#
# coding: utf-8
# Copyright (c) 2018 DATADVANCE
#
# Permission is hereby granted, free of charge, to any person obtaining
# a copy of this software and associated documentation files (the
# "Software"), to deal in the Software without restriction, including
# without limitation the rights to use, copy, modify, merge, publish,
# distribute, sublicense, and/or sell copies of the Software, and to
# permit persons to whom the Software is furnished to do so, subject to
# the following conditions:
#
# The above copyright notice and this permission notice shall be
# included in all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
# IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
# CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
# TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
# SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

"""GraphQL over WebSockets implementation with subscriptions.

This module contains implementation of GraphQL WebSocket protocol. The
implementation bases on the Graphene and the Channels 2.

The `Subscription` class itself is a "creative" copy of `Mutation` class
from the Graphene (`graphene/types/mutation.py`).

The `GraphqlWsConsumer` is a Channels WebSocket consumer which maintains
WebSocket connection with the client.

Implementation assumes that client uses the protocol implemented by the
library `subscription-transport-ws` (which is used by Apollo).
"""

# NOTE: The motivation is that currently there is no viable Python-based
# GraphQL subscriptions implementation out of the box. Hopefully there
# is a promising GraphQL WS https://github.com/graphql-python/graphql-ws
# library by the Graphene authors. In particular this pull request
# https://github.com/graphql-python/graphql-ws/pull/9 gives a hope that
# implementation in the current file can be replaced with GraphQL WS one
# day.

# NOTE: Links based on which this functionality is implemented:
# - Protocol description:
# https://github.com/apollographql/subscriptions-transport-ws/blob/master/PROTOCOL.md
# https://github.com/apollographql/subscriptions-transport-ws/blob/master/src/message-types.ts
# - ASGI specification for WebSockets:
# https://github.com/django/asgiref/blob/master/specs/www.rst#websocket
# - GitHubGist with the root of inspiration:
# https://gist.github.com/tricoder42/af3d0337c1b33d82c1b32d12bd0265ec


import asyncio
import collections
import concurrent
import hashlib
import traceback
import types

import asgiref.sync
import channels.db
import channels.generic.websocket as ch_websocket
import channels.layers
import django.core.serializers
import django.db
import graphene
import graphene.types.objecttype
import graphene.types.utils
import graphene.utils.get_unbound_function
import graphene.utils.props
import graphql
import graphql.error
import graphql.execution.executors.asyncio
import msgpack
from namedlist import namedlist
import promise
import rx


class Subscription(graphene.ObjectType):
    """Subscription type definition.

    Subclass this class to define a GraphQL subscription. The class
    works with `GraphqlWsConsumer` which maintains a WebSocket
    connection with the client.

    The subclass specifies the following methods. You can define each of
    them as a `@classmethod`, as a `@staticmethod`, or even as a regular
    method (like Graphene typically does). It shall work fine either
    way. NOTE, if you define the method as a regular method (not a
    classmethod or a staticmethod) you will receive the first argument
    (`payload`/`root`) into the `self` argument.

        publish(payload, info, *args, **kwds):
            This method invoked each time subscription "triggers".
            Raising an exception here will lead to sending the
            notification with the error. To suppress the notification
            return `Subscription.SKIP`.
            Required.

            Args:
                payload: The `payload` from the `broadcast` invocation.
                info: The value of `info.context` is a Channels
                    websocket context with all the connection
                    information.
                args, kwds: Values of the GraphQL subscription inputs.
            Returns:
                The same the any Graphene resolver returns. Returning
                a special object `Subscription.SKIP` indicates that this
                notification shall not be sent to the client at all.

        subscribe(root, info, *args, **kwds):
            Called when client subscribes. Define this to do some extra
            work when client subscribes and to group subscriptions into
            different subscription groups. Method signature is the same
            as in other GraphQL "resolver" methods but it may return
            the subscription groups names to put the subscription into.
            Optional.

