def remove_from_aggregate(self, context, aggregate, host, slave_info=None):
        """Remove a compute host from an aggregate."""
        slave_info = slave_info or dict()
        if not pool_states.is_hv_pool(aggregate.metadata):
            return

        invalid = {pool_states.CREATED: _('no hosts to remove'),
                   pool_states.CHANGING: _('setup in progress'),
                   pool_states.DISMISSED: _('aggregate deleted')}
        if aggregate.metadata[pool_states.KEY] in invalid.keys():
            raise exception.InvalidAggregateActionDelete(
                    aggregate_id=aggregate.id,
                    reason=invalid[aggregate.metadata[pool_states.KEY]])

        master_compute = aggregate.metadata['master_compute']
        if master_compute == CONF.host and master_compute != host:
            # this is the master -> instruct it to eject a host from the pool
            host_uuid = aggregate.metadata[host]
            self._eject_slave(aggregate.id,
                              slave_info.get('compute_uuid'), host_uuid)
            aggregate.update_metadata({host: None})
        elif master_compute == host:
            # Remove master from its own pool -> destroy pool only if the
            # master is on its own, otherwise raise fault. Destroying a
            # pool made only by master is fictional
            if len(aggregate.hosts) > 1:
                # NOTE: this could be avoided by doing a master
                # re-election, but this is simpler for now.
                raise exception.InvalidAggregateActionDelete(
                                    aggregate_id=aggregate.id,
                                    reason=_('Unable to eject %s '
                                             'from the pool; pool not empty')
                                             % host)
            self._clear_pool(aggregate.id)
            aggregate.update_metadata({'master_compute': None, host: None})
        elif master_compute and master_compute != host:
            # A master exists -> forward pool-eject request to master
            slave_info = self._create_slave_info()

            self.compute_rpcapi.remove_aggregate_host(
                context, aggregate.id, host, master_compute, slave_info)
        else:
            # this shouldn't have happened
            raise exception.AggregateError(aggregate_id=aggregate.id,
                                           action='remove_from_aggregate',
                                           reason=_('Unable to eject %s '
                                           'from the pool; No master found')
                                           % host)
dataset/ETHPy150Open BU-NU-CLOUD-SP16/Trusted-Platform-Module-nova/nova/virt/xenapi/pool.py/ResourcePool.remove_from_aggregate/original