Re: [PATCH v2 10/13] qdev: make properties array "const"

[Paolo Bonzini <pbonzini@redhat.com>]

On Tue, Oct 22, 2024 at 6:31 AM Philippe Mathieu-Daudé
<philmd@linaro.org> wrote:
> > -void qdev_property_add_static(DeviceState *dev, Property *prop)
> > +void qdev_property_add_static(DeviceState *dev, const Property *prop)
> >   {
> >       Object *obj = OBJECT(dev);
> >       ObjectProperty *op;
> > @@ -980,7 +980,7 @@ void qdev_property_add_static(DeviceState *dev, Property *prop)
> >                                field_prop_getter(prop->info),
> >                                field_prop_setter(prop->info),
> >                                prop->info->release,
> > -                             prop);
> > +                             (Property *)prop);
>
> I like the overall patch idea, but I'm not keen on casting
> const to non-const. Should we adapt the callee -- here
> object_property_add() -- to also take a const argument?

This argument goes into prop->opaque and is passed to all
accessor/resolver/finalizers functions. So it would be a much larger
change because it needs to change all those functions from "void
*opaque" to "const void *opaque".

It would also be an issue because some finalizers write to the opaque
for good reason:

static void object_finalize_child_property(
    Object *obj, const char *name, void *opaque)
{
    Object *child = opaque;

    if (child->class->unparent) {
        (child->class->unparent)(child);
    }
    child->parent = NULL; // <--- here
    object_unref(child);
}

So, it's not great but it seems necessary. At least keeping the const
within qdev properties makes things "safer" within that realm.

Paolo