desktop-shell: Fail if get_xdg_surface is called on an xdg_surface

If a client calls xdg_shell.get_xdg_surface on a surface that is already
an xdg_surface would, prior to this patch, succeed, but cause weston to
crash later when trying to configure. This patch instead sends a role
error to the client complaining that it already is an xdg_surface.

Note that .._set_role() only fails when changing roles, not when setting
the same role twice.

The same is done for xdg_popup.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

desktop-shell/shell.c

@@ -3978,6 +3978,14 @@ xdg_get_xdg_surface(struct wl_client *client,
 	struct desktop_shell *shell = sc->shell;
 	struct shell_surface *shsurf;
 
+	shsurf = get_shell_surface(surface);
+	if (shsurf && shell_surface_is_xdg_surface(shsurf)) {
+		wl_resource_post_error(resource, XDG_SHELL_ERROR_ROLE,
+				       "This wl_surface is already an "
+				       "xdg_surface");
+		return;
+	}
+
 	if (weston_surface_set_role(surface, "xdg_surface",
 				    resource, XDG_SHELL_ERROR_ROLE) < 0)
 		return;
@@ -4071,6 +4079,14 @@ xdg_get_xdg_popup(struct wl_client *client,
 	struct weston_surface *parent;
 	struct shell_seat *seat;
 
+	shsurf = get_shell_surface(surface);
+	if (shsurf && shell_surface_is_xdg_popup(shsurf)) {
+		wl_resource_post_error(resource, XDG_SHELL_ERROR_ROLE,
+				       "This wl_surface is already an "
+				       "xdg_popup");
+		return;
+	}
+
 	if (weston_surface_set_role(surface, "xdg_popup",
 				    resource, XDG_SHELL_ERROR_ROLE) < 0)
 		return;