libweston: set the seat automatically based on the XDG_SEAT environment variable

This will allow the seat to be set by the environment as pam_systemd typically
sets the XDG_SEAT variable

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

libweston/compositor-drm.c

@@ -6029,8 +6029,16 @@ drm_backend_create(struct weston_compositor *compositor,
 	struct udev_device *drm_device;
 	struct wl_event_loop *loop;
 	const char *seat_id = default_seat;
+	const char *session_seat;
 	int ret;
 
+	session_seat = getenv("XDG_SEAT");
+	if (session_seat)
+		seat_id = session_seat;
+
+	if (config->seat_id)
+		seat_id = config->seat_id;
+
 	weston_log("initializing drm backend\n");
 
 	b = zalloc(sizeof *b);
@@ -6062,9 +6070,6 @@ drm_backend_create(struct weston_compositor *compositor,
 	if (parse_gbm_format(config->gbm_format, GBM_FORMAT_XRGB8888, &b->gbm_format) < 0)
 		goto err_compositor;
 
-	if (config->seat_id)
-		seat_id = config->seat_id;
-
 	/* Check if we run drm-backend using weston-launch */
 	compositor->launcher = weston_launcher_connect(compositor, config->tty,
 						       seat_id, true);

libweston/compositor-drm.h

@@ -106,7 +106,8 @@ struct weston_drm_backend_config {
 
 	/** The seat to be used for input and output.
 	 *
-	 * If NULL the default "seat0" will be used.  The backend will
+	 * If seat_id is NULL, the seat is taken from XDG_SEAT environment
+	 * variable. If neither is set, "seat0" is used. The backend will
 	 * take ownership of the seat_id pointer and will free it on
 	 * backend destruction.
 	 */