MDEV-30924 Server crashes in MYSQL_LOG::is_open upon ALTER vs FUNCTION

ASAN showed use-after-free in binlog_online_alter_end_trans, during
running through thd->online_alter_cache_list.

In online_alter_binlog_get_cache_data, new_cache_data was allocated on
thd->mem_root, in case of autocommit=1, but this mem_root could be freed
in sp_head::execute, upon using stored functions.

It appears that thd->transaction->mem_root exists even in single-stmt
transaction mode (i.e autocommit=1), so it can be used in all cases.
This mem_root will remain valid till the end of transaction, including
commit phase.

mysql-test/main/alter_table_online_debug.result

@@ -1190,5 +1190,27 @@ set debug_sync= 'now signal goon';
 connection default;
 drop table t;
 #
+# MDEV-30924 Server crashes in MYSQL_LOG::is_open upon ALTER vs FUNCTION
+#
+create table t (a int);
+insert into t values (1),(2);
+create function f () returns int
+begin
+update t set a = 10;
+return 0;
+end $
+set debug_sync= 'alter_table_online_downgraded signal downgraded wait_for goon';
+alter table t force, algorithm=copy;
+connection con1;
+set debug_sync= 'now wait_for downgraded';
+select f();
+f()
+0
+set debug_sync= 'now signal goon';
+connection default;
+drop table t;
+drop function f;
+disconnect con1;
+#
 # End of 11.2 tests
 #

mysql-test/main/alter_table_online_debug.test

@@ -1370,6 +1370,34 @@ set debug_sync= 'now signal goon';
 --reap
 drop table t;
 
+--echo #
+--echo # MDEV-30924 Server crashes in MYSQL_LOG::is_open upon ALTER vs FUNCTION
+--echo #
+create table t (a int);
+insert into t values (1),(2);
+
+--delimiter $
+create function f () returns int
+begin
+  update t set a = 10;
+  return 0;
+end $
+--delimiter ;
+
+set debug_sync= 'alter_table_online_downgraded signal downgraded wait_for goon';
+send alter table t force, algorithm=copy;
+
+--connection con1
+set debug_sync= 'now wait_for downgraded';
+select f();
+set debug_sync= 'now signal goon';
+
+--connection default
+--reap
+drop table t;
+drop function f;
+--disconnect con1
+
 --echo #
 --echo # End of 11.2 tests
 --echo #

sql/log.cc

@@ -6449,8 +6449,7 @@ online_alter_cache_data *online_alter_binlog_get_cache_data(THD *thd, TABLE *tab
       return &cache;
   }
 
-  MEM_ROOT *root= thd->in_multi_stmt_transaction_mode()
-                  ? &thd->transaction->mem_root : thd->mem_root;
+  MEM_ROOT *root= &thd->transaction->mem_root;
   auto *new_cache_data= binlog_setup_cache_data(root, table->s);
   list.push_back(*new_cache_data);