This document describes the current stable version of Celery (5.6). For development docs, go here.

# Source code for celery.worker.consumer.consumer

    """Worker Consumer Blueprint.

    This module contains the components responsible for consuming messages
    from the broker, processing the messages and keeping the broker connections
    up and running.
    """
    import errno
    import logging
    import os
    import warnings
    from collections import defaultdict
    from time import sleep

    from billiard.common import restart_state
    from billiard.exceptions import RestartFreqExceeded
    from kombu.asynchronous.semaphore import DummyLock
    from kombu.exceptions import ContentDisallowed, DecodeError
    from kombu.utils.compat import _detect_environment
    from kombu.utils.encoding import safe_repr
    from kombu.utils.limits import TokenBucket
    from vine import ppartial, promise

    from celery import bootsteps, signals
    from celery.app.trace import build_tracer
    from celery.exceptions import (CPendingDeprecationWarning, InvalidTaskError, NotRegistered, WorkerShutdown,
                                   WorkerTerminate)
    from celery.utils.functional import noop
    from celery.utils.log import get_logger
    from celery.utils.nodenames import gethostname
    from celery.utils.objects import Bunch
    from celery.utils.text import truncate
    from celery.utils.time import humanize_seconds, rate
    from celery.worker import loops
    from celery.worker.state import (active_requests, maybe_shutdown, requests, reserved_requests, successful_requests,
                                     task_reserved)

    __all__ = ('Consumer', 'Evloop', 'dump_body')

    CLOSE = bootsteps.CLOSE
    TERMINATE = bootsteps.TERMINATE
    STOP_CONDITIONS = {CLOSE, TERMINATE}
    logger = get_logger(__name__)
    debug, info, warn, error, crit = (logger.debug, logger.info, logger.warning,
                                      logger.error, logger.critical)

    CONNECTION_RETRY = """\
    consumer: Connection to broker lost. \
    Trying to re-establish the connection...\
    """

    CONNECTION_RETRY_STEP = """\
    Trying again {when}... ({retries}/{max_retries})\
    """

    CONNECTION_ERROR = """\
    consumer: Cannot connect to %s: %s.
    %s
    """

    CONNECTION_FAILOVER = """\
    Will retry using next failover.\
    """

    UNKNOWN_FORMAT = """\
    Received and deleted unknown message.  Wrong destination?!?

    The full contents of the message body was: %s
    """

    #: Error message for when an unregistered task is received.
    UNKNOWN_TASK_ERROR = """\
    Received unregistered task of type %s.
    The message has been ignored and discarded.

    Did you remember to import the module containing this task?
    Or maybe you're using relative imports?

    Please see
    https://docs.celeryq.dev/en/latest/internals/protocol.html
    for more information.

    The full contents of the message body was:
    %s

    The full contents of the message headers:
    %s

    The delivery info for this task is:
    %s
    """

    #: Error message for when an invalid task message is received.
    INVALID_TASK_ERROR = """\
    Received invalid task message: %s
    The message has been ignored and discarded.

    Please ensure your message conforms to the task
    message protocol as described here:
    https://docs.celeryq.dev/en/latest/internals/protocol.html

    The full contents of the message body was:
    %s
    """

    MESSAGE_DECODE_ERROR = """\
    Can't decode message body: %r [type:%r encoding:%r headers:%s]

    body: %s
    """

    MESSAGE_REPORT = """\
    body: {0}
    {{content_type:{1} content_encoding:{2}
      delivery_info:{3} headers={4}}}
    """

    TERMINATING_TASK_ON_RESTART_AFTER_A_CONNECTION_LOSS = """\
    Task %s cannot be acknowledged after a connection loss since late acknowledgement is enabled for it.
    Terminating it instead.
    """

