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

[Eli Zaretskii <eliz@gnu.org>]

> From: Christopher Wellons <wellons@nullprogram.com>
> Date: Sat, 21 Mar 2015 16:46:16 -0400
> 
> 
>     ;;; -*- lexical-binding: t; -*-
>     (let ((calls 0))
>       (url-retrieve "http://localhost:1/"
>                     (lambda (_)
>                       (assert (= 1 (cl-incf calls)))
>                       (kill-buffer))))
> 
> When I eval the above expression, the assertion fails asynchronously.
> 
>     error in process sentinel: Assertion failed: (= 1 (cl-incf calls))
> 
> I expect the callback given to `url-retrieve' to never be invoked more
> than once each time it's passed to `url-retrieve'. Ideally I expect it
> to be called exactly once for each call to `url-retrieve', always
> delivering errors asynchronously.
> 
> I also tested this on a Windows build of Emacs. The callback is invoked
> zero times because the connection error is delivered synchronously at
> the call to `url-retrieve' (which I now realize explains why connection
> queuing doesn't work right on Windows). So even if the double-invoke
> problem is fixed for Linux, the behavior still differs across platforms.

I don't understand what you mean by "error is delivered synchronously
at the call", and by "connection queuing doesn't work right".  Please
consider elaborating on those 2 points.

Thanks.