In the case that the user has no callback for g_dbus_connection_call() then we should do a bit of cleverness and instruct the remote side not to reply to us.

We should do the same for _call() methods on proxies as well, but not for any calls that deal with GDBusMessage directly.  For those methods, the user has direct access to the GDBusMessage and can set the flag for themselves and also the user can see the serial number (so they might be processing the reply in a filter).

We discussed the possibility of instead adding a new GDBusCallFlags to make the setting of the flag more explicit, but after thinking about it, this raises some ugly edge cases: what if the user sets the flag, but gives a callback anyway?

After a discussion of the risks, David and I agree that the best way to proceed is to automatically set the no-reply flag if the callback is NULL.  If this new behaviour causes problems for anybody (and it's hard to imagine how it could) then they can always work around it by giving a do-nothing callback.