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Re: [Libvir] Re: Asynchronous notifications of domain start/stop

Daniel P. Berrange wrote:
Instead we should provide an API which lets application
register callbacks for adding/removing file handles to an event loop.

Understood ...

We in fact already have this - look at the src/event.h file. So if we make
this part of the official public API, then applications merely need provide
an impl of the virEventAddHandleFunc and virEventRemoveHandleFunc. The underlying libvirt drivers can thus invoke these callbacks as required,
be it with the XenStored filehandle, a XenAPI filehandle, a libvirtd fd
or something else.

If I understand what you're saying here, you are proposing that libvirt provides a call like this following (don't worry about specifics):

  virEventDomainNotify (virConnectPtr, int state, void (*f)(...));

The implementation of this call is that it sets up a xenstore watch, and registers the xenstore fd with the previously defined virEventAddHandleFunc (implemented by the main app / app's event loop code).

When the event happens, xenstore fd gets some data available, app's event loop triggers this, and calls into libvirt, which then calls the app's callback function ("f").

Is that right?

In the remote case it's more tricky. What does the remote's callback function do when it gets an event? (This could happen pretty asynchronously relative to other communications going on -- eg. in the middle of a round of TLS chatter). Choices seem to be: (1) Send the data OOB, unencrypted[1]. (2) Piggy-back the notification on to the reply to the next call [not asynchronous]. (3) Try to work out where we are in the TLS message and if necessary queue the event up and wait for a "safe" place to insert the message in the protocol. (3) seems fairly do-able.


[1] TLS 1.1 includes support for OOB data, but it's not really clear if GnuTLS supports that.

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