unixsocket test of MariaDB (mysql) with unix_socket authentication plugin installed

[David Fletcher]

Hi,

After tracing an issue that was preventing Monit correctly monitoring
MariaDB, would a small update in the Monit manual be possible? I've
found this while updating an old installation on which the following
entry within a MySQL monitoring section worked perfectly:

if failed unixsocket /tmp/mysql.sock protocol mysql timeout 90 seconds
then restart

On a new installation with MariaDB version 10.11.3 on Raspbian 12 (based
on Debian 12), Monit (version 5.33.0) kept reporting that MariaDB could
not be started. Actually it was being started and then restarted
generating a mess, because Monit could not connect to the unix socket.

I found that disabling the unixsocket test enabled startup. But the cause
is that MariaDB 10.4.3 and later have a unix_socket authentication
plugin installed by default. This aims to check if the unix user
connecting to the socket has been granted permission on the database
resource they are trying to access.

Whether Monit's connection would succeed if I created a MariaDB grant
for the unix user Monit runs under I don't know. But I found it was
simpler to disable this plugin by inserting unix_socket=OFF into the
[maraidbd] section of /etc/mysql/my.cnf.

Since Ubuntu 15.10 based on Debian 9 this unix_socket authentication has
been a default so probably it's catching many people out. Could
something about this be added to the manual page to alert people to this
issue and work-around?
https://mmonit.com/monit/documentation/monit.html#MYSQL

Best regards, David.