Re: [Qemu-devel] [PATCH 07/11] qapi: add json output visitor

[Eric Blake <eblake@redhat.com>]

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On 12/10/2015 04:53 PM, Eric Blake wrote:
> We have several places that want to go from qapi to JSON; right now,
> they have to create an intermediate QObject to do the work.  That
> also has the drawback that the JSON formatting of a QDict will
> rearrange keys (according to a deterministic, but unpredictable,
> hash), when humans have an easier time if dicts are produced in
> the same order as the qapi type.
> 
> For these reasons, it is time to add a new JSON output visitor.
> This patch just adds the basic visitor and tests that it works;
> later patches will add pretty-printing, and convert clients to
> use the visitor.
> 
> Design choices: Unlike the QMP output visitor, the JSON visitor
> refuses to visit a required string with a NULL value (abort), as
> well as a non-finite number (raises an error message).  Reusing
> QString to grow the contents means that we easily share code with
> both qobject-json.c and qjson.c; although it might be nice to
> enhance things to take an optional output callback function so
> that the output can truly be streamed instead of collected in
> memory.
> 

> +static void json_output_type_number(Visitor *v, double *obj, const char *name,
> +                                    Error **errp)
> +{
> +    JsonOutputVisitor *jov = to_jov(v);
> +    json_output_name(jov, name);
> +    qstring_append_json_number(jov->str, *obj, errp);
> +}

Hmm. The generated qapi callers always pass a non-NULL local errp, and
fail the overall iteration even if the upper-level caller passed NULL.
My intent here was to allow the choice between diagnosing NaN, or
ignoring the problem and producing output anyways; but for that to work,
I may need to add a configuration parameter 'bool strict' at the
creation of the visitor, that determines whether I pass errp or NULL on
to qstring_append_json_number() here.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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