def maintain_leases(self):
        """Maintain all of the leases being managed.

        This method modifies the ack deadline for all of the managed
        ack IDs, then waits for most of that time (but with jitter), and
        repeats.
        """
        while self._manager.is_active and not self._stop_event.is_set():
            # Determine the appropriate duration for the lease. This is
            # based off of how long previous messages have taken to ack, with
            # a sensible default and within the ranges allowed by Pub/Sub.
            p99 = self._manager.ack_histogram.percentile(99)
            _LOGGER.debug("The current p99 value is %d seconds.", p99)

            # Make a copy of the leased messages. This is needed because it's
            # possible for another thread to modify the dictionary while
            # we're iterating over it.
            leased_messages = copy.copy(self._leased_messages)

            # Drop any leases that are well beyond max lease time. This
            # ensures that in the event of a badly behaving actor, we can
            # drop messages and allow Pub/Sub to resend them.
            cutoff = time.time() - self._manager.flow_control.max_lease_duration
            to_drop = [
                requests.DropRequest(ack_id, item.size)
                for ack_id, item in six.iteritems(leased_messages)
                if item.added_time < cutoff
            ]

            if to_drop:
                _LOGGER.warning(
                    "Dropping %s items because they were leased too long.", len(to_drop)
                )
                self._manager.dispatcher.drop(to_drop)

            # Remove dropped items from our copy of the leased messages (they
            # have already been removed from the real one by
            # self._manager.drop(), which calls self.remove()).
            for item in to_drop:
                leased_messages.pop(item.ack_id)

            # Create a streaming pull request.
            # We do not actually call `modify_ack_deadline` over and over
            # because it is more efficient to make a single request.
            ack_ids = leased_messages.keys()
            if ack_ids:
                _LOGGER.debug("Renewing lease for %d ack IDs.", len(ack_ids))

                # NOTE: This may not work as expected if ``consumer.active``
                #       has changed since we checked it. An implementation
                #       without any sort of race condition would require a
                #       way for ``send_request`` to fail when the consumer
                #       is inactive.
                self._manager.dispatcher.modify_ack_deadline(
                    [requests.ModAckRequest(ack_id, p99) for ack_id in ack_ids]
                )

            # Now wait an appropriate period of time and do this again.
            #
            # We determine the appropriate period of time based on a random
            # period between 0 seconds and 90% of the lease. This use of
            # jitter (http://bit.ly/2s2ekL7) helps decrease contention in cases
            # where there are many clients.
            snooze = random.uniform(0.0, p99 * 0.9)
            _LOGGER.debug("Snoozing lease management for %f seconds.", snooze)
            self._stop_event.wait(timeout=snooze)

        _LOGGER.info("%s exiting.", _LEASE_WORKER_NAME)
Maintain all of the leases being managed.

        This method modifies the ack deadline for all of the managed
        ack IDs, then waits for most of that time (but with jitter), and
        repeats.