bug#16630: desktop-auto-save seems confusing

[Glenn Morris]

Juri Linkov wrote:

>> Why doesn't enabling desktop-save-mode at any time just turn it on?
>> Why does it require the desktop file to exist beforehand?
>
> desktop-auto-save is an noninteractive operation, so first
> you need to select a directory where you want to save the desktop
> using M-x desktop-save or M-x desktop-save-in-desktop-dir
> that asks interactively where to save the desktop.
> This allows desktop-auto-save to know the desktop file name.

emacs -Q
M-x desktop-save-mode RET
M-x desktop-save RET /tmp RET
C-h v desktop-auto-save-timer  
   -> still nil, still no auto-save

As I said, nothing enables desktop-auto-save except after-init-hook.

Whereas if I happen to customize desktop-auto-save-timeout, suddenly it
will start auto-saving. This doesn't make much sense.

(Also, I guess the ignore-errors in the :set of desktop-auto-save-timeout
isn't really needed, rather an :initialize is.)