Re: backup-buffer-copy loops if old backup can't be deleted

[Ulrich Mueller <ulm@gentoo.org>]

>>>>> On Wed, 22 Aug 2007, Martin von Gagern wrote:

> I can think of at least two scenarios where this could be a problem:

> 1. The dir is not writable, but the backup file is. Here the current
>    behaviour will loop and even with the suggested fix it will fall
>    back to ~/%backup%~ unnecessarily.

> [...]

> I originally assumed that emacs would try backup-buffer-copy only
> after figuring out that it could not write to the existing backup
> file, but it seems I was wrong there, at least if I read
> backup-buffer correctly.

It seems that your assumption is the intended behaviour. At least the
problem was noticed and a fix was installed in March 2005:
<http://lists.gnu.org/archive/html/emacs-devel/2005-03/msg00842.html>

However, it no longer works due to the introduction of the loop in
backup-buffer-copy one month later (this change was in CVS revision
1.757 of files.el). The relevant part is:

 (defun backup-buffer-copy (from-name to-name modes)
-  (condition-case ()
-      (copy-file from-name to-name t t)
-    (file-error
-     ;; If copying fails because file TO-NAME
-     ;; is not writable, delete that file and try again.
-     (if (and (file-exists-p to-name)
-	      (not (file-writable-p to-name)))
-	 (delete-file to-name))
-     (copy-file from-name to-name t t)))
+  (let ((umask (default-file-modes)))
+    (unwind-protect
+	(progn
+	  ;; Create temp files with strict access rights.  It's easy to
+	  ;; loosen them later, whereas it's impossible to close the
+	  ;; time-window of loose permissions otherwise.
+	  (set-default-file-modes ?\700)
+	  (while (condition-case ()
+		     (progn
+		       (condition-case nil
+			   (delete-file to-name)
+			 (file-error nil))
+		       (write-region "" nil to-name nil 'silent nil 'excl)
+		       nil)
+		   (file-already-exists t))
+	    ;; the file was somehow created by someone else between
+	    ;; `make-temp-name' and `write-region', let's try again.
+	    nil)
+	  (copy-file from-name to-name t t 'excl))
+      ;; Reset the umask.
+      (set-default-file-modes umask)))
   (and modes
        (set-file-modes to-name (logand modes #o1777))))

with the following ChangeLog entry:

2005-04-23  Richard M. Stallman  <rms@gnu.org>

	* files.el [...]
	(backup-buffer-copy, basic-save-buffer-2): Take care against
	writing thru an unexpected existing symlink.
	[...]

There should be easier ways to achieve protection against writing
through symlinks.

Ulrich