stand: lua: enhance lfs.dir() to speed up kernels_autodetect

This eliminates a lot of stat() calls that happen when lualoader renders the
menu with the default settings, and greatly speeds up rendering on my
laptop.

ftype is nil if loader/loader.efi hasn't been updated yet, falling back to
lfs.attributes() to test.

This is technically incompatible with lfs, but not in a particularly
terrible way.

Reviewed-by:	cem
MFC-after:	4 days
Differential Revision:	https://reviews.freebsd.org/D27542

libexec/flua/modules/lfs.c

@@ -122,6 +122,27 @@ __FBSDID("$FreeBSD$");
 
 #define DIR_METATABLE "directory iterator metatable"
 
+static int
+lua_dir_iter_pushtype(lua_State *L __unused, const struct dirent *ent __unused)
+{
+
+	/*
+	 * This is a non-standard extension to luafilesystem for loader's
+	 * benefit.  The extra stat() calls to determine the entry type can
+	 * be quite expensive on some systems, so this speeds up enumeration of
+	 * /boot greatly by providing the type up front.
+	 *
+	 * This extension is compatible enough with luafilesystem, in that we're
+	 * just using an extra return value for the iterator.
+	 */
+#ifdef _STANDALONE
+	lua_pushinteger(L, ent->d_type);
+	return 1;
+#else
+	return 0;
+#endif
+}
+
 static int
 lua_dir_iter_next(lua_State *L)
 {
@@ -144,7 +165,7 @@ lua_dir_iter_next(lua_State *L)
 	}
 
 	lua_pushstring(L, entry->d_name);
-	return 1;
+	return 1 + lua_dir_iter_pushtype(L, entry);
 }
 
 static int
@@ -420,5 +441,10 @@ luaopen_lfs(lua_State *L)
 {
 	register_metatable(L);
 	luaL_newlib(L, fslib);
+#ifdef _STANDALONE
+	/* Non-standard extension for loader, used with lfs.dir(). */
+	lua_pushinteger(L, DT_DIR);
+	lua_setfield(L, -2, "DT_DIR");
+#endif
 	return 1;
 }

stand/lua/core.lua

@@ -240,14 +240,18 @@ function core.kernelList()
 	-- Automatically detect other bootable kernel directories using a
 	-- heuristic.  Any directory in /boot that contains an ordinary file
 	-- named "kernel" is considered eligible.
-	for file in lfs.dir("/boot") do
+	for file, ftype in lfs.dir("/boot") do
 		local fname = "/boot/" .. file
 
 		if file == "." or file == ".." then
 			goto continue
 		end
 
-		if lfs.attributes(fname, "mode") ~= "directory" then
+		if ftype then
+			if ftype ~= lfs.DT_DIR then
+				goto continue
+			end
+		elseif lfs.attributes(fname, "mode") ~= "directory" then
 			goto continue
 		end