[gtk+] wayland: Handle after-paint invocations when nothing was painted

[Ray Strode <halfline src gnome org>]

commit 64c9ec14fea2a68377f7a9c5297fb24b365bf50e
Author: Jonas Ådahl <jadahl gmail com>
Date:   Tue Feb 16 12:41:57 2016 +0800

    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

 gdk/wayland/gdkwindow-wayland.c |   18 ++++++++++++------
 1 files changed, 12 insertions(+), 6 deletions(-)
---
diff --git a/gdk/wayland/gdkwindow-wayland.c b/gdk/wayland/gdkwindow-wayland.c
index abc390c..bafeb71 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;
 }