Subject: bug#21766: 25.0.50; delete-trailing-whitespace sometimes deletes non-whitespace

From: Juanma Barranquero <[email-masked]>
Date: Tue, 27 Oct 2015 01:15:18 +0100

On Tue, Oct 27, 2015 at 12:07 AM, Markus Triska <[email-masked]> wrote:
>
>
> Steps to reproduce:
>
> 1) wget http://www.metalevel.at/ei/fault1.html
>
> 2) emacs -Q fault1.html
>
> 3) M-x delete-trailing-whitespace RET
>
> This removes several non-white-space characters, including the whole
> line containing "and use it like" (line 83 in the original file),

Apparently, the call to skip-syntax-backward is altering the match data. If
you try this

--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -609,7 +609,7 @@ delete-trailing-whitespace
             (start (or start (point-min))))
         (goto-char start)
         (while (re-search-forward "\\s-$" end-marker t)
-          (skip-syntax-backward "-" (line-beginning-position))
+          (save-match-data (skip-syntax-backward "-"
(line-beginning-position)))
           ;; Don't delete formfeeds, even if they are considered
whitespace.
           (if (looking-at-p ".*\f")
               (goto-char (match-end 0)))

or, alternatively,

@@ -609,11 +609,12 @@ delete-trailing-whitespace
             (start (or start (point-min))))
         (goto-char start)
         (while (re-search-forward "\\s-$" end-marker t)
-          (skip-syntax-backward "-" (line-beginning-position))
-          ;; Don't delete formfeeds, even if they are considered
whitespace.
-          (if (looking-at-p ".*\f")
-              (goto-char (match-end 0)))
-          (delete-region (point) (match-end 0)))
+          (let ((end (match-end 0)))
+            (skip-syntax-backward "-" (line-beginning-position))
+            ;; Don't delete formfeeds, even if they are considered
whitespace.
+            (if (looking-at-p ".*\f")
+                (goto-char end))
+            (delete-region (point) end)))
         ;; Delete trailing empty lines.
         (goto-char end-marker)
         (when (and (not end)

it works. Now, the question is, should skip-syntax-backward preserve the
match data, or must delete-trailing-whitespace be coded defensively? The
usual policy has been that code that needs the match data preserved should
make sure itself that it is so.