sysvar cache for globals in show variables is incorrect.

[morgo]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

SHOW VARIABLES LIKE 'max_connections'; # shows 151
SET GLOBAL max_connections=1234;
SHOW VARIABLES LIKE 'max_connections'; # shows 151; should show 1234

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

Session 1 should show the new value because the variable is GLOBAL-only scope.

3. What did you see instead (Required)

Session 1 shows the global variable unchanged, but because the setting is global-only scope it is expected to show the new value.

This is caused by the following code which caches the global variables into the session's systems cache, which is unsafe:

tidb/sessionctx/variable/varsutil.go

Lines 161 to 167 in 86b6429

	gVal, err = s.GlobalVarsAccessor.GetGlobalSysVar(key)
	if err != nil {
	return "", err
	}
	s.systems[key] = gVal
	return gVal, nil
	}

Unfortunately this caching is required to make subsequent calls to SHOW VARIABLES fast, but assuming #24359 merges all calls can be fast, and compatibility can be ensured.

4. What is your TiDB version? (Required)

mysql> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v4.0.0-beta.2-2749-gdbecb9e74-dirty
Edition: Community
Git Commit Hash: dbecb9e74538393906ac844082e60d520d73bacb
Git Branch: master
UTC Build Time: 2021-04-29 03:20:04
GoVersion: go1.13.15
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

[ti-srebot]

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