Re: [Qemu-devel] [PATCH] iotests: fix wait_until_completed()

[Peter Xu <peterx@redhat.com>]

On Tue, Apr 03, 2018 at 01:59:18PM +0100, Stefan Hajnoczi wrote:
> On Tue, Mar 27, 2018 at 10:21:55AM +0800, Peter Xu wrote:
> > On Mon, Mar 26, 2018 at 12:47:39PM +0200, Kevin Wolf wrote:
> > > Am 26.03.2018 um 08:11 hat Peter Xu geschrieben:
> > > > If there are more than one events, wait_until_completed() might return
> > > > the 2nd event even if the 1st event is JOB_COMPLETED, since the for loop
> > > > will continue to run even if completed is set to True.
> > > > 
> > > > It never happened before, but it can be triggered when OOB is enabled
> > > > due to the RESUME startup message. Fix that up by removing the boolean
> > > > and make sure we return the correct event.
> > > > 
> > > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > > > ---
> > > >  tests/qemu-iotests/iotests.py | 20 ++++++++------------
> > > >  1 file changed, 8 insertions(+), 12 deletions(-)
> > > > 
> > > > diff --git a/tests/qemu-iotests/iotests.py b/tests/qemu-iotests/iotests.py
> > > > index b5d7945af8..11704e6583 100644
> > > > --- a/tests/qemu-iotests/iotests.py
> > > > +++ b/tests/qemu-iotests/iotests.py
> > > > @@ -470,18 +470,14 @@ class QMPTestCase(unittest.TestCase):
> > > >  
> > > >      def wait_until_completed(self, drive='drive0', check_offset=True):
> > > >          '''Wait for a block job to finish, returning the event'''
> > > > -        completed = False
> > > > -        while not completed:
> > > > -            for event in self.vm.get_qmp_events(wait=True):
> > > > -                if event['event'] == 'BLOCK_JOB_COMPLETED':
> > > > -                    self.assert_qmp(event, 'data/device', drive)
> > > > -                    self.assert_qmp_absent(event, 'data/error')
> > > > -                    if check_offset:
> > > > -                        self.assert_qmp(event, 'data/offset', event['data']['len'])
> > > > -                    completed = True
> > > > -
> > > > -        self.assert_no_active_block_jobs()
> > > > -        return event
> > > > +        for event in self.vm.get_qmp_events(wait=True):
> > > > +            if event['event'] == 'BLOCK_JOB_COMPLETED':
> > > > +                self.assert_qmp(event, 'data/device', drive)
> > > > +                self.assert_qmp_absent(event, 'data/error')
> > > > +                if check_offset:
> > > > +                    self.assert_qmp(event, 'data/offset', event['data']['len'])
> > > > +                self.assert_no_active_block_jobs()
> > > > +                return event
> > > >  
> > > >      def wait_ready(self, drive='drive0'):
> > > >          '''Wait until a block job BLOCK_JOB_READY event'''
> > > 
> > > If an event is pending, but it's not the expected event, won't we return
> > > None now instead of waiting for the BLOCK_JOB_COMPLETED event?
> > 
> > If so, we'll return none.
> 
> Kevin is pointing out that this patch is broken.  Previously the
> function waited for BLOCK_JOB_COMPLETED, even when other events were
> pending when we entered the function.  Now it returns None and does not
> wait for BLOCK_JOB_COMPLETED!

Aha!  Surely I missed that. :)

> 
> > The patch fixes the other case when there
> > are two events: one JOB_COMPLETED plus another (e.g., RESUME) event.
> > When that happens, logically we should return one JOB_COMPLETED event,
> > but the old code will return the other event (e.g., RESUME).
> > 
> > > 
> > > Wouldn't it be much easier to just add a 'break'?
> > 
> > Yes, it's the same.  But IMHO those logics (e.g., the completed
> > variable) are not really needed at all.  This one is simpler.
> 
> No, the outer loop is needed so that the function waits until
> BLOCK_JOB_COMPLETED is received.  It's not possible to do it with a
> single for loop.

Indeed.  But then I would still slightly prefer removing the
"completed" var:

    def wait_until_completed(self, drive='drive0', check_offset=True):
        '''Wait for a block job to finish, returning the event'''
        while True:
            for event in self.vm.get_qmp_events(wait=True):
                if event['event'] == 'BLOCK_JOB_COMPLETED':
                    self.assert_qmp(event, 'data/device', drive)
                    self.assert_qmp_absent(event, 'data/error')
                    if check_offset:
                        self.assert_qmp(event, 'data/offset', event['data']['len'])
                    self.assert_no_active_block_jobs()
                    return event

Or a single break would work too.  Do either of you have any
preference?  I can repost in either way.  Thanks,

-- 
Peter Xu