            Args:
                root: Root resolver object. Typically `None`.
                info: The value of `info.context` is a Channels
                    websocket context with all the connection
                    information.
                args, kwds: Values of the GraphQL subscription inputs.

            Returns:
                The list or tuple of subscription group names this
                subscription instance belongs to. Later the subscription
                will trigger on publishes to any of that groups. If method
                returns None (default behavior) then the subscription is
                only put to the default group (the one which corresponds to
                the `Subscription` subclass).

        unsubscribed(root, info, *args, **kwds):
            Called when client unsubscribes. Define this to be notified
            when client unsubscribes. Optional.

            Args:
                root: Always `None`.
                info: The value of `info.context` is a Channels
                    websocket context with all the connection
                    information.
                args, kwds: Values of the GraphQL subscription inputs.

    The methods enlisted above receives "standard" set of GraphQL
    resolver arguments. The `info` field has `context` which can be used
    to transmit some useful payload between these methods. For example
    if `subscribe` sets `info.context.zen=42` then `publish` will have
    access to this value as `info.context.zen`.

    Static methods of subscription subclass:
        broadcast: Call this method to notify all subscriptions in the
            group.
        unsubscribe: Call this method to stop all subscriptions in the
            group.
    """

    # ----------------------------------------------------------------------- PUBLIC API

    # Return this from the `publish` to suppress the notification.
    SKIP = object()

    @classmethod
    def broadcast(cls, *, group=None, payload=None):
        """Call this method to notify all subscriptions in the group.

        NOTE: The `payload` argument will be serialized before sending
        to the subscription group. Serialization is made by MessagePack
        and if `payload` contains Django models, then it is serialized
        by the Django serialization utilities. For details see:
        Django serialization:
            https://docs.djangoproject.com/en/dev/topics/serialization/
        MessagePack:
            https://github.com/msgpack/msgpack-python

        Args:
            group: Name of the subscription group which members must be
                notified. `None` means that all the subscriptions of
                type will be triggered.
            payload: The payload delivered to the `publish` handler.
        """

        # Manually serialize the payload with the MessagePack
        # (https://msgpack.org) like Redis channel layer backend does.
        # We do this here to allow user to transfer Django models inside
        # the `payload`.

        def encode_django_model(obj):
            """MessagePack hook to serialize the Django model."""

            if isinstance(obj, django.db.models.Model):
                return {
                    "__djangomodel__": True,
                    "as_str": django.core.serializers.serialize("json", [obj]),
                }
            return obj

        serialized_payload = msgpack.packb(
            payload, default=encode_django_model, use_bin_type=True
        )

        # Send the message to the Channels group.
        group_send = asgiref.sync.async_to_sync(
            channels.layers.get_channel_layer().group_send
        )
        group = cls._group_name(group)
        group_send(
            group=group,
            message={
                "type": "broadcast",
                "group": group,
                "payload": serialized_payload,
            },
        )

    @classmethod
    def unsubscribe(cls, *, group=None):
        """Call this method to stop all subscriptions in the group.

        Args:
            group: Name of the subscription group which members must be
                unsubscribed. `None` means that all the client of the
                subscription will be unsubscribed.
        """

        # Send the message to the Channels group.
        group_send = asgiref.sync.async_to_sync(
            channels.layers.get_channel_layer().group_send
        )
        group = cls._group_name(group)
        group_send(group=group, message={"type": "unsubscribe", "group": group})

    @classmethod
    def Field(
        cls, name=None, description=None, deprecation_reason=None, required=False
    ):
        """Represent subscription as a field to "deploy" it."""
        return graphene.Field(
            cls._meta.output,
            args=cls._meta.arguments,
            resolver=cls._meta.resolver,
            name=name,
            description=description,
            deprecation_reason=deprecation_reason,
            required=required,
        )

    # ------------------------------------------------------------------- IMPLEMENTATION

    @classmethod
    def __init_subclass_with_meta__(
        cls,
        subscribe=None,
        publish=None,
        unsubscribed=None,
        output=None,
        arguments=None,
        _meta=None,
        **options,
    ):  # pylint: disable=arguments-differ
        """Prepare subscription when on subclass creation.

