Re: [Qemu-devel] [PATCH 08/11] QMP: Asynchronous messages enable/disable support

[Luiz Capitulino <lcapitulino@redhat.com>]

On Sun, 24 Jan 2010 18:35:14 +0000
Jamie Lokier <jamie@shareable.org> wrote:

> Anthony Liguori wrote:
> > >I'd like to be able to connect and be sure not to receive any async
> > >messages, from simple scripts with simple output parsing.
> > 
> > You can't have simple output parsing with QMP.  You need a full JSON 
> > stack.  The simplest script would be a python script that uses the 
> > builtin json support.  Having async messages means that you'll have to 
> > loop on recv in order make sure that the response is a command response 
> > vs. an async message.  It's just a few lines more of code so I have a 
> > hard time believing it's really a problem.
> > 
> > But what you probably want is a python QMP library and that would mean 
> > you wouldn't need the few more lines of code.
> 
> You're right.  To be honest, parsing JSON can be done in a single Perl
> regexp; a "full JSON stack" isn't much.
> 
> On that note, it'd be good if the end of a QMP message is framed with
> something that can't appear inside the JSON value, without having to
> parse the JSON incrementally on each partial read().  There are plenty
> of short character sequences to choose from that can't appear.  Some
> JSON parsers expect a whole well-formed expression or throw a parse
> error - it's what people do over HTTP after all.  So you wait until
> you think you've read a whole one, then pass it to the JSON parser.

 The Monitor sends a CRLF sequence at the end of each line and I have
maintained that behaivor for QMP, but it's hard to _guarantee_ that this
sequence won't appear inside a json-string.

 In practice it doesn't, afaik.