forked-synapse/synapse/rest/client/sync.py

#
# This file is licensed under the Affero General Public License (AGPL) version 3.
#
# Copyright 2015, 2016 OpenMarket Ltd
# Copyright (C) 2023 New Vector, Ltd
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as
# published by the Free Software Foundation, either version 3 of the
# License, or (at your option) any later version.
#
# See the GNU Affero General Public License for more details:
# <https://www.gnu.org/licenses/agpl-3.0.html>.
#
# Originally licensed under the Apache License, Version 2.0:
# <http://www.apache.org/licenses/LICENSE-2.0>.
#
# [This file includes modifications made by New Vector Limited]
#
#
import itertools
import logging
from collections import defaultdict
from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple, Union

from synapse.api.constants import AccountDataTypes, EduTypes, Membership, PresenceState
from synapse.api.errors import Codes, StoreError, SynapseError
from synapse.api.filtering import FilterCollection
from synapse.api.presence import UserPresenceState
from synapse.events.utils import (
    SerializeEventConfig,
    format_event_for_client_v2_without_room_id,
    format_event_raw,
)
from synapse.handlers.presence import format_user_presence_state
from synapse.handlers.sliding_sync import SlidingSyncConfig, SlidingSyncResult
from synapse.handlers.sync import (
    ArchivedSyncResult,
    InvitedSyncResult,
    JoinedSyncResult,
    KnockedSyncResult,
    SyncConfig,
    SyncResult,
    SyncVersion,
)
from synapse.http.server import HttpServer
from synapse.http.servlet import (
    RestServlet,
    parse_and_validate_json_object_from_request,
    parse_boolean,
    parse_integer,
    parse_string,
)
from synapse.http.site import SynapseRequest
from synapse.logging.opentracing import trace_with_opname
from synapse.types import JsonDict, Requester, StreamToken
from synapse.types.rest.client import SlidingSyncBody
from synapse.util import json_decoder
from synapse.util.caches.lrucache import LruCache

from ._base import client_patterns, set_timeline_upper_limit

if TYPE_CHECKING:
    from synapse.server import HomeServer

logger = logging.getLogger(__name__)


class SyncRestServlet(RestServlet):
    """

    GET parameters::
        timeout(int): How long to wait for new events in milliseconds.
        since(batch_token): Batch token when asking for incremental deltas.
        set_presence(str): What state the device presence should be set to.
            default is "online".
        filter(filter_id): A filter to apply to the events returned.

    Response JSON::
        {
          "next_batch": // batch token for the next /sync
          "presence": // presence data for the user.
          "rooms": {
            "join": { // Joined rooms being updated.
              "${room_id}": { // Id of the room being updated
                "event_map": // Map of EventID -> event JSON.
                "timeline": { // The recent events in the room if gap is "true"
                  "limited": // Was the per-room event limit exceeded?
                             // otherwise the next events in the room.
                  "events": [] // list of EventIDs in the "event_map".
                  "prev_batch": // back token for getting previous events.
                }
                "state": {"events": []} // list of EventIDs updating the
                                        // current state to be what it should
                                        // be at the end of the batch.
                "ephemeral": {"events": []} // list of event objects
              }
            },
            "invite": {}, // Invited rooms being updated.
            "leave": {} // Archived rooms being updated.
          }
        }
    """

    PATTERNS = client_patterns("/sync$")
    ALLOWED_PRESENCE = {"online", "offline", "unavailable"}
    CATEGORY = "Sync requests"

    def __init__(self, hs: "HomeServer"):
        super().__init__()
        self.hs = hs
        self.auth = hs.get_auth()
        self.store = hs.get_datastores().main
        self.sync_handler = hs.get_sync_handler()
        self.clock = hs.get_clock()
        self.filtering = hs.get_filtering()
        self.presence_handler = hs.get_presence_handler()
        self._server_notices_sender = hs.get_server_notices_sender()
        self._event_serializer = hs.get_event_client_serializer()
        self._msc2654_enabled = hs.config.experimental.msc2654_enabled
        self._msc3773_enabled = hs.config.experimental.msc3773_enabled

