Re: [Qemu-devel] [PATCH v2 4/5] monitor: add object-add (QMP) and object_add (HMP) command

[Peter Crosthwaite]

On Tue, Dec 17, 2013 at 10:24 PM, Paolo Bonzini <address@hidden> wrote:
> Il 17/12/2013 12:54, Peter Crosthwaite ha scritto:
>>> +    visit_start_struct(opts_get_visitor(ov), &dummy, NULL, NULL, 0, &err);
>>> +    if (error_is_set(&err)) {
>>> +        goto out_clean;
>>> +    }
>>
>> So I have been thinking about repeated if(error_is_set(&err)) { goto
>> foo; } and how to reduce its verbosity in situations like this. Can it
>> be solved with a simple semantic:
>>
>> "Error ** accepting APIs will perform no action if the Error **
>> argument is already set."
>
> I think this is a case where verbosity <<< ease of use.
>
> In this case, the caller code is particularly simple, but what if I
> needed to dereference the return value of the first called function, to
> get the argument to the second?  You would still need an "if".
>

Yes thats right. This isn't going to work universally and callers will
always have the responsibility of knowing whether they can continue or
not. But it will help a lot for repetitive collections of similar
independent functions calls. The ultimate example is probably the
device tree API calls in hw/ppc/e500.c.

I want to patch the device tree API to be nice and Error**ified (for
my own future reasons) but I sure don't want to have to patch e500 to
check every qemu_devtree_foo API call with these 3 LOC. TBH i'll
probably just preserve current behavior using &error_abort in next rev
of that series, but it should be possible to do less verbose
caller-customized collective error handling in some way.

Regards,
Peter