Since [1], when a new agent is added, the event (SEGMENT_HOST_MAPPING, AFTER_CREATE) is called. That calls the method ``auto_schedule_new_network_segments`` [2]. If the ``segments`` plugin is not loaded, that will raise the following exception: [3]

"""

ERROR neutron_lib.callbacks.manager [None req-7a8042b5-ec95-432e-a03c-3770628cc424 None None] Error during notification for neutron.plugins.ml2.plugin.DhcpAgentSchedulerDbMixin.auto_schedule_new_network_segments-2354089 segment_host_mapping, after_create: AttributeError: 'NoneType' object has no attribute 'get_segments'

ERROR neutron_lib.callbacks.manager Traceback (most recent call last):

ERROR neutron_lib.callbacks.manager File "/opt/stack/neutron-lib/neutron_lib/callbacks/manager.py", line 189, in _notify_loop

ERROR neutron_lib.callbacks.manager callback.method(resource, event, trigger, payload=payload)

ERROR neutron_lib.callbacks.manager File "/opt/stack/neutron/neutron/db/agentschedulers_db.py", line 494, in auto_schedule_new_network_segments

ERROR neutron_lib.callbacks.manager segments = segment_plugin.get_segments(

ERROR neutron_lib.callbacks.manager AttributeError: 'NoneType' object has no attribute 'get_segments'

ERROR neutron_lib.callbacks.manager

"""

How to reproduce:

• Remove if present the ``segments`` plugin from the Neutron API configuration.
• Delete any OVN agent using the CLI: $ openstack network agent delete <>. For example, an OVN controller.
• Check the Neutron API logs.

[1]https://review.opendev.org/q/I8112076f8acb821752941396e7aa39ecb1352ca3
[2]https://github.com/openstack/neutron/blob/e47aed1508badcf2f52e0a5ced932abf9ff73655/neutron/db/agentschedulers_db.py#L487-L509
[3]https://paste.opendev.org/show/bZRtBWdRNT8o8roOSlm1/