        self._json_filter_cache: LruCache[str, bool] = LruCache(
            max_size=1000,
            cache_name="sync_valid_filter",
        )

    async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
        # This will always be set by the time Twisted calls us.
        assert request.args is not None

        if b"from" in request.args:
            # /events used to use 'from', but /sync uses 'since'.
            # Lets be helpful and whine if we see a 'from'.
            raise SynapseError(
                400, "'from' is not a valid query parameter. Did you mean 'since'?"
            )

        requester = await self.auth.get_user_by_req(request, allow_guest=True)
        user = requester.user
        device_id = requester.device_id

        timeout = parse_integer(request, "timeout", default=0)
        since = parse_string(request, "since")
        set_presence = parse_string(
            request,
            "set_presence",
            default="online",
            allowed_values=self.ALLOWED_PRESENCE,
        )
        filter_id = parse_string(request, "filter")
        full_state = parse_boolean(request, "full_state", default=False)

        logger.debug(
            "/sync: user=%r, timeout=%r, since=%r, "
            "set_presence=%r, filter_id=%r, device_id=%r",
            user,
            timeout,
            since,
            set_presence,
            filter_id,
            device_id,
        )

        # Stream position of the last ignored users account data event for this user,
        # if we're initial syncing.
        # We include this in the request key to invalidate an initial sync
        # in the response cache once the set of ignored users has changed.
        # (We filter out ignored users from timeline events, so our sync response
        # is invalid once the set of ignored users changes.)
        last_ignore_accdata_streampos: Optional[int] = None
        if not since:
            # No `since`, so this is an initial sync.
            last_ignore_accdata_streampos = await self.store.get_latest_stream_id_for_global_account_data_by_type_for_user(
                user.to_string(), AccountDataTypes.IGNORED_USER_LIST
            )

        request_key = (
            user,
            timeout,
            since,
            filter_id,
            full_state,
            device_id,
            last_ignore_accdata_streampos,
        )

        if filter_id is None:
            filter_collection = self.filtering.DEFAULT_FILTER_COLLECTION
        elif filter_id.startswith("{"):
            try:
                filter_object = json_decoder.decode(filter_id)
            except Exception:
                raise SynapseError(400, "Invalid filter JSON", errcode=Codes.NOT_JSON)

            # We cache the validation, as this can get quite expensive if people use
            # a literal json blob as a query param.
            if not self._json_filter_cache.get(filter_id):
                self.filtering.check_valid_filter(filter_object)
                self._json_filter_cache[filter_id] = True

            set_timeline_upper_limit(
                filter_object, self.hs.config.server.filter_timeline_limit
            )
            filter_collection = FilterCollection(self.hs, filter_object)
        else:
            try:
                filter_collection = await self.filtering.get_user_filter(
                    user, filter_id
                )
            except StoreError as err:
                if err.code != 404:
                    raise
                # fix up the description and errcode to be more useful
                raise SynapseError(400, "No such filter", errcode=Codes.INVALID_PARAM)

        sync_config = SyncConfig(
            user=user,
            filter_collection=filter_collection,
            is_guest=requester.is_guest,
            device_id=device_id,
        )

        since_token = None
        if since is not None:
            since_token = await StreamToken.from_string(self.store, since)

        # send any outstanding server notices to the user.
        await self._server_notices_sender.on_user_syncing(user.to_string())

        affect_presence = set_presence != PresenceState.OFFLINE

        context = await self.presence_handler.user_syncing(
            user.to_string(),
            requester.device_id,
            affect_presence=affect_presence,
            presence_state=set_presence,
        )
        with context:
            sync_result = await self.sync_handler.wait_for_sync_for_user(
                requester,
                sync_config,
                SyncVersion.SYNC_V2,
                request_key,
                since_token=since_token,
                timeout=timeout,
                full_state=full_state,
            )

