ddl: the use of sys var `tidb_scatter_region` seems incorrect

[fzzf678]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a int);

-- global scope
mysql> set @@global.tidb_scatter_region='table';
Query OK, 0 rows affected (0.02 sec)

mysql> exit
Bye
(base) ➜  ~ mysql -h 127.0.0.1 -P 4001 --local-infile=1  -u root
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 44040196
Server version: 8.0.11-TiDB-v8.4.0-this-is-a-placeholder TiDB Server (Apache License 2.0) Community Edition, MySQL 8.0 compatible

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Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> truncate table t;
Query OK, 0 rows affected (32.54 sec)

-- session scope
mysql> set @@session.tidb_scatter_region='table';
Query OK, 0 rows affected (0.00 sec)

mysql> truncate table t;
Query OK, 0 rows affected (2 min 50.06 sec)

After set the sys var tidb_scatter_region and truncate a table, in function preSplitAndScatter we can't get the val, it's both empty in global and session scope:

• global:

• sesssion:

Maybe introduce by #56157

2. What did you expect to see? (Required)

Get tidb_scatter_region value

3. What did you see instead (Required)

Can't get tidb_scatter_region value

4. What is your TiDB version? (Required)

v8.5.1