Global variables are not refreshed immediately in a new session

[xhebox]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

[xhe@PC tidb-test]$ cat t5.sh 
#!/bin/sh

mycli -h 127.0.0.1 -P 4000 -u root --no-warn << "EOF"
set global tidb_enable_alter_placement=on;
EOF

mycli -h 127.0.0.1 -P 4000 -u root --no-warn << "EOF"
select @@global.tidb_enable_alter_placement;
drop table if exists test.t1;
create table test.t1 (a int) partition by hash(a) partitions 2;
alter table test.t1 alter partition p0 alter placement policy role=leader;
EOF
[xhe@PC tidb-test]$ cat t4.sh 
#!/bin/sh
mycli -h 127.0.0.1 -P 4000 -u root --no-warn << "EOF"
select @@global.tidb_enable_alter_placement;
drop table if exists test.t1;
create table test.t1 (a int) partition by hash(a) partitions 2;
alter table test.t1 alter partition p0 alter placement policy role=leader;
EOF

# start a new tiup playground
[xhe@PC tidb-test]$ ./t5.sh 
@@global.tidb_enable_alter_placement
1
(1105, 'alter partition alter placement is experimental and it is switched off by tidb_enable_alter_placement')
[xhe@PC tidb-test]$ ./t4.sh 
@@global.tidb_enable_alter_placement
1

# start a new tiup playground
[xhe@PC tidb-test]$ ./t5.sh 
./@@global.tidb_enable_alter_placement
1
(1105, 'alter partition alter placement is experimental and it is switched off by tidb_enable_alter_placement')
[xhe@PC tidb-test]$ ./t4.sh 
@@global.tidb_enable_alter_placement
1
(1105, 'alter partition alter placement is experimental and it is switched off by tidb_enable_alter_placement')
[xhe@PC tidb-test]$ ./t4.sh 
@@global.tidb_enable_alter_placement
1

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

If I see select @@global.tidb_enable_alter_placement == 1, then the command should execute successfully. In another word, the sessionVars of session should be updated.

3. What did you see instead (Required)

An outdated session variable is causing SQL failure.

4. What is your TiDB version? (Required)

59ccb29, the current/latest master commit.

[xhebox]

a duplicated of #14531

[ti-srebot]

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[xhebox]

/label type/duplicate