        This method is invoked by the superclass `__init__subclass__`.
        It is needed to process class fields, `Meta` and inheritance
        parameters. This is genuine Graphene approach.
        """
        if not _meta:
            _meta = SubscriptionOptions(cls)

        output = output or getattr(cls, "Output", None)
        # Collect fields if output class is not explicitly defined.
        fields = {}
        if not output:
            fields = collections.OrderedDict()
            for base in reversed(cls.__mro__):
                fields.update(
                    graphene.types.utils.yank_fields_from_attrs(
                        base.__dict__, _as=graphene.Field
                    )
                )
            output = cls

        if not arguments:
            input_class = getattr(cls, "Arguments", None)

            if input_class:
                arguments = graphene.utils.props.props(input_class)
            else:
                arguments = {}

        # Get `publish`, `subscribe`, and `unsubscribe` handlers.
        subscribe = subscribe or getattr(cls, "subscribe", None)
        publish = publish or getattr(cls, "publish", None)
        assert publish is not None, (
            f"Subscription `{cls.__qualname__}` does not define a"
            " method `publish`! All subscriptions must define"
            " `publish` which processes a GraphQL query!"
        )

        unsubscribed = unsubscribed or getattr(cls, "unsubscribed", None)

        if _meta.fields:
            _meta.fields.update(fields)
        else:
            _meta.fields = fields

        # Auxiliary alias.
        get_function = graphene.utils.get_unbound_function.get_unbound_function

        # pylint: disable=attribute-defined-outside-init
        _meta.arguments = arguments
        _meta.output = output
        _meta.resolver = get_function(cls._subscribe)
        _meta.subscribe = get_function(subscribe)
        _meta.publish = get_function(publish)
        _meta.unsubscribed = get_function(unsubscribed)

        super().__init_subclass_with_meta__(_meta=_meta, **options)

    @classmethod
    def _subscribe(cls, obj, info, *args, **kwds):
        """Subscription request received.

        This is called by the Graphene when a client subscribes.
        """

        # Extract function which associates the callback with the groups
        # and remove it from the context so extra field does not pass to
        # the `subscribe` method of a subclass.
        register = info.context.register
        del info.context.register

        # Attach current subscription to the group corresponding to the
        # concrete class. This allows to trigger all the subscriptions
        # of the current type, by invoking `publish` without setting
        # the `group` argument.
        groups = [cls._group_name()]

        # Invoke the subclass-specified `subscribe` method to get the
        # groups subscription must be attached to.
        if cls._meta.subscribe is not None:
            subclass_groups = cls._meta.subscribe(obj, info, *args, **kwds)
            assert subclass_groups is None or isinstance(
                subclass_groups, (list, tuple)
            ), (
                f"Method `subscribe` returned a value of an incorrect type"
                f" {type(subclass_groups)}! A list, a tuple, or `None` expected."
            )
            subclass_groups = subclass_groups or []
        else:
            subclass_groups = []

        groups += [cls._group_name(group) for group in subclass_groups]

        # Register callbacks to call `publish` and `unsubscribed`.
        # Function `register` provides an observable which must
        # be returned from here, cause that is what GraphQL expects from
        # the subscription "resolver" functions.
        def publish_callback(payload):
            """Call `publish` with the payload."""
            return cls._meta.publish(payload, info, *args, **kwds)

        def unsubscribed_callback():
            """Call `unsubscribed` with `None`."""
            if cls._meta.unsubscribed is not None:
                cls._meta.unsubscribed(None, info, *args, **kwds)

        return register(groups, publish_callback, unsubscribed_callback)

    @classmethod
    def _group_name(cls, group=None):
        """Group name based on the name of the subscription class."""

