Fix scratchpad fullscreen behavior and crash

When setting fullscreen on a hidden scratchpad container, there was a
check to see if there was an existing fullscreen container on the
workspace so it could be fullscreen disabled first. Since the workspace
is NULL, it would cause a SIGSEGV. This adds a NULL check to avoid the
crash.

This also changes the behavior of how fullscreen is handled when adding
a container to the scratchpad or changing visibility of a scratchpad
container to match i3's. The behavior is as follows:
- When adding a container to the scratchpad or hiding a container back
  into the scratchpad, there is an implicit fullscreen disable
- When setting fullscreen on a container that is hidden in the
  scratchpad, it will be fullscreen when shown (and fullscreen disabled
  when hidden as stated above)
- When setting fullscreen global on a container that is hidden in the
  scratchpad, it will be shown immediately as fullscreen global. The
  container is not moved to a workspace and remains in the
  scratchpad. The container will be visible until fullscreen disabled
  or killed. Since the container is in the scratchpad, running
  `scratchpad show` or `move container to scratchpad` will have no
  effect

This also changes `container_replace` to transfer fullscreen and
scratchpad status.

include/sway/input/seat.h

@@ -143,6 +143,11 @@ struct sway_node *seat_get_focus(struct sway_seat *seat);
 
 struct sway_workspace *seat_get_focused_workspace(struct sway_seat *seat);
 
+// If a scratchpad container is fullscreen global, this can be used to try to
+// determine the last focused workspace. Otherwise, this should yield the same
+// results as seat_get_focused_workspace.
+struct sway_workspace *seat_get_last_known_workspace(struct sway_seat *seat);
+
 struct sway_container *seat_get_focused_container(struct sway_seat *seat);
 
 /**

sway/commands/floating.c

@@ -32,6 +32,11 @@ struct cmd_results *cmd_floating(int argc, char **argv) {
 		seat_set_focus_container(config->handler_context.seat, container);
 	}
 
+	if (container_is_scratchpad_hidden(container)) {
+		return cmd_results_new(CMD_INVALID,
+				"Can't change floating on hidden scratchpad container");
+	}
+
 	// If the container is in a floating split container,
 	// operate on the split container instead of the child.
 	if (container_is_floating_or_child(container)) {

sway/commands/focus.c

@@ -182,6 +182,10 @@ static struct sway_node *node_get_in_direction_floating(
 	double closest_distance = DBL_MAX;
 	struct sway_container *closest_con = NULL;
 
+	if (!con->workspace) {
+		return NULL;
+	}
+
 	for (int i = 0; i < con->workspace->floating->length; i++) {
 		struct sway_container *floater = con->workspace->floating->items[i];
 		if (floater == con) {

sway/commands/fullscreen.c

@@ -26,6 +26,13 @@ struct cmd_results *cmd_fullscreen(int argc, char **argv) {
 				"Can't fullscreen an empty workspace");
 	}
 
+	// If in the scratchpad, operate on the highest container
+	if (container && !container->workspace) {
+		while (container->parent) {
+			container = container->parent;
+		}
+	}
+
 	bool is_fullscreen = container &&
 		container->fullscreen_mode != FULLSCREEN_NONE;
 	bool global = false;

sway/commands/layout.c

@@ -147,8 +147,12 @@ struct cmd_results *cmd_layout(int argc, char **argv) {
 			workspace->layout = new_layout;
 			workspace_update_representation(workspace);
 		}
+		if (root->fullscreen_global) {
+			arrange_root();
+		} else {
 			arrange_workspace(workspace);
 		}
+	}
 
 	return cmd_results_new(CMD_SUCCESS, NULL);
 }

sway/commands/move.c

@@ -395,6 +395,11 @@ static struct cmd_results *cmd_move_container(int argc, char **argv) {
 		container = workspace_wrap_children(workspace);
 	}
 
