Re: [Qemu-devel] [PATCH] Hack qmp.py to support reading a JSON multi-line response

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Aug 20, 2012 at 03:03:11PM -0300, Luiz Capitulino wrote:
> On Mon, 20 Aug 2012 15:01:37 +0100
> "Daniel P. Berrange" <berrange@redhat.com> wrote:
> 
> > From: "Daniel P. Berrange" <berrange@redhat.com>
> > 
> > The qmp-shell code assumes the JSON response is only on a single
> > line. If the QEMU monitor is configured in "pretty print" mode
> > the JSON response can be multi-line. The basic Python JSON APIs
> > do not appear to support a streaming mode, so the simple hack
> > here is to try parsing a line, and if it fails, then read another
> > line, append it, and try parsing again. Keep reading lines until
> > we can successfully parse
> > 
> > Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
> > ---
> >  QMP/qmp.py | 16 ++++++++++++----
> >  1 file changed, 12 insertions(+), 4 deletions(-)
> > 
> > diff --git a/QMP/qmp.py b/QMP/qmp.py
> > index 36ecc1d..464a01a 100644
> > --- a/QMP/qmp.py
> > +++ b/QMP/qmp.py
> > @@ -61,10 +61,18 @@ class QEMUMonitorProtocol:
> >  
> >      def __json_read(self, only_event=False):
> >          while True:
> > -            data = self.__sockfile.readline()
> > -            if not data:
> > -                return
> > -            resp = json.loads(data)
> > +            data = ""
> > +            while True:
> > +                moredata = self.__sockfile.readline()
> > +                if not moredata:
> > +                    return
> > +                data = data + moredata
> > +                try:
> > +                    resp = json.loads(data)
> > +                    break
> > +                except ValueError:
> > +                    pass
> > +
> 
> I'm reluctant about this, because it makes it impossible to detect bad
> json from qemu (not that qmp-shell handles this gracefully today).

Yeah, it is slightly unpleasant.

> At the same time I can't think of anything simpler than your hack. What
> about calling json.loads() when closing } matches the number of opening ones?
> QMP responses are always dictionaries.

How about making multi-line processing an optional feature, by adding
a '-m' flag to qmp-shell. That way we still get  error checking by
default


Daniel
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