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Re: [Qemu-devel] [libvirt] virDomainBlockJobAbort and block_job_cancel
From: |
Daniel Veillard |
Subject: |
Re: [Qemu-devel] [libvirt] virDomainBlockJobAbort and block_job_cancel |
Date: |
Thu, 24 Nov 2011 13:31:12 +0800 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Wed, Nov 23, 2011 at 09:04:50AM -0700, Eric Blake wrote:
> On 11/23/2011 07:48 AM, Stefan Hajnoczi wrote:
> > This means that virDomainBlockJobAbort() returns to the client without
> > a guarantee that the job has completed. If the client enumerates jobs
> > it may still see a job that has not finished cancelling. The client
> > must register a handler for the BLOCK_JOB_CANCELLED event if it wants
> > to know when the job really goes away. The BLOCK_JOB_CANCELLED event
> > has the same fields as the BLOCK_JOB_COMPLETED event, except it lacks
> > the optional "error" message field.
> >
> > The impact on clients is that they need to add a BLOCK_JOB_CANCELLED
> > handler if they really want to wait. Most clients today (not many
> > exist) will be fine without waiting for cancellation.
> >
> > Any objections or thoughts on this?
>
> virDomainBlockJobAbort() thankfully has an 'unsigned int flags'
> argument. For backwards-compatibility, I suggest we use it:
>
> calling virDomainBlockJobAbort(,0) maintains old blocking behavior, and
> we document that blocking until things abort may render the rest of
> interactions with the domain unresponsive.
>
> The new virDomainBlockJobAbort(,VIR_DOMAIN_BLOCK_JOB_ABORT_ASYNC) would
> then implement your new proposed semantics of returning immediately once
> the cancellation has been requested, even if it hasn't been acted on yet.
>
> Maybe you could convince me to swap the flags: have 0 change semantics
> to non-blocking, and a new flag to request blocking, where callers that
> care have to try the flag, and if the flag is unsupported then they know
> they are talking to older libvirtd where the behavior is blocking by
> default, but that's a bit riskier.
Agreed, I would rather not change the current call semantic,
but an ASYNC flag would be a really good addition. We can document
the risk of not using it in the function description and suggest
new applications use ASYNC flag.
Daniel
--
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