#
# This file is licensed under the Affero General Public License (AGPL) version 3.
#
# Copyright (C) 2024 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 enum
import logging
import typing
from collections import ChainMap
from enum import Enum
from itertools import chain
from typing import (
    TYPE_CHECKING,
    Any,
    Callable,
    Dict,
    Final,
    Generic,
    List,
    Literal,
    Mapping,
    MutableMapping,
    Optional,
    Sequence,
    Set,
    Tuple,
    TypeVar,
    Union,
    cast,
)

import attr
from immutabledict import immutabledict
from prometheus_client import Histogram
from typing_extensions import assert_never

from synapse.api.constants import (
    AccountDataTypes,
    Direction,
    EventContentFields,
    EventTypes,
    Membership,
)
from synapse.api.errors import SlidingSyncUnknownPosition
from synapse.events import EventBase, StrippedStateEvent
from synapse.events.utils import parse_stripped_state_event, strip_event
from synapse.handlers.receipts import ReceiptEventSource
from synapse.handlers.relations import BundledAggregations
from synapse.logging.opentracing import (
    SynapseTags,
    log_kv,
    set_tag,
    start_active_span,
    tag_args,
    trace,
)
from synapse.storage.databases.main.roommember import extract_heroes_from_room_summary
from synapse.storage.databases.main.state import (
    ROOM_UNKNOWN_SENTINEL,
    Sentinel as StateSentinel,
)
from synapse.storage.databases.main.stream import (
    CurrentStateDeltaMembership,
    PaginateFunction,
)
from synapse.storage.roommember import MemberSummary
from synapse.types import (
    DeviceListUpdates,
    JsonDict,
    JsonMapping,
    MultiWriterStreamToken,
    MutableStateMap,
    PersistedEventPosition,
    Requester,
    RoomStreamToken,
    SlidingSyncStreamToken,
    StateMap,
    StrCollection,
    StreamKeyType,
    StreamToken,
    UserID,
)
from synapse.types.handlers import OperationType, SlidingSyncConfig, SlidingSyncResult
from synapse.types.state import StateFilter
from synapse.util.async_helpers import concurrently_execute
from synapse.visibility import filter_events_for_client

if TYPE_CHECKING:
    from synapse.server import HomeServer

logger = logging.getLogger(__name__)


sync_processing_time = Histogram(
    "synapse_sliding_sync_processing_time",
    "Time taken to generate a sliding sync response, ignoring wait times.",
    ["initial"],
)


class Sentinel(enum.Enum):
    # defining a sentinel in this way allows mypy to correctly handle the
    # type of a dictionary lookup and subsequent type narrowing.
    UNSET_SENTINEL = object()


# The event types that clients should consider as new activity.
DEFAULT_BUMP_EVENT_TYPES = {
    EventTypes.Create,
    EventTypes.Message,
    EventTypes.Encrypted,
    EventTypes.Sticker,
    EventTypes.CallInvite,
    EventTypes.PollStart,
    EventTypes.LiveLocationShareStart,
}


@attr.s(slots=True, frozen=True, auto_attribs=True)
class _RoomMembershipForUser:
    """
    Attributes:
        room_id: The room ID of the membership event
        event_id: The event ID of the membership event
        event_pos: The stream position of the membership event
        membership: The membership state of the user in the room
        sender: The person who sent the membership event
        newly_joined: Whether the user newly joined the room during the given token
            range and is still joined to the room at the end of this range.
        newly_left: Whether the user newly left (or kicked) the room during the given
            token range and is still "leave" at the end of this range.
        is_dm: Whether this user considers this room as a direct-message (DM) room
    """

    room_id: str
    # Optional because state resets can affect room membership without a corresponding event.
    event_id: Optional[str]
    # Even during a state reset which removes the user from the room, we expect this to
    # be set because `current_state_delta_stream` will note the position that the reset
    # happened.
    event_pos: PersistedEventPosition
    # Even during a state reset which removes the user from the room, we expect this to
    # be set to `LEAVE` because we can make that assumption based on the situaton (see
    # `get_current_state_delta_membership_changes_for_user(...)`)
    membership: str
    # Optional because state resets can affect room membership without a corresponding event.
    sender: Optional[str]
    newly_joined: bool
    newly_left: bool
    is_dm: bool

    def copy_and_replace(self, **kwds: Any) -> "_RoomMembershipForUser":
        return attr.evolve(self, **kwds)


def filter_membership_for_sync(
    *, user_id: str, room_membership_for_user: _RoomMembershipForUser
) -> bool:
    """
    Returns True if the membership event should be included in the sync response,
    otherwise False.

    Attributes:
        user_id: The user ID that the membership applies to
        room_membership_for_user: Membership information for the user in the room
    """

    membership = room_membership_for_user.membership
    sender = room_membership_for_user.sender
    newly_left = room_membership_for_user.newly_left

    # We want to allow everything except rooms the user has left unless `newly_left`
    # because we want everything that's *still* relevant to the user. We include
    # `newly_left` rooms because the last event that the user should see is their own
    # leave event.
    #
    # A leave != kick. This logic includes kicks (leave events where the sender is not
    # the same user).
    #
    # When `sender=None`, it means that a state reset happened that removed the user
    # from the room without a corresponding leave event. We can just remove the rooms
    # since they are no longer relevant to the user but will still appear if they are
    # `newly_left`.
    return (
        # Anything except leave events
        membership != Membership.LEAVE
        # Unless...
        or newly_left
        # Allow kicks
        or (membership == Membership.LEAVE and sender not in (user_id, None))
    )


# We can't freeze this class because we want to update it in place with the
# de-duplicated data.
@attr.s(slots=True, auto_attribs=True)
class RoomSyncConfig:
    """
    Holds the config for what data we should fetch for a room in the sync response.

