The type of the introspect function in GDBusSubtreeVTable
Subtrees are flat. node, if non-NULL, is always exactly one segment of the object path (ie: it never contains a slash).
This function should return NULL to indicate that there is no object at this node.
If this function returns non-NULL, the return value is expected to be a NULL-terminated array of pointers to gio.DBusInterfaceInfo structures describing the interfaces implemented by node. This array will have g_dbus_interface_info_unref() called on each item before being freed with g_free().
The difference between returning NULL and an array containing zero items is that the standard DBus interfaces will returned to the remote introspector in the empty array case, but not in the NULL case.