Re: Questionable aspects of QEMU Error's design

[Daniel P . Berrangé]

On Thu, Apr 02, 2020 at 07:54:11AM +0200, Markus Armbruster wrote:
> Peter Maydell <address@hidden> writes:
> 
> > On Wed, 1 Apr 2020 at 10:03, Markus Armbruster <address@hidden> wrote:
> >>
> >> QEMU's Error was patterned after GLib's GError.  Differences include:
> >
> > From my POV the major problem with Error as we have it today
> > is that it makes the simple process of writing code like
> > device realize functions horrifically boilerplate heavy;
> > for instance this is from hw/arm/armsse.c:
> >
> >         object_property_set_link(cpuobj, OBJECT(&s->cpu_container[i]),
> >                                  "memory", &err);
> >         if (err) {
> >             error_propagate(errp, err);
> >             return;
> >         }
> >         object_property_set_link(cpuobj, OBJECT(s), "idau", &err);
> >         if (err) {
> >             error_propagate(errp, err);
> >             return;
> >         }
> >         object_property_set_bool(cpuobj, true, "realized", &err);
> >         if (err) {
> >             error_propagate(errp, err);
> >             return;
> >         }
> >
> > 16 lines of code just to set 2 properties on an object
> > and realize it. It's a lot of boilerplate and as
> > a result we frequently get it wrong or take shortcuts
> > (eg forgetting the error-handling entirely, calling
> > error_propagate just once for a whole sequence of
> > calls, taking the lazy approach and using err_abort
> > or err_fatal when we ought really to be propagating
> > an error, etc). I haven't looked at 'auto propagation'
> > yet, hopefully it will help?
> 
> With that, you can have
> 
>         object_property_set_link(cpuobj, OBJECT(&s->cpu_container[i]),
>                                  "memory", errp);
>         if (*errp) {
>             return;
>         }
>         object_property_set_link(cpuobj, OBJECT(s), "idau", errp);
>         if (*errp) {
>             return;
>         }
>         object_property_set_bool(cpuobj, true, "realized", errp);
>         if (*errp) {
>             return;
>         }
> 
> but you have to add
> 
>         ERRP_AUTO_PROPAGATE();
> 
> right at the beginning of the function.
> 
> It's a small improvement.  A bigger one is
> 
>         if (object_property_set_link(cpuobj, OBJECT(&s->cpu_container[i]),
>                                      "memory", errp)) {
>             return;
>         }
>         if (object_property_set_link(cpuobj, OBJECT(s), "idau", errp)) {
>             return;
>         }
>         if (object_property_set_bool(cpuobj, true, "realized", errp)) {
>             return;
>         }
> 
> This is item "Return value conventions" in the message you replied to.

Even better, we can then string the checks together

        if (object_property_set_link(cpuobj, OBJECT(&s->cpu_container[i]),
                                      "memory", errp) ||
            object_property_set_link(cpuobj, OBJECT(s), "idau", errp) ||
            object_property_set_bool(cpuobj, true, "realized", errp)) {
             return;
        }
 
Regards,
Daniel
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