    CANCEL_TASKS_BY_DEFAULT = """
    In Celery 5.1 we introduced an optional breaking change which
    on connection loss cancels all currently executed tasks with late acknowledgement enabled.
    These tasks cannot be acknowledged as the connection is gone, and the tasks are automatically redelivered
    back to the queue. You can enable this behavior using the worker_cancel_long_running_tasks_on_connection_loss
    setting. In Celery 5.1 it is set to False by default. The setting will be set to True by default in Celery 6.0.
    """

    def dump_body(m, body):
        """Format message body for debugging purposes."""
        # v2 protocol does not deserialize body
        body = m.body if body is None else body
        return '{} ({}b)'.format(truncate(safe_repr(body), 1024),
                                 len(m.body))

    class Consumer:
        """Consumer blueprint."""

        Strategies = dict

        #: Optional callback called the first time the worker
        #: is ready to receive tasks.
        init_callback = None

        #: The current worker pool instance.
        pool = None

        #: A timer used for high-priority internal tasks, such
        #: as sending heartbeats.
        timer = None

        restart_count = -1  # first start is the same as a restart

        #: This flag will be turned off after the first failed
        #: connection attempt.
        first_connection_attempt = True

        #: Counter to track number of conn retry attempts
        #: to broker. Will be reset to 0 once successful
        broker_connection_retry_attempt = 0

        class Blueprint(bootsteps.Blueprint):
            """Consumer blueprint."""

            name = 'Consumer'
            default_steps = [
                'celery.worker.consumer.connection:Connection',
                'celery.worker.consumer.mingle:Mingle',
                'celery.worker.consumer.events:Events',
                'celery.worker.consumer.gossip:Gossip',
                'celery.worker.consumer.heart:Heart',
                'celery.worker.consumer.control:Control',
                'celery.worker.consumer.tasks:Tasks',
                'celery.worker.consumer.delayed_delivery:DelayedDelivery',
                'celery.worker.consumer.consumer:Evloop',
                'celery.worker.consumer.agent:Agent',
            ]

            def shutdown(self, parent):
                self.send_all(parent, 'shutdown')

        def __init__(self, on_task_request,
                     init_callback=noop, hostname=None,
                     pool=None, app=None,
                     timer=None, controller=None, hub=None, amqheartbeat=None,
                     worker_options=None, disable_rate_limits=False,
                     initial_prefetch_count=2, prefetch_multiplier=1, **kwargs):
            self.app = app
            self.controller = controller
            self.init_callback = init_callback
            self.hostname = hostname or gethostname()
            self.pid = os.getpid()
            self.pool = pool
            self.timer = timer
            self.strategies = self.Strategies()
            self.conninfo = self.app.connection_for_read()
            self.connection_errors = self.conninfo.connection_errors
            self.channel_errors = self.conninfo.channel_errors
            self._restart_state = restart_state(maxR=5, maxT=1)

            self._does_info = logger.isEnabledFor(logging.INFO)
            self._limit_order = 0
            self.on_task_request = on_task_request
            self.on_task_message = set()
            self.amqheartbeat_rate = self.app.conf.broker_heartbeat_checkrate
            self.disable_rate_limits = disable_rate_limits
            self.initial_prefetch_count = initial_prefetch_count
            self.prefetch_multiplier = prefetch_multiplier
            self._maximum_prefetch_restored = True

            # this contains a tokenbucket for each task type by name, used for
            # rate limits, or None if rate limits are disabled for that task.
            self.task_buckets = defaultdict(lambda: None)
            self.reset_rate_limits()

            self.hub = hub
            if self.hub or getattr(self.pool, 'is_green', False):
                self.amqheartbeat = amqheartbeat
                if self.amqheartbeat is None:
                    self.amqheartbeat = self.app.conf.broker_heartbeat
            else:
                self.amqheartbeat = 0

            if not hasattr(self, 'loop'):
                self.loop = loops.asynloop if hub else loops.synloop