+	if (container->fullscreen_mode == FULLSCREEN_GLOBAL) {
+		return cmd_results_new(CMD_FAILURE,
+				"Can't move fullscreen global container");
+	}
+
 	bool no_auto_back_and_forth = false;
 	while (strcasecmp(argv[0], "--no-auto-back-and-forth") == 0) {
 		no_auto_back_and_forth = true;
@@ -646,6 +651,10 @@ static struct cmd_results *cmd_move_workspace(int argc, char **argv) {
 	}
 
 	struct sway_workspace *workspace = config->handler_context.workspace;
+	if (!workspace) {
+		return cmd_results_new(CMD_FAILURE, "No workspace to move");
+	}
+
 	struct sway_output *old_output = workspace->output;
 	int center_x = workspace->width / 2 + workspace->x,
 		center_y = workspace->height / 2 + workspace->y;

sway/commands/split.c

@@ -13,7 +13,8 @@ static struct cmd_results *do_split(int layout) {
 	struct sway_container *con = config->handler_context.container;
 	struct sway_workspace *ws = config->handler_context.workspace;
 	if (con) {
-		if (container_is_scratchpad_hidden(con)) {
+		if (container_is_scratchpad_hidden(con) &&
+				con->fullscreen_mode != FULLSCREEN_GLOBAL) {
 			return cmd_results_new(CMD_FAILURE,
 					"Cannot split a hidden scratchpad container");
 		}

sway/desktop/output.c

@@ -251,14 +251,11 @@ static void output_for_each_surface(struct sway_output *output,
 	};
 
 	struct sway_workspace *workspace = output_get_active_workspace(output);
+	struct sway_container *fullscreen_con = root->fullscreen_global;
+	if (!fullscreen_con) {
 		if (!workspace) {
 			return;
 		}
-	struct sway_container *fullscreen_con = root->fullscreen_global;
-	if (fullscreen_con && container_is_scratchpad_hidden(fullscreen_con)) {
-		fullscreen_con = NULL;
-	}
-	if (!fullscreen_con) {
 		fullscreen_con = workspace->current.fullscreen;
 	}
 	if (fullscreen_con) {

sway/desktop/render.c

@@ -997,9 +997,6 @@ void output_render(struct sway_output *output, struct timespec *when,
 	}
 
 	struct sway_container *fullscreen_con = root->fullscreen_global;
-	if (fullscreen_con && container_is_scratchpad_hidden(fullscreen_con)) {
-		fullscreen_con = NULL;
-	}
 	if (!fullscreen_con) {
 		fullscreen_con = workspace->current.fullscreen;
 	}

sway/input/seat.c

@@ -172,14 +172,14 @@ static void handle_seat_node_destroy(struct wl_listener *listener, void *data) {
 
 	seat_node_destroy(seat_node);
 
-	if (!parent) {
+	if (!parent && !needs_new_focus) {
 		// Destroying a container that is no longer in the tree
 		return;
 	}
 
 	// Find new focus_inactive (ie. sibling, or workspace if no siblings left)
 	struct sway_node *next_focus = NULL;
-	while (next_focus == NULL) {
+	while (next_focus == NULL && parent != NULL) {
 		struct sway_container *con =
 			seat_get_focus_inactive_view(seat, parent);
 		next_focus = con ? &con->node : NULL;
@@ -192,6 +192,16 @@ static void handle_seat_node_destroy(struct wl_listener *listener, void *data) {
 		parent = node_get_parent(parent);
 	}
 
+	if (!next_focus) {
+		struct sway_workspace *ws = seat_get_last_known_workspace(seat);
+		if (!ws) {
+			return;
+		}
+		struct sway_container *con =
+			seat_get_focus_inactive_view(seat, &ws->node);
+		next_focus = con ? &(con->node) : &(ws->node);
+	}
+
 	if (next_focus->type == N_WORKSPACE &&
 			!workspace_is_visible(next_focus->sway_workspace)) {
 		// Do not change focus to a non-visible workspace
@@ -199,6 +209,10 @@ static void handle_seat_node_destroy(struct wl_listener *listener, void *data) {
 	}
 
