bug#20159: 24.4; url-retrieve invokes same callback twice with kill-buffer

[Eli Zaretskii]

> Date: Fri, 27 Mar 2015 22:22:31 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 20159@debbugs.gnu.org
> 
> > From: Andy Moreton <andrewjmoreton@gmail.com>
> > Date: Fri, 27 Mar 2015 16:59:39 +0000
> > 
> > > No one volunteered, so I did it myself, see commit d133cf8 on master.
> > 
> > This change broke gnus for me on a mingw64 x64 build of emacs, as gnutls
> > seems unable to connect to the NNTP server at gmane.org.
> 
> Sorry about that.  Does Gnus do something windows-specific for
> connecting?  Perhaps that specific treatment relied on something that
> is no longer true on Windows.

Another, perhaps more probable culprit is the emacs_gnutls_pull
function.  It calls sys_read, where I made some changes to support the
non-blocking connect.  Perhaps emacs_gnutls_pull cannot cope with some
results of those changes, and itself needs to be amended.