Scheduled backups skip hidden courses, even if they have been modified

backup/backup_scheduled.php contains code that prevents hidden courses from being backed up if they have not been modified in the last month:

// Skip backup of unavailable courses that have remained unmodified in a month

$skipped = false;

if (!$course->visible && ($now - $course->timemodified) > 31*24*60*60) {  //Hidden + unmodified last month

    mtrace("            SKIPPING - hidden+unmodified");

    $DB->set_field("backup_courses","laststatus","3", array("courseid"=>$backup_course->courseid));

    $skipped = true;

}

However, $course->timemodified is only changed when the function "update_course" is called. This only happens when the course's "settings" page (course/edit.php) has been submitted with changes.

If a user is editing a course while it is hidden from students, it will not be backed up unless they change a setting.

Steps to reproduce:

1. Hide a course
2. In the database, change mdl_course.timemodified to a Unix timestamp older than 31 days, e.g. UPDATE mdl_course SET timemodified = UNIX_TIMESTAMP() - 32 * 86400
3. Make some changes to the course, e.g. add some resources.
4. Trigger a scheduled backup

Expected outcome: The course is backed up

Observed outcome: The course is skipped during scheduled backup, and no backup file is created.

I'm not sure what the best fix is. Something that queries mdl_log, perhaps, e.g.

SELECT MAX(time) from mdl_log WHERE course = <course id> AND action IN (<list of "update"-type actions>)

Alternatively, perhaps there should be a mechanism for updating timemodified in mdl_course when such changes are made.