ERROR log messages don't get sent to configured output file at startup

While having MySQL shut down on localhost, start kea-dhcp6 with:

{
  "Dhcp6": {
    "lease-database": {
      "name": "keatest",
      "password": "keatest",
      "user": "keatest",
      "type": "mysql"
    },
    "loggers": [
      {
        "debuglevel": 0,
        "name": "kea-dhcp6",
        "output_options": [
          {
            "output": "/var/log/kea-dhcp6.log"
          }
        ],
        "severity": "INFO"
      }
    ]
  }
}

You'll see the following messages in the log file:

INFO HOSTS_BACKENDS_REGISTERED
INFO DHCP6_CONFIG_COMPLETE
INFO DHCPSRV_MYSQL_DB opening

But the following messages are only sent to standard output and standard error:

INFO DHCP6_STARTING
WARN DHCP6_DEVELOPMENT_VERSION
ERROR DHCP6_CONFIG_LOAD_FAIL
ERROR DHCP6_INIT_FAIL

The INFO message logs predate the ERROR message logs so it's not like the logger isn't properly initialized when ERROR messages are logged.

Anyway, the suggestions are:

• initialize the logger as early as possible, maybe in a separate first pass of the configuration
• send as many logs as possible to the configured output

This is affecting customers who run Kea as a systemd service. Most packages install a systemd service. They can't see the reason of the failure unless they look in /var/log/messages or syslog instead of the configured /var/log/kea-dhcp6.log.

RT#18588