            if _detect_environment() == 'gevent':
                # there's a gevent bug that causes timeouts to not be reset,
                # so if the connection timeout is exceeded once, it can NEVER
                # connect again.
                self.app.conf.broker_connection_timeout = None

            self._pending_operations = []

            self.steps = []
            self.blueprint = self.Blueprint(
                steps=self.app.steps['consumer'],
                on_close=self.on_close,
            )
            self.blueprint.apply(self, **dict(worker_options or {}, **kwargs))

        def call_soon(self, p, *args, **kwargs):
            p = ppartial(p, *args, **kwargs)
            if self.hub:
                return self.hub.call_soon(p)
            self._pending_operations.append(p)
            return p

        def perform_pending_operations(self):
            if not self.hub:
                while self._pending_operations:
                    try:
                        self._pending_operations.pop()()
                    except Exception as exc:  # pylint: disable=broad-except
                        logger.exception('Pending callback raised: %r', exc)

        def bucket_for_task(self, type):
            limit = rate(getattr(type, 'rate_limit', None))
            return TokenBucket(limit, capacity=1) if limit else None

        def reset_rate_limits(self):
            self.task_buckets.update(
                (n, self.bucket_for_task(t)) for n, t in self.app.tasks.items()
            )

        def _update_prefetch_count(self, index=0):
            """Update prefetch count after pool/shrink grow operations.

            Index must be the change in number of processes as a positive
            (increasing) or negative (decreasing) number.

            Note:
                Currently pool grow operations will end up with an offset
                of +1 if the initial size of the pool was 0 (e.g.
                :option:`--autoscale=1,0 <celery worker --autoscale>`).
            """
            num_processes = self.pool.num_processes
            if not self.initial_prefetch_count or not num_processes:
                return  # prefetch disabled
            self.initial_prefetch_count = (
                self.pool.num_processes * self.prefetch_multiplier
            )
            return self._update_qos_eventually(index)

        def _update_qos_eventually(self, index):
            return (self.qos.decrement_eventually if index < 0
                    else self.qos.increment_eventually)(
                abs(index) * self.prefetch_multiplier)

        def _limit_move_to_pool(self, request):
            task_reserved(request)
            self.on_task_request(request)

        def _schedule_bucket_request(self, bucket):
            while True:
                try:
                    request, tokens = bucket.pop()
                except IndexError:
                    # no request, break
                    break

                if bucket.can_consume(tokens):
                    self._limit_move_to_pool(request)
                    continue
                else:
                    # requeue to head, keep the order.
                    bucket.contents.appendleft((request, tokens))

                    pri = self._limit_order = (self._limit_order + 1) % 10
                    hold = bucket.expected_time(tokens)
                    self.timer.call_after(
                        hold, self._schedule_bucket_request, (bucket,),
                        priority=pri,
                    )
                    # no tokens, break
                    break

        def _limit_task(self, request, bucket, tokens):
            bucket.add((request, tokens))
            return self._schedule_bucket_request(bucket)

        def _limit_post_eta(self, request, bucket, tokens):
            self.qos.decrement_eventually()
            bucket.add((request, tokens))
            return self._schedule_bucket_request(bucket)