        prefix = GraphqlWsConsumer.group_name_prefix
        suffix = f"{cls.__module__}.{cls.__qualname__}"
        if group is not None:
            suffix += "-" + group

        # Wrap the suffix into MD5 to guarantee that the length of the
        # group name is limited. Otherwise Channels will complain about
        # that the group name is wrong (actually is too long).
        suffix_md5 = hashlib.md5()
        suffix_md5.update(suffix.encode("utf-8"))
        suffix_md5 = suffix_md5.hexdigest()

        return f"{prefix}-{suffix_md5}"


class SubscriptionOptions(graphene.types.objecttype.ObjectTypeOptions):
    """Options stored in the Subscription's `_meta` field."""

    arguments = None
    output = None
    subscribe = None
    publish = None


class GraphqlWsConsumer(ch_websocket.AsyncJsonWebsocketConsumer):
    """Channels consumer for the WebSocket GraphQL backend.

    NOTE: Each instance of this class maintains one WebSocket
    connection to a single client.

    This class implements the WebSocket-based GraphQL protocol used by
    `subscriptions-transport-ws` library (used by Apollo):
    https://github.com/apollographql/subscriptions-transport-ws/blob/master/PROTOCOL.md
    """

    # ----------------------------------------------------------------- PUBLIC INTERFACE

    # Overwrite this in the subclass to specify the GraphQL schema which
    # processes GraphQL queries.
    schema = None

    # The interval to send keepalive messages to the clients (seconds).
    send_keepalive_every = None

    # Set to `True` to process requests (i.e. GraphQL documents) from
    # a client in order of arrival, which is the same as sending order,
    # as guaranteed by the WebSocket protocol. This means that request
    # processing for this particular client becomes serial - in other
    # words, the server will not start processing another request
    # before it finishes the current one. Note that requests from
    # different clients (within different WebSocket connections)
    # are still processed asynchronously. Useful for tests.
    strict_ordering = False

    # When set to `True` the server will send an empty data message in
    # response to the subscription. This is needed to let client know
    # when the subscription activates, so he can be sure he doesn't miss
    # any notifications. Disabled by default, cause this is an extension
    # to the original protocol and the client must be tuned accordingly.
    confirm_subscriptions = False

    # A prefix of the Channel group names used for the subscription
    # notifications. You may change this to avoid name clashes in the
    # ASGI backend, e.g. in the Redis.
    group_name_prefix = "GRAPHQL_WS_SUBSCRIPTION"

    async def on_connect(self, payload):
        """Called after CONNECTION_INIT message from client.

        Overwrite to raise an Exception to tell the server
        to reject the connection when it's necessary.

        Args:
            payload: Payload from CONNECTION_INIT message.
        """
        pass

    # ------------------------------------------------------------------- IMPLEMENTATION

    # Threadpool used to process requests.
    _workers = concurrent.futures.ThreadPoolExecutor(thread_name_prefix="GraphqlWs.")

    # Disable promise's "trampoline" (whatever it means), cause when it
    # is enabled (which is by default) the promise is not thread-safe.
    # This leads to hanging threads and unanswered requests.
    # https://github.com/syrusakbary/promise/issues/57#issuecomment-406778476
    promise.promise.async_instance.disable_trampoline()

    # Subscription WebSocket subprotocol.
    _GRAPHQL_WS_SUBPROTOCOL = "graphql-ws"

    # Structure that holds subscription information.
    _SubInf = namedlist("_SubInf", ["groups", "op_id", "trigger", "unsubscribed"])

    def __init__(self, *args, **kwargs):
        assert self.schema is not None, (
            "An attribute `schema` is not set! Subclasses must specify "
            "the schema which processes GraphQL subscription queries."
        )

        # Registry of active (subscribed) subscriptions.
        self._subscriptions = {}  # {'<sid>': '<SubInf>', ...}
        self._sids_by_group = {}  # {'<grp>': ['<sid0>', '<sid1>', ...], ...}

        # Task that sends keepalive messages periodically.
        self._keepalive_task = None

        # Background tasks. When `strict_ordering` is `False` we spawn
        # an asyncio task per request or incoming broadcast message.
        self._background_tasks = []

        super().__init__(*args, **kwargs)

    # ---------------------------------------------------------- CONSUMER EVENT HANDLERS

    async def connect(self):
        """Handle new WebSocket connection."""