        # the client may have disconnected by now; don't bother to serialize the
        # response if so.
        if request._disconnected:
            logger.info("Client has disconnected; not serializing response.")
            return 200, {}

        time_now = self.clock.time_msec()
        # We know that the the requester has an access token since appservices
        # cannot use sync.
        response_content = await self.encode_response(
            time_now, sync_result, requester, filter_collection
        )

        logger.debug("Event formatting complete")
        return 200, response_content

    @trace_with_opname("sync.encode_response")
    async def encode_response(
        self,
        time_now: int,
        sync_result: SyncResult,
        requester: Requester,
        filter: FilterCollection,
    ) -> JsonDict:
        logger.debug("Formatting events in sync response")
        if filter.event_format == "client":
            event_formatter = format_event_for_client_v2_without_room_id
        elif filter.event_format == "federation":
            event_formatter = format_event_raw
        else:
            raise Exception("Unknown event format %s" % (filter.event_format,))

        serialize_options = SerializeEventConfig(
            event_format=event_formatter,
            requester=requester,
            only_event_fields=filter.event_fields,
        )
        stripped_serialize_options = SerializeEventConfig(
            event_format=event_formatter,
            requester=requester,
            include_stripped_room_state=True,
        )

        joined = await self.encode_joined(
            sync_result.joined, time_now, serialize_options
        )

        invited = await self.encode_invited(
            sync_result.invited, time_now, stripped_serialize_options
        )

        knocked = await self.encode_knocked(
            sync_result.knocked, time_now, stripped_serialize_options
        )

        archived = await self.encode_archived(
            sync_result.archived, time_now, serialize_options
        )

        logger.debug("building sync response dict")

        response: JsonDict = defaultdict(dict)
        response["next_batch"] = await sync_result.next_batch.to_string(self.store)

        if sync_result.account_data:
            response["account_data"] = {"events": sync_result.account_data}
        if sync_result.presence:
            response["presence"] = SyncRestServlet.encode_presence(
                sync_result.presence, time_now
            )

        if sync_result.to_device:
            response["to_device"] = {"events": sync_result.to_device}

        if sync_result.device_lists.changed:
            response["device_lists"]["changed"] = list(sync_result.device_lists.changed)
        if sync_result.device_lists.left:
            response["device_lists"]["left"] = list(sync_result.device_lists.left)

        # We always include this because https://github.com/vector-im/element-android/issues/3725
        # The spec isn't terribly clear on when this can be omitted and how a client would tell
        # the difference between "no keys present" and "nothing changed" in terms of whole field
        # absent / individual key type entry absent
        # Corresponding synapse issue: https://github.com/matrix-org/synapse/issues/10456
        response["device_one_time_keys_count"] = sync_result.device_one_time_keys_count

        # https://github.com/matrix-org/matrix-doc/blob/54255851f642f84a4f1aaf7bc063eebe3d76752b/proposals/2732-olm-fallback-keys.md
        # states that this field should always be included, as long as the server supports the feature.
        response["org.matrix.msc2732.device_unused_fallback_key_types"] = (
            sync_result.device_unused_fallback_key_types
        )
        response["device_unused_fallback_key_types"] = (
            sync_result.device_unused_fallback_key_types
        )

        if joined:
            response["rooms"][Membership.JOIN] = joined
        if invited:
            response["rooms"][Membership.INVITE] = invited
        if knocked:
            response["rooms"][Membership.KNOCK] = knocked
        if archived:
            response["rooms"][Membership.LEAVE] = archived

        return response

    @staticmethod
    def encode_presence(events: List[UserPresenceState], time_now: int) -> JsonDict:
        return {
            "events": [
                {
                    "type": EduTypes.PRESENCE,
                    "sender": event.user_id,
                    "content": format_user_presence_state(
                        event, time_now, include_user_id=False
                    ),
                }
                for event in events
            ]
        }

    @trace_with_opname("sync.encode_joined")
    async def encode_joined(
        self,
        rooms: List[JoinedSyncResult],
        time_now: int,
        serialize_options: SerializeEventConfig,
    ) -> JsonDict:
        """
        Encode the joined rooms in a sync result

