MDEV-24452 ALTER TABLE event take infinite time which for example breaks mysql_upgrade

The problem was that update_timing_fields_for_event() didn't release all
MDL locks it took.

mysql-test/main/alter_events.result

+SET GLOBAL event_scheduler=1;
+CREATE TABLE t1 (a int primary key, b int) engine=innodb;
+insert into t1 values (1,1),(2,2),(3,3);
+CREATE TABLE t2 (a int primary key) engine=innodb;
+CREATE EVENT e_t1 ON SCHEDULE EVERY 1 SECOND DO insert ignore into test.t2 select a from test.t1;
+select * from t2;
+a
+1
+2
+3
+drop event e_t1;
+ALTER TABLE mysql.event DROP PRIMARY KEY, ADD PRIMARY KEY(db,name);
+drop table t1,t2;
+SET GLOBAL event_scheduler=default;

mysql-test/main/alter_events.test

+--source include/not_embedded.inc
+--source include/have_innodb.inc
+#--source include/have_metadata_lock_info.inc
+
+#
+# Testing of problems with ALTER TABLE and events on the same table(s)
+#
+
+#
+# MDEV-24452 ALTER TABLE event take infinite time which for example breaks
+#            mysql_upgrade
+#
+
+SET GLOBAL event_scheduler=1;
+
+CREATE TABLE t1 (a int primary key, b int) engine=innodb;
+insert into t1 values (1,1),(2,2),(3,3);
+CREATE TABLE t2 (a int primary key) engine=innodb;
+
+CREATE EVENT e_t1 ON SCHEDULE EVERY 1 SECOND DO insert ignore into test.t2 select a from test.t1;
+
+let $wait_condition=SELECT count(*) > 0 from t2;
+--source include/wait_condition.inc
+select * from t2;
+drop event e_t1;
+
+# select * from information_schema.metadata_lock_info;
+ALTER TABLE mysql.event DROP PRIMARY KEY, ADD PRIMARY KEY(db,name);
+drop table t1,t2;
+
+SET GLOBAL event_scheduler=default;

sql/event_db_repository.cc

@@ -1130,6 +1130,12 @@ update_timing_fields_for_event(THD *thd,
 
   DBUG_ASSERT(thd->security_ctx->master_access & PRIV_IGNORE_READ_ONLY);
 
+  /*
+    Take a savepoint to release only the lock on mysql.event
+    table at the end but keep the global read lock and
+    possible other locks taken by the caller.
+  */
+  MDL_savepoint mdl_savepoint= thd->mdl_context.mdl_savepoint();
   if (open_event_table(thd, TL_WRITE, &table))
     DBUG_RETURN(1);
 
@@ -1162,6 +1168,7 @@ update_timing_fields_for_event(THD *thd,
 end:
   if (thd->commit_whole_transaction_and_close_tables())
     ret= 1;
+  thd->mdl_context.rollback_to_savepoint(mdl_savepoint);
 
   DBUG_RETURN(MY_TEST(ret));
 }