shell: Store parent–child links between shsurfs for window stacking

This ensures transient surfaces are included in the layer of their
parent, even if the parent later changes layers. It achieves this by
recursively changing the layers of all children of a surface when that
surface’s layer is changed. The recursion is unbounded unless transient
surfaces are restricted to not being children of other popup or transient
surfaces.

This fixes a bug whereby a surface which is transient for a fullscreen
surface could end up being stacked below it.

https://bugs.freedesktop.org/show_bug.cgi?id=69443

src/shell.c

@@ -251,6 +251,44 @@ struct ping_timer {
 	uint32_t serial;
 };
 
+/*
+ * Surface stacking and ordering.
+ *
+ * This is handled using several linked lists of surfaces, organised into
+ * ‘layers’. The layers are ordered, and each of the surfaces in one layer are
+ * above all of the surfaces in the layer below. The set of layers is static and
+ * in the following order (top-most first):
+ *  • Lock layer (only ever displayed on its own)
+ *  • Cursor layer
+ *  • Fullscreen layer
+ *  • Panel layer
+ *  • Input panel layer
+ *  • Workspace layers
+ *  • Background layer
+ *
+ * The list of layers may be manipulated to remove whole layers of surfaces from
+ * display. For example, when locking the screen, all layers except the lock
+ * layer are removed.
+ *
+ * A surface’s layer is modified on configuring the surface, in
+ * set_surface_type() (which is only called when the surface’s type change is
+ * _committed_). If a surface’s type changes (e.g. when making a window
+ * fullscreen) its layer changes too.
+ *
+ * In order to allow popup and transient surfaces to be correctly stacked above
+ * their parent surfaces, each surface tracks both its parent surface, and a
+ * linked list of its children. When a surface’s layer is updated, so are the
+ * layers of its children. Note that child surfaces are *not* the same as
+ * subsurfaces — child/parent surfaces are purely for maintaining stacking
+ * order.
+ *
+ * The children_link list of siblings of a surface (i.e. those surfaces which
+ * have the same parent) only contains weston_surfaces which have a
+ * shell_surface. Stacking is not implemented for non-shell_surface
+ * weston_surfaces. This means that the following implication does *not* hold:
+ *     (shsurf->parent != NULL) ⇒ !wl_list_is_empty(shsurf->children_link)
+ */
+
 struct shell_surface {
 	struct wl_resource *resource;
 	struct wl_signal destroy_signal;
@@ -259,6 +297,8 @@ struct shell_surface {
 	struct weston_view *view;
 	struct wl_listener surface_destroy_listener;
 	struct weston_surface *parent;
+	struct wl_list children_list;  /* child surfaces of this one */
+	struct wl_list children_link;  /* sibling surfaces of this one */
 	struct desktop_shell *shell;
 
 	enum shell_surface_type type, next_type;
@@ -369,6 +409,9 @@ shell_fade_startup(struct desktop_shell *shell);
 static struct shell_seat *
 get_shell_seat(struct weston_seat *seat);
 
+static void
+shell_surface_update_child_surface_layers(struct shell_surface *shsurf);
+
 static bool
 shell_surface_is_top_fullscreen(struct shell_surface *shsurf)
 {
@@ -1263,6 +1306,8 @@ move_surface_to_workspace(struct desktop_shell *shell,
 	wl_list_remove(&view->layer_link);
 	wl_list_insert(&to->layer.view_list, &view->layer_link);
 
+	shell_surface_update_child_surface_layers(shsurf);
+
 	drop_focus_state(shell, from, view->surface);
 	wl_list_for_each(seat, &shell->compositor->seat_list, link) {
 		if (!seat->keyboard)
@@ -1302,6 +1347,8 @@ take_surface_to_workspace_by_seat(struct desktop_shell *shell,
 	wl_list_insert(&to->layer.view_list, &view->layer_link);
 
 	shsurf = get_shell_surface(surface);
+	if (shsurf != NULL)
+		shell_surface_update_child_surface_layers(shsurf);
 
 	replace_focus_state(shell, to, seat);
 	drop_focus_state(shell, from, surface);
@@ -2083,8 +2130,36 @@ shell_surface_calculate_layer_link (struct shell_surface *shsurf)
 	return &ws->layer.view_list;
 }
 
