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

[Jamie Lokier <jamie@shareable.org>]

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.

> >If async messages can only be received as a result of commands which
> >trigger individual messages, that will be achieved.
> >
> >But it would be a nice little bonus if disabling async messages caused
> >all slow commands to be synchronous - that is, the async result
> >message becomes the command's synchronous result.
> 
> I know what you're getting at but I think it's the wrong target for 
> QMP.  The goal is not to allow simple, hacky clients, but to provide a 
> robust management API.  It should not be difficult to use, but parsing 
> it in a shell with awk is not a requirement in my mind :-)

I agree - not a requirement.  Just seemed nice if that turned out to
be a natural thing to do anyway.

-- Jamie