url-retrieve may cause hang

[David Reitter <david.reitter@gmail.com>]

url-retrieve may cause Emacs to hang for around 3 minutes when the  
firewall is configured to delay packets. This is unwanted behavior,  
as url-retrieve claims to work asynchronously.

Example:

Configure firewall to delay (rather than deny) packets:

sudo ipfw add 1 pipe 7 tcp from any to any 80

(url-retrieve  "http://www.google.com" 'print)
% or use url-http for this

-> Hang until time-out (several minutes).

You may have to install and/or turn on the ipfw firewall on your  
system to reproduce.

I can produce this with an up-to-date CVS Emacs (Carbon port) on OS  
X, a BSD like system that comes with the "ipfw" firewall - I don't  
have root privileges on my GNU/Linux machine to try it out there.

You may rightfully argue that the above way to set up a firewall is  
flawed. However, I keep getting bug reports from people inside the  
".mil" domain who seem to have a firewall that swallows all outgoing  
traffic. I don't know what their firewall setup is, but the above  
rule leads to the same symptoms for me.


PS.:
sudo ipfw del 1 pipe 7 tcp from any to any 80
to get rid of the block.