+static void
+shell_surface_update_child_surface_layers (struct shell_surface *shsurf)
+{
+	struct shell_surface *child;
+
+	/* Move the child layers to the same workspace as shsurf. They will be
+	 * stacked above shsurf. */
+	wl_list_for_each_reverse(child, &shsurf->children_list, children_link) {
+		if (shsurf->view->layer_link.prev != &child->view->layer_link) {
+			weston_view_geometry_dirty(child->view);
+			wl_list_remove(&child->view->layer_link);
+			wl_list_insert(shsurf->view->layer_link.prev,
+			               &child->view->layer_link);
+			weston_view_geometry_dirty(child->view);
+			weston_surface_damage(child->surface);
+
+			/* Recurse. We don’t expect this to recurse very far (if
+			 * at all) because that would imply we have transient
+			 * (or popup) children of transient surfaces, which
+			 * would be unusual. */
+			shell_surface_update_child_surface_layers(child);
+		}
+	}
+}
+
 /* Update the surface’s layer. Mark both the old and new views as having dirty
- * geometry to ensure the changes are redrawn. */
+ * geometry to ensure the changes are redrawn.
+ *
+ * If any child surfaces exist and are mapped, ensure they’re in the same layer
+ * as this surface. */
 static void
 shell_surface_update_layer(struct shell_surface *shsurf)
 {
@@ -2100,6 +2175,8 @@ shell_surface_update_layer(struct shell_surface *shsurf)
 	wl_list_insert(new_layer_link, &shsurf->view->layer_link);
 	weston_view_geometry_dirty(shsurf->view);
 	weston_surface_damage(shsurf->surface);
+
+	shell_surface_update_child_surface_layers(shsurf);
 }
 
 static void
@@ -2107,6 +2184,17 @@ shell_surface_set_parent(struct shell_surface *shsurf,
                          struct weston_surface *parent)
 {
 	shsurf->parent = parent;
+
+	wl_list_remove(&shsurf->children_link);
+	wl_list_init(&shsurf->children_link);
+
+	/* Insert into the parent surface’s child list. */
+	if (parent != NULL) {
+		struct shell_surface *parent_shsurf = get_shell_surface(parent);
+		if (parent_shsurf != NULL)
+			wl_list_insert(&parent_shsurf->children_list,
+			               &shsurf->children_link);
+	}
 }
 
 static void
@@ -2131,6 +2219,9 @@ set_toplevel(struct shell_surface *shsurf)
 	shell_surface_set_parent(shsurf, NULL);
 
 	shsurf->next_type = SHELL_SURFACE_TOPLEVEL;
+
+	/* The layer_link is updated in set_surface_type(),
+	 * called from configure. */
 }
 
 static void
@@ -2155,6 +2246,9 @@ set_transient(struct shell_surface *shsurf,
 	shell_surface_set_parent(shsurf, parent);
 
 	shsurf->next_type = SHELL_SURFACE_TRANSIENT;
+
+	/* The layer_link is updated in set_surface_type(),
+	 * called from configure. */
 }
 
 static void
@@ -2189,6 +2283,9 @@ set_fullscreen(struct shell_surface *shsurf,
 	shsurf->client->send_configure(shsurf->surface, 0,
 				       shsurf->output->width,
 				       shsurf->output->height);
+
+	/* The layer_link is updated in set_surface_type(),
+	 * called from configure. */
 }
 
 static void
@@ -2851,6 +2948,8 @@ destroy_shell_surface(struct shell_surface *shsurf)
 
 	weston_view_destroy(shsurf->view);
 
+	wl_list_remove(&shsurf->children_link);
+
 	wl_list_remove(&shsurf->link);
 	free(shsurf);
 }
@@ -2941,6 +3040,10 @@ create_shell_surface(void *shell, struct weston_surface *surface,
 
 	wl_list_init(&shsurf->workspace_transform.link);
 
+	wl_list_init(&shsurf->children_link);
+	wl_list_init(&shsurf->children_list);
+	shsurf->parent = NULL;
+
 	shsurf->type = SHELL_SURFACE_NONE;
 	shsurf->next_type = SHELL_SURFACE_NONE;