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

[Alan Mackenzie]

Hello, Eli.

On Tue, Dec 01, 2015 at 05:43:13PM +0200, Eli Zaretskii wrote:
> > Date: Tue, 1 Dec 2015 12:19:41 +0000
> > Cc: 22025@debbugs.gnu.org
> > From: Alan Mackenzie <acm@muc.de>

> > OK, here is a patch.  It introduces a customizable variable,
> > `desktop-version-strategy' which specifies in which version (206 or 208),
> > the desktop file should be saved.  Until that variable is set to non-nil,
> > the user is prompted each time the desktop is loaded.  She is given the
> > opportunity at that time to customize it immediately via a `yes-or-no-p'.

> Thanks.  I'd ask Juanma to review the patch, but I have a comment on
> this design: is it really a good idea to nag the user with these
> questions?  This will happen to every single Emacs user once they
> upgrade to Emacs 25.1.  Do we really want that?

I think we do, yes.  "Nag" might be a bit of an overstatement,
considering that the user need only be subjected to the questions once,
and is provided with an easy means of avoiding them in the future.  We
are, after all, talking about a backwards incompatible change here.

> How about the following strategy instead: by default always save in
> backward compatible way, and give the user a command (or a prefix
> argument to desktop-save, perhaps) to switch to the new format?  I
> think this will be nicer to our users, and will also save us
> yet-another defcustom.

Yes, I'm not too keen of the defcustom either.  But we could save the
desktop file in the same format it was read in, and offer facilities to
convert to the new format (or back to the old one, even).

> WDYT?

I think (but I don't know for sure) that explicitly invoking
`desktop-save' is quite rare - most desktop files will be saved at Emacs
shutdown, so a prefix argument to `desktop-save' might not be optimal.
But thinking about it again, that would be a conscious deliberate action
which couldn't be done accidentally.

I think we need some way of alerting all users to the change.  Otherwise
we could end up with lots of users never upgrading their desktops at
all, which would be a shame.  The trouble is, we also need some way of
not alerting the users too often, and then we're coming back to some
sort of variable, if not a defcustom.

-- 
Alan Mackenzie (Nuremberg, Germany).