Re: [Qemu-devel] [PATCH 01/13] qapi: Add default-variant for flat unions

[Eric Blake]

On 05/10/2018 08:12 AM, Eric Blake wrote:

Oh, I just had a thought:

> > +++ b/scripts/qapi/visit.py
> > @@ -40,10 +40,20 @@ def gen_visit_object_members(name, base, members, 

> >       if variants:
> > +        if variants.default_tag_value is None:
> > +            ret += mcgen('''
> > +    %(c_name)s = obj->%(c_name)s;
> > +''',
> > +                         c_name=c_name(variants.tag_member.name))
> > +        else:
> > +            ret += mcgen('''
> > +    if (obj->has_%(c_name)s) {
> > +        %(c_name)s = obj->%(c_name)s;
> > +    } else {
> > +        %(c_name)s = %(enum_const)s;

In this branch of code, is it worth also generating:

%has_(c_name)s = true;

That way, the rest of the C code does not have to check 
has_discriminator, because the process of assigning the default will 
ensure that has_discriminator is always true later on.  It does have the 
effect that output would never omit the discriminator - but that might 
be a good thing: if we ever have an output union that used to have a 
mandatory discriminator and want to now make it optional, we don't want 
to break older clients that expected the discriminator to be present. 
It does obscure whether input relied on the default, but I don't think 
that hurts.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org