kea4 crash when "remote-network4-del" command is issued with "subnets-action":"delete"

1. kea configuration:

    "Dhcp4": {
        "config-control": {
            "config-databases": [
                {
                    "name": "keadb",
                    "password": "keapass",
                    "type": "mysql",
                    "user": "keauser"
                }
            ]
        },
        "control-socket": {
            "socket-name": "/home/wlodek/installed/git/var/run/kea/control_socket",
            "socket-type": "unix"
        },
        "hooks-libraries": [
            {
                "library": "/home/wlodek/installed/git/lib/kea/hooks/libdhcp_cb_cmds.so"
            },
            {
                "library": "/home/wlodek/installed/git/lib/kea/hooks/libdhcp_mysql_cb.so"
            }
        ],
        "interfaces-config": {
            "interfaces": [
                "enp0s9"
            ]
        },
        "lease-database": {
            "type": "memfile"
        },
        "loggers": [
            {
                "debuglevel": 99,
                "name": "kea-dhcp4",
                "output_options": [
                    {
                        "output": "/home/wlodek/installed/git/var/log/kea.log"
                    }
                ],
                "severity": "DEBUG"
            }
        ],
        "server-tag": "abc"
    }

wlodek@debian9-64-2:kea (master) $ /home/wlodek/installed/git/sbin/kea-dhcp4 -V
1.6.0-beta-git
git 5511725555138213de4f48dc1091d65b5db47034
linked with:
log4cplus 1.1.2
OpenSSL 1.0.2s  28 May 2019
database:
MySQL backend 8.1, library 10.1.37-MariaDB
PostgreSQL backend 5.1, library 90613
Memfile backend 2.1

2. added server-tag "abc" to database
3. added network "net1" with server-tag "abc"

{'arguments': {'remote': {'type': 'mysql'},
               'server-tags': ['abc'],
               'shared-networks': [{'interface': 'enp0s9', 'name': 'net1'}]},
 'command': 'remote-network4-set',
 'service': ['dhcp4']}

  {
    "arguments": {
      "shared-networks": [
        {
          "name": "net1"
        }
      ]
    },
    "result": 0,
    "text": "IPv4 shared network successfully set."
  }

all is cool 4. now let's delete this network:

{'arguments': {'remote': {'type': 'mysql'},
               'shared-networks': [{'name': 'net1'}],
               'subnets-action': 'delete'},
 'command': 'remote-network4-del',
 'service': ['dhcp4']}

and we already getting message about dhcp4 service offline

  {
    "result": 1,
    "text": "unable to forward command to the dhcp4 service: End of file. The server is likely to be offline"
  }

but kea-ctrl-agent is actually forwarding this message, which kea logs shows:

2019-07-16 10:24:35.097 INFO  [kea-dhcp4.commands/2207] COMMAND_RECEIVED Received command 'remote-network4-del'
2019-07-16 10:24:35.097 DEBUG [kea-dhcp4.callouts/2207] HOOKS_CALLOUTS_BEGIN begin all callouts for hook $remote_network4_del
2019-07-16 10:24:35.097 DEBUG [kea-dhcp4.mysql-cb-hooks/2207] MYSQL_CB_GET_TYPE4 get type
2019-07-16 10:24:35.097 DEBUG [kea-dhcp4.mysql-cb-hooks/2207] MYSQL_CB_DELETE_SHARED_NETWORK_SUBNETS4 delete shared network: net1 subnets
2019-07-16 10:24:35.097 DEBUG [kea-dhcp4.database/2207] DATABASE_MYSQL_START_TRANSACTION starting new MySQL transaction

but it died during this performing -del; data from database was NOT removed, there is NO kea error log, message at the end of tests indicate that kea-dhcp4 is not working

FAILED[192.168.51.3] sudo: /home/wlodek/installed/git/sbin/keactrl stop
[192.168.51.3] out: INFO/keactrl: kea-dhcp4 isn't running.
[192.168.51.3] out: INFO/keactrl: kea-dhcp6 isn't running.
[192.168.51.3] out: INFO/keactrl: kea-dhcp-ddns isn't running.
[192.168.51.3] out: INFO/keactrl: Stopping kea-ctrl-agent...
[192.168.51.3] out: INFO/keactrl: kea-netconf isn't running.

and in fact there are leftovers:

wlodek@debian9-64-2:kea (master) $ ls -l /home/wlodek/installed/git/var/run/kea/
total 4
srwxr-xr-x 1 root root 0 Jul 16 10:24 control_socket
-rw-r--r-- 1 root root 5 Jul 16 10:24 kea.kea-dhcp4.pid

I don't have time to debug it but journalctl shows this error: debian9-64-2 kernel: kea-dhcp4[2207]: segfault at 4a1 ip 00007f28f6569484 sp 00007ffef7962d90 error 4 in libmariadbclient.so.18.0.0[7f28f653b000+37f000]

• problem occur no matter if there is or isn't subnet configured for network.
• problem NOT occur on "subnets-action":"keep"