        Args:
            rooms: list of sync results for rooms this user is joined to
            time_now: current time - used as a baseline for age calculations
            serialize_options: Event serializer options
        Returns:
            The joined rooms list, in our response format
        """
        joined = {}
        for room in rooms:
            joined[room.room_id] = await self.encode_room(
                room, time_now, joined=True, serialize_options=serialize_options
            )

        return joined

    @trace_with_opname("sync.encode_invited")
    async def encode_invited(
        self,
        rooms: List[InvitedSyncResult],
        time_now: int,
        serialize_options: SerializeEventConfig,
    ) -> JsonDict:
        """
        Encode the invited rooms in a sync result

        Args:
            rooms: list of sync results for rooms this user is invited to
            time_now: current time - used as a baseline for age calculations
            serialize_options: Event serializer options

        Returns:
            The invited rooms list, in our response format
        """
        invited = {}
        for room in rooms:
            invite = await self._event_serializer.serialize_event(
                room.invite, time_now, config=serialize_options
            )
            unsigned = dict(invite.get("unsigned", {}))
            invite["unsigned"] = unsigned
            invited_state = list(unsigned.pop("invite_room_state", []))
            invited_state.append(invite)
            invited[room.room_id] = {"invite_state": {"events": invited_state}}

        return invited

    @trace_with_opname("sync.encode_knocked")
    async def encode_knocked(
        self,
        rooms: List[KnockedSyncResult],
        time_now: int,
        serialize_options: SerializeEventConfig,
    ) -> Dict[str, Dict[str, Any]]:
        """
        Encode the rooms we've knocked on in a sync result.

        Args:
            rooms: list of sync results for rooms this user is knocking on
            time_now: current time - used as a baseline for age calculations
            serialize_options: Event serializer options

        Returns:
            The list of rooms the user has knocked on, in our response format.
        """
        knocked = {}
        for room in rooms:
            knock = await self._event_serializer.serialize_event(
                room.knock, time_now, config=serialize_options
            )

            # Extract the `unsigned` key from the knock event.
            # This is where we (cheekily) store the knock state events
            unsigned = knock.setdefault("unsigned", {})

            # Duplicate the dictionary in order to avoid modifying the original
            unsigned = dict(unsigned)

            # Extract the stripped room state from the unsigned dict
            # This is for clients to get a little bit of information about
            # the room they've knocked on, without revealing any sensitive information
            knocked_state = list(unsigned.pop("knock_room_state", []))

            # Append the actual knock membership event itself as well. This provides
            # the client with:
            #
            # * A knock state event that they can use for easier internal tracking
            # * The rough timestamp of when the knock occurred contained within the event
            knocked_state.append(knock)

            # Build the `knock_state` dictionary, which will contain the state of the
            # room that the client has knocked on
            knocked[room.room_id] = {"knock_state": {"events": knocked_state}}

        return knocked

    @trace_with_opname("sync.encode_archived")
    async def encode_archived(
        self,
        rooms: List[ArchivedSyncResult],
        time_now: int,
        serialize_options: SerializeEventConfig,
    ) -> JsonDict:
        """
        Encode the archived rooms in a sync result

        Args:
            rooms: list of sync results for rooms this user is joined to
            time_now: current time - used as a baseline for age calculations
            serialize_options: Event serializer options
        Returns:
            The archived rooms list, in our response format
        """
        joined = {}
        for room in rooms:
            joined[room.room_id] = await self.encode_room(
                room, time_now, joined=False, serialize_options=serialize_options
            )

        return joined

    async def encode_room(
        self,
        room: Union[JoinedSyncResult, ArchivedSyncResult],
        time_now: int,
        joined: bool,
        serialize_options: SerializeEventConfig,
    ) -> JsonDict:
        """
        Args:
            room: sync result for a single room
            time_now: current time - used as a baseline for age calculations
            token_id: ID of the user's auth token - used for namespacing
                of transaction IDs
            joined: True if the user is joined to this room - will mean
                we handle ephemeral events
            only_fields: Optional. The list of event fields to include.
            event_formatter: function to convert from federation format
                to client format
        Returns:
            The room, encoded in our response format
        """
        state_dict = room.state
        timeline_events = room.timeline.events

