Steps to reproduce

1. Create a large course, say with 10,000 users, a few dozen groups each with a few thousand members, and several quizzes. (The OU has courses this big.)
2. Time doing a course reset to remove all group members. (Also look at the DB query count in the footer.)

Do this for Moodle versions both before and after MDL-39958 was integrated. (Note that MDL-39958 was committed with MDL-39959 is the issue id in the commit comment.)

That is the perofrmance problem. Now the nasty race condition.

1. Work out which order $DB->get_recordset('groups', array('courseid' => $courseid)); in groups_delete_group_members, in particular identify which goup will be deleted first, and which last.
2. For one quiz, set Time limits as follows:
   • Quiz closes: Now + 1 day
   • Quiz closes group override for last group to be processed: Now - 1 day
   • Quiz closes group override for first group to be processed: Now + 1 day
3. Set the quiz to have attempts become overdue when time expires, and to send an email notification to student when this happens.
4. Start an attempt as one student that is a member of both those groups. Answer some questions but don't submit.
5. Now, try to work out when cron will next run, and just beofre that, do a course reset to remove all the group members, Now also need to be between T + 1 minute and T + 1 day.

Expected result: The student does not get a notification, because initially the close date is Now + 1 day, and when they are removed from both groups it is Now + 1 day.

Possible result:
Cron runs between time the student is removed from the first group, and the time they are removed from the last group. Hence cron sees them as due to submit T - 1 day ago, and so makes their attempt overdue, and sends them an incorrect email.