Syntax:
RESTART

This statement stops and restarts the MySQL server. It requires the
SHUTDOWN privilege.

One use for RESTART is when it is not possible or convenient to gain
command-line access to the MySQL server on the server host to restart
it. For example, SET PERSIST_ONLY can be used at runtime to make
configuration changes to system variables that can be set only at
server startup, but the server must still be restarted for those
changes to take effect. The RESTART statement provides a way to do so
from within client sessions, without requiring command-line access on
the server host.

*Note*:

After executing a RESTART statement, the client can expect the current
connection to be lost. If auto-reconnect is enabled, the connection is
reestablished after the server restarts. Otherwise, the connection must
be reestablished manually.

A successful RESTART operation requires mysqld to be running in an
environment that has a monitoring process available to detect a server
shutdown performed for restart purposes:

o In the presence of a monitoring process, RESTART causes mysqld to
  terminate such that the monitoring process can determine that it
  should start a new mysqld instance.

o If no monitoring process is present, RESTART fails with an error.

These platforms provide the necessary monitoring support for the
RESTART statement:

o Windows, when mysqld is started as a Windows service or standalone.
  (mysqld forks, and one process acts as a monitor to the other, which
  acts as the server.)

o Unix and Unix-like systems that use systemd or mysqld_safe to manage
  mysqld.

To configure a monitoring environment such that mysqld enables the
RESTART statement:

1. Set the MYSQLD_PARENT_PID environment variable to the value of the
   process ID of the process that starts mysqld, before starting
   mysqld.

2. When mysqld performs a shutdown due to use of the RESTART statement,
   it returns exit code 16.

3. When the monitoring process detects an exit code of 16, it starts
   mysqld again. Otherwise, it exits.

Here is a minimal example as implemented in the bash shell:

#!/bin/bash

export MYSQLD_PARENT_PID=$$

export MYSQLD_RESTART_EXIT=16

while true ; do
  bin/mysqld mysqld options here
  if [ $? -ne $MYSQLD_RESTART_EXIT ]; then
    break
  fi
done

On Windows, the forking used to implement RESTART makes determining the
server process to attach to for debugging more difficult. To alleviate
this, starting the server with --gdb suppresses forking, in addition to
its other actions done to set up a debugging environment. In non-debug
settings, --no-monitor may be used for the sole purpose of suppressing
forking the monitor process. For a server started with either --gdb or
--no-monitor, executing RESTART causes the server to simply exit
without restarting.

URL: https://dev.mysql.com/doc/refman/8.4/en/restart.html

