Support querying MVCC information through builtin function

[tangenta]

Enhancement

MVCC information is useful for debugging data inconsistenct issue. For now, we have HTTP API mvcc/key/... to query MVCC. However, it is not always convenient to construct a HTTP query. For example, when there are complex data types in index columns or clustered primary key columns like timestamp, datetime or even binary, query parameters cannot represent these values.

Maybe we can provide a builtin function like tidb_mvcc_info() achieve this purpose.

Note: the priviege should be considered to make sure there are no security issues.

[tangenta]

@kennytm What do you think about this design?

[kennytm]

@tangenta LGTM, but I'm not sure if the input to tidb_mvcc_info should be a hex-encoded varchar or a direct varbinary (i.e. tidb_mvcc_info(X'7480000000000000705f728000000000000001')). Maybe too late to use varbinary given how tidb_decode_key works 🤷.

[kennytm]

BTW should these functions require some privilege check to be invoked

EDIT: No it definitely needs privilege check. Consider this scenario:

-- as root:
create user alice;
create table test.secrets (id bigint primary key, secret varchar(200) not null);
insert into test.secrets values (1, 'confidential data');

mysql> -- as alice:

mysql> select * from test.secrets; -- no privilege, good
ERROR 1142 (42000): SELECT command denied to user 'alice'@'%' for table 'secrets'

mysql> select tidb_encode_record_key('test', 'secrets', 1);  -- 🤔
+----------------------------------------------+
| tidb_encode_record_key('test', 'secrets', 1) |
+----------------------------------------------+
| 7480000000000000685f728000000000000001       |
+----------------------------------------------+
1 row in set (0.00 sec)

mysql> select tidb_mvcc_info('7480000000000000685f728000000000000001');  -- 🤔🤔🤔🤔🤔🤔🤔🤔🤔🤔🤔
+-------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_mvcc_info('7480000000000000685f728000000000000001')                                                                                  |
+-------------------------------------------------------------------------------------------------------------------------------------------+
| {"info":{"writes":[{"start_ts":450462836357332993,"commit_ts":450462836357332994,"short_value":"gAABAAAAAhEAY29uZmlkZW50aWFsIGRhdGE="}]}} |
+-------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> select cast(from_base64('gAABAAAAAhEAY29uZmlkZW50aWFsIGRhdGE=') as char charset latin1) a; -- 💣💣💣💣💣💣💣
+----------------------------+
| a                          |
+----------------------------+
|      confidential data  |
+----------------------------+
1 row in set (0.00 sec)

The simplest solution is probably require SUPER privilege for tidb_mvcc_info.