        # Check the subprotocol told by the client.
        #
        # NOTE: In Python 3.6 `scope["subprotocols"]` was a string, but
        # starting with Python 3.7 it is a bytes. This can be a proper
        # change or just a bug in the Channels to be fixed. So let's
        # accept both variants until it becomes clear.
        assert self._GRAPHQL_WS_SUBPROTOCOL in (
            (sp.decode() if isinstance(sp, bytes) else sp)
            for sp in self.scope["subprotocols"]
        ), (
            f"WebSocket client does not request for the subprotocol "
            f"{self._GRAPHQL_WS_SUBPROTOCOL}!"
        )

        # Accept connection with the GraphQL-specific subprotocol.
        await self.accept(subprotocol=self._GRAPHQL_WS_SUBPROTOCOL)

    async def disconnect(self, code):
        """WebSocket disconnection handler."""
        del code

        # Remove itself from the Channels groups, clear triggers and
        # stop sending keepalive messages.

        # The list of awaitables to simultaneously wait at the end.
        waitlist = []

        # Unsubscribe from the Channels groups.
        waitlist += [
            self.channel_layer.group_discard(group, self.channel_name)
            for group in self._sids_by_group
        ]

        # Cancel all currently running background tasks.
        for bg_task in self._background_tasks:
            bg_task.cancel()
        waitlist += self._background_tasks

        # Stop sending keepalive messages (if enabled).
        if self._keepalive_task is not None:
            self._keepalive_task.cancel()
            waitlist += [self._keepalive_task]

        if waitlist:
            await asyncio.wait(waitlist)

        self._subscriptions.clear()
        self._sids_by_group.clear()
        self._background_tasks.clear()

    async def receive_json(self, content):  # pylint: disable=arguments-differ
        """Process WebSocket message received from the client."""

        async def process_message():
            """Process the message. Can be awaited or spawned."""
            # Extract message type based on which we select how to proceed.
            msg_type = content["type"].upper()

            if msg_type == "CONNECTION_INIT":
                await self._on_gql_connection_init(payload=content["payload"])

            elif msg_type == "CONNECTION_TERMINATE":
                await self._on_gql_connection_terminate()

            elif msg_type == "START":
                await self._on_gql_start(
                    operation_id=content["id"], payload=content["payload"]
                )

            elif msg_type == "STOP":
                await self._on_gql_stop(operation_id=content["id"])

            else:
                await self._send_gql_error(
                    content["id"], f"Message of unknown type '{msg_type}' received!"
                )

        # If strict ordering is required then simply wait until the
        # message processing is finished. Otherwise spawn a task so
        # Channels may continue calling `receive_json` while requests
        # (i.e. GraphQL documents) are being processed.
        if self.strict_ordering:
            await process_message()
        else:
            background_task = asyncio.create_task(process_message())
            self._background_tasks.append(background_task)
            # Schedule automatic removal from the list when finishes.
            background_task.add_done_callback(self._background_tasks.remove)

    async def broadcast(self, message):
        """The broadcast message handler.

