Display mDDLTableVersion in mysql.tidb table

[Defined2014]

Enhancement

We add mDDLTableVersion to control ddl related version update, like concurrency ddl and MDL. But it's hard to get current value by SQL now. I think we could display mDDLTableVersion in mysql.tidb just like currentBootstrapVersion.

[xinxiao]

/assign

[ivange94]

@xinxiao are you still working on this? I wanted to have a look if you aren't.

@Defined2014 is this issue still relevant? I see that it's now 7 months old. And there's this function that checks the mDDLTableVersion which is also tested in bootstrap_test.go. So I'm exactly sure what is required here.

// CheckDDLTableVersion check if the tables related to concurrent DDL exists.
func (m *Meta) CheckDDLTableVersion() (DDLTableVersion, error) {
	v, err := m.txn.Get(mDDLTableVersion)
	if err != nil {
		return -1, errors.Trace(err)
	}
	if string(v) == "" {
		return InitDDLTableVersion, nil
	}
	ver, err := strconv.Atoi(string(v))
	if err != nil {
		return -1, errors.Trace(err)
	}
	return DDLTableVersion(ver), nil
}

[Defined2014]

My idea is to display this variable in the mysql.tidb table, just like tidb_server_version shown below, @ivange94

mysql> select * from mysql.tidb;
+-----------------------+----------------+----------------------------------------------------+
| VARIABLE_NAME         | VARIABLE_VALUE | COMMENT                                            |
+-----------------------+----------------+----------------------------------------------------+
| bootstrapped          | True           | Bootstrap flag. Do not delete.                     |
| tidb_server_version   | 169            | Bootstrap version. Do not delete.                  |
| system_tz             | Asia/Shanghai  | TiDB Global System Timezone.                       |
| new_collation_enabled | True           | If the new collations are enabled. Do not edit it. |
+-----------------------+----------------+----------------------------------------------------+
4 rows in set (0.00 sec)

[lkshminarayanan]

/assign

[lkshminarayanan]

@Defined2014 I have raised a PR that implements what you have suggested. Can you review it?

[tangenta]

Add bug label in order to cherry-pick to v7.5.