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

[Jan Kiszka]

Gerd Hoffmann wrote:
> 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.

Sounds reasonable to me as well.

> 
>> 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.

But doesn't it have to _loop_ anyway? If wait returned multiple events,
the management app would have to loop over the results and then anyway
over the actual wait to get the next chunk - thus twice. To me, one
event per wait invocation looks simpler to handle.

> 
> BTW: "wait" is quite generic.  Maybe we should name the commands
> notify-*, i.e. have
> 
>   notify-enable $class
>   notify-disable $class
>   notify-getevents

My 2 cents:

  event_enable $class1[,...]
  event_disable $class1[,...]

with a special class 'all' and

  event_wait

to finally collect the queued and enabled events. There is just the
question what to do with queued events of a certain class that gets
disabled before the events were dequeued. Purge them selectively or let
the user do this via event_event? I'm not a fan of cleanup via magic
timeouts / event aging.

Jan

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