[libvirt] [Qemu-devel] virDomainBlockJobAbort and block_job_cancel

[Daniel Veillard]

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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