diff --git a/luaa.c b/luaa.c
index e5cd0556..332105d1 100644
--- a/luaa.c
+++ b/luaa.c
@@ -54,6 +54,7 @@
 #include <basedir_fs.h>
 
 #include <xcb/xcb_atom.h>
+#include <xcb/xcb_aux.h>
 
 #include <unistd.h> /* for gethostname() */
 
@@ -214,6 +215,17 @@ luaA_kill(lua_State *L)
     return 1;
 }
 
+/** Synchronize with the X11 server. This is needed in the test suite to avoid
+ * some race conditions. You should never need to use this function.
+ * @function sync
+ */
+static int
+luaA_sync(lua_State *L)
+{
+    xcb_aux_sync(globalconf.connection);
+    return 0;
+}
+
 /** Load an image from a given path.
  *
  * @param name The file name.
@@ -647,6 +659,7 @@ luaA_init(xdgHandle* xdg, string_array_t *searchpath)
         { "xkb_get_group_names", luaA_xkb_get_group_names},
         { "xrdb_get_value", luaA_xrdb_get_value},
         { "kill", luaA_kill},
+        { "sync", luaA_sync},
         { NULL, NULL }
     };