Set default lock level to READ COMMITTED
Drupal recently changed the default lock level slightly down from REPEATABLE READ to READ COMMITTED, which has the effect of avoiding some db locks. See https://www.drupal.org/node/3269885 We have seen over recent years some issues with deadlocks, primarily on rebuilding group contact cache when there are hierarchies of smart groups. Should CiviCRM be more explicit about what isolation level we expect in order to get more repeatable results, and should we follow Drupal in its move to READ COMMITTED? I tend to think so.
See responses in MM chat from @demeritcowboy https://chat.civicrm.org/civicrm/pl/gnmhfdxes7rgb8k7ppoaezo45a