in xoops directory there is a file name: mainfile.dist.php, which seems to have a similar content with mainfile.php.

should this file be patched too (with the include(XOOPS_ROOT_PATH . '/modules/protector/include/precheck.inc.php');)? what is file for anyway?

No, you don't need to.

mainfile.dist.php comes with Xoops and its only purpose is for the installation. When you first started installation, mainfile.php was empty and after you have finished the configuration process, Xoops created/overwrote mainfile.php by copying alll the information from mainfile.dist.php with your server, user and database information added.

If you compare the two files carefully, you will notice mainfile.dist.php does not have your server, user, and database information.

Hope this clarifies for you.

Thanks tl, for the clarification. it's clear no. thanks once again!