Re: [Qemu-devel] [PATCH v3 07/18] qapi: Add json output visitor

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 05/04/2016 09:45 AM, Markus Armbruster wrote:
>>>>>> +void json_output_visitor_reset(JsonOutputVisitor *v);
>>>>>
>>>>> Hmm.  Why is "reset" not a Visitor method?
>>>>>
>>>>> I think this would let us put the things enforced by your "qmp: Tighten
>>>>> output visitor rules" in the Visitor contract.
>>>>
>>>> I thought about that, and now that you've mentioned it, I'll probably
>>>> give it a try (that is, make visit_reset() a top-level construct that
>>>> ALL visitors must support, rather than just qmp-output and json-output).
>>>
>>> Yes, please.
>> 
>> Same question for "cleanup".  Stupid name for a destructor, by the way.
>
> Interface question - all of the FOO_visitor_new() functions return a
> subtype pointer Foo*, rather than Visitor*; along with a Visitor
> *FOO_get_visitor(FOO*) for up-casting, so that FOO* can be used in the
> per-type cleanup function; the FOO* pointers are also useful for two
> additional output functions in the two output visitors.  We're proposing
> hiding the per-type cleanup function behind a simpler visit_free(Visitor
> *v).  So all that's left are the two output functions.  Can we get rid
> of those, and make Visitor* the only public interface, rather than
> making every caller have to do upcasts?

The two output functions are

    QObject *qmp_output_get_qobject(QmpOutputVisitor *v);
    char *string_output_get_string(StringOutputVisitor *v);

> It looks like outside of the testsuite, all uses of these visitors are
> local to a single function; and improving the testsuite is not the end
> of the world.  Particularly if only the testsuite is using reset, it may
> be easier to just patch the testsuite to use a new visitor in the places
> where it currently does a reset.

I'm okay with replacing reset by destroy + new in the test suite.

>                                   How ugly would it be to require that
> the caller pass in a pointer to the result as part of creating the
> visitor, with the promise that the result is populated at the end of a
> successful visit, and left NULL if the visit is reset early?  Or maybe a
> visit_complete() function that is a no-op on input visitors, but
> populates the parameter passed at creation for output visitors.  If we
> do that, we could rewrite things like the existing:
>
> QObject *object_property_get_qobject(Object *obj, const char *name,
>                                      Error **errp)
> {
>     QObject *ret = NULL;
>     Error *local_err = NULL;
>     QmpOutputVisitor *qov;
>
>     qov = qmp_output_visitor_new();
>     object_property_get(obj, qmp_output_get_visitor(qov), name, &local_err);
>     if (!local_err) {
>         ret = qmp_output_get_qobject(qov);
>     }
>     error_propagate(errp, local_err);
>     qmp_output_visitor_cleanup(qov);
>     return ret;
> }
>
> to instead be:
>
> QObject *object_property_get_qobject(Object *obj, const char *name,
>                                      Error **errp)
> {
>     QObject *ret = NULL;
>     Error *local_err = NULL;
>     Visitor *v;
>
>     v = qmp_output_visitor_new(&ret);
>     object_property_get(obj, v, name, &local_err);
>     if (!local_err) {
>         visit_complete(v); /* populates ret */
>     }
>     error_propagate(errp, local_err);
>     visit_free(v);
>     return ret;
> }
>
> Slightly shorter, but populating 'ret' at a distance feels a bit weird.

I like not having to deal with the QmpOutputVisitor type, but like you,
I don't like action at a distance.

>  Maybe we need to keep the FOO_new() functions returning FOO* rather
> than Visitor*, along with the FOO_get_visitor() functions, after all.

I can think of a other ways to hide the QmpOutputVisitor type, but they
have drawbacks, too.

You can let the two output functions take Visitor *.  Drawback: now the
compiler lets you pass the wrong kind of visitor.

You can let visit_complete() return the output (if any) as void *.
Drawback: now the compiler lets you misinterpret the output's type.