calling vif hotplug scripts more than once

[Olaf Hering <olaf@aepfle.de>]

There is a regression in libxl in its handling of hotplug scripts. Its
not clear to me how to solve it.

If in a Windows VM the network interface is disabled and reenabled in
the "Network and Sharing Center", the backend remains in state 2 and the
vif is not reconnected to the bridge.  With xend all that was done via
udev. If run_hotplug_scripts in /etc/xen/xl.conf is set to 0 it works as
well.

Looking through libxl it appears that libxl__device_nic_add ->
libxl__wait_device_connection -> device_backend_callback ->
device_hotplug does all the work. Unfortunately, device_backend_callback
unregisters the watch, so no further events will be received. It seems
device_backend_callback should call device_hotplug, and then register
yet another watch to wait for state changes so that it eventually
catches such a disable/enable event from the guest.

Since all that is generic code, it seems this use case was not
considered. It looks like a one-shot approach to me.

I'm not sure how to fix this. Should device_backend_callback call into
libxl__wait_device_connection again to wait (forever) for the next 'state==2'?


Olaf