        def start(self):
            blueprint = self.blueprint
            while blueprint.state not in STOP_CONDITIONS:
                maybe_shutdown()
                if self.restart_count:
                    try:
                        self._restart_state.step()
                    except RestartFreqExceeded as exc:
                        crit('Frequent restarts detected: %r', exc, exc_info=1)
                        sleep(1)
                self.restart_count += 1
                if self.app.conf.broker_channel_error_retry:
                    recoverable_errors = (self.connection_errors + self.channel_errors)
                else:
                    recoverable_errors = self.connection_errors
                try:
                    blueprint.start(self)
                except recoverable_errors as exc:
                    # If we're not retrying connections, we need to properly shutdown or terminate
                    # the Celery main process instead of abruptly aborting the process without any cleanup.
                    is_connection_loss_on_startup = self.first_connection_attempt
                    self.first_connection_attempt = False
                    connection_retry_type = self._get_connection_retry_type(is_connection_loss_on_startup)
                    connection_retry = self.app.conf[connection_retry_type]
                    if not connection_retry:
                        crit(
                            f"Retrying to {'establish' if is_connection_loss_on_startup else 're-establish'} "
                            f"a connection to the message broker after a connection loss has "
                            f"been disabled (app.conf.{connection_retry_type}=False). Shutting down..."
                        )
                        raise WorkerShutdown(1) from exc
                    if isinstance(exc, OSError) and exc.errno == errno.EMFILE:
                        crit("Too many open files. Aborting...")
                        raise WorkerTerminate(1) from exc
                    maybe_shutdown()
                    if blueprint.state not in STOP_CONDITIONS:
                        if self.connection:
                            self.on_connection_error_after_connected(exc)
                        else:
                            self.on_connection_error_before_connected(exc)
                        self.on_close()
                        blueprint.restart(self)

        def _get_connection_retry_type(self, is_connection_loss_on_startup):
            return ('broker_connection_retry_on_startup'
                    if (is_connection_loss_on_startup
                        and self.app.conf.broker_connection_retry_on_startup is not None)
                    else 'broker_connection_retry')

        def on_connection_error_before_connected(self, exc):
            error(CONNECTION_ERROR, self.conninfo.as_uri(), exc,
                  'Trying to reconnect...')

        def on_connection_error_after_connected(self, exc):
            warn(CONNECTION_RETRY, exc_info=True)
            try:
                self.connection.collect()
            except Exception:  # pylint: disable=broad-except
                pass

            if self.app.conf.worker_cancel_long_running_tasks_on_connection_loss:
                for request in tuple(active_requests):
                    if request.task.acks_late and not request.acknowledged:
                        warn(TERMINATING_TASK_ON_RESTART_AFTER_A_CONNECTION_LOSS,
                             request)
                        request.cancel(self.pool)
            else:
                warnings.warn(CANCEL_TASKS_BY_DEFAULT, CPendingDeprecationWarning)

            if self.app.conf.worker_enable_prefetch_count_reduction:
                self.initial_prefetch_count = max(
                    self.prefetch_multiplier,
                    self.max_prefetch_count - len(tuple(active_requests)) * self.prefetch_multiplier
                )

                self._maximum_prefetch_restored = self.initial_prefetch_count == self.max_prefetch_count
                if not self._maximum_prefetch_restored:
                    logger.info(
                        f"Temporarily reducing the prefetch count to {self.initial_prefetch_count} to avoid "
                        f"over-fetching since {len(tuple(active_requests))} tasks are currently being processed.\n"
                        f"The prefetch count will be gradually restored to {self.max_prefetch_count} as the tasks "
                        "complete processing."
                    )

        def register_with_event_loop(self, hub):
            self.blueprint.send_all(
                self, 'register_with_event_loop', args=(hub,),
                description='Hub.register',
            )

        def shutdown(self):
            self.perform_pending_operations()
            self.blueprint.shutdown(self)

        def stop(self):
            self.blueprint.stop(self)

        def on_ready(self):
            callback, self.init_callback = self.init_callback, None
            if callback:
                callback(self)

        def loop_args(self):
            return (self, self.connection, self.task_consumer,
                    self.blueprint, self.hub, self.qos, self.amqheartbeat,
                    self.app.clock, self.amqheartbeat_rate)

        def on_decode_error(self, message, exc):
            """Callback called if an error occurs while decoding a message.

            Simply logs the error and acknowledges the message so it
            doesn't enter a loop.

