Re: save-buffer: avoid data loss on interrupt

[Paul Eggert <eggert@cs.ucla.edu>]

On 12/13/11 09:13, Jim Meyering wrote:

> -      (if (or (and file-precious-flag dir-writable)
> +      (if (or (and dir-writable
> +		   (or file-precious-flag
> +		       (= (file-nlinks buffer-file-name) 1)))

I like the general idea, but this solution seems a bit aggressive,
as it will cause the file's ownership to change
if the file's link count is 1, and often that isn't what is wanted.
Instead, how about extending the semantics of break-hardlink-on-save,
with something like the following (untested) patch?

--- lisp/files.el	2011-12-04 08:02:42 +0000
+++ lisp/files.el	2011-12-13 19:46:33 +0000
@@ -4469,8 +4469,11 @@ Before and after saving the buffer, this
            (dir-writable (file-writable-p dir)))
       (if (or (and file-precious-flag dir-writable)
               (and break-hardlink-on-save
-                   (file-exists-p buffer-file-name)
-                   (> (file-nlinks buffer-file-name) 1)
+		   (let ((nlinks (file-nlinks buffer-file-name)))
+		     (and nlinks
+			  (> nlinks (if (numberp break-hardlink-on-save)
+					break-hardlink-on-save
+				      1))))
                    (or dir-writable
                        (error (concat (format
                                        "Directory %s write-protected; " dir)

That way, you can set break-hardlink-on-save to -1 to get
the behavior that you want.

While we're on the subject, break-hardlink-on-save is not
documented; I wonder why not?  Also, the current code
does not work if the directory is sticky and writable
and the file is owned by someone else (a common situation
in /tmp); this should get fixed too.