On Tue, 10 Dec 2013 19:15:05 +0100
Paolo Bonzini <pbonz...@redhat.com> wrote:
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> Il 10/12/2013 19:00, Eric Blake ha scritto:
> >>> + 'data': {'qom-type': 'str', 'id': 'str', '*props': 'dict'},
> >>> + 'gen': 'no' }
> >
> > This feels VERY open-coded. No where else in qapi-schema do we
> > have 'dict' as a type
>
> Yes, in fact the "data" field is entirely skipped by the code
> generator (that's 'gen':'no').
>
> > ; using it violates all sorts of type-safety (which, I guess, is
> > the point), making it impossible to introspect what keys are valid
> > for use in the "props":{...} dictionary. Do we really want to
> > play this fast and loose with the type system, or should we try
> > harder to make this a robust self-describing union of types?
> >
> > That is, why can't we have object-add use a discriminated union,
> > where qom-type is the discriminator, and where props is an
> > appropriate JSON struct type that corresponds to the branch of the
> > union, so that we get full introspection on the set of valid keys
> > to put in props for any given qom-type?
>
> The point of "props" is passing arbitrary data to a QOM object. We
> should indeed have introspection for QOM objects, where each QOM class
> name can be introspected separately. However, the union of all
> possible QOM objects need not have a "C struct" representation.
The "props" key was added to represent the "O" argument type of
early QMP (which is used by commands like device_add), so that
we could convert them to the QAPI. IIRC, we didn't plan for it
to be used by new commands... But I don't have anything better
to suggest, so I won't object to its usage here.