            Arguments:
                message (kombu.Message): The message received.
                exc (Exception): The exception being handled.
            """
            crit(MESSAGE_DECODE_ERROR,
                 exc, message.content_type, message.content_encoding,
                 safe_repr(message.headers), dump_body(message, message.body),
                 exc_info=1)
            message.ack()

        def on_close(self):
            # Clear internal queues to get rid of old messages.
            # They can't be acked anyway, as a delivery tag is specific
            # to the current channel.
            if self.controller and self.controller.semaphore:
                self.controller.semaphore.clear()
            for bucket in self.task_buckets.values():
                if bucket:
                    bucket.clear_pending()
            for request_id in reserved_requests:
                if request_id in requests:
                    del requests[request_id]
            reserved_requests.clear()
            if self.pool and self.pool.flush:
                self.pool.flush()

        def connect(self):
            """Establish the broker connection used for consuming tasks.

            Retries establishing the connection if the
            :setting:`broker_connection_retry` setting is enabled
            """
            conn = self.connection_for_read(heartbeat=self.amqheartbeat)
            if self.hub:
                conn.transport.register_with_event_loop(conn.connection, self.hub)
            return conn

        def connection_for_read(self, heartbeat=None):
            return self.ensure_connected(
                self.app.connection_for_read(heartbeat=heartbeat))

        def connection_for_write(self, url=None, heartbeat=None):
            return self.ensure_connected(
                self.app.connection_for_write(url=url, heartbeat=heartbeat))

        def ensure_connected(self, conn):
            # Callback called for each retry while the connection
            # can't be established.
            def _error_handler(exc, interval, next_step=CONNECTION_RETRY_STEP):
                if getattr(conn, 'alt', None) and interval == 0:
                    next_step = CONNECTION_FAILOVER
                elif interval > 0:
                    self.broker_connection_retry_attempt += 1
                next_step = next_step.format(
                    when=humanize_seconds(interval, 'in', ' '),
                    retries=self.broker_connection_retry_attempt,
                    max_retries=self.app.conf.broker_connection_max_retries)
                error(CONNECTION_ERROR, conn.as_uri(), exc, next_step)

            # Remember that the connection is lazy, it won't establish
            # until needed.

            # TODO: Rely only on broker_connection_retry_on_startup to determine whether connection retries are disabled.
            #       We will make the switch in Celery 6.0.

            retry_disabled = False

            if self.app.conf.broker_connection_retry_on_startup is None:
                # If broker_connection_retry_on_startup is not set, revert to broker_connection_retry
                # to determine whether connection retries are disabled.
                retry_disabled = not self.app.conf.broker_connection_retry

                if retry_disabled:
                    warnings.warn(
                        CPendingDeprecationWarning(
                            "The broker_connection_retry configuration setting will no longer determine\n"
                            "whether broker connection retries are made during startup in Celery 6.0 and above.\n"
                            "If you wish to refrain from retrying connections on startup,\n"
                            "you should set broker_connection_retry_on_startup to False instead.")
                    )
            else:
                if self.first_connection_attempt:
                    retry_disabled = not self.app.conf.broker_connection_retry_on_startup
                else:
                    retry_disabled = not self.app.conf.broker_connection_retry

            if retry_disabled:
                # Retry disabled, just call connect directly.
                conn.connect()
                self.first_connection_attempt = False
                return conn

            conn = conn.ensure_connection(
                _error_handler, self.app.conf.broker_connection_max_retries,
                callback=maybe_shutdown,
            )
            self.first_connection_attempt = False
            self.broker_connection_retry_attempt = 0
            return conn

        def _flush_events(self):
            if self.event_dispatcher:
                self.event_dispatcher.flush()

        def on_send_event_buffered(self):
            if self.hub:
                self.hub._ready.add(self._flush_events)

        def add_task_queue(self, queue, exchange=None, exchange_type=None,
                           routing_key=None, **options):
            cset = self.task_consumer
            queues = self.app.amqp.queues
            # Must use in' here, as __missing__ will automatically
            # create queues when :setting:`task_create_missing_queues` is enabled.
            # (Issue #1079)
            if queue in queues:
                q = queues[queue]
            else:
                exchange = queue if exchange is None else exchange
                exchange_type = ('direct' if exchange_type is None
                                 else exchange_type)
                q = queues.select_add(queue,
                                      exchange=exchange,
                                      exchange_type=exchange_type,
                                      routing_key=routing_key, **options)
            if not cset.consuming_from(queue):
                cset.add_queue(q)
                cset.consume()
                info('Started consuming from %s', queue)

