bug#19822: url-retrieve: allow to fail when no document is associated with the URI

[Lars Ingebrigtsen <larsi@gnus.org>]

Ivan Shmakov <ivan@siamics.net> writes:

> The handling of the news: (nntp:), irc:, and (as it seems) ftp:
> (file:) URI schemes is implemented in such a way that a
> /successful/ url-retrieve call is /not/ in fact guaranteed to
> return a “retrieved document” of any kind.  Consider, e. g.:
>
> (let ((url-proxy-services nil))
> (list (url-retrieve "news://news.aioe.org/alt.sources"
> (lambda (&rest any) (message "news: %S" any)))
> (url-retrieve "irc://irc.freenode.net:6667/x-test-channel"
> (lambda (&rest any) (message "irc:  %S" any)))))
>
> Here, the first call starts up Gnus and opens a *Summary* buffer
> for the group; the second starts Rcirc by default; either call
> returns nil.

I think all these non-http{s,} things in the URL library should be
marked obsolete and removed.  There is very little utility to them (some
have been broken for years without anybody noticing), and they are very
fiddly to maintain.

Anybody mind if I remove them for Emacs 25.2?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no