 	if (needs_new_focus) {
+		// Make sure the workspace IPC event gets sent
+		if (node->type == N_CONTAINER && node->sway_container->scratchpad) {
+			seat_set_focus(seat, NULL);
+		}
 		// The structure change might have caused it to move up to the top of
 		// the focus stack without sending focus notifications to the view
 		seat_send_focus(next_focus, seat);
@@ -207,7 +221,7 @@ static void handle_seat_node_destroy(struct wl_listener *listener, void *data) {
 		// Setting focus_inactive
 		focus = seat_get_focus_inactive(seat, &root->node);
 		seat_set_raw_focus(seat, next_focus);
-		if (focus->type == N_CONTAINER) {
+		if (focus->type == N_CONTAINER && focus->sway_container->workspace) {
 			seat_set_raw_focus(seat, &focus->sway_container->workspace->node);
 		}
 		seat_set_raw_focus(seat, focus);
@@ -795,7 +809,13 @@ void seat_set_raw_focus(struct sway_seat *seat, struct sway_node *node) {
 	wl_list_remove(&seat_node->link);
 	wl_list_insert(&seat->focus_stack, &seat_node->link);
 	node_set_dirty(node);
-	node_set_dirty(node_get_parent(node));
+
+	// If focusing a scratchpad container that is fullscreen global, parent
+	// will be NULL
+	struct sway_node *parent = node_get_parent(node);
+	if (parent) {
+		node_set_dirty(parent);
+	}
 }
 
 void seat_set_focus(struct sway_seat *seat, struct sway_node *node) {
@@ -850,7 +870,8 @@ void seat_set_focus(struct sway_seat *seat, struct sway_node *node) {
 		}
 	}
 
-	struct sway_output *new_output = new_workspace->output;
+	struct sway_output *new_output =
+		new_workspace ? new_workspace->output : NULL;
 
 	if (last_workspace != new_workspace && new_output) {
 		node_set_dirty(&new_output->node);
@@ -894,7 +915,8 @@ void seat_set_focus(struct sway_seat *seat, struct sway_node *node) {
 	}
 
 	// Move sticky containers to new workspace
-	if (new_output_last_ws && new_workspace != new_output_last_ws) {
+	if (new_workspace && new_output_last_ws
+			&& new_workspace != new_output_last_ws) {
 		for (int i = 0; i < new_output_last_ws->floating->length; ++i) {
 			struct sway_container *floater =
 				new_output_last_ws->floating->items[i];
@@ -940,7 +962,7 @@ void seat_set_focus(struct sway_seat *seat, struct sway_node *node) {
 
 	seat->has_focus = true;
 
-	if (config->smart_gaps) {
+	if (config->smart_gaps && new_workspace) {
 		// When smart gaps is on, gaps may change when the focus changes so
 		// the workspace needs to be arranged
 		arrange_workspace(new_workspace);
@@ -1134,6 +1156,20 @@ struct sway_workspace *seat_get_focused_workspace(struct sway_seat *seat) {
 	return NULL; // output doesn't have a workspace yet
 }
 
+struct sway_workspace *seat_get_last_known_workspace(struct sway_seat *seat) {
+	struct sway_seat_node *current;
+	wl_list_for_each(current, &seat->focus_stack, link) {
+		struct sway_node *node = current->node;
+		if (node->type == N_CONTAINER &&
+				node->sway_container->workspace) {
+			return node->sway_container->workspace;
+		} else if (node->type == N_WORKSPACE) {
+			return node->sway_workspace;
+		}
+	}
+	return NULL;
+}
+
 struct sway_container *seat_get_focused_container(struct sway_seat *seat) {
 	struct sway_node *focus = seat_get_focus(seat);
 	if (focus && focus->type == N_CONTAINER) {

sway/tree/container.c

@@ -95,6 +95,10 @@ void container_begin_destroy(struct sway_container *con) {
 	if (con->fullscreen_mode == FULLSCREEN_WORKSPACE && con->workspace) {
 		con->workspace->fullscreen = NULL;
 	}
+	if (con->scratchpad && con->fullscreen_mode == FULLSCREEN_GLOBAL) {
+		container_fullscreen_disable(con);
+	}
+
 	wl_signal_emit(&con->node.events.destroy, &con->node);
 
 	container_end_mouse_operation(con);
@@ -128,8 +132,10 @@ void container_reap_empty(struct sway_container *con) {
 		container_begin_destroy(con);
 		con = parent;
 	}
+	if (ws) {
 		workspace_consider_destroy(ws);
 	}
+}
 
 struct sway_container *container_flatten(struct sway_container *container) {
 	if (container->view) {
@@ -954,12 +960,13 @@ static void container_fullscreen_workspace(struct sway_container *con) {
 	set_fullscreen_iterator(con, &enable);
 	container_for_each_child(con, set_fullscreen_iterator, &enable);
 
