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

[Markus Armbruster <armbru@redhat.com>]

Luiz Capitulino <lcapitulino@redhat.com> writes:

> 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.

JSON requires control characters in strings to be escaped.  RFC 4627
section 2.5:

   A string begins and ends with quotation marks.  All Unicode
   characters may be placed within the quotation marks except for the
   characters that must be escaped: quotation mark, reverse solidus, and
   the control characters (U+0000 through U+001F).

>  In practice it doesn't, afaik.

If it doesn't, then it's not proper JSON, is it?