Add some extra protection when 'preparing' a group that doesn't yet
contain any menus. This can happen if you subscribe to a group that
doesn't yet exist.
It was possible to crash any application using
g_dbus_connection_export_menu_model() by requesting a non-existent
subscription group over the bus.
In practice this only happened in races -- where the proxy sees a group
that exists and queries it, but by the time it does, it's already gone.
https://bugzilla.gnome.org/show_bug.cgi?id=687089
GBusNameVanishedCallback is called with a NULL GDBusConnection in the
case that the connection has vanished. We were doing an assert to
verify that it was the same as we had exported the menu on and that
assert was failing.
https://bugzilla.gnome.org/show_bug.cgi?id=685995
Give it the same treatment as the exporter for GActionGroup just got.
There is a wart here: the exporter attempt to re-enter GDBusConnection
when it is freed in order to cancel outstanding name watches.
GDBusConnection holds its own lock while calling the destroy notify, so
the attempt at reentrancy results in a deadlock.
We have a workaround to deal with that for now...
There are no public 'exporter' objects, so don't allude to them
in the function names. At the same time, we want to make it clear
that these functions are D-Bus specific.
The new APIs are
g_action_group_dbus_export_start
g_action_group_dbus_export_query
g_action_group_dbus_export_stop
g_menu_model_dbus_export_start
g_menu_model_dbus_export_query
g_menu_model_dbus_export_stop