-	con->workspace->fullscreen = con;
 	con->saved_x = con->x;
 	con->saved_y = con->y;
 	con->saved_width = con->width;
 	con->saved_height = con->height;
 
+	if (con->workspace) {
+		con->workspace->fullscreen = con;
 		struct sway_seat *seat;
 		struct sway_workspace *focus_ws;
 		wl_list_for_each(seat, &server.input->seats, link) {
@@ -968,6 +975,7 @@ static void container_fullscreen_workspace(struct sway_container *con) {
 				seat_set_focus_container(seat, con);
 			}
 		}
+	}
 
 	con->fullscreen_mode = FULLSCREEN_WORKSPACE;
 	container_end_mouse_operation(con);
@@ -1019,13 +1027,16 @@ void container_fullscreen_disable(struct sway_container *con) {
 	con->height = con->saved_height;
 
 	if (con->fullscreen_mode == FULLSCREEN_WORKSPACE) {
+		if (con->workspace) {
 			con->workspace->fullscreen = NULL;
 			if (container_is_floating(con)) {
-			struct sway_output *output = container_floating_find_output(con);
+				struct sway_output *output =
+					container_floating_find_output(con);
 				if (con->workspace->output != output) {
 					container_floating_move_to_center(con);
 				}
 			}
+		}
 	} else {
 		root->fullscreen_global = NULL;
 	}
@@ -1040,6 +1051,17 @@ void container_fullscreen_disable(struct sway_container *con) {
 	con->fullscreen_mode = FULLSCREEN_NONE;
 	container_end_mouse_operation(con);
 	ipc_event_window(con, "fullscreen_mode");
+
+	if (con->scratchpad) {
+		struct sway_seat *seat;
+		wl_list_for_each(seat, &server.input->seats, link) {
+			struct sway_container *focus = seat_get_focused_container(seat);
+			if (focus == con || container_has_ancestor(focus, con)) {
+				seat_set_focus(seat,
+						seat_get_focus_inactive(seat, &root->node));
+			}
+		}
+	}
 }
 
 void container_set_fullscreen(struct sway_container *con,
@@ -1056,7 +1078,7 @@ void container_set_fullscreen(struct sway_container *con,
 		if (root->fullscreen_global) {
 			container_fullscreen_disable(root->fullscreen_global);
 		}
-		if (con->workspace->fullscreen) {
+		if (con->workspace && con->workspace->fullscreen) {
 			container_fullscreen_disable(con->workspace->fullscreen);
 		}
 		container_fullscreen_workspace(con);
@@ -1171,6 +1193,11 @@ void container_add_gaps(struct sway_container *c) {
 			c->current_gaps.bottom > 0 || c->current_gaps.left > 0) {
 		return;
 	}
+	// Fullscreen global scratchpad containers cannot have gaps
+	struct sway_workspace *ws = c->workspace;
+	if (!ws) {
+		return;
+	}
 	// Linear containers don't have gaps because it'd create double gaps
 	if (!c->view && c->layout != L_TABBED && c->layout != L_STACKED) {
 		return;
@@ -1199,8 +1226,6 @@ void container_add_gaps(struct sway_container *c) {
 		}
 	}
 
-	struct sway_workspace *ws = c->workspace;
-
 	c->current_gaps.top = c->y == ws->y ? ws->gaps_inner : 0;
 	c->current_gaps.right = ws->gaps_inner;
 	c->current_gaps.bottom = ws->gaps_inner;
@@ -1308,6 +1333,10 @@ void container_add_child(struct sway_container *parent,
 	child->parent = parent;
 	child->workspace = parent->workspace;
 	container_for_each_child(child, set_workspace, NULL);
+	bool fullscreen = child->fullscreen_mode != FULLSCREEN_NONE ||
+		parent->fullscreen_mode != FULLSCREEN_NONE;
+	set_fullscreen_iterator(child, &fullscreen);
+	container_for_each_child(child, set_fullscreen_iterator, &fullscreen);
 	container_handle_fullscreen_reparent(child);
 	container_update_representation(parent);
 	node_set_dirty(&child->node);
@@ -1347,8 +1376,31 @@ void container_detach(struct sway_container *child) {
 