        Method is called when new `broadcast` message received from the
        Channels group. The message is sent by `Subscription.broadcast`.
        Here we figure out the group message received from and trigger
        the observable which makes the subscription process the query
        and notify the client.
        """

        async def process_broadcast():
            """Process the message. Can be awaited or spawned."""
            group = message["group"]
            serialized_payload = message["payload"]

            def decode_django_model(obj):
                """MessagePack hook to deserialize the Django model."""
                if "__djangomodel__" in obj:
                    obj = next(
                        django.core.serializers.deserialize("json", obj["as_str"])
                    ).object
                return obj

            payload = msgpack.unpackb(
                serialized_payload, object_hook=decode_django_model, raw=False
            )

            # Offload trigger to the thread (in threadpool) cause it may
            # work slowly and do DB operations.
            db_sync_to_async = channels.db.database_sync_to_async
            triggers = (
                db_sync_to_async(self._subscriptions[op_id].trigger)
                for op_id in self._sids_by_group[group]
            )
            await asyncio.wait([trigger(payload) for trigger in triggers])

        # If strict ordering is required then simply wait until all the
        # broadcast messages are sent. Otherwise spawn a task so this
        # consumer will continue receiving messages.
        if self.strict_ordering:
            await process_broadcast()
        else:
            background_task = asyncio.create_task(process_broadcast())
            self._background_tasks.append(background_task)
            # Schedule automatic removal from the list when finishes.
            background_task.add_done_callback(self._background_tasks.remove)

    async def unsubscribe(self, message):
        """The unsubscribe message handler.

        Method is called when new `_unsubscribe` message received from
        the Channels group. The message is typically sent by the method
        `Subscription.unsubscribe`. Here we figure out the group message
        received from and stop all the active subscriptions in this
        group.
        """
        group = message["group"]

        # Unsubscribe all active subscriptions current client has in
        # the subscription group `group`.
        await asyncio.wait(
            [self._on_gql_stop(sid) for sid in self._sids_by_group[group]]
        )

    # ---------------------------------------------------------- GRAPHQL PROTOCOL EVENTS

    async def _on_gql_connection_init(self, payload):
        """Process the CONNECTION_INIT message.

        Start sending keepalive messages if `send_keepalive_every` set.
        Respond with either CONNECTION_ACK or CONNECTION_ERROR message.
        """

        try:
            # Notify subclass a new client is connected.
            await self.on_connect(payload)
        except Exception as e:  # pylint: disable=broad-except
            await self._send_gql_connection_error(self._format_error(e))
            # Close the connection.
            # NOTE: We use the 4000 code because there are two reasons:
            # A) We can not use codes greater than 1000 and less than
            # 3000 because daphne and autobahn do not allow this
            # (see `sendClose` from `autobahn/websocket/protocol.py`
            # and `daphne/ws_protocol.py`).
            # B) https://developer.mozilla.org/en-US/docs/Web/API/CloseEvent
            # So mozilla offers us the following codes:
            # 4000–4999 - Available for use by applications.
            await self.close(code=4000)
        else:
            # Send CONNECTION_ACK message.
            await self._send_gql_connection_ack()
            # If keepalive enabled then send one message immediately and
            # schedule periodic messages.
            if self.send_keepalive_every is not None:

                async def keepalive_sender():
                    """Send keepalive messages periodically."""
                    while True:
                        await asyncio.sleep(self.send_keepalive_every)
                        await self._send_gql_connection_keep_alive()

                self._keepalive_task = asyncio.ensure_future(keepalive_sender())
                # Immediately send keepalive message cause it is
                # required by the protocol description.
                await self._send_gql_connection_keep_alive()

    async def _on_gql_connection_terminate(self):
        """Process the CONNECTION_TERMINATE message."""

        # Close the connection.
        await self.close(code=1000)

    async def _on_gql_start(self, operation_id, payload):
        """Process the START message.