        state_events = state_dict.values()

        for event in itertools.chain(state_events, timeline_events):
            # We've had bug reports that events were coming down under the
            # wrong room.
            if event.room_id != room.room_id:
                logger.warning(
                    "Event %r is under room %r instead of %r",
                    event.event_id,
                    room.room_id,
                    event.room_id,
                )

        serialized_state = await self._event_serializer.serialize_events(
            state_events, time_now, config=serialize_options
        )
        serialized_timeline = await self._event_serializer.serialize_events(
            timeline_events,
            time_now,
            config=serialize_options,
            bundle_aggregations=room.timeline.bundled_aggregations,
        )

        account_data = room.account_data

        result: JsonDict = {
            "timeline": {
                "events": serialized_timeline,
                "prev_batch": await room.timeline.prev_batch.to_string(self.store),
                "limited": room.timeline.limited,
            },
            "state": {"events": serialized_state},
            "account_data": {"events": account_data},
        }

        if joined:
            assert isinstance(room, JoinedSyncResult)
            ephemeral_events = room.ephemeral
            result["ephemeral"] = {"events": ephemeral_events}
            result["unread_notifications"] = room.unread_notifications
            if room.unread_thread_notifications:
                result["unread_thread_notifications"] = room.unread_thread_notifications
                if self._msc3773_enabled:
                    result["org.matrix.msc3773.unread_thread_notifications"] = (
                        room.unread_thread_notifications
                    )
            result["summary"] = room.summary
            if self._msc2654_enabled:
                result["org.matrix.msc2654.unread_count"] = room.unread_count

        return result


class SlidingSyncE2eeRestServlet(RestServlet):
    """
    API endpoint for MSC3575 Sliding Sync `/sync/e2ee`. This is being introduced as part
    of Sliding Sync but doesn't have any sliding window component. It's just a way to
    get E2EE events without having to sit through a big initial sync (`/sync` v2). And
    we can avoid encryption events being backed up by the main sync response.

    Having To-Device messages split out to this sync endpoint also helps when clients
    need to have 2 or more sync streams open at a time, e.g a push notification process
    and a main process. This can cause the two processes to race to fetch the To-Device
    events, resulting in the need for complex synchronisation rules to ensure the token
    is correctly and atomically exchanged between processes.

    GET parameters::
        timeout(int): How long to wait for new events in milliseconds.
        since(batch_token): Batch token when asking for incremental deltas.

    Response JSON::
        {
            "next_batch": // batch token for the next /sync
            "to_device": {
                // list of to-device events
                "events": [
                    {
                        "content: { "algorithm": "m.olm.v1.curve25519-aes-sha2", "ciphertext": { ... }, "org.matrix.msgid": "abcd", "session_id": "abcd" },
                        "type": "m.room.encrypted",
                        "sender": "@alice:example.com",
                    }
                    // ...
                ]
            },
            "device_lists": {
                "changed": ["@alice:example.com"],
                "left": ["@bob:example.com"]
            },
            "device_one_time_keys_count": {
                "signed_curve25519": 50
            },
            "device_unused_fallback_key_types": [
                "signed_curve25519"
            ]
        }
    """

    PATTERNS = client_patterns(
        "/org.matrix.msc3575/sync/e2ee$", releases=[], v1=False, unstable=True
    )

    def __init__(self, hs: "HomeServer"):
        super().__init__()
        self.hs = hs
        self.auth = hs.get_auth()
        self.store = hs.get_datastores().main
        self.sync_handler = hs.get_sync_handler()

