Re: [Qemu-devel] Confused by QOM: /machine/unattached/device[5]/dr-connector[255]/fdt

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/09/2015 18:04, Andreas Färber wrote:
>> > Should qom-list really fail DeviceNotFound?  I find it rather confusing.
>> > For what it's worth, attempting to read a directory fails with EISDIR,
>> > not ENOENT.
> Listing a non-existing directory on my system results in:
> 
>   ls: cannot access foo: No such file or directory

This is more like

$ ls vl.c/*
ls: cannot access vl.c/*: Not a directory

So it's ENOTDIR.  Not really DeviceNotFound, but perhaps close enough.

FWIW, "struct" isn't a well-defined QAPI name.  The type probably should
be changed to "any" (right?).

> > Moving on to my next confusion: qom-get.
> > 
> >     QMP> { "execute": "qom-get", "arguments": { "path": "/machine/unattached/device[5]/dr-connector[255]", "property": "fdt" } }
> >     {"return": {}}
> > 
> > The return value {} is weird.  Let's see where it comes from.
> > 
> > qmp_qom_get() first calls object_resolve_path() on the path, then
> > object_property_get_qobject() on the property.  For our test case,
> > object_resolve_path() succeeds.  Now have a closer look at
> > object_property_get_qobject()'s behavior:
> > 
> >     QObject *object_property_get_qobject(Object *obj, const char *name,
> >                                          Error **errp)
> >     {
> >         QObject *ret = NULL;
> >         Error *local_err = NULL;
> >         QmpOutputVisitor *mo;
> > 
> >         mo = qmp_output_visitor_new();
> >         object_property_get(obj, qmp_output_get_visitor(mo), name, &local_err);
> > 
> > This call succeeds, and we enter the conditional.
> > 
> >         if (!local_err) {
> >             ret = qmp_output_get_qobject(mo);
> > 
> > This call returns NULL.
> > 
> > Function returns NULL without setting an error.  Its function comment:
> > 
> > /*
> >  * object_property_get_qobject:
> >  * @obj: the object
> >  * @name: the name of the property
> >  * @errp: returns an error if this function fails
> >  *
> >  * Returns: the value of the property, converted to QObject, or NULL if
> >  * an error occurs.
> >  */
> > 
> > Is returning NULL without setting an error okay?
> > 
> > Should it return qnull() instead?  Then the QMP return value would be
> > JSON null.

JSON null support in QObject is new, it should be the result of
object_property_get_qobject() or even qmp_output_get_qobject().  Needs
auditing the code.

Paolo