Elena Stepanova added a comment - 2021-01-12 12:18 - edited Thanks for the report. I am raising the priority, because it is likely to affect all users who use event scheduler. Starting from this commit in 10.5 commit 4102f1589c23309de968a5bf9511d3228a1b9319 Author: Monty Date: Sat May 2 13:19:53 2020 +0300   Aria will now register it's transactions after the first event is executed, the server acquires and keeps holding backup/metadata locks on mysql.event table – seemingly forever, as long as the event scheduler is enabled. It doesn't prevent further creation of events, but it blocks DDL on the table, hence the problem. I cannot tell whether this new locking routine was intentional; but even if it is, mysql_upgrade should somehow take it into account, maybe temporarily disable event_scheduler while it handles system tables. The test case below is dirty and shouldn't go as is into the regression suite, it's just a demonstration of the problem. install soname 'metadata_lock_info' ; --let $ts= `SELECT DATE_ADD(CURRENT_TIMESTAMP(), INTERVAL 3 SECOND)` eval create event ev1 on schedule every 1 minute starts '$ts' do set @a=1; set global event_scheduler= on ; sleep 2; show status like 'Executed_events' ; select * from information_schema.metadata_lock_info; sleep 2; show status like 'Executed_events' ; select * from information_schema.metadata_lock_info;   echo #; echo # If you now run ALTER TABLE mysql.event <something non-void>, it will hang; echo #; 10.5 8de233af set global event_scheduler= on ; show status like 'Executed_events' ; Variable_name Value Executed_events 0 select * from information_schema.metadata_lock_info; THREAD_ID LOCK_MODE LOCK_DURATION LOCK_TYPE TABLE_SCHEMA TABLE_NAME show status like 'Executed_events' ; Variable_name Value Executed_events 1 select * from information_schema.metadata_lock_info; THREAD_ID LOCK_MODE LOCK_DURATION LOCK_TYPE TABLE_SCHEMA TABLE_NAME 5 MDL_BACKUP_SYS_DML NULL Backup lock 5 MDL_SHARED_WRITE NULL Table metadata lock mysql event

Sergei Golubchik added a comment - 2021-01-13 16:32 I agree it's a serious bug. event scheduler should close the transaction after reading the event table.

Michael Widenius added a comment - 2021-01-21 16:35 The problem was that update_timing_fields_for_event() didn't release all MDL locks it took. (No problem with reading the event table or closing transactions

Michael Widenius added a comment - 2021-01-21 17:09 Fix pushed