fix: make SET system type case-insensitive

[fanyang01]

Resolves #2701

[jycor]

Thanks for the contribution!
MySQL does use the utf8mb4_0900_ai_ci collation for @@sql_mode, so we should change that, but only that variable and not all system set type

[jycor]

We shouldn't change all our system types to use that collation, and they should still use collation_default. Also it should be utf8mb4_0900_ai_ci and not utf8mb4_ascii_general_ci.

Instead, NewSystemSetType() should take in a sql.CollationID and we should pass it sql.Collation_utf8mb4_0900_ai_ci for sql_mode in go-msql-server/sql/variables/system_variables.go.

Fortunately, NewSystemSetType() is only used in a few places, so there won't be too many changes.

[fanyang01]

Hi @jycor, thanks for your review! I'm uncertain about which collation should be used for other system variables. Could you please direct me to where I can find MySQL's implementation?

mysql> set global log_output = 'table,file';
Query OK, 0 rows affected (0.00 sec)

mysql> select @@global.log_output;
+---------------------+
| @@global.log_output |
+---------------------+
| FILE,TABLE          |
+---------------------+
1 row in set (0.00 sec)

mysql> set global log_output = default;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@global.log_output;
+---------------------+
| @@global.log_output |
+---------------------+
| FILE                |
+---------------------+
1 row in set (0.00 sec)

mysql> set global protocol_compression_algorithms = 'zLIB,Zstd';
Query OK, 0 rows affected (0.01 sec)

mysql> select @@global.protocol_compression_algorithms;
+------------------------------------------+
| @@global.protocol_compression_algorithms |
+------------------------------------------+
| zLIB,Zstd                                |
+------------------------------------------+
1 row in set (0.01 sec)

mysql> set global protocol_compression_algorithms = default;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@global.protocol_compression_algorithms;
+------------------------------------------+
| @@global.protocol_compression_algorithms |
+------------------------------------------+
| zlib,zstd,uncompressed                   |
+------------------------------------------+
1 row in set (0.00 sec)

[fanyang01]

I have learned the following facts from MySQL's source code:

• protocol_compression_algorithms is defined to be a Sys_var_charptr and is validated using the case-insensitive collation of latin1.
• sql_mode and log_output are defined as Sys_var_set and are parsed using the to_upper function of system_charset_info.

[jycor]

Thanks for the changes and even digging into the MySQL source code.
LGTM!