bug#22025: Emacs 25 corrupts Emacs 24 .emacs.desktop.

[Andy Moreton <andrewjmoreton@gmail.com>]

On Fri 27 Nov 2015, Eli Zaretskii wrote:

>> I think we are enormously cavalier with the .desktop file - no backup of
>> any type (such as .emacs.desktop~) is kept, and should there be trouble
>> reading .desktop, it is difficult to exit Emacs without having this
>> .desktop overwritten.
>
> Maybe having backups for it would be a good and safe measure, indeed.
> At least as an option.  Does anyone see any reasons why not?

I add this to desktop-save-hook to avoid problems with lost or corrupted
desktop files:

  (defun ajm:backup-desktop-file ()
    "Save a backup of the desktop file."
    (let* ((version-control     t)
           (delete-old-versions t)
           (kept-new-versions   8)
           (kept-old-versions   2)
           (file   (desktop-full-file-name))
           (names  (find-backup-file-name file))
           (backup (pop names)))
      (when (and (file-exists-p file) backup)
        (copy-file file backup nil t)
        (message "Saved backup of desktop file '%s' as '%s'." file backup)
        (mapc #'delete-file names))))

Doubtless this is hacky and could be improved, but it seems to work for me.

>> > Of course, if making that happen is relatively easy, it would be good
>> > to have such a compatibility mode.  Patches welcome.
>> 
>> I will work on this.

I haven't checked recently, but this worked for me the last time I used
emacs-25 desktop files with emacs-24:

  (when (< emacs-major-version 25)
    (defun ajm:fixup-desktop-create-buffer (args)
      "Fixup for emacs-24 to use emacs-25 desktop files."
      (let ((nargs (length args)))
        (if (> nargs 10)
            (butlast args (- nargs 10))
          args)))
    (advice-add 'desktop-create-buffer :filter-args
                'ajm:fixup-desktop-create-buffer))

With that advice installed, both emacs versions can read either version
of desktop file. If emacs-24 reads a version 208 desktop file and then
saves a version 206 desktop file, then there may be minor information
loss.

    AndyM