Re: [Qemu-devel] [PATCH] qmp: fix object-add assert() without props

[Markus Armbruster <armbru@redhat.com>]

Marc-André Lureau <marcandre.lureau@redhat.com> writes:

> Since commit ad739706bbadee49, user_creatable_add_type() expects to be
> given a qdict. However, if object-add is called without props, you reach
> the assert: "qemu/qom/object_interfaces.c:115: user_creatable_add_type:
> Assertion `qdict' failed.", because the qdict isn't created in this
> case (it's optional).
>
> Furthermore, qmp_input_visitor_new() is not meant to be called without a
> dict, and a further commit will assert in this situation.
>
> If none given, create an empty qdict in qmp to avoid the
> user_creatable_add_type() assert(qdict).
>
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  qmp.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/qmp.c b/qmp.c
> index 6733463..8078038 100644
> --- a/qmp.c
> +++ b/qmp.c
> @@ -665,7 +665,7 @@ void qmp_add_client(const char *protocol, const char *fdname,
>  void qmp_object_add(const char *type, const char *id,
>                      bool has_props, QObject *props, Error **errp)
>  {
> -    const QDict *pdict = NULL;
> +    QDict *pdict;
>      Visitor *v;
>      Object *obj;
>  
> @@ -675,14 +675,19 @@ void qmp_object_add(const char *type, const char *id,
>              error_setg(errp, QERR_INVALID_PARAMETER_TYPE, "props", "dict");
>              return;
>          }
> +    } else {
> +        pdict = qdict_new();
>      }
>  
> -    v = qmp_input_visitor_new(props, true);
> +    v = qmp_input_visitor_new(QOBJECT(pdict), true);
>      obj = user_creatable_add_type(type, id, pdict, v, errp);
>      visit_free(v);
>      if (obj) {
>          object_unref(obj);
>      }
> +    if (!props) {
> +        qobject_decref(QOBJECT(pdict));
> +    }
>  }
>  
>  void qmp_object_del(const char *id, Error **errp)

The reference counting here is needlessly complex.

Case props != NULL:

* The caller holds a reference to props

* pdict = qobject_to_qdict(props) is a weak reference (not counted)

Case props == NULL:

* pdict = qdict_new() is a counted reference

* When it goes out of scope, we need to decrement the reference count.

I recommend to avoid the dual nature of pdict, by converting the weak
reference into a strong one, then decrement the reference count
unconditionally:

    if (props) {
        pdict = qobject_to_qdict(props);
        if (!pdict) {
            error_setg(errp, QERR_INVALID_PARAMETER_TYPE, "props", "dict");
            return;
        }
        QINCREF(pdict);
    } else {
        pdict = qdict_new();
    }

    v = qmp_input_visitor_new(QOBJECT(pdict), true);
    obj = user_creatable_add_type(type, id, pdict, v, errp);
    visit_free(v);
    if (obj) {
        object_unref(obj);
    }
    QDECREF(pdict);