Deleting a group with group messaging enabled can leave orphaned favourites

Master and 3.6 Testing (test both, patch differs!) Setup Create a fresh instance, but don't install. When testing master, checkout 9d4f4f0051290d0dc and install. When checking 3.6, checkout dcaa783741fe and install. Create a course with 2 students Create a group and place both students in it Enable group messaging for the group As student 1, log in and star the group conversation Do the same for student 2 Check the database for the favourite records: SELECT * FROM mdl_favourite WHERE component = 'core_message' and itemtype = 'message_conversations'; There should be 2 records.  Record the ids somewhere for BOTH master and 36! Now, as the admin, delete the course. Again, check the favourites records are present: SELECT * FROM mdl_favourite WHERE component = 'core_message' and itemtype = 'message_conversations'; There should be 2 records (these are the stale records). Now, upgrade. Now, checkout either master or MOODLE_36_STABLE (depending on which version you're currently testing) and upgrade. SELECT * FROM mdl_favourite WHERE id in (xxx); Replacing the xxx with the list of ids you stored earlier, eg. '1,2'. Verify that there are 0 records returned now. The stale records have been cleaned up. Create a course with 2 students Create a group and place both students in it Enable group messaging for the group As student 1, log in and star the group conversation Do the same for student 2 Check the database for the favourite records for both users: SELECT * FROM mdl_favourite WHERE component = 'core_message' and itemtype = 'message_conversations' and userid = X; SELECT * FROM mdl_favourite WHERE component = 'core_message' and itemtype = 'message_conversations' and userid = Y; Replacing 'X' and 'Y' with the ids of each user, respectively. On 36, verify there is 1 record for each student. On master,  verify there are 2 records for each student (master has self conversations automatically starred for the user) Now, as the admin, delete the course. Again, check the favourites records are present: SELECT * FROM mdl_favourite WHERE component = 'core_message' and itemtype = 'message_conversations' and userid = X; SELECT * FROM mdl_favourite WHERE component = 'core_message' and itemtype = 'message_conversations' and userid = Y; Replacing 'X' and 'Y' with the ids of each user, respectively. On 36, verify there are 0 records for each student. These were removed on course deletion. On master,  verify there is 1 records for each student (master has self conversations automatically starred for the user)