Re: [Qemu-devel] [PATCH v11 25/28] qapi: Simplify visits of optional fields

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 11/12/2015 08:11 AM, Markus Armbruster wrote:
>> Eric Blake <address@hidden> writes:
>> 
>>> None of the visitor callbacks would set an error when testing
>>> if an optional field was present; make this part of the interface
>>> contract by eliminating the errp argument.  Then, for less code,
>>> reflect the determined boolean value back to the caller instead
>>> of making the caller read the boolean after the fact.
>>>
>>> The resulting generated code has a nice diff:
>>>
>>> |-    visit_optional(v, &has_fdset_id, "fdset-id", &err);
>>> |-    if (err) {
>>> |-        goto out;
>>> |-    }
>>> |-    if (has_fdset_id) {
>>> |+    if (visit_optional(v, &has_fdset_id, "fdset-id")) {
>>> |         visit_type_int(v, &fdset_id, "fdset-id", &err);
>>> |         if (err) {
>>> |             goto out;
>>> |         }
>>> |     }
>> 
>> Any particular reason not to do
>> 
>>         has_fdset_id = visit_optional(v, "fdset-id");
>>         if (has_fdset_id) {
>
> We can't. Output visitors do not implement visit_optional() callbacks,
> but must rely on the incoming value of has_fdset_id.  Which means
> assigning to has_fdset_id without an incoming value will do the wrong
> thing.  Or worded differently, &has_fdset_id is modified as an output
> parameter by input visitors, and read unchanged as an input parameter by
> output visitors.

I guess I'd limit myself to just dropping the useless error checking
then.  Results in

    visit_optional(v, &has_fdset_id, "fdset-id")
    if (has_fdset_id) {

which is slightly more verbose but also slightly more obvious.  But it's
all in generated code, so it doesn't really matter much either way.

>>> +++ b/qapi/qapi-visit-core.c
>>> @@ -73,12 +73,12 @@ void visit_end_union(Visitor *v, bool data_present, 
>>> Error **errp)
>>>      }
>>>  }
>>>
>>> -void visit_optional(Visitor *v, bool *present, const char *name,
>>> -                    Error **errp)
>>> +bool visit_optional(Visitor *v, bool *present, const char *name)
>>>  {
>>>      if (v->optional) {
>>> -        v->optional(v, present, name, errp);
>>> +        v->optional(v, present, name);
>>>      }
>>> +    return *present;
>>>  }
>> 
>> Slightly ugly: struct Visitor method optional returns void, but the
>> wrapper returns bool.
>
> I could make all the callbacks (all 3 of them: opts-visitor,
> qmp-input-visitor, string-input-visitor) return bool, but didn't see the
> point in the churn, especially since the contract of the return value is
> easy to do in the wrapper.