[Twisted-Python] design issue wrt exceptional events

[Dennis Allison]

I need a front-end to a complex of Zope systems.  The front-end listens on
an address/port and examines incoming http transactions, and, based upon
http headers and other information, connects to one or another of the Zope
systems.  The underlying approach is straightforward -- a Listener waits 
for a http transaction, it spawns a receiver which gathers the message, 
decides what to do, spawns a sender to the destination.  When the sender
responds, the process is reversed.  

I currently use an adaptation of Antony Baxter's pydirector to do the job.
I use his asynchat version, but have found that it leaks file descriptors
in my application.  I've not tried the twisted version.

So I thought I'd try to craft a tailored, light-weight version using 
the twisted framework.  I see how to manage things when everything works, 
but what if one or another of the endpoints fails?  How do I get 
everything cleaned up?