`Invalid client serial`when nesting proxies

[uninsane]

xdg-dbus-proxy generally fails to proxy messages when its upstream dbus path is another instance of xdg-dbus-proxy instead of the root dbus session.

repro

configure a session bus at /tmp/bus1, a filtering proxy from /tmp/bus2 to that session bus, and then another proxy at /tmp/bus3 to /tmp/bus2:

# start a dbus session (or skip this step if already running a dbus session, and replace /tmp/bus1 with $DBUS_SESSION_BUS_ADDRESS in the later invocations)
$ dbus-daemon --session --address=unix:path=/tmp/bus1
$ xdg-dbus-proxy unix:path=/tmp/bus1 /tmp/bus2 --log --filter '--call=org.freedesktop.portal.Desktop=*' &
$ xdg-dbus-proxy unix:path=/tmp/bus2 /tmp/bus3 --log --filter '--call=org.freedesktop.portal.Desktop=*' &

applications can generally speak to the outer xdg-dbus-proxy at /tmp/bus2, but not the inner proxy at /tmp/bus3:

$ DBUS_SESSION_BUS_ADDRESS=unix:path=/tmp/bus2 busctl --user list
NAME                     PID PROCESS        USER  CONNECTION UNIT              SESSION DESCRIPTION
:1.1                 1277488 xdg-dbus-proxy colin :1.1       session-167.scope 167     -
org.freedesktop.DBus 1277488 xdg-dbus-proxy colin -          session-167.scope 167     -
$ DBUS_SESSION_BUS_ADDRESS=unix:path=/tmp/bus3 busctl --user list
Failed to list names: Transport endpoint is not connected

upon connecting to /tmp/bus3, the xdg-dbus-proxy proxying from /tmp/bus2 to /tmp/bus1 logs:

WARNING **: Invalid client serial: Exceeds maximum value of 4294901759

[uninsane]

4f081c7 looks relevant to this. in this we initialize the proxy with

client->last_fake_serial = MAX_CLIENT_SERIAL; 

then we generate fake messages to the upstream bus like:

  client->last_fake_serial++;
  g_dbus_message_set_serial (message, client->last_fake_serial);
  buffer = message_to_buffer (message);
  queue_outgoing_buffer (&client->bus_side, buffer);

when receiving client messages in got_buffer_from_client (FlatpakProxyClient *client, ProxySide *side, Buffer *buffer), we decode the serial:

header = parse_header (buffer, &error);
if (header->serial > MAX_CLIENT_SERIAL) {
  g_warning ("Invalid client serial: Exceeds maximum value of %u", MAX_CLIENT_SERIAL);
  // and terminate the client 
}  

xdg-dbus-proxy intentionally generates messages above MAX_CLIENT_SERIAL and sends these bus-side. but since it expects to never receive such messages from the client side, xdg-dbus-proxy cannot operate as a client to another xdg-dbus-proxy.