 void container_replace(struct sway_container *container,
 		struct sway_container *replacement) {
+	enum sway_fullscreen_mode fullscreen = container->fullscreen_mode;
+	bool scratchpad = container->scratchpad;
+	if (fullscreen != FULLSCREEN_NONE) {
+		container_fullscreen_disable(container);
+	}
+	if (scratchpad) {
+		root_scratchpad_show(container);
+		root_scratchpad_remove_container(container);
+	}
 	container_add_sibling(container, replacement, 1);
 	container_detach(container);
+	if (scratchpad) {
+		root_scratchpad_add_container(replacement);
+	}
+	switch (fullscreen) {
+	case FULLSCREEN_WORKSPACE:
+		container_fullscreen_workspace(replacement);
+		break;
+	case FULLSCREEN_GLOBAL:
+		container_fullscreen_global(replacement);
+		break;
+	case FULLSCREEN_NONE:
+		// noop
+		break;
+	}
 }
 
 struct sway_container *container_split(struct sway_container *child,
@@ -1369,8 +1421,12 @@ struct sway_container *container_split(struct sway_container *child,
 
 	if (set_focus) {
 		seat_set_raw_focus(seat, &cont->node);
+		if (cont->fullscreen_mode == FULLSCREEN_GLOBAL) {
+			seat_set_focus(seat, &child->node);
+		} else {
 			seat_set_raw_focus(seat, &child->node);
 		}
+	}
 
 	return cont;
 }
@@ -1529,7 +1585,7 @@ void container_raise_floating(struct sway_container *con) {
 	while (floater->parent) {
 		floater = floater->parent;
 	}
-	if (container_is_floating(floater)) {
+	if (container_is_floating(floater) && floater->workspace) {
 		list_move_to_end(floater->workspace->floating, floater);
 		node_set_dirty(&floater->workspace->node);
 	}

sway/tree/node.c

@@ -142,11 +142,19 @@ list_t *node_get_children(struct sway_node *node) {
 }
 
 bool node_has_ancestor(struct sway_node *node, struct sway_node *ancestor) {
+	if (ancestor->type == N_ROOT && node->type == N_CONTAINER &&
+			node->sway_container->fullscreen_mode == FULLSCREEN_GLOBAL) {
+		return true;
+	}
 	struct sway_node *parent = node_get_parent(node);
 	while (parent) {
 		if (parent == ancestor) {
 			return true;
 		}
+		if (ancestor->type == N_ROOT && parent->type == N_CONTAINER &&
+				parent->sway_container->fullscreen_mode == FULLSCREEN_GLOBAL) {
+			return true;
+		}
 		parent = node_get_parent(parent);
 	}
 	return false;

sway/tree/root.c

@@ -62,6 +62,11 @@ void root_scratchpad_add_container(struct sway_container *con) {
 	struct sway_container *parent = con->parent;
 	struct sway_workspace *workspace = con->workspace;
 
+	// Clear the fullscreen mode when sending to the scratchpad
+	if (con->fullscreen_mode != FULLSCREEN_NONE) {
+		container_fullscreen_disable(con);
+	}
+
 	// When a tiled window is sent to scratchpad, center and resize it.
 	if (!container_is_floating(con)) {
 		container_set_floating(con, true);
@@ -143,6 +148,15 @@ void root_scratchpad_hide(struct sway_container *con) {
 	struct sway_node *focus = seat_get_focus_inactive(seat, &root->node);
 	struct sway_workspace *ws = con->workspace;
 
+	if (con->fullscreen_mode == FULLSCREEN_GLOBAL && !con->workspace) {
+		// If the container was made fullscreen global while in the scratchpad,
+		// it should be shown until fullscreen has been disabled
+		return;
+	}
+
+	if (con->fullscreen_mode != FULLSCREEN_NONE) {
+		container_fullscreen_disable(con);
+	}
 	container_detach(con);
 	arrange_workspace(ws);
 	if (&con->node == focus || node_has_ancestor(focus, &con->node)) {

sway/tree/view.c

@@ -197,10 +197,11 @@ static bool gaps_to_edge(struct sway_view *view) {
 
 void view_autoconfigure(struct sway_view *view) {
 	struct sway_container *con = view->container;
-	if (container_is_scratchpad_hidden(con)) {
+	if (container_is_scratchpad_hidden(con) &&
+			con->fullscreen_mode != FULLSCREEN_GLOBAL) {
 		return;
 	}
-	struct sway_output *output = con->workspace->output;
+	struct sway_output *output = con->workspace ? con->workspace->output : NULL;
 