        This message holds query, mutation or subscription request.
        """

        try:
            if operation_id in self._subscriptions:
                raise graphql.error.GraphQLError(
                    f"Subscription with the given `id={operation_id}` "
                    "already exists! Would you like to unsubscribe first?"
                )

            # Get the message data.
            op_id = operation_id
            query = payload["query"]
            op_name = payload.get("operationName")
            variables = payload.get("variables", {})

            # Local alias for convenience.
            async_to_sync = asgiref.sync.async_to_sync

            # The subject we will trigger on the `broadcast` message.
            broadcasts = rx.subjects.Subject()

            # This function is called by subscription to associate a
            # callback (which publishes notifications) with the groups.
            def register(groups, publish_callback, unsubscribed_callback):
                """Associate publish callback with the groups."""
                # Put subscription information into the registry and
                # start listening to the subscription groups.
                trigger = broadcasts.on_next
                subinf = self._SubInf(
                    groups=groups,
                    op_id=op_id,
                    trigger=trigger,
                    unsubscribed=unsubscribed_callback,
                )
                self._subscriptions[op_id] = subinf
                for group in groups:
                    self._sids_by_group.setdefault(group, []).append(op_id)
                    async_to_sync(self.channel_layer.group_add)(
                        group, self.channel_name
                    )

                # Enqueue the `publish` method execution.
                stream = broadcasts.map(publish_callback)
                # Do not notify clients when `publish` returns `SKIP` by
                # filtering out such items from the stream.
                stream = stream.filter(
                    lambda publish_returned: publish_returned is not Subscription.SKIP
                )
                return stream

            # Create object-like context (like in `Query` or `Mutation`)
            # from the dict-like one provided by the Channels. The
            # subscriptions groups must be set by the subscription using
            # the `register` function.
            context = types.SimpleNamespace(**self.scope)
            context.register = register

            # Process the request with Graphene/GraphQL.
            # NOTE: We offload the GraphQL processing to the worker
            # thread (with `channels.db.database_sync_to_async`) because
            # according to our experiments even GraphQL document parsing
            # may take a while (and depends approx. linearly on the size
            # of the selection set). Moreover it is highly probable that
            # resolvers will invoke blocking DB operations so it is
            # better to offload the whole thing to a worker thread.
            def thread_func():
                """Execute schema in a worker thread."""

                # Create an eventloop if this worker thread does not
                # have one yet. Eventually each worker will have its own
                # eventloop so the `AsyncioExecutor` will use it.
                try:
                    loop = asyncio.get_event_loop()
                except RuntimeError:
                    loop = asyncio.new_event_loop()
                    asyncio.set_event_loop(loop)

                try:
                    return graphql.graphql(
                        self.schema,
                        request_string=query,
                        operation_name=op_name,
                        variable_values=variables,
                        context_value=context,
                        allow_subscriptions=True,
                        executor=graphql.execution.executors.asyncio.AsyncioExecutor(
                            loop
                        ),
                    )
                finally:
                    # Resolvers can open the connections to the database
                    # so close them. Django Channels does the same in
                    # the `channels.db.database_sync_to_async`.
                    django.db.close_old_connections()

            result = await asyncio.wait_for(
                asyncio.get_running_loop().run_in_executor(self._workers, thread_func),
                timeout=None,
            )

        except Exception:  # pylint: disable=broad-except
            # Something is wrong - send error message.
            await self._send_gql_error(op_id, traceback.format_exc())

        else:
            # Receiving an observer means the subscription has been
            # processed. Otherwise it is just regular query or mutation.
            if isinstance(result, rx.Observable):
                # Client subscribed to a subscription so we subscribe to
                # the observable returned.

                # Subscribe to the observable.
                # NOTE: Function `on_next` is called from a thread
                # where trigger runs, so it is necessary to wrap our
                # async method into `async_to_sync`, which carefully
                # runs given a coroutine in the current eventloop.
                # (See the implementation of `async_to_sync`
                # for details.)
                send_gql_data = async_to_sync(self._send_gql_data)
                result.subscribe(lambda r: send_gql_data(op_id, r.data, r.errors))

                # Respond with the subscription activation message if
                # enabled in the consumer configuration.
                if self.confirm_subscriptions:
                    await self._send_gql_data(op_id, data=None, errors=None)
            else:
                # Query or mutation received - send a response
                # immediately.

                # We respond with a message of type `data`.
                # If Graphene complains that the request is invalid
                # (e.g. query contains syntax error), then the `data`
                # argument is None and the 'errors' argument contains
                # all errors that occurred before or during execution.
                # `result` is instance of ExecutionResult
                # Respond with data.
                await self._send_gql_data(op_id, result.data, result.errors)
                # Tell the client that the request processing is over.
                await self._send_gql_complete(op_id)

    async def _on_gql_stop(self, operation_id):
        """Process the STOP message."""
        # Currently only subscriptions can be stopped. But we see but
        # some clients (e.g. GraphiQL) send the stop message even for
        # queries and mutations. We also see that the Apollo server
        # ignores such messages, so we ignore them as well.
        if not operation_id in self._subscriptions:
            return

        # Unsubscribe: stop listening corresponding groups and
        # subscription from the registry.
        waitlist = []
        subinf = self._subscriptions.pop(operation_id)
        for group in subinf.groups:
            waitlist.append(self.channel_layer.group_discard(group, self.channel_name))
            # Remove operation if from groups it belongs to. And remove
            # group from `_sids_by_group` if there is not subscriptions
            # in it.
            assert self._sids_by_group[group].count(operation_id) == 1, (
                f"Registry is inconsistent: group `{group}` has "
                f"`{self._sids_by_group[group].count(operation_id)}` "
                "occurrences of operation_id=`{operation_id}`!"
            )

            self._sids_by_group[group].remove(operation_id)
            if not self._sids_by_group[group]:
                del self._sids_by_group[group]
        await asyncio.wait(waitlist)

        # Call subscription class `unsubscribed` handler.
        subinf.unsubscribed()

        # Send the unsubscription confirmation message.
        await self._send_gql_complete(operation_id)

    # -------------------------------------------------------- GRAPHQL PROTOCOL MESSAGES

    async def _send_gql_connection_ack(self):
        """Sent in reply to the `connection_init` request."""
        await self.send_json({"type": "connection_ack"})

    async def _send_gql_connection_error(self, error):
        """Connection error sent in reply to the `connection_init`."""
        await self.send_json({"type": "connection_error", "payload": error})

    async def _send_gql_data(self, operation_id, data, errors):
        """Send GraphQL `data` message to the client.

        Args:
            data: Dict with GraphQL query response.
            errors: List with exceptions occurred during processing the
                GraphQL query. (Errors happened in the resolvers.)
        """
        await self.send_json(
            {
                "type": "data",
                "id": operation_id,
                "payload": {
                    "data": data,
                    **(
                        {"errors": [self._format_error(e) for e in errors]}
                        if errors
                        else {}
                    ),
                },
            }
        )

    async def _send_gql_error(self, operation_id, error):
        """Tell client there is a query processing error.

        Server sends this message upon a failing operation.
        It can be an unexpected or unexplained GraphQL execution error
        or a bug in the code. It is unlikely that this is GraphQL
        validation errors (such errors are part of data message and
        must be sent by the `_send_gql_data` method).

        Args:
            operation_id: Id of the operation that failed on the server.
            error: String with the information about the error.
        """
        await self.send_json(
            {"type": "error", "id": operation_id, "payload": {"errors": [error]}}
        )

    async def _send_gql_complete(self, operation_id):
        """Send GraphQL `complete` message to the client.

        Args:
            operation_id: If of the corresponding operation.
        """
        await self.send_json({"type": "complete", "id": operation_id})

    async def _send_gql_connection_keep_alive(self):
        """Send the keepalive (ping) message."""
        await self.send_json({"type": "ka"})

    # ------------------------------------------------------------------------ AUXILIARY

    def _format_error(self, error):
        """Format exception `error` to send over a network."""

        if isinstance(error, graphql.error.GraphQLError):
            return graphql.error.format_error(error)

        return {"message": f"{type(error).__name__}: {str(error)}"}
