Regarding select/poll bugs, for example this one: https://sourceware.org/bugzilla/show_bug.cgi?id=10352 (which is closed, but I believe I've seen active bugs too). I doubt MariaDB is actually affected by this, but didn't check.

Conversion script as it is now is only meaningful for upgrades, while original report is about clean installation.

I believe if we add extra pre-script it will further complicate our systemd script, while major purpose of all this effort was simplicity.

Right, let's increase the limit to 16K.

There's, of course, an option of chaging to User=root and mysqld --user=mysql, that is, starting mysqld as root. Then it'll be able to change open-files-limit and do memlock too, if requested.

Fixed by increasing the limit up to 16K.

If think that there should be a warning or an error if someone is trying to set the max_connection or table_(open|definition)_cache to a value too high which could lead the server to exhaust all its file descriptors.

And is there a warning thrown if the open_files_limit is set on my.cnf on a system using the systemd script?

There're two warnings, which are issued if --log-warnings is set and --open-files-limit is not set:

Changed limits: max_open_files: %u  max_connections: %ld  table_cache: %ld

Could not increase number of max_open_files to more than %u (request: %u)

There is no warning thrown if open_files_limit is set on a system using the systemd script.