Unncessary session locks in ajax.php

[marxjohnson]

While doing some log analysis for a large Moodle site with session lock debugging enabled, I noticed a large proportion of the messages about long session locks came from mod/turnitintooltwo/ajax.php. For locks lasting more than 1 second, this script accounted for about 75%. For those lasting more than 3 seconds, 33%, and those lasting more than 5 seconds, 15%.

I took a loot at this script, and while some of the actions is provides do alter the session, most do not, so do not need to hold a session lock for the whole request.

This can be improved by releasing the session lock if it will not be needed, by adding the following after $action = optional_param():

if (!in_array($action, ['sync_all_submissions', 'get_submissions'])) {
    // No changes will be made to the session.
    \core\session\manager::write_close();
}

[aspark21]

this one also worth implementing @carl-hostrander

It will probably provide inspiration for turnitin/moodle-plagiarism_turnitin#503