Testing the newly wrapped Gst::Query classes

[José Alburquerque <jaalburquerque cox net>]

I'm running into something a bit peculiar while testing the newly 
wrapped Gst::Query classes.  To make sure that queries work, I decided 
to modify the ogg player gui version to use the Element::query() method 
(with a Gst::Query) instead of Element::query_position() to get the 
stream position in the PlayerWindow::on_forward() method when the 
forward button is pressed.

The problem arises if I attempt to cast the Glib::RefPtr<> of the 
Gst::Query class to a Glib::RefPtr<> of the derived type 
(Gst::QueryPosition) before "querying the element" (calling 
Gst::Element::query()) in the following way:

void PlayerWindow::on_forward(void)
{
   ...
   Gst::Format fmt = Gst::FORMAT_TIME;

   Glib::RefPtr<Gst::Query> query = Gst::QueryPosition::create(fmt);

   Glib::RefPtr<Gst::QueryPosition> posQuery =
       Glib::RefPtr<Gst::QueryPosition>::cast_dynamic(query);

   if (mainPipeline->query(posQuery))
   {
        ...
   }
}

When the forward button is pressed, gstreamer gives two warnings which 
essentially mean that the Gst::Query is refcounted more than once (I 
found this out by looking at the definition of the IS_MUTABLE macro in 
gststructure.h):

(lt-oggplayer:7134): GStreamer-CRITICAL **: gst_structure_id_set_valist: 
assertion `IS_MUTABLE (structure)' failed

(lt-oggplayer:7134): GStreamer-CRITICAL **: gst_structure_id_set_valist: 
assertion `IS_MUTABLE (structure)' failed

If, on the other hand, I don't do the cast until after the element query 
(in the if block), the warnings are gone which tells me that gstreamer 
has a problem with the Gst::Query being referenced more than once before 
the query is sent (the cast is necessary to "parse" the 
Gst::QueryPosition).  Is this something I should worry about or should 
it be overlooked?  I've checked in the changes to svn with the cast so 
it can be tested.  After, I'll simply place the cast in the if block to 
avoid the warnings.  Suggestions appreciated.  Thanks.

-Jose