Re: Questionable aspects of QEMU Error's design

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

01.04.2020 23:15, Peter Maydell wrote:
> On Wed, 1 Apr 2020 at 10:03, Markus Armbruster <armbru@redhat.com> 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?

Yes, after it the code above will look like this:

... some_func(..., errp)
{
     ERRP_AUTO_PROPAGATE(); # magic macro at function start, and no "Error *err" definition

...
           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;
           }
...
}

  - propagation is automatic, errp is used directly and may be safely dereferenced.



-- 
Best regards,
Vladimir