Ensure the given identities are in the iam role bindings for the topic.

def ensure_iam(self, publisher=None):
        """Ensure the given identities are in the iam role bindings for the topic.
        """
        topic = self.get_topic_param()
        client = self.session.client('pubsub', 'v1', 'projects.topics')
        policy = client.execute_command('getIamPolicy', {'resource': topic})
        policy.pop('etag')
        found = False
        for binding in policy.get('bindings', {}):
            if binding['role'] != 'roles/pubsub.publisher':
                continue
            if publisher in binding['members']:
                return
            found = binding

        if not found:
            policy.setdefault(
                'bindings', {'members': [publisher], 'role': 'roles/pubsub.publisher'})
        else:
            found['members'].append(publisher)

        client.execute_command('setIamPolicy', {'resource': topic, 'body': {'policy': policy}})