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

[Alan Mackenzie]

Hello, Drew.

On Thu, Dec 03, 2015 at 06:33:01AM -0800, Drew Adams wrote:
> Again, apologies for not really reading the bug thread.

:-)

> Just wondering whether this format change will be mentioned
> in NEWS, and what the reason for the change is.

I have written a NEWS entry for it.  The reason for the change seems to
be twofold: (i) to store the mark ring of each buffer; (ii) to allow
further information to be added later, without there being another
backward incompatible change.

> In NEWS, I find only this when looking for "desktop":

> *** Partial state of the eww buffers (the URIs and the titles of the
> pages visited) is now preserved in the desktop file.

> Is that it?

No, that's not it.  My NEWS entry will be undergoing review, together
with my patch to desktop.el, in the next few hours/days.

> Does just _adding_ info to the file really change the format in an
> incompatible way?

Yes, it does.  Once a desktop file has been upgraded to version 208, it
can no longer be read by Emacs < 25.1.

> If so, has it been fixed now, so that future additions (i.e., without
> changes that also change the existing fields) don't result in an
> incompatible change?

Yes, it has.

> If this NEWS entry is in fact the announcement of the format
> change, then please consider changing the wording to make
> clear that this is not just a benign addition of info but
> is an incompatible format change for the file.

There's a new NEWS entry in the pipeline.

> (A priori, I have nothing against a change.  But it would
> be good to know what this is about.)

The current plan is as follows: a desktop file will stay in format
version 206 (which works with any Emacs from 22.1) until the user
positively decides to upgrade it to 208, which she will do with C-u M-x
save-desktop.  If, for any reason, a desktop needs to be put back to
206, this can be done with C-u C-u M-x save-desktop.  There are
programmatic equivalents to these command options.

-- 
Alan Mackenzie (Nuremberg, Germany).