protocol: set_desync should flush

wl_subsurface.set_desync should apply the cached wl_surface state.
Otherwise, the sub-surface may be stuck: a commit on the parent surface,
if desynchronized, will not commit the sub-surface because it is
desynchronized, too. A commit on the sub-surface may not happen, if it
is waiting for the frame callback from the previous commit.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

protocol/subsurface.xml

@@ -229,6 +229,9 @@
 	Note: even if a sub-surface is set to desynchronized, a parent
 	sub-surface may override it to behave as synchronized. For details,
 	see wl_subsurface.
+
+	If a surface's parent surface behaves as desynchronized, then
+	the cached state is applied on set_desync.
       </description>
     </request>
 

src/compositor.c

@@ -1851,21 +1851,11 @@ weston_subsurface_commit(struct weston_subsurface *sub)
 }
 
 static void
-weston_subsurface_parent_commit(struct weston_subsurface *sub,
-				int parent_is_synchronized)
+weston_subsurface_synchronized_commit(struct weston_subsurface *sub)
 {
 	struct weston_surface *surface = sub->surface;
 	struct weston_subsurface *tmp;
 
-	if (sub->position.set) {
-		weston_surface_set_position(sub->surface,
-					    sub->position.x, sub->position.y);
-		sub->position.set = 0;
-	}
-
-	if (!parent_is_synchronized && !sub->synchronized)
-		return;
-
 	/* From now on, commit_from_cache the whole sub-tree, regardless of
 	 * the synchronized mode of each child. This sub-surface or some
 	 * of its ancestors were synchronized, so we are synchronized
@@ -1881,6 +1871,20 @@ weston_subsurface_parent_commit(struct weston_subsurface *sub,
 	}
 }
 
+static void
+weston_subsurface_parent_commit(struct weston_subsurface *sub,
+				int parent_is_synchronized)
+{
+	if (sub->position.set) {
+		weston_surface_set_position(sub->surface,
+					    sub->position.x, sub->position.y);
+		sub->position.set = 0;
+	}
+
+	if (parent_is_synchronized || sub->synchronized)
+		weston_subsurface_synchronized_commit(sub);
+}
+
 static void
 subsurface_configure(struct weston_surface *surface, int32_t dx, int32_t dy,
 		     int32_t width, int32_t height)
@@ -2050,8 +2054,13 @@ subsurface_set_desync(struct wl_client *client, struct wl_resource *resource)
 {
 	struct weston_subsurface *sub = resource->data;
 
-	if (sub)
+	if (sub && sub->synchronized) {
 		sub->synchronized = 0;
+
+		/* If sub became effectively desynchronized, flush. */
+		if (!weston_subsurface_is_synchronized(sub))
+			weston_subsurface_synchronized_commit(sub);
+	}
 }
 
 static void