        def cancel_task_queue(self, queue):
            info('Canceling queue %s', queue)
            self.app.amqp.queues.deselect(queue)
            self.task_consumer.cancel_by_queue(queue)

        def apply_eta_task(self, task):
            """Method called by the timer to apply a task with an ETA/countdown."""
            task_reserved(task)
            self.on_task_request(task)
            self.qos.decrement_eventually()

        def _message_report(self, body, message):
            return MESSAGE_REPORT.format(dump_body(message, body),
                                         safe_repr(message.content_type),
                                         safe_repr(message.content_encoding),
                                         safe_repr(message.delivery_info),
                                         safe_repr(message.headers))

        def on_unknown_message(self, body, message):
            warn(UNKNOWN_FORMAT, self._message_report(body, message))
            message.reject_log_error(logger, self.connection_errors)
            signals.task_rejected.send(sender=self, message=message, exc=None)

        def on_unknown_task(self, body, message, exc):
            error(UNKNOWN_TASK_ERROR,
                  exc,
                  dump_body(message, body),
                  message.headers,
                  message.delivery_info,
                  exc_info=True)
            try:
                id_, name = message.headers['id'], message.headers['task']
                root_id = message.headers.get('root_id')
            except KeyError:  # proto1
                payload = message.payload
                id_, name = payload['id'], payload['task']
                root_id = None
            request = Bunch(
                name=name, chord=None, root_id=root_id,
                correlation_id=message.properties.get('correlation_id'),
                reply_to=message.properties.get('reply_to'),
                errbacks=None,
            )
            message.reject_log_error(logger, self.connection_errors)
            self.app.backend.mark_as_failure(
                id_, NotRegistered(name), request=request,
            )
            if self.event_dispatcher:
                self.event_dispatcher.send(
                    'task-failed', uuid=id_,
                    exception=f'NotRegistered({name!r})',
                )
            signals.task_unknown.send(
                sender=self, message=message, exc=exc, name=name, id=id_,
            )

        def on_invalid_task(self, body, message, exc):
            error(INVALID_TASK_ERROR, exc, dump_body(message, body),
                  exc_info=True)
            message.reject_log_error(logger, self.connection_errors)
            signals.task_rejected.send(sender=self, message=message, exc=exc)

        def update_strategies(self):
            loader = self.app.loader
            for name, task in self.app.tasks.items():
                self.strategies[name] = task.start_strategy(self.app, self)
                task.__trace__ = build_tracer(name, task, loader, self.hostname,
                                              app=self.app)

        def create_task_handler(self, promise=promise):
            strategies = self.strategies
            on_unknown_message = self.on_unknown_message
            on_unknown_task = self.on_unknown_task
            on_invalid_task = self.on_invalid_task
            callbacks = self.on_task_message
            call_soon = self.call_soon

            def on_task_received(message):
                # payload will only be set for v1 protocol, since v2
                # will defer deserializing the message body to the pool.
                payload = None
                try:
                    type_ = message.headers['task']  # protocol v2
                except TypeError:
                    return on_unknown_message(None, message)
                except KeyError:
                    try:
                        payload = message.decode()
                    except Exception as exc:  # pylint: disable=broad-except
                        return self.on_decode_error(message, exc)
                    try:
                        type_, payload = payload['task'], payload  # protocol v1
                    except (TypeError, KeyError):
                        return on_unknown_message(payload, message)
                try:
                    strategy = strategies[type_]
                except KeyError as exc:
                    return on_unknown_task(None, message, exc)
                else:
                    try:
                        ack_log_error_promise = promise(
                            call_soon,
                            (message.ack_log_error,),
                            on_error=self._restore_prefetch_count_after_connection_restart,
                        )
                        reject_log_error_promise = promise(
                            call_soon,
                            (message.reject_log_error,),
                            on_error=self._restore_prefetch_count_after_connection_restart,
                        )

