[libvirt] Re: [PATCH 2/3] Introduce monitor 'wait' command

[Gerd Hoffmann]

On 04/08/09 16:33, Daniel P. Berrange wrote:
> On Wed, Apr 08, 2009 at 09:16:43AM -0500, Anthony Liguori wrote:
>> The wait command will pause the monitor the command was issued in until a new
>> event becomes available.  Events are queued if there isn't a waiter present.
>> The wait command completes after a single event is available.
>>
>> Today, we queue events indefinitely but in the future, I suspect we'll drop
>> events that are older than a certain amount of time to avoid infinitely
>> allocating memory for long running VMs.
>>
>> To make use of the new notification mechanism, this patch introduces a
>> qemu_notify_event() API.  This API takes three parameters: a class which is
>> meant to classify the type of event being generated, a name which is meant to
>> distinguish which event in the class that has been generated, and a details
>> parameters which is meant to allow events to send arbitrary data with a given
>> event.
>
> Perhaps we should have the ability to turn on/off events, via a 'notify EVENT'
> command, and a way turn off the prompt on the channel used for receiving
> events.

That would nicely solve the "queue events indefinitely" issue.  By 
default no events are generated.  Apps which want receive them (and thus 
receive them) can enable them as needed.

> And then in the 2nd monitor channel, a single 'wait' command would turn
> off the monitor prompt and make the channel dedicated for just events,
> one per line
>
>    (qemu) wait
>    rtc-change UTC+0100
>    vnc-client connect 192.46.12.4:9353
>    vnc-client disconnect 192.46.12.4:9353
>    vnc-client connect 192.46.12.2:9353
>    vnc-client disconnect 192.46.12.2:9353

IMHO this is more useful than having "wait" just get one event.  You'll 
need a dedicated monitor channel for events anyway, so with 
one-event-per-wait the management app would have to issue wait in a loop.

BTW: "wait" is quite generic.  Maybe we should name the commands 
notify-*, i.e. have

   notify-enable $class
   notify-disable $class
   notify-getevents

cheers,
   Gerd