wayland: Handle after-paint invocations when nothing was painted

If a after-paint was scheduled but nothing was painted, for example when
the it was scheduled by a subsurface wanting to update its position,
we'd still try to read back from the backfill cairo surface and update
the committed cairo surface reference even though no buffer was
attached.

Fix this by adding a new state, 'pending_buffer_attached', which is only
true if a buffer was attached during frame. Only when this is true will
the backfill be read back and the committed cairo surface reference be
updated.

https://bugzilla.gnome.org/show_bug.cgi?id=762120

1 files changed, 12 insertions, 6 deletions

diff --git a/gdk/wayland/gdkwindow-wayland.c b/gdk/wayland/gdkwindow-wayland.c
index abc390c066..bafeb715d8 100644
--- a/gdk/wayland/gdkwindow-wayland.c
+++ b/gdk/wayland/gdkwindow-wayland.c
@@ -113,6 +113,7 @@ struct _GdkWindowImplWayland
 
   unsigned int mapped : 1;
   unsigned int use_custom_surface : 1;
+  unsigned int pending_buffer_attached : 1;
   unsigned int pending_commit : 1;
   unsigned int awaiting_frame : 1;
   unsigned int position_set : 1;
@@ -482,10 +483,6 @@ on_frame_clock_after_paint (GdkFrameClock *clock,
   if (!impl->pending_commit)
     return;
 
-  impl->pending_commit = FALSE;
-  impl->pending_frame_counter = gdk_frame_clock_get_frame_counter (clock);
-  impl->awaiting_frame = TRUE;
-
   callback = wl_surface_frame (impl->display_server.wl_surface);
   wl_callback_add_listener (callback, &frame_listener, window);
   _gdk_frame_clock_freeze (clock);
@@ -493,7 +490,8 @@ on_frame_clock_after_paint (GdkFrameClock *clock,
   /* Before we commit a new buffer, make sure we've backfilled
    * undrawn parts from any old committed buffer
    */
-  read_back_cairo_surface (window);
+  if (impl->pending_buffer_attached)
+    read_back_cairo_surface (window);
 
   /* From this commit forward, we can't write to the buffer,
    * it's "live".  In the future, if we need to stage more changes
@@ -504,7 +502,14 @@ on_frame_clock_after_paint (GdkFrameClock *clock,
    * use it again.
    */
   wl_surface_commit (impl->display_server.wl_surface);
-  impl->committed_cairo_surface = g_steal_pointer (&impl->staging_cairo_surface);
+
+  if (impl->pending_buffer_attached)
+    impl->committed_cairo_surface = g_steal_pointer (&impl->staging_cairo_surface);
+
+  impl->pending_buffer_attached = FALSE;
+  impl->pending_commit = FALSE;
+  impl->pending_frame_counter = gdk_frame_clock_get_frame_counter (clock);
+  impl->awaiting_frame = TRUE;
 
   g_signal_emit (impl, signals[COMMITTED], 0);
 }
@@ -640,6 +645,7 @@ gdk_wayland_window_attach_image (GdkWindow *window)
   if (display->compositor_version >= WL_SURFACE_HAS_BUFFER_SCALE)
     wl_surface_set_buffer_scale (impl->display_server.wl_surface, impl->scale);
 
+  impl->pending_buffer_attached = TRUE;
   impl->pending_commit = TRUE;
 }