Comment 2 for bug 1849802

I've repeated this today and searched through all logs on all nodes in the cloud, but the only further lines of relevance were on the nova-conductor service:

/var/log/nova/nova-conductor.log:2019-10-26 06:40:21.915 1042354 ERROR nova.network.neutronv2.api [req-2772ad1b-488b-41c7-9916-2808293415a4 349e52d708d542bab601d9698a3e4880 df9337688f7f4041bb2db4c4942de6a0 - 3e15c9d8c5254828a3fa73e76af71f75 3e15c9d8c5254828a3fa73e76af71f75] [instance: 8703d9db-81b0-4e86-a2ef-c4ba5250556c] Binding failed for port 5a3c5d23-8727-47d2-af72-a53b495358d2 and host shinx. Error: (409 {"NeutronError": {"type": "PortBindingAlreadyExists", "message": "Binding for port 5a3c5d23-8727-47d2-af72-a53b495358d2 on host shinx already exists.", "detail": ""}})
/var/log/nova/nova-conductor.log:2019-10-26 06:40:22.909 1042354 WARNING nova.scheduler.utils [req-2772ad1b-488b-41c7-9916-2808293415a4 349e52d708d542bab601d9698a3e4880 df9337688f7f4041bb2db4c4942de6a0 - 3e15c9d8c5254828a3fa73e76af71f75 3e15c9d8c5254828a3fa73e76af71f75] Failed to compute_task_migrate_server: Migration pre-check error: Binding failed for port 5a3c5d23-8727-47d2-af72-a53b495358d2, please check neutron logs for more information.: nova.exception.MigrationPreCheckError: Migration pre-check error: Binding failed for port 5a3c5d23-8727-47d2-af72-a53b495358d2, please check neutron logs for more information.
/var/log/nova/nova-conductor.log:2019-10-26 06:40:22.911 1042354 WARNING nova.scheduler.utils [req-2772ad1b-488b-41c7-9916-2808293415a4 349e52d708d542bab601d9698a3e4880 df9337688f7f4041bb2db4c4942de6a0 - 3e15c9d8c5254828a3fa73e76af71f75 3e15c9d8c5254828a3fa73e76af71f75] [instance: 8703d9db-81b0-4e86-a2ef-c4ba5250556c] Setting instance to ACTIVE state.: nova.exception.MigrationPreCheckError: Migration pre-check error: Binding failed for port 5a3c5d23-8727-47d2-af72-a53b495358d2, please check neutron logs for more information.

So I'm guessing perhaps an earlier attempt at migration got as far as attaching one of the ports for this instance on the destination host (shinx) but not detaching it on the source host, and now it has an entry for both hosts (I'm not particularly familiar with the schema, so I could be way off here).