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

[Andy Moreton <andrewjmoreton@gmail.com>]

On Wed 02 Dec 2015, Alan Mackenzie wrote:

> Hello, Eli.
>
> On Tue, Dec 01, 2015 at 08:46:51PM +0200, Eli Zaretskii wrote:
>> > Date: Tue, 1 Dec 2015 17:01:38 +0000
>> > Cc: 22025@debbugs.gnu.org
>> > From: Alan Mackenzie <acm@muc.de>
>
>> > 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.
>
>> I think NEWS is the appropriate place to alert them to this.
>
> I'm a bit wary about NEWS.  How many people actually read it thoroughly?
> At each new release, I try to read NEWS, but my eyes start glazing over
> before I even get half way through.
>
> But that aside, how about the following NEWS entry (to be followed by
> actual code)?
>
>
> ** Desktop
>
> ---
> *** The desktop format version has been upgraded from 206 to 208.
> Although Emacs 25.1 can read a version 206 desktop, earlier Emacsen
> cannot read a version 208 desktop.  To upgrade your desktop file, you
> must explicitly request the upgrade by C-u M-x desktop-save.  You are
> recommended to do this as soon as you have firmly upgraded to Emacs
> 25.1 (or later).  Should you ever need to downgrade your desktop file
> to version 206, you can do this with C-u C-u M-x desktop-save.
>
> An additional possibility would by to nag the user with a prompt in the
> echo area after loading a 206 desktop.  Something like:
>
> You have loaded an old format desktop file.  To upgrade it, see NEWS (C-h n).
>
> , together with a variable to disable it.  Maybe.

Another possibility is an ELPA package for older emacsen to allow them
to read and write version 208 format desktop files.

    AndyM