        # Filtering only matters for the `device_lists` because it requires a bunch of
        # derived information from rooms (see how `_generate_sync_entry_for_rooms()`
        # prepares a bunch of data for `_generate_sync_entry_for_device_list()`).
        self.only_member_events_filter_collection = FilterCollection(
            self.hs,
            {
                "room": {
                    # We only care about membership events for the `device_lists`.
                    # Membership will tell us whether a user has joined/left a room and
                    # if there are new devices to encrypt for.
                    "timeline": {
                        "types": ["m.room.member"],
                    },
                    "state": {
                        "types": ["m.room.member"],
                    },
                    # We don't want any extra account_data generated because it's not
                    # returned by this endpoint. This helps us avoid work in
                    # `_generate_sync_entry_for_rooms()`
                    "account_data": {
                        "not_types": ["*"],
                    },
                    # We don't want any extra ephemeral data generated because it's not
                    # returned by this endpoint. This helps us avoid work in
                    # `_generate_sync_entry_for_rooms()`
                    "ephemeral": {
                        "not_types": ["*"],
                    },
                },
                # We don't want any extra account_data generated because it's not
                # returned by this endpoint. (This is just here for good measure)
                "account_data": {
                    "not_types": ["*"],
                },
                # We don't want any extra presence data generated because it's not
                # returned by this endpoint. (This is just here for good measure)
                "presence": {
                    "not_types": ["*"],
                },
            },
        )

    async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
        requester = await self.auth.get_user_by_req(request, allow_guest=True)
        user = requester.user
        device_id = requester.device_id

        timeout = parse_integer(request, "timeout", default=0)
        since = parse_string(request, "since")

        sync_config = SyncConfig(
            user=user,
            filter_collection=self.only_member_events_filter_collection,
            is_guest=requester.is_guest,
            device_id=device_id,
        )

        since_token = None
        if since is not None:
            since_token = await StreamToken.from_string(self.store, since)

        # Request cache key
        request_key = (
            SyncVersion.E2EE_SYNC,
            user,
            timeout,
            since,
        )

        # Gather data for the response
        sync_result = await self.sync_handler.wait_for_sync_for_user(
            requester,
            sync_config,
            SyncVersion.E2EE_SYNC,
            request_key,
            since_token=since_token,
            timeout=timeout,
            full_state=False,
        )

        # The client may have disconnected by now; don't bother to serialize the
        # response if so.
        if request._disconnected:
            logger.info("Client has disconnected; not serializing response.")
            return 200, {}

        response: JsonDict = defaultdict(dict)
        response["next_batch"] = await sync_result.next_batch.to_string(self.store)

        if sync_result.to_device:
            response["to_device"] = {"events": sync_result.to_device}

        if sync_result.device_lists.changed:
            response["device_lists"]["changed"] = list(sync_result.device_lists.changed)
        if sync_result.device_lists.left:
            response["device_lists"]["left"] = list(sync_result.device_lists.left)

        # We always include this because https://github.com/vector-im/element-android/issues/3725
        # The spec isn't terribly clear on when this can be omitted and how a client would tell
        # the difference between "no keys present" and "nothing changed" in terms of whole field
        # absent / individual key type entry absent
        # Corresponding synapse issue: https://github.com/matrix-org/synapse/issues/10456
        response["device_one_time_keys_count"] = sync_result.device_one_time_keys_count

        # https://github.com/matrix-org/matrix-doc/blob/54255851f642f84a4f1aaf7bc063eebe3d76752b/proposals/2732-olm-fallback-keys.md
        # states that this field should always be included, as long as the server supports the feature.
        response["device_unused_fallback_key_types"] = (
            sync_result.device_unused_fallback_key_types
        )

        return 200, response


class SlidingSyncRestServlet(RestServlet):
    """
    API endpoint for MSC3575 Sliding Sync `/sync`. Allows for clients to request a
    subset (sliding window) of rooms, state, and timeline events (just what they need)
    in order to bootstrap quickly and subscribe to only what the client cares about.
    Because the client can specify what it cares about, we can respond quickly and skip
    all of the work we would normally have to do with a sync v2 response.

    Request query parameters:
        timeout: How long to wait for new events in milliseconds.
        pos: Stream position token when asking for incremental deltas.

