docker-registry hook fail nrpe-external-master-relation-joined

[Andy Wu]

when adding relation between docker-registry and nrpe charm, docker-registry failed with error
hook failed: "nrpe-external-master-relation-joined"

The debug log shows

Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-docker-registry-0/.venv/lib/python3.8/site-packages/charms/reactive/__init__.py", line 74, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-docker-registry-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-docker-registry-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-docker-registry-0/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-docker-registry-0/charm/reactive/docker_registry.py", line 376, in setup_nagios
    nagios.add_check(['/usr/lib/nagios/plugins/check_http',
  File "/var/lib/juju/agents/unit-docker-registry-0/charm/hooks/relations/nrpe-external-master/provides.py", line 21, in add_check
    unit = unit.replace('/', '-')
AttributeError: 'NoneType' object has no attribute 'replace'

In docker-registry code line 376 setup_nagios function has following content

    check_args = {}
    if charm_config.get('nagios_context'):
        check_args['context'] = charm_config['nagios_context']
    if charm_config.get('nagios_servicegroups'):
        check_args['servicegroups'] = charm_config['nagios_servicegroups']
    nagios.add_check(['/usr/lib/nagios/plugins/check_http',
                      '-I', '127.0.0.1', '-p', str(charm_config.get('registry-port')),
                      '-e', " 200 OK", '-u', '/'],
                     name="check_http",
                     description="Verify docker-registry is responding",
                     **check_args)

The check_args dict doesn't contain the unit argument that nagios.add_check is expecting , cause the hook fail

    # line 21 in var/lib/juju/agents/unit-docker-registry-0/charm/hooks/relations/nrpe-external-master/provides.py

    def add_check(self, args, name=None, description=None, context=None,
                  servicegroups=None, unit=None):
        unit = unit.replace('/', '-')

Charm version:
docker-registry 1.24/stable , rev 24
nrpe : latest/stable rev 96