Re: Questionable aspects of QEMU Error's design

[Markus Armbruster <armbru@redhat.com>]

Peter Maydell <peter.maydell@linaro.org> writes:

> 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?

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.

Elsewhere in this thread, I discussed the difficulties of automating the
conversion to this style.  I think I know how to automate converting the
calls to use the bool return value, but converting the functions to
return it looks hard.  We could do that manually for a modest set of
frequently used functions.  object.h would top my list.