ui/dbus: filter out pending messages when scanout

The "Listener" connection, being private and under the control of the
qemu display, allows for the optimization of discarding pending
intermediary messages when queuing a new scanout. This ensures that the
client receives only the latest scanout update, improving communication
efficiency.

While the current implementation does not provide a mechanism for
clients who may wish to receive all updates, making this behavior
optional could be considered in the future. For now, adopting this new
default behavior accelerates the communication process without a
guarantee of delivering all updates.

The filter is removed when the connection is dropped.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

ui/dbus-listener.c

@@ -83,6 +83,9 @@ struct _DBusDisplayListener {
     egl_fb fb;
 #endif
 #endif
+
+    guint dbus_filter;
+    guint32 out_serial_to_discard;
 };
 
 G_DEFINE_TYPE(DBusDisplayListener, dbus_display_listener, G_TYPE_OBJECT)
@@ -90,6 +93,12 @@ G_DEFINE_TYPE(DBusDisplayListener, dbus_display_listener, G_TYPE_OBJECT)
 static void dbus_gfx_update(DisplayChangeListener *dcl,
                             int x, int y, int w, int h);
 
+static void ddl_discard_pending_messages(DBusDisplayListener *ddl)
+{
+    ddl->out_serial_to_discard = g_dbus_connection_get_last_serial(
+        g_dbus_proxy_get_connection(G_DBUS_PROXY(ddl->proxy)));
+}
+
 #ifdef CONFIG_OPENGL
 static void dbus_scanout_disable(DisplayChangeListener *dcl)
 {
@@ -276,6 +285,8 @@ static void dbus_scanout_dmabuf(DisplayChangeListener *dcl,
         return;
     }
 
+    ddl_discard_pending_messages(ddl);
+
     /* FIXME: add missing x/y/w/h support */
     qemu_dbus_display1_listener_call_scanout_dmabuf(
         ddl->proxy,
@@ -323,6 +334,8 @@ static bool dbus_scanout_map(DBusDisplayListener *ddl)
         return false;
     }
 
+    ddl_discard_pending_messages(ddl);
+
     if (!qemu_dbus_display1_listener_win32_map_call_scanout_map_sync(
             ddl->map_proxy,
             GPOINTER_TO_UINT(target_handle),
@@ -384,6 +397,8 @@ dbus_scanout_share_d3d_texture(
         return false;
     }
 
+    ddl_discard_pending_messages(ddl);
+
     qemu_dbus_display1_listener_win32_d3d11_call_scanout_texture2d(
         ddl->d3d11_proxy,
         GPOINTER_TO_INT(target_handle),
@@ -639,6 +654,8 @@ static void ddl_scanout(DBusDisplayListener *ddl)
         surface_stride(ddl->ds) * surface_height(ddl->ds), TRUE,
         (GDestroyNotify)pixman_image_unref, pixman_image_ref(ddl->ds->image));
 
+    ddl_discard_pending_messages(ddl);
+
     qemu_dbus_display1_listener_call_scanout(
         ddl->proxy, surface_width(ddl->ds), surface_height(ddl->ds),
         surface_stride(ddl->ds), surface_format(ddl->ds), v_data,
@@ -963,6 +980,28 @@ dbus_display_listener_setup_shared_map(DBusDisplayListener *ddl)
 #endif
 }
 
+static GDBusMessage *
+dbus_filter(GDBusConnection *connection,
+            GDBusMessage    *message,
+            gboolean         incoming,
+            gpointer         user_data)
+{
+    DBusDisplayListener *ddl = DBUS_DISPLAY_LISTENER(user_data);
+    guint32 serial;
+
+    if (incoming) {
+        return message;
+    }
+
+    serial = g_dbus_message_get_serial(message);
+    if (serial <= ddl->out_serial_to_discard) {
+        trace_dbus_filter(serial, ddl->out_serial_to_discard);
+        return NULL;
+    }
+
+    return message;
+}
+
 DBusDisplayListener *
 dbus_display_listener_new(const char *bus_name,
                           GDBusConnection *conn,
@@ -987,6 +1026,7 @@ dbus_display_listener_new(const char *bus_name,
         return NULL;
     }
 
+    ddl->dbus_filter = g_dbus_connection_add_filter(conn, dbus_filter, g_object_ref(ddl), g_object_unref);
     ddl->bus_name = g_strdup(bus_name);
     ddl->conn = conn;
     ddl->console = console;

ui/trace-events

@@ -161,6 +161,7 @@ dbus_clipboard_register(const char *bus_name) "peer %s"
 dbus_clipboard_unregister(const char *bus_name) "peer %s"
 dbus_scanout_texture(uint32_t tex_id, bool backing_y_0_top, uint32_t backing_width, uint32_t backing_height, uint32_t x, uint32_t y, uint32_t w, uint32_t h) "tex_id:%u y0top:%d back:%ux%u %u+%u-%ux%u"
 dbus_gl_gfx_switch(void *p) "surf: %p"
+dbus_filter(unsigned int serial, unsigned int filter) "serial=%u (<= %u)"
 
 # egl-helpers.c
 egl_init_d3d11_device(void *p) "d3d device: %p"