  --prefix=/opt/homebrew/Cellar/emacs-plus@30/30.2
  --with-native-compilation=aot --with-xml2 --with-gnutls
  --without-compress-install --with-dbus --with-imagemagick
  --with-modules --with-rsvg --with-webp --with-xwidgets --with-ns
  --disable-ns-self-contained 'CFLAGS=-O2 -DFD_SETSIZE=10000
  -DDARWIN_UNLIMITED_SELECT -I/opt/homebrew/opt/sqlite/include
  -I/opt/homebrew/opt/gcc/include -I/opt/homebrew/opt/libgccjit/include'
  'LDFLAGS=-L/opt/homebrew/opt/sqlite/lib -L/opt/homebrew/lib/gcc/15
  -Wl,-rpath,/opt/homebrew/lib/gcc/15''

Configured features:
ACL DBUS GIF GLIB GMP GNUTLS IMAGEMAGICK JPEG LCMS2 LIBXML2 MODULES
NATIVE_COMP NOTIFY KQUEUE NS PDUMPER PNG RSVG SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP XIM XWIDGETS ZLIB

Important settings:
   value of $LANG: en_US.UTF-8
   locale-coding-system: utf-8-unix

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From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Kevin Montag <kmontag@HIDDEN>
Subject: bug#80040: Acknowledgement (30.2; diff-sanity-check-hunk false
 positive on "\ No newline at end, of file")
Message-ID: <handler.80040.B.176621804127727.ack <at> debbugs.gnu.org>
References: <ff66c0c2-f630-4438-9981-5d3a234404ba@HIDDEN>
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Date: Sat, 20 Dec 2025 08:08:02 +0000

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Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#80040: 30.2; diff-sanity-check-hunk false positive on "\ No newline at end, of file"
Resent-From: Sean Whitton <spwhitton@HIDDEN>
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From: Sean Whitton <spwhitton@HIDDEN>
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Date: Sat, 20 Dec 2025 15:02:34 +0000
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Hello,

On Sat 20 Dec 2025 at 12:39am +01, Kevin Montag wrote:

> When running `diff-sanity-check-hunk` on a diff hunk containing "\ No
> newline at end of file" in the middle (like, for example, the shell
> output of `diff -u <(printf 'old') <(printf 'new')`), the user is
> prompted to "Try to auto-fix word wrap damage?".
>
> If the user says yes to this prompt, the "\ No newline at end of file"
> is simply moved up to the end of the previous line, i.e. treated as if
> the previous line was accidentally wrapped.
>
> This indirectly breaks e.g. `diff-apply-hunk`, which fails if the user
> chooses to "fix" the damage, or aborts if they choose not to.

Thanks.  I think this is an appropriate fix, though Stefan's review
would be appreciated:

--8<---------------cut here---------------start------------->8---
diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index a7d92039889..d6c7ae46a1c 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -1913,6 +1913,7 @@ diff-sanity-check-hunk
                           (eq (char-after) ?\n)
                           (> before 0) (> after 0))
                      (decf before) (decf after) t)
+                    ((looking-at "\\\\ No newline at end of file$") t)
                     ((and (zerop before) (zerop after)) nil)
                     ((or (< before 0) (< after 0))
                      (error (if (or (zerop before) (zerop after))
--8<---------------cut here---------------end--------------->8---

-- 
Sean Whitton

Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#80040: 30.2; diff-sanity-check-hunk false positive on "\ No newline at end, of file"
Resent-From: Stefan Monnier <monnier@HIDDEN>
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To: Sean Whitton <spwhitton@HIDDEN>
Cc: 80040 <at> debbugs.gnu.org, Kevin Montag <kmontag@HIDDEN>
From: Stefan Monnier <monnier@HIDDEN>
In-Reply-To: <875xa1gprp.fsf@HIDDEN>
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References: <ff66c0c2-f630-4438-9981-5d3a234404ba@HIDDEN>
 <875xa1gprp.fsf@HIDDEN>
Date: Sat, 20 Dec 2025 13:43:25 -0500
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> Thanks.  I think this is an appropriate fix, though Stefan's review
> would be appreciated:
>
> --8<---------------cut here---------------start------------->8---
> diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
> index a7d92039889..d6c7ae46a1c 100644
> --- a/lisp/vc/diff-mode.el
> +++ b/lisp/vc/diff-mode.el
> @@ -1913,6 +1913,7 @@ diff-sanity-check-hunk
>                            (eq (char-after) ?\n)
>                            (> before 0) (> after 0))
>                       (decf before) (decf after) t)
> +                    ((looking-at "\\\\ No newline at end of file$") t)
>                      ((and (zerop before) (zerop after)) nil)
>                      ((or (< before 0) (< after 0))
>                       (error (if (or (zerop before) (zerop after))
> --8<---------------cut here---------------end--------------->8---

Ideally, we'd check that it occurs at the end of the hunk, but the
definition of "end" is a bit subtle, so it's probably not worth
the trouble.

IOW: LGTM, thanks,


        Stefan

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On Dez 20 2025, Sean Whitton wrote:

> diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
> index a7d92039889..d6c7ae46a1c 100644
> --- a/lisp/vc/diff-mode.el
> +++ b/lisp/vc/diff-mode.el
> @@ -1913,6 +1913,7 @@ diff-sanity-check-hunk
>                            (eq (char-after) ?\n)
>                            (> before 0) (> after 0))
>                       (decf before) (decf after) t)
> +                    ((looking-at "\\\\ No newline at end of file$") t)

The text is locale dependent, it should only check for the backslash.

-- 
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