screen-cast: Always wait for an update before sending cursor-only frames

With the unthrottled input emission, we ended up often getting the
cursor updates long before any damage had been posted, meaning that if
you moved around the mouse pointer where the mouse had a high enough
refresh rate, we'd effectively stall the screen cast stream by only
sending cursor updates and nothing else.

Fix this by scheduling an update when we get a cursor update, then
sending a cursor-only frame after any damage and relayout has been
processed, but only if there is no queued damage that will cause an
actual repaint.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2393>

src/backends/meta-screen-cast-area-stream-src.c

@@ -44,6 +44,7 @@ struct _MetaScreenCastAreaStreamSrc
 
   gulong position_invalidated_handler_id;
   gulong cursor_changed_handler_id;
+  gulong prepare_frame_handler_id;
 
   guint maybe_record_idle_id;
 };
@@ -174,7 +175,9 @@ static void
 pointer_position_invalidated (MetaCursorTracker           *cursor_tracker,
                               MetaScreenCastAreaStreamSrc *area_src)
 {
-  sync_cursor_state (area_src);
+  ClutterStage *stage = get_stage (area_src);
+
+  clutter_stage_schedule_update (stage);
 }
 
 static void
@@ -185,6 +188,14 @@ cursor_changed (MetaCursorTracker           *cursor_tracker,
   sync_cursor_state (area_src);
 }
 
+static void
+on_prepare_frame (ClutterStage                *stage,
+                  ClutterStageView            *stage_view,
+                  MetaScreenCastAreaStreamSrc *area_src)
+{
+  sync_cursor_state (area_src);
+}
+
 static void
 inhibit_hw_cursor (MetaScreenCastAreaStreamSrc *area_src)
 {
@@ -371,6 +382,10 @@ meta_screen_cast_area_stream_src_enable (MetaScreenCastStreamSrc *src)
         g_signal_connect_after (cursor_tracker, "cursor-changed",
                                 G_CALLBACK (cursor_changed),
                                 area_src);
+      area_src->prepare_frame_handler_id =
+        g_signal_connect_after (stage, "prepare-frame",
+                                G_CALLBACK (on_prepare_frame),
+                                area_src);
       meta_cursor_tracker_track_position (cursor_tracker);
       G_GNUC_FALLTHROUGH;
     case META_SCREEN_CAST_CURSOR_MODE_HIDDEN:
@@ -420,6 +435,8 @@ meta_screen_cast_area_stream_src_disable (MetaScreenCastStreamSrc *src)
                           cursor_tracker);
   g_clear_signal_handler (&area_src->cursor_changed_handler_id,
                           cursor_tracker);
+  g_clear_signal_handler (&area_src->prepare_frame_handler_id,
+                          stage);
 
   g_clear_handle_id (&area_src->maybe_record_idle_id, g_source_remove);
 

src/backends/meta-screen-cast-monitor-stream-src.c

@@ -48,6 +48,7 @@ struct _MetaScreenCastMonitorStreamSrc
 
   gulong position_invalidated_handler_id;
   gulong cursor_changed_handler_id;
+  gulong stage_prepare_frame_handler_id;
 
   guint maybe_record_idle_id;
 };
@@ -269,7 +270,9 @@ static void
 pointer_position_invalidated (MetaCursorTracker              *cursor_tracker,
                               MetaScreenCastMonitorStreamSrc *monitor_src)
 {
-  sync_cursor_state (monitor_src);
+  ClutterStage *stage = get_stage (monitor_src);
+
+  clutter_stage_schedule_update (stage);
 }
 
 static void
@@ -280,6 +283,14 @@ cursor_changed (MetaCursorTracker              *cursor_tracker,
   sync_cursor_state (monitor_src);
 }
 
