data-device: only allow one drag at a time

types/data_device/wlr_drag.c

@@ -122,6 +122,9 @@ static void drag_end(struct wlr_drag *drag) {
 			drag_icon_set_mapped(drag->icon, false);
 		}
 
+		assert(drag->seat->drag == drag);
+		drag->seat->drag = NULL;
+
 		wlr_signal_emit_safe(&drag->events.destroy, drag);
 		free(drag);
 	}
@@ -434,6 +437,13 @@ bool seat_client_start_drag(struct wlr_seat_client *client,
 		return true;
 	}
 
+	if (seat->drag != NULL) {
+		wlr_log(WLR_DEBUG, "Refusing to start drag, "
+			"another drag is already in progress");
+		free(drag);
+		return true;
+	}
+
 	if (icon_surface) {
 		int32_t touch_id = (point ? point->touch_id : 0);
 		struct wlr_drag_icon *icon =