Comment 0 for bug 1917645

We use independent RabbitMQ clusters for each OpenStack project, Nova Cells and also for notifications. Recently, I noticed in our test infrastructure that if the RabbitMQ cluster for notifications has an outage, Nova can't create new instances. Possibly other operations will also hang.

Not being able to send a notification/connect to the RabbitMQ cluster shouldn't stop new instances to be created. (If this is actually an use-case for some deployments, the operator should have the possibility to configure it.)

Tested against the master branch.

If the notification RabbitMQ is stooped, when creating an instance, nova-scheduler is stuck with:

```
Mar 01 21:16:28 devstack nova-scheduler[18384]: DEBUG nova.scheduler.request_filter [None req-353318d1-f4bd-499d-98db-a0919d28ecf7 demo demo] Request filter 'accelerators_filter' took 0.0 seconds {{(pid=18384) wrapper /opt/stack/nova/nova/scheduler/request_filter.py:46}}
Mar 01 21:16:32 devstack nova-scheduler[18384]: ERROR oslo.messaging._drivers.impl_rabbit [None req-353318d1-f4bd-499d-98db-a0919d28ecf7 demo demo] Connection failed: [Errno 113] EHOSTUNREACH (retrying in 2.0 seconds): OSError: [Errno 113] EHOSTUNREACH
Mar 01 21:16:35 devstack nova-scheduler[18384]: ERROR oslo.messaging._drivers.impl_rabbit [None req-353318d1-f4bd-499d-98db-a0919d28ecf7 demo demo] Connection failed: [Errno 113] EHOSTUNREACH (retrying in 4.0 seconds): OSError: [Errno 113] EHOSTUNREACH
Mar 01 21:16:42 devstack nova-scheduler[18384]: ERROR oslo.messaging._drivers.impl_rabbit [None req-353318d1-f4bd-499d-98db-a0919d28ecf7 demo demo] Connection failed: [Errno 113] EHOSTUNREACH (retrying in 6.0 seconds): OSError: [Errno 113] EHOSTUNREACH
Mar 01 21:16:51 devstack nova-scheduler[18384]: ERROR oslo.messaging._drivers.impl_rabbit [None req-353318d1-f4bd-499d-98db-a0919d28ecf7 demo demo] Connection failed: [Errno 113] EHOSTUNREACH (retrying in 8.0 seconds): OSError: [Errno 113] EHOSTUNREACH
Mar 01 21:17:02 devstack nova-scheduler[18384]: ERROR oslo.messaging._drivers.impl_rabbit [None req-353318d1-f4bd-499d-98db-a0919d28ecf7 demo demo] Connection failed: [Errno 113] EHOSTUNREACH (retrying in 10.0 seconds): OSError: [Errno 113] EHOSTUNREACH
(...)
```

Because the notification RabbitMQ cluster is down, it fails to send:

https://github.com/openstack/nova/blob/5b66caab870558b8a7f7b662c01587b959ad3d41/nova/scheduler/filter_scheduler.py#L85

because oslo messaging never gives up:

https://github.com/openstack/oslo.messaging/blob/5aa645b38b4c1cf08b00e687eb6c7c4b8a0211fc/oslo_messaging/_drivers/impl_rabbit.py#L736