    Request body::
        {
            // Sliding Window API
            "lists": {
                "foo-list": {
                    "ranges": [ [0, 99] ],
                    "sort": [ "by_notification_level", "by_recency", "by_name" ],
                    "required_state": [
                        ["m.room.join_rules", ""],
                        ["m.room.history_visibility", ""],
                        ["m.space.child", "*"]
                    ],
                    "timeline_limit": 10,
                    "filters": {
                        "is_dm": true
                    },
                    "bump_event_types": [ "m.room.message", "m.room.encrypted" ],
                }
            },
            // Room Subscriptions API
            "room_subscriptions": {
                "!sub1:bar": {
                    "required_state": [ ["*","*"] ],
                    "timeline_limit": 10,
                    "include_old_rooms": {
                        "timeline_limit": 1,
                        "required_state": [ ["m.room.tombstone", ""], ["m.room.create", ""] ],
                    }
                }
            },
            // Extensions API
            "extensions": {}
        }

    Response JSON::
        {
            "next_pos": "s58_224_0_13_10_1_1_16_0_1",
            "lists": {
                "foo-list": {
                    "count": 1337,
                    "ops": [{
                        "op": "SYNC",
                        "range": [0, 99],
                        "room_ids": [
                            "!foo:bar",
                            // ... 99 more room IDs
                        ]
                    }]
                }
            },
            // Aggregated rooms from lists and room subscriptions
            "rooms": {
                // Room from room subscription
                "!sub1:bar": {
                    "name": "Alice and Bob",
                    "avatar": "mxc://...",
                    "initial": true,
                    "required_state": [
                        {"sender":"@alice:example.com","type":"m.room.create", "state_key":"", "content":{"creator":"@alice:example.com"}},
                        {"sender":"@alice:example.com","type":"m.room.join_rules", "state_key":"", "content":{"join_rule":"invite"}},
                        {"sender":"@alice:example.com","type":"m.room.history_visibility", "state_key":"", "content":{"history_visibility":"joined"}},
                        {"sender":"@alice:example.com","type":"m.room.member", "state_key":"@alice:example.com", "content":{"membership":"join"}}
                    ],
                    "timeline": [
                        {"sender":"@alice:example.com","type":"m.room.create", "state_key":"", "content":{"creator":"@alice:example.com"}},
                        {"sender":"@alice:example.com","type":"m.room.join_rules", "state_key":"", "content":{"join_rule":"invite"}},
                        {"sender":"@alice:example.com","type":"m.room.history_visibility", "state_key":"", "content":{"history_visibility":"joined"}},
                        {"sender":"@alice:example.com","type":"m.room.member", "state_key":"@alice:example.com", "content":{"membership":"join"}},
                        {"sender":"@alice:example.com","type":"m.room.message", "content":{"body":"A"}},
                        {"sender":"@alice:example.com","type":"m.room.message", "content":{"body":"B"}},
                    ],
                    "prev_batch": "t111_222_333",
                    "joined_count": 41,
                    "invited_count": 1,
                    "notification_count": 1,
                    "highlight_count": 0
                },
                // rooms from list
                "!foo:bar": {
                    "name": "The calculated room name",
                    "avatar": "mxc://...",
                    "initial": true,
                    "required_state": [
                        {"sender":"@alice:example.com","type":"m.room.join_rules", "state_key":"", "content":{"join_rule":"invite"}},
                        {"sender":"@alice:example.com","type":"m.room.history_visibility", "state_key":"", "content":{"history_visibility":"joined"}},
                        {"sender":"@alice:example.com","type":"m.space.child", "state_key":"!foo:example.com", "content":{"via":["example.com"]}},
                        {"sender":"@alice:example.com","type":"m.space.child", "state_key":"!bar:example.com", "content":{"via":["example.com"]}},
                        {"sender":"@alice:example.com","type":"m.space.child", "state_key":"!baz:example.com", "content":{"via":["example.com"]}}
                    ],
                    "timeline": [
                        {"sender":"@alice:example.com","type":"m.room.join_rules", "state_key":"", "content":{"join_rule":"invite"}},
                        {"sender":"@alice:example.com","type":"m.room.message", "content":{"body":"A"}},
                        {"sender":"@alice:example.com","type":"m.room.message", "content":{"body":"B"}},
                        {"sender":"@alice:example.com","type":"m.room.message", "content":{"body":"C"}},
                        {"sender":"@alice:example.com","type":"m.room.message", "content":{"body":"D"}},
                    ],
                    "prev_batch": "t111_222_333",
                    "joined_count": 4,
                    "invited_count": 0,
                    "notification_count": 54,
                    "highlight_count": 3
                },
                 // ... 99 more items
            },
            "extensions": {}
        }
    """

