Thanks to Joseph's pointer I changed the default timeout to 0.05 and
ran demo.py. I also added print statements to all the exception code
near the patch.

When downloading a 25MB file (loaded prior to changing the timeout)

I now see:

  In handler socket.error
  timed out
  raising exception

indicating that the timeout exception is re-raised by the existing
code. The "timed out" is from printing the exception (print err).

I applied patch (with a mod to define err) and I get:

  In handler socket.timeout
  timed out

Note that I am not seeing any attempt to send email in either case
(the tracker is configured with MAIL_DEBUG=log.email.txt). So I am not
sure if something else is broken. But if raising the socket.error does
generate email, this patch does seem to stop it.

Bernhard is this a good enough investigation to let the patch proceed?

-- rouilj