 	if (con->fullscreen_mode == FULLSCREEN_WORKSPACE) {
 		con->content_x = output->lx;
@@ -226,6 +227,7 @@ void view_autoconfigure(struct sway_view *view) {
 
 	con->border_top = con->border_bottom = true;
 	con->border_left = con->border_right = true;
+	if (ws) {
 		if (config->hide_edge_borders == E_BOTH
 				|| config->hide_edge_borders == E_VERTICAL
 				|| (smart && !other_views && no_gaps)) {
@@ -240,6 +242,7 @@ void view_autoconfigure(struct sway_view *view) {
 			int bottom_y = con->y + con->height + con->current_gaps.bottom;
 			con->border_bottom = bottom_y != ws->y + ws->height;
 		}
+	}
 
 	double y_offset = 0;
 
@@ -247,7 +250,8 @@ void view_autoconfigure(struct sway_view *view) {
 	// title area. We have to offset the surface y by the height of the title,
 	// bar, and disable any top border because we'll always have the title bar.
 	list_t *siblings = container_get_siblings(con);
-	bool show_titlebar = siblings->length > 1 || !config->hide_lone_tab;
+	bool show_titlebar = (siblings && siblings->length > 1)
+		|| !config->hide_lone_tab;
 	if (show_titlebar && !container_is_floating(con)) {
 		enum sway_container_layout layout = container_parent_layout(con);
 		if (layout == L_TABBED) {
@@ -538,6 +542,10 @@ static bool should_focus(struct sway_view *view) {
 	struct sway_workspace *prev_ws = seat_get_focused_workspace(seat);
 	struct sway_workspace *map_ws = view->container->workspace;
 
+	if (view->container->fullscreen_mode == FULLSCREEN_GLOBAL) {
+		return true;
+	}
+
 	// Views can only take focus if they are mapped into the active workspace
 	if (prev_ws != map_ws) {
 		return false;
@@ -581,7 +589,8 @@ void view_map(struct sway_view *view, struct wlr_surface *wlr_surface,
 	}
 
 	struct sway_seat *seat = input_manager_current_seat();
-	struct sway_node *node = seat_get_focus_inactive(seat, &ws->node);
+	struct sway_node *node = ws ? seat_get_focus_inactive(seat, &ws->node)
+		: seat_get_focus_inactive(seat, &root->node);
 	struct sway_container *target_sibling = node->type == N_CONTAINER ?
 		node->sway_container : NULL;
 
@@ -589,12 +598,13 @@ void view_map(struct sway_view *view, struct wlr_surface *wlr_surface,
 	// launch it as a tiled view in the root of the workspace instead.
 	if (target_sibling && container_is_floating(target_sibling)) {
 		target_sibling = NULL;
+		ws = seat_get_last_known_workspace(seat);
 	}
 
 	view->container = container_create(view);
 	if (target_sibling) {
 		container_add_sibling(target_sibling, view->container, 1);
-	} else {
+	} else if (ws) {
 		workspace_add_tiling(ws, view->container);
 	}
 	ipc_event_window(view->container, "new");
@@ -1032,16 +1042,26 @@ bool view_is_visible(struct sway_view *view) {
 		return false;
 	}
 	struct sway_workspace *workspace = view->container->workspace;
-	if (!workspace) {
+	if (!workspace && view->container->fullscreen_mode != FULLSCREEN_GLOBAL) {
+		bool fs_global_descendant = false;
+		struct sway_container *parent = view->container->parent;
+		while (parent) {
+			if (parent->fullscreen_mode == FULLSCREEN_GLOBAL) {
+				fs_global_descendant = true;
+			}
+			parent = parent->parent;
+		}
+		if (!fs_global_descendant) {
 			return false;
 		}
+	}
 	// Determine if view is nested inside a floating container which is sticky
 	struct sway_container *floater = view->container;
 	while (floater->parent) {
 		floater = floater->parent;
 	}
 	bool is_sticky = container_is_floating(floater) && floater->is_sticky;
-	if (!is_sticky && !workspace_is_visible(workspace)) {
+	if (!is_sticky && workspace && !workspace_is_visible(workspace)) {
 		return false;
 	}
 	// Check view isn't in a tabbed or stacked container on an inactive tab