    PATTERNS = client_patterns(
        "/org.matrix.msc3575/sync$", releases=[], v1=False, unstable=True
    )

    def __init__(self, hs: "HomeServer"):
        super().__init__()
        self.auth = hs.get_auth()
        self.store = hs.get_datastores().main
        self.filtering = hs.get_filtering()
        self.sliding_sync_handler = hs.get_sliding_sync_handler()

    # TODO: Update this to `on_GET` once we figure out how we want to handle params
    async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
        requester = await self.auth.get_user_by_req(request, allow_guest=True)
        user = requester.user
        device_id = requester.device_id

        timeout = parse_integer(request, "timeout", default=0)
        # Position in the stream
        from_token_string = parse_string(request, "pos")

        from_token = None
        if from_token_string is not None:
            from_token = await StreamToken.from_string(self.store, from_token_string)

        # TODO: We currently don't know whether we're going to use sticky params or
        # maybe some filters like sync v2  where they are built up once and referenced
        # by filter ID. For now, we will just prototype with always passing everything
        # in.
        body = parse_and_validate_json_object_from_request(request, SlidingSyncBody)
        logger.info("Sliding sync request: %r", body)

        sync_config = SlidingSyncConfig(
            user=user,
            device_id=device_id,
            # FIXME: Currently, we're just manually copying the fields from the
            # `SlidingSyncBody` into the config. How can we gurantee into the future
            # that we don't forget any? I would like something more structured like
            # `copy_attributes(from=body, to=config)`
            lists=body.lists,
            room_subscriptions=body.room_subscriptions,
            extensions=body.extensions,
        )

        sliding_sync_results = await self.sliding_sync_handler.wait_for_sync_for_user(
            requester,
            sync_config,
            from_token,
            timeout,
        )

        # The client may have disconnected by now; don't bother to serialize the
        # response if so.
        if request._disconnected:
            logger.info("Client has disconnected; not serializing response.")
            return 200, {}

        response_content = await self.encode_response(sliding_sync_results)

        return 200, response_content

    # TODO: Is there a better way to encode things?
    async def encode_response(
        self,
        sliding_sync_result: SlidingSyncResult,
    ) -> JsonDict:
        response: JsonDict = defaultdict(dict)

        response["next_pos"] = await sliding_sync_result.next_pos.to_string(self.store)
        serialized_lists = self.encode_lists(sliding_sync_result.lists)
        if serialized_lists:
            response["lists"] = serialized_lists
        response["rooms"] = {}  # TODO: sliding_sync_result.rooms
        response["extensions"] = {}  # TODO: sliding_sync_result.extensions

        return response

    def encode_lists(
        self, lists: Dict[str, SlidingSyncResult.SlidingWindowList]
    ) -> JsonDict:
        def encode_operation(
            operation: SlidingSyncResult.SlidingWindowList.Operation,
        ) -> JsonDict:
            return {
                "op": operation.op.value,
                "range": operation.range,
                "room_ids": operation.room_ids,
            }

        serialized_lists = {}
        for list_key, list_result in lists.items():
            serialized_lists[list_key] = {
                "count": list_result.count,
                "ops": [encode_operation(op) for op in list_result.ops],
            }

        return serialized_lists


def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
    SyncRestServlet(hs).register(http_server)

    if hs.config.experimental.msc3575_enabled:
        SlidingSyncRestServlet(hs).register(http_server)
        SlidingSyncE2eeRestServlet(hs).register(http_server)