bug#16630: desktop-auto-save seems confusing

[Juri Linkov <juri@jurta.org>]

> Why doesn't desktop-save-mode simply enable the auto-save timer?
> The timer function itself seems to take the necessary steps to ensure it
> only actually does something when appropriate.

This would be simpler, and I guess there should not be much
performance overhead of firing the timer that does nothing
in some rare cases (when the desktop filename is not yet known
or another instance acquires the lock).

But `desktop-auto-save-set-timer' still needs to be called
from :set of `desktop-auto-save-timeout' defcustom for the case
when the user customizes the timeout either from 0 to a positive integer,
or back to 0 that needs to cancel the timer.

=== modified file 'lisp/desktop.el'
--- lisp/desktop.el	2014-01-01 07:43:34 +0000
+++ lisp/desktop.el	2014-02-05 07:47:45 +0000
@@ -162,7 +162,10 @@ (define-minor-mode desktop-save-mode
 one session to another.  See variable `desktop-save' and function
 `desktop-read' for details."
   :global t
-  :group 'desktop)
+  :group 'desktop
+  (if desktop-save-mode
+      (desktop-auto-save-set-timer)
+    (desktop-auto-save-cancel-timer)))
 
 (defun desktop-save-mode-off ()
   "Disable `desktop-save-mode'.  Provided for use in hooks."
@@ -1216,6 +1219,11 @@ (defun desktop-auto-save-set-timer ()
 	  (run-with-idle-timer desktop-auto-save-timeout t
 			       'desktop-auto-save))))
 
+(defun desktop-auto-save-cancel-timer ()
+  (when desktop-auto-save-timer
+    (cancel-timer desktop-auto-save-timer)
+    (setq desktop-auto-save-timer nil)))
+
 ;; ----------------------------------------------------------------------------
 ;;;###autoload
 (defun desktop-revert ()
@@ -1465,10 +1471,9 @@ (add-hook
     (let ((key "--no-desktop"))
       (when (member key command-line-args)
         (setq command-line-args (delete key command-line-args))
-        (setq desktop-save-mode nil)))
+        (desktop-save-mode 0)))
     (when desktop-save-mode
       (desktop-read)
-      (desktop-auto-save-set-timer)
       (setq inhibit-startup-screen t))))
 
 ;; So we can restore vc-dir buffers.