Use the nix path for mysql/mariadb 'basedir'. Fixes #2524

[jefft]

Summary

As described on #2524, mysqld are using the local /usr for basedir (e.g. reading /usr/lib/mysql/plugins if present), when it should be using the nix equivalent.

How was it tested?

Warning: this is untested. I don't know how to recompile flake.nix, so I cheated and edited the mysqld wrapper with sudo vim $(which mysqld). This patch may be wrong, e.g. I'm not sure if $out will be expanded in those single-quotes.

But if the patch does what I hope, this is the effect it should have:

cd /tmp
devbox create mysqltest --template mysql
devbox run mysqld --verbose --help | grep ^basedir

With this patch, the command above should print something like:

basedir                                                      /nix/store/15pzxq00jcraacaxancf3zm6h444azpm-mysql-wrapped/

previously it would print:

basedir                                                      /usr/

Also, one can fully launch mysqld, connect with mysql and show the basedir with:

mysql> SHOW VARIABLES LIKE '%dir';
+-----------------------------+---------------------------------------------------------------------------------+
| Variable_name               | Value                                                                           |
+-----------------------------+---------------------------------------------------------------------------------+
| basedir                     | /nix/store/15pzxq00jcraacaxancf3zm6h444azpm-mysql-wrapped/                      |
| character_sets_dir          | /nix/store/15pzxq00jcraacaxancf3zm6h444azpm-mysql-wrapped/share/mysql/charsets/ |
| datadir                     | /tmp/mysqltest/.devbox/virtenv/mysql80/data/                                    |
| innodb_data_home_dir        |                                                                                 |
| innodb_doublewrite_dir      |                                                                                 |
| innodb_log_group_home_dir   | ./                                                                              |
| innodb_temp_tablespaces_dir | ./#innodb_temp/                                                                 |
| innodb_tmpdir               |                                                                                 |
| lc_messages_dir             | /nix/store/15pzxq00jcraacaxancf3zm6h444azpm-mysql-wrapped/share/mysql/          |
| plugin_dir                  | /nix/store/15pzxq00jcraacaxancf3zm6h444azpm-mysql-wrapped/lib/mysql/plugin/     |
| replica_load_tmpdir         | /tmp                                                                            |
| slave_load_tmpdir           | /tmp                                                                            |
| tmpdir                      | /tmp                                                                            |
+-----------------------------+---------------------------------------------------------------------------------+
13 rows in set (0.00 sec)

The basedir and related paths are now correctly pointing to /nix/store paths, and everything still works!