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

[Drew Adams <drew.adams@oracle.com>]

> I think (but I don't know for sure) that explicitly invoking
> `desktop-save' is quite rare

I and other Bookmark+ users use it all the time.  We create
desktops as bookmarks, and switch among them - any number
of them - by "jumping" to them.

> "- most desktop files will be saved at Emacs shutdown,

Are you sure?  My guess is that most users who only do
that content themselves with a single desktop file, or
perhaps 2 or 3.  Users of desktop bookmarks might have
dozens of them.

And saving is independent of Emacs shutdown, logically,
and in reality for desktop bookmarks.

IMHO, it is a big mistake to assume that desktops are
used only in the way provided out of the box.  This is
one example of that.

Another example:

It is an unfortunate mistake that desktop.el is written in
such a way that it hard-codes an assumption that there is
only one desktop file per directory.  The code uses a
desktop directory (argument or global var) everywhere,
instead of a desktop file argument.  Yet the directory is
used only to find the file.

The basic functions, such as `desktop-change-dir' and
`desktop-read', assume this, so they don't take a
DESKTOP-FILE as an optional arg.  I needed to write simple
wrapper functions for them, to let users have multiple
desktop files in the same directory.  There is no reason
why a desktop file must be associated with a directory -
users should be able to store the files anywhere.

For example:

(defun bmkp-desktop-change-dir (desktop-file)
  "Change to desktop saved in DESKTOP-FILE.
Kill the desktop as specified by variables `desktop-save-mode' and
 `desktop-save'.
Clear the desktop and load DESKTOP-FILE."
  (interactive (list (read-file-name "Change to desktop file: ")))
  (unless (file-name-absolute-p desktop-file)
    (setq desktop-file  (expand-file-name desktop-file)))
  (let ((desktop-base-file-name (file-name-nondirectory desktop-file))
        (desktop-dir            (file-name-directory desktop-file))
        (desktop-restore-eager  t) ; Don't bother with lazy restore.
        (desktop-globals-to-save
         (bmkp-remove-if
           (lambda (elt) (memq elt bmkp-desktop-no-save-vars))
           desktop-globals-to-save)))
    (bmkp-desktop-kill)
    (desktop-clear)
    (desktop-read desktop-dir)))

Fiddling to bind `desktop-base-file-name' etc. is silly,
but necessary because of the desktop.el implementation's
assumption about desktop files.

This is the kind of thing that can result from thinking
things like "explicitly invoking `desktop-save' is quite
rare".  Don't assume that the originally intended use case
is the only one.

This said, I don't have anything particular to say now about
a change in desktop format or prompting the user (once only)
about upgrading the format used.  And I haven't looked at the
proposed change.

FWIW, bookmark.el handles evolution of the bookmark-file
format across 3 versions in a way that is transparent to
users.  Dunno whether the format change for desktop files is
similar, but if it is, you might want to take a look at how
bookmark.el handles it.  Search bookmark.el for "IMPORTANT
NOTICE" to see a description of the changes and how they are
dealt with.

HTH.