MDEV-21476: auth_socket: add support for illumos with getpeerucred()

[citrus-it]

I have been packaging MariaDB 10.4 for the OmniOS operating system, which is a distribution of illumos (née OpenSolaris), and wanted to enable support for the auth_socket plugin.

illumos has getpeerucred() and so here's a patch to make auth_socket work on illumos platforms.
(It may also allow it to work on Solaris, which also has getpeerucred(), but I have not tested)

root@database:~# uname -a
SunOS sparse 5.11 omnios-r151032-702376803e i86pc i386 i86pc

root@database:~# mysql
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 8
Server version: 10.4.11-MariaDB OmniOS MariaDB Server

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> select current_user() from dual;
+----------------+
| current_user() |
+----------------+
| root@localhost |
+----------------+
1 row in set (0.000 sec)

MariaDB [(none)]> select user, password from mysql.user where user != '';
+-------+----------+
| User  | Password |
+-------+----------+
| root  | invalid  |
| mysql | invalid  |
+-------+----------+
2 rows in set (0.001 sec)

MariaDB [(none)]> show plugins soname where name = 'unix_socket';
+-------------+--------+----------------+---------+---------+
| Name        | Status | Type           | Library | License |
+-------------+--------+----------------+---------+---------+
| unix_socket | ACTIVE | AUTHENTICATION | NULL    | GPL     |
+-------------+--------+----------------+---------+---------+
1 row in set (0.014 sec)

[claassistantio]

All committers have signed the CLA.

[an3l]

Hi @citrus-it,
thank you for the patch.
The patch is for 10.5, but you said you are doing packaging for 10.4, which version is from interest?

[citrus-it]

Hi @citrus-it,
thank you for the patch.
The patch is for 10.5, but you said you are doing packaging for 10.4, which version is from interest?

I'm using this patch on 10.4 but I was intending to contribute the patch to trunk so that it will be in the next version. I could do a separate PR against 10.4 too if it's of interest (the code hasn't changed between the two).

[an3l]

Not needed, just asking about the intention of the patch. Someone will take a look on this.
Thanks.

[vuvova]

Looks good to me.

[grooverdan]

Visually inspected flows of code and cmake definations (and laughed at existing comments).

Looks good to me.