mysqldump: Got error: 1016: Can't open file: 'some_file' (errno: 24 - Too many open files) when using LOCK TABLES

This is a report and also the fix. Please patch this so I don't have to any more.

Error:
During run of automysqlbackup I get the following email:
mysqldump: Got error: 1016: Can't open file: 'some_file.frm' (errno: 24 - Too many open files) when using LOCK TABLES
mysqldump: Got error: 1016: Can't open file: 'some_file.frm' (errno: 24 - Too many open files) when using LOCK TABLES
mysqldump: Couldn't execute 'show events': Out of resources when opening file './mysql/event.MYD' (Errcode: 24 - Too many open files) (23)

Solution:
There is an error in the maximum number of open files. The source is not in the kernel configuration!

This solves this problem:
Error: mysql: too much open files error

Changing /lib/systemd/system/mysql.service and adding in the Service section 2 extra lines:

[Service]
... ... ...
LimitNOFILE=infinity
LimitMEMLOCK=infinity

After that:

systemctl daemon-reload
service mysql restart

Problem goes away (at least until I run an apt update again :-) )

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: mysql-server 5.7.22-0ubuntu0.16.04.1
ProcVersionSignature: Ubuntu 4.4.0-119.143-generic 4.4.114
Uname: Linux 4.4.0-119-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
Date: Wed Jun 13 10:06:48 2018
InstallationDate: Installed on 2012-07-01 (2172 days ago)
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
Logs.var.log.daemon.log:

MySQLConf.etc.mysql.conf.d.mysql.cnf: [mysql]
MySQLConf.etc.mysql.conf.d.mysqldump.cnf:
 [mysqldump]
 quick
 quote-names
 max_allowed_packet = 16M
MySQLConf.etc.mysql.mysql.conf.d.mysqld_safe_syslog.cnf:
 [mysqld_safe]
 syslog
MySQLVarLibDirListing: False
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=nl_NL.UTF-8
 SHELL=/bin/bash
SourcePackage: mysql-5.7
UpgradeStatus: Upgraded to xenial on 2016-09-24 (627 days ago)