layers: arrange layers on destroy

...rather than unmap because that is the approach of sway and it also
avoids wshowkeys crashing.

Fixes: #1153

include/layers.h

@@ -10,6 +10,8 @@ struct output;
 struct lab_layer_surface {
 	struct wlr_scene_layer_surface_v1 *scene_layer_surface;
 	struct server *server;
+	struct output *output;
+	struct wlr_scene_tree *tree;
 
 	bool mapped;
 

src/layers.c

@@ -43,6 +43,13 @@ arrange_one_layer(const struct wlr_box *full_area, struct wlr_box *usable_area,
 {
 	struct wlr_scene_node *node;
 	wl_list_for_each(node, &tree->children, link) {
+		/*
+		 * Could be null during destruction when called from
+		 * handle_node_destroy()
+		 */
+		if (!node->data) {
+			continue;
+		}
 		struct lab_layer_surface *surface = node_layer_surface_from_node(node);
 		struct wlr_scene_layer_surface_v1 *scene = surface->scene_layer_surface;
 		if (!!scene->layer_surface->current.exclusive_zone != exclusive) {
@@ -101,6 +108,7 @@ handle_output_destroy(struct wl_listener *listener, void *data)
 	struct lab_layer_surface *layer =
 		wl_container_of(listener, layer, output_destroy);
 	layer->scene_layer_surface->layer_surface->output = NULL;
+	layer->output = NULL;
 	wlr_layer_surface_v1_destroy(layer->scene_layer_surface->layer_surface);
 }
 
@@ -196,6 +204,16 @@ handle_node_destroy(struct wl_listener *listener, void *data)
 	 * focus to.
 	 */
 
+	/*
+	 * Nullify node descriptor early to avoid functions such as
+	 * arrange_one_layer() trying to dereference it.
+	 */
+	layer->tree->node.data = NULL;
+
+	if (layer->output) {
+		output_update_usable_area(layer->output);
+	}
+
 	wl_list_remove(&layer->map.link);
 	wl_list_remove(&layer->unmap.link);
 	wl_list_remove(&layer->surface_commit.link);
@@ -211,9 +229,6 @@ handle_unmap(struct wl_listener *listener, void *data)
 	struct lab_layer_surface *layer = wl_container_of(listener, layer, unmap);
 	struct wlr_layer_surface_v1 *layer_surface =
 		layer->scene_layer_surface->layer_surface;
-	if (layer_surface->output) {
-		output_update_usable_area(layer_surface->output->data);
-	}
 	struct seat *seat = &layer->server->seat;
 	if (seat->focused_layer == layer_surface) {
 		seat_set_focus_layer(seat, NULL);
@@ -376,6 +391,7 @@ handle_new_layer_surface(struct wl_listener *listener, void *data)
 	struct lab_layer_surface *surface = znew(*surface);
 
 	struct output *output = layer_surface->output->data;
+	surface->output = output;
 
 	struct wlr_scene_tree *selected_layer =
 		output->layer_tree[layer_surface->current.layer];
@@ -387,6 +403,7 @@ handle_new_layer_surface(struct wl_listener *listener, void *data)
 		wlr_log(WLR_ERROR, "could not create layer surface");
 		return;
 	}
+	surface->tree = surface->scene_layer_surface->tree;
 
 	node_descriptor_create(&surface->scene_layer_surface->tree->node,
 		LAB_NODE_DESC_LAYER_SURFACE, surface);

src/node.c

@@ -11,7 +11,7 @@ descriptor_destroy(struct node_descriptor *node_descriptor)
 		return;
 	}
 	wl_list_remove(&node_descriptor->destroy.link);
-	free(node_descriptor);
+	zfree(node_descriptor);
 }
 
 static void