[libvirt] [PATCH] events: Don't fail on registering events for two different domains

[Eric Blake]

On 06/27/2012 06:45 AM, Daniel P. Berrange wrote:

>> If we are happy with double registration, then your patch looks correct
>> on the registration side (although it may be incomplete, if we mishandle
>> double firing).  On the other hand, if we want to prevent double
>> registration, then you need to further forbid registration when ((dom &&
>> !cbList->callbacks[i]->dom) || (!dom && cblist->callbacks[i]->dom)).
> 
> You are correct that this patch doesn't actually fix the problem
> of registering handlers for multiple domains. The core issue is
> that libvirtd tracks all registered callbacks, so that it can later
> unregister them. But it only tracks 1 callback per event type. So
> I don't see how this patch could have actually been tested.
> 
> Indeed the wire protocol does not even pass the 'virDomainPtr' over
> the network, so this can't possibly work with a non-NULL domain;

In the case of qemu, there's two layers of handlers:

- libvirt.so registers a local handler (either global or per-domain),
and remembers which user callback to fire
- the remote hypervisor driver then sends an RPC to register a global
handler, but with the callback being a function in the remote driver
- when an event is triggered remotely, the global handler registered via
RPC sends an RPC event back
- libvirt.so then looks at the event (which _does_ have domain
information) and decides whether to fire the local callback(s) based on
whether the local callback is global or has a per-domain match

In other words, I see no reason why the current RPC design shouldn't
work; you can still have both global and per-domain callbacks from the
point of view of the application, even though the RPC side is using only
a single global callback (which is basically installed on a
reference-counting principle - if there is one or more local handlers,
then the global remote handler exists)

-- 
Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org



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