Desktop: protect users against inadvertant upgrading of desktop file.

An upgraded (version 208) desktop file cannot be read in Emacs < 25.

* etc/NEWS: Add an entry about upgrading a desktop file.

* lisp/desktop.el (desktop-file-version): Amend doc string.
(desktop-native-file-version, desktop-io-file-version): new variables.
(desktop-clear): Set desktop-io-file-version to nil.
(desktop-buffer-info): make the presence of the last item on the list
conditional on (>= desktop-io-file-version 208).
(desktop-save): Add extra parameter VERSION to take user's C-u or C-u C-u.
Amend the doc string.  Add code to determine the output file version.
(desktop-create-buffer): Set desktop-io-file-version to the input file's
version.

1 files changed, 11 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index c0be46f5ec8..e9b539a3785 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -315,6 +315,17 @@ current package keywords are recognized.  Set the new option
 It's meant for use together with `compile':
 emacs -batch --eval "(checkdoc-file \"subr.el\")"
 
+** 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.
+
 +++
 ** New function `bookmark-set-no-overwrite' bound to C-x r M.
 It raises an error if a bookmark of that name already exists,