GDBus: Only rewrite serial number if g_dbus_message_get_serial() returns 0

While this a dangerous thing to allow (collissions, reply_serial not
matching up etc.), the added flexibility makes this a good trade-off -
for example, with this feature, it's now a lot easier to build message
routers.

Signed-off-by: David Zeuthen <davidz@redhat.com>

gio/gdbusconnection.c

@@ -1026,7 +1026,11 @@ g_dbus_connection_send_message_unlocked (GDBusConnection   *connection,
   if (blob == NULL)
     goto out;
 
-  serial_to_use = ++connection->priv->last_serial; /* TODO: handle overflow */
+  serial_to_use = g_dbus_message_get_serial (message);
+  if (serial_to_use == 0)
+    {
+      serial_to_use = ++connection->priv->last_serial; /* TODO: handle overflow */
+    }
 
   switch (blob[0])
     {
@@ -1079,8 +1083,11 @@ g_dbus_connection_send_message_unlocked (GDBusConnection   *connection,
  *
  * Asynchronously sends @message to the peer represented by @connection.
  *
- * If @out_serial is not %NULL, then the serial number assigned to
+ * If g_dbus_message_get_serial() returns non-zero for @message, then
- * @message by @connection will be written to this location prior to
+ * that value is used for the message serial number. Otherwise a
+ * serial number will be assigned by @connection and set on @message
+ * via g_dbus_message_set_serial(). If @out_serial is not %NULL, then
+ * the serial number used will be written to this location prior to
  * submitting the message to the underlying transport.
  *
  * If @connection is closed then the operation will fail with
@@ -1392,8 +1399,11 @@ g_dbus_connection_send_message_with_reply_unlocked (GDBusConnection     *connect
  *
  * Asynchronously sends @message to the peer represented by @connection.
  *
- * If @out_serial is not %NULL, then the serial number assigned to
+ * If g_dbus_message_get_serial() returns non-zero for @message, then
- * @message by @connection will be written to this location prior to
+ * that value is used for the message serial number. Otherwise a
+ * serial number will be assigned by @connection and set on @message
+ * via g_dbus_message_set_serial(). If @out_serial is not %NULL, then
+ * the serial number used will be written to this location prior to
  * submitting the message to the underlying transport.
  *
  * If @connection is closed then the operation will fail with
@@ -1525,8 +1535,11 @@ send_message_with_reply_sync_cb (GDBusConnection *connection,
  * timeout is reached. See g_dbus_connection_send_message_with_reply()
  * for the asynchronous version of this method.
  *
- * If @out_serial is not %NULL, then the serial number assigned to
+ * If g_dbus_message_get_serial() returns non-zero for @message, then
- * @message by @connection will be written to this location prior to
+ * that value is used for the message serial number. Otherwise a
+ * serial number will be assigned by @connection and set on @message
+ * via g_dbus_message_set_serial(). If @out_serial is not %NULL, then
+ * the serial number used will be written to this location prior to
  * submitting the message to the underlying transport.
  *
  * If @connection is closed then the operation will fail with

gio/tests/gdbus-connection.c

@@ -618,12 +618,14 @@ test_connection_filter (void)
   while (data.num_handled == 0)
     g_thread_yield ();
 
+  g_dbus_message_set_serial (m, 0);
   g_dbus_connection_send_message (c, m, &data.serial, &error);
   g_assert_no_error (error);
 
   while (data.num_handled == 1)
     g_thread_yield ();
 
+  g_dbus_message_set_serial (m, 0);
   r = g_dbus_connection_send_message_with_reply_sync (c,
                                                       m,
                                                       -1,
@@ -637,6 +639,7 @@ test_connection_filter (void)
 
   g_dbus_connection_remove_filter (c, filter_id);
 
+  g_dbus_message_set_serial (m, 0);
   r = g_dbus_connection_send_message_with_reply_sync (c,
                                                       m,
                                                       -1,