+static void
+on_prepare_frame (ClutterStage                   *stage,
+                  ClutterStageView               *stage_view,
+                  MetaScreenCastMonitorStreamSrc *monitor_src)
+{
+  sync_cursor_state (monitor_src);
+}
+
 static void
 inhibit_hw_cursor (MetaScreenCastMonitorStreamSrc *monitor_src)
 {
@@ -402,6 +413,7 @@ meta_screen_cast_monitor_stream_src_enable (MetaScreenCastStreamSrc *src)
   MetaMonitorManager *monitor_manager =
     meta_backend_get_monitor_manager (backend);
   MetaCursorTracker *cursor_tracker = meta_backend_get_cursor_tracker (backend);
+  ClutterStage *stage = get_stage (monitor_src);
   MetaScreenCastStream *stream;
 
   stream = meta_screen_cast_stream_src_get_stream (src);
@@ -417,6 +429,10 @@ meta_screen_cast_monitor_stream_src_enable (MetaScreenCastStreamSrc *src)
         g_signal_connect_after (cursor_tracker, "cursor-changed",
                                 G_CALLBACK (cursor_changed),
                                 monitor_src);
+      monitor_src->stage_prepare_frame_handler_id =
+        g_signal_connect_after (stage, "prepare-frame",
+                                G_CALLBACK (on_prepare_frame),
+                                monitor_src);
       meta_cursor_tracker_track_position (cursor_tracker);
       break;
     case META_SCREEN_CAST_CURSOR_MODE_HIDDEN:
@@ -465,6 +481,8 @@ meta_screen_cast_monitor_stream_src_disable (MetaScreenCastStreamSrc *src)
                           cursor_tracker);
   g_clear_signal_handler (&monitor_src->cursor_changed_handler_id,
                           cursor_tracker);
+  g_clear_signal_handler (&monitor_src->stage_prepare_frame_handler_id,
+                          stage);
 
   g_clear_handle_id (&monitor_src->maybe_record_idle_id, g_source_remove);
 

src/backends/meta-screen-cast-virtual-stream-src.c

@@ -42,6 +42,7 @@ struct _MetaScreenCastVirtualStreamSrc
 
   gulong position_invalidated_handler_id;
   gulong cursor_changed_handler_id;
+  gulong prepare_frame_handler_id;
 
   gulong monitors_changed_handler_id;
 };
@@ -127,9 +128,11 @@ sync_cursor_state (MetaScreenCastVirtualStreamSrc *virtual_src)
 
 static void
 pointer_position_invalidated (MetaCursorTracker       *cursor_tracker,
-                              MetaScreenCastVirtualStreamSrc *virtual_src)
+                              MetaScreenCastStreamSrc *src)
 {
-  sync_cursor_state (virtual_src);
+  ClutterStage *stage = stage_from_src (src);
+
+  clutter_stage_schedule_update (stage);
 }
 
 static void
@@ -140,6 +143,14 @@ cursor_changed (MetaCursorTracker              *cursor_tracker,
   sync_cursor_state (virtual_src);
 }
 
+static void
+on_prepare_frame (ClutterStage                   *stage,
+                  ClutterStageView               *stage_view,
+                  MetaScreenCastVirtualStreamSrc *virtual_src)
+{
+  sync_cursor_state (virtual_src);
+}
+
 static void
 inhibit_hw_cursor (MetaScreenCastVirtualStreamSrc *virtual_src)
 {
@@ -220,6 +231,7 @@ init_record_callbacks (MetaScreenCastVirtualStreamSrc *virtual_src)
     meta_backend_get_monitor_manager (backend);
   MetaCursorTracker *cursor_tracker =
     meta_backend_get_cursor_tracker (backend);
+  ClutterStage *stage = stage_from_src (src);
 
   switch (meta_screen_cast_stream_get_cursor_mode (stream))
     {
@@ -232,6 +244,10 @@ init_record_callbacks (MetaScreenCastVirtualStreamSrc *virtual_src)
         g_signal_connect_after (cursor_tracker, "cursor-changed",
                                 G_CALLBACK (cursor_changed),
                                 virtual_src);
+      virtual_src->prepare_frame_handler_id =
+        g_signal_connect_after (stage, "prepare-frame",
+                                G_CALLBACK (on_prepare_frame),
+                                virtual_src);
       G_GNUC_FALLTHROUGH;
     case META_SCREEN_CAST_CURSOR_MODE_EMBEDDED:
     case META_SCREEN_CAST_CURSOR_MODE_HIDDEN:
@@ -284,6 +300,7 @@ meta_screen_cast_virtual_stream_src_disable (MetaScreenCastStreamSrc *src)
   MetaCursorTracker *cursor_tracker = meta_backend_get_cursor_tracker (backend);
   MetaMonitorManager *monitor_manager =
     meta_backend_get_monitor_manager (backend);
+  ClutterStage *stage = stage_from_src (src);
 