                        if (
                            not self._maximum_prefetch_restored
                            and self.restart_count > 0
                            and self._new_prefetch_count <= self.max_prefetch_count
                        ):
                            ack_log_error_promise.then(self._restore_prefetch_count_after_connection_restart,
                                                       on_error=self._restore_prefetch_count_after_connection_restart)
                            reject_log_error_promise.then(self._restore_prefetch_count_after_connection_restart,
                                                          on_error=self._restore_prefetch_count_after_connection_restart)

                        strategy(
                            message, payload,
                            ack_log_error_promise,
                            reject_log_error_promise,
                            callbacks,
                        )
                    except (InvalidTaskError, ContentDisallowed) as exc:
                        return on_invalid_task(payload, message, exc)
                    except DecodeError as exc:
                        return self.on_decode_error(message, exc)

            return on_task_received

        def _restore_prefetch_count_after_connection_restart(self, p, *args):
            with self.qos._mutex:
                if any((
                    not self.app.conf.worker_enable_prefetch_count_reduction,
                    self._maximum_prefetch_restored,
                )):
                    return

                new_prefetch_count = min(self.max_prefetch_count, self._new_prefetch_count)
                self.qos.value = self.initial_prefetch_count = new_prefetch_count
                self.qos.set(self.qos.value)

                already_restored = self._maximum_prefetch_restored
                self._maximum_prefetch_restored = new_prefetch_count == self.max_prefetch_count

                if already_restored is False and self._maximum_prefetch_restored is True:
                    logger.info(
                        "Resuming normal operations following a restart.\n"
                        f"Prefetch count has been restored to the maximum of {self.max_prefetch_count}"
                    )

        @property
        def max_prefetch_count(self):
            return self.pool.num_processes * self.prefetch_multiplier

        @property
        def _new_prefetch_count(self):
            return self.qos.value + self.prefetch_multiplier

        def __repr__(self):
            """``repr(self)``."""
            return '<Consumer: {self.hostname} ({state})>'.format(
                self=self, state=self.blueprint.human_state(),
            )

        def cancel_active_requests(self):
            """Cancel active requests during shutdown.

            Cancels all active requests that either do not require late acknowledgments or,
            if they do, have not been acknowledged yet.

            Does not cancel successful tasks, even if they have not been acknowledged yet.
            """

            def should_cancel(request):
                if not request.task.acks_late:
                    # Task does not require late acknowledgment, cancel it.
                    return True

                if not request.acknowledged:
                    # Task is late acknowledged, but it has not been acknowledged yet, cancel it.
                    if request.id in successful_requests:
                        # Unless it was successful, in which case we don't want to cancel it.
                        return False
                    return True

                # Task is late acknowledged, but it has already been acknowledged.
                return False  # Do not cancel and allow it to gracefully finish as it has already been acknowledged.

            requests_to_cancel = tuple(filter(should_cancel, active_requests))

            if requests_to_cancel:
                for request in requests_to_cancel:
                    # For acks_late tasks, don't emit RETRY signal since broker will handle redelivery
                    # For non-acks_late tasks, emit RETRY signal as usual
                    emit_retry = not request.task.acks_late
                    request.cancel(self.pool, emit_retry=emit_retry)

    class Evloop(bootsteps.StartStopStep):
        """Event loop service.

        Note:
            This is always started last.
        """

        label = 'event loop'
        last = True

        def start(self, c):
            self.patch_all(c)
            c.loop(*c.loop_args())

        def patch_all(self, c):
            c.qos._mutex = DummyLock()