DND on GTK - cancel attempt

[Veronika Irvine]

To allow a widget to participate as a drag source I have two options 1) gtk_drag_source_set and 2) gtk_drag_begin.

What I want to be able to do is when the conditions are right for a drag to begin, I want to check if I indeed want to perform a drag (e.g. do I have anything selected to drag).
If I have nothing to drag, I do not want to proceed.

 As far as I can gather, if I use gtk_drag_source_set I will be notified in my drag_begin callback that the conditions for a drag to start have been met.  But I can see no way in the drag_begin callback to abort the operation (in fact calling gdk_drag_abort does not work and there is no return value to indicate failure).  

Alternatively, I can detect that a DND operation may begin and then call gtk_drag_begin but since the user can possibly configure the timing, button used, distance moved etc I really can't do a good job of figuring out when the DND operation may begin.

Is there a better solution?  What am I missing?

Thanks in advance,
Veronika