   if (virtual_src->hw_cursor_inhibited)
     uninhibit_hw_cursor (virtual_src);
@@ -299,6 +316,8 @@ meta_screen_cast_virtual_stream_src_disable (MetaScreenCastStreamSrc *src)
                           cursor_tracker);
   g_clear_signal_handler (&virtual_src->cursor_changed_handler_id,
                           cursor_tracker);
+  g_clear_signal_handler (&virtual_src->prepare_frame_handler_id,
+                          stage);
 
   g_clear_signal_handler (&virtual_src->monitors_changed_handler_id,
                           monitor_manager);

src/backends/meta-screen-cast-window-stream-src.c

@@ -39,6 +39,7 @@ struct _MetaScreenCastWindowStreamSrc
   gulong screen_cast_window_destroyed_handler_id;
   gulong position_invalidated_handler_id;
   gulong cursor_changed_handler_id;
+  gulong prepare_frame_handler_id;
 
   gboolean cursor_bitmap_invalid;
 };
@@ -59,6 +60,12 @@ get_backend (MetaScreenCastWindowStreamSrc *window_src)
   return meta_screen_cast_get_backend (screen_cast);
 }
 
+static ClutterStage *
+get_stage (MetaScreenCastWindowStreamSrc *window_src)
+{
+  return CLUTTER_STAGE (meta_backend_get_stage (get_backend (window_src)));
+}
+
 static MetaScreenCastWindowStream *
 get_window_stream (MetaScreenCastWindowStreamSrc *window_src)
 {
@@ -320,6 +327,7 @@ meta_screen_cast_window_stream_src_stop (MetaScreenCastWindowStreamSrc *window_s
   MetaScreenCastStream *stream = meta_screen_cast_stream_src_get_stream (src);
   MetaBackend *backend = get_backend (window_src);
   MetaCursorTracker *cursor_tracker = meta_backend_get_cursor_tracker (backend);
+  ClutterStage *stage = get_stage (window_src);
 
   if (!window_src->screen_cast_window)
     return;
@@ -332,6 +340,8 @@ meta_screen_cast_window_stream_src_stop (MetaScreenCastWindowStreamSrc *window_s
                           cursor_tracker);
   g_clear_signal_handler (&window_src->cursor_changed_handler_id,
                           cursor_tracker);
+  g_clear_signal_handler (&window_src->prepare_frame_handler_id,
+                          stage);
 
   switch (meta_screen_cast_stream_get_cursor_mode (stream))
     {
@@ -380,7 +390,9 @@ static void
 pointer_position_invalidated (MetaCursorTracker             *cursor_tracker,
                               MetaScreenCastWindowStreamSrc *window_src)
 {
-  sync_cursor_state (window_src);
+  ClutterStage *stage = get_stage (window_src);
+
+  clutter_stage_schedule_update (stage);
 }
 
 static void
@@ -391,12 +403,21 @@ cursor_changed (MetaCursorTracker             *cursor_tracker,
   sync_cursor_state (window_src);
 }
 
+static void
+on_prepare_frame (ClutterStage                  *stage,
+                  ClutterStageView              *stage_view,
+                  MetaScreenCastWindowStreamSrc *window_src)
+{
+  sync_cursor_state (window_src);
+}
+
 static void
 meta_screen_cast_window_stream_src_enable (MetaScreenCastStreamSrc *src)
 {
   MetaScreenCastWindowStreamSrc *window_src =
     META_SCREEN_CAST_WINDOW_STREAM_SRC (src);
   MetaBackend *backend = get_backend (window_src);
+  ClutterStage *stage = get_stage (window_src);
   MetaCursorTracker *cursor_tracker = meta_backend_get_cursor_tracker (backend);
   MetaWindowActor *window_actor;
   MetaScreenCastStream *stream;
@@ -433,6 +454,10 @@ meta_screen_cast_window_stream_src_enable (MetaScreenCastStreamSrc *src)
         g_signal_connect_after (cursor_tracker, "cursor-changed",
                                 G_CALLBACK (cursor_changed),
                                 window_src);
+      window_src->prepare_frame_handler_id =
+        g_signal_connect_after (stage, "prepare_frame",
+                                G_CALLBACK (on_prepare_frame),
+                                window_src);
       meta_cursor_tracker_track_position (cursor_tracker);
       break;
     case META_SCREEN_CAST_CURSOR_MODE_HIDDEN: