window: destroy the 2nd shm buffer, if both released

Handle the case when we the compositor somehow migrates from requiring
double buffering into working on single buffering, so we release the
extra shm buffer.

Currently, I do not think this can happen in practice, but in the future
it may happen with sub-surfaces.

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

clients/window.c

@@ -807,6 +807,8 @@ shm_surface_leaf_release(struct shm_surface_leaf *leaf)
 
 	if (leaf->resize_pool)
 		shm_pool_destroy(leaf->resize_pool);
+
+	memset(leaf, 0, sizeof *leaf);
 }
 
 struct shm_surface {
@@ -829,17 +831,17 @@ to_shm_surface(struct toysurface *base)
 static void
 shm_surface_buffer_release(void *data, struct wl_buffer *buffer)
 {
-	struct shm_surface_leaf *leaf = data;
+	struct shm_surface *surface = data;
 
-	leaf->busy = 0;
+	if (surface->leaf[0].data->buffer == buffer)
+		surface->leaf[0].busy = 0;
+	else if (surface->leaf[1].data->buffer == buffer)
+		surface->leaf[1].busy = 0;
+	else
+		assert(0 && "shm_surface_buffer_release: unknown buffer");
 
-	/* If both leaves are now free, we should call
-	 * shm_surface_leaf_release(shm_surface::leaf[1]).
-	 * However, none of Weston's backends switch dynamically
-	 * between early buffer release and requiring double-buffering,
-	 * so if both leaves are free, we never used the second
-	 * leaf to begin with.
-	 */
+	if (!surface->leaf[0].busy && !surface->leaf[1].busy)
+		shm_surface_leaf_release(&surface->leaf[1]);
 }
 
 static const struct wl_buffer_listener shm_surface_buffer_listener = {
@@ -908,7 +910,7 @@ shm_surface_prepare(struct toysurface *base, int dx, int dy,
 					   leaf->resize_pool,
 					   &leaf->data);
 	wl_buffer_add_listener(leaf->data->buffer,
-			       &shm_surface_buffer_listener, leaf);
+			       &shm_surface_buffer_listener, surface);
 
 out:
 	surface->current = leaf;