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[Matt Riedemann <mriedem.os@xxxxxxxxx>]

** Also affects: nova/rocky
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1746863

Title:
  scheduler affinity doesn't work with multiple cells

Status in OpenStack Compute (nova):
  In Progress
Status in OpenStack Compute (nova) pike series:
  Confirmed
Status in OpenStack Compute (nova) queens series:
  Confirmed
Status in OpenStack Compute (nova) rocky series:
  New

Bug description:
  I happened upon this while hacking on my WIP CellDatabases fixture
  patch.

  Some of the nova/tests/functional/test_server_group.py tests started
  failing with multiple cells and I found that it's because there's a
  database query 'objects.InstanceList.get_by_filters' for all instances
  who are members of the server group to do the affinity check. The
  query for instances doesn't check all cells, so it fails to return any
  hosts that group members are currently on.

  This makes the ServerGroup[Anti|]AffinityFilter a no-op for multiple
  cells. Affinity is checked again via the late-affinity check in
  compute, but compute is using the same InstanceGroup.get_hosts method
  and will only find group member's hosts that are in its cell.

  This is the code that populates the RequestSpec.instance_group.hosts via a
  lazy-load on first access:

  nova/objects/instance_group.py:

      def obj_load_attr(self, attrname):
          ...
          self.hosts = self.get_hosts()
          self.obj_reset_changes(['hosts'])

      ...

      @base.remotable
      def get_hosts(self, exclude=None):
          """Get a list of hosts for non-deleted instances in the group
          This method allows you to get a list of the hosts where instances in
          this group are currently running.  There's also an option to exclude
          certain instance UUIDs from this calculation.
          """
          filter_uuids = self.members
          if exclude:
              filter_uuids = set(filter_uuids) - set(exclude)
          filters = {'uuid': filter_uuids, 'deleted': False}
          instances = objects.InstanceList.get_by_filters(self._context,
                                                          filters=filters)
          return list(set([instance.host for instance in instances
                           if instance.host]))

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