/var/run/mysqld not present after reboot

Symptoms: MySQL starts correctly after installation, but fails to start after the first reboot because it cannot create the PID/socket in /var/run/mysqld.

Root cause: the Debian package creates /var/run/mysqld during installation.

percona-xtradb-cluster-server-5.7_5.7.25-31.35-1.stretch_amd64.deb/deb://DEBIAN/preinst
...
MYSQLRUN=/var/run/mysqld
...
if [ ! -d ${MYSQLRUN} -a ! -L ${MYSQLRUN} ]; then
    mkdir ${MYSQLRUN}
    chown mysql:mysql ${MYSQLRUN}
    chmod 755 ${MYSQLRUN}
fi
...

After a reboot, /run (symlinked from /var/run) is recreated from scratch, so /var/run/mysqld no longer exists. The correct way to fix this on Debian/Ubuntu/CentOS is to use tmpfiles.d:

# cat /etc/tmpfiles.d/mysqld.conf
d    /var/run/mysqld    0755    mysql    mysql    -    -

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