    Attributes:
        timeline_limit: The maximum number of events to return in the timeline.

        required_state_map: Map from state event type to state_keys requested for the
            room. The values are close to `StateKey` but actually use a syntax where you
            can provide `*` wildcard and `$LAZY` for lazy-loading room members.
    """

    timeline_limit: int
    required_state_map: Dict[str, Set[str]]

    @classmethod
    def from_room_config(
        cls,
        room_params: SlidingSyncConfig.CommonRoomParameters,
    ) -> "RoomSyncConfig":
        """
        Create a `RoomSyncConfig` from a `SlidingSyncList`/`RoomSubscription` config.

        Args:
            room_params: `SlidingSyncConfig.SlidingSyncList` or `SlidingSyncConfig.RoomSubscription`
        """
        required_state_map: Dict[str, Set[str]] = {}
        for (
            state_type,
            state_key,
        ) in room_params.required_state:
            # If we already have a wildcard for this specific `state_key`, we don't need
            # to add it since the wildcard already covers it.
            if state_key in required_state_map.get(StateValues.WILDCARD, set()):
                continue

            # If we already have a wildcard `state_key` for this `state_type`, we don't need
            # to add anything else
            if StateValues.WILDCARD in required_state_map.get(state_type, set()):
                continue

            # If we're getting wildcards for the `state_type` and `state_key`, that's
            # all that matters so get rid of any other entries
            if state_type == StateValues.WILDCARD and state_key == StateValues.WILDCARD:
                required_state_map = {StateValues.WILDCARD: {StateValues.WILDCARD}}
                # We can break, since we don't need to add anything else
                break

            # If we're getting a wildcard for the `state_type`, get rid of any other
            # entries with the same `state_key`, since the wildcard will cover it already.
            elif state_type == StateValues.WILDCARD:
                # Get rid of any entries that match the `state_key`
                #
                # Make a copy so we don't run into an error: `dictionary changed size
                # during iteration`, when we remove items
                for (
                    existing_state_type,
                    existing_state_key_set,
                ) in list(required_state_map.items()):
                    # Make a copy so we don't run into an error: `Set changed size during
                    # iteration`, when we filter out and remove items
                    for existing_state_key in existing_state_key_set.copy():
                        if existing_state_key == state_key:
                            existing_state_key_set.remove(state_key)

                    # If we've the left the `set()` empty, remove it from the map
                    if existing_state_key_set == set():
                        required_state_map.pop(existing_state_type, None)

            # If we're getting a wildcard `state_key`, get rid of any other state_keys
            # for this `state_type` since the wildcard will cover it already.
            if state_key == StateValues.WILDCARD:
                required_state_map[state_type] = {state_key}
            # Otherwise, just add it to the set
            else:
                if required_state_map.get(state_type) is None:
                    required_state_map[state_type] = {state_key}
                else:
                    required_state_map[state_type].add(state_key)

        return cls(
            timeline_limit=room_params.timeline_limit,
            required_state_map=required_state_map,
        )

    def deep_copy(self) -> "RoomSyncConfig":
        required_state_map: Dict[str, Set[str]] = {
            state_type: state_key_set.copy()
            for state_type, state_key_set in self.required_state_map.items()
        }

        return RoomSyncConfig(
            timeline_limit=self.timeline_limit,
            required_state_map=required_state_map,
        )

    def combine_room_sync_config(
        self, other_room_sync_config: "RoomSyncConfig"
    ) -> None:
        """
        Combine this `RoomSyncConfig` with another `RoomSyncConfig` and take the
        superset union of the two.
        """
        # Take the highest timeline limit
        if self.timeline_limit < other_room_sync_config.timeline_limit:
            self.timeline_limit = other_room_sync_config.timeline_limit

        # Union the required state
        for (
            state_type,
            state_key_set,
        ) in other_room_sync_config.required_state_map.items():
            # If we already have a wildcard for everything, we don't need to add
            # anything else
            if StateValues.WILDCARD in self.required_state_map.get(
                StateValues.WILDCARD, set()
            ):
                break

            # If we already have a wildcard `state_key` for this `state_type`, we don't need
            # to add anything else
            if StateValues.WILDCARD in self.required_state_map.get(state_type, set()):
                continue

            # If we're getting wildcards for the `state_type` and `state_key`, that's
            # all that matters so get rid of any other entries
            if (
                state_type == StateValues.WILDCARD
                and StateValues.WILDCARD in state_key_set
            ):
                self.required_state_map = {state_type: {StateValues.WILDCARD}}
                # We can break, since we don't need to add anything else
                break

            for state_key in state_key_set:
                # If we already have a wildcard for this specific `state_key`, we don't need
                # to add it since the wildcard already covers it.
                if state_key in self.required_state_map.get(
                    StateValues.WILDCARD, set()
                ):
                    continue

                # If we're getting a wildcard for the `state_type`, get rid of any other
                # entries with the same `state_key`, since the wildcard will cover it already.
                if state_type == StateValues.WILDCARD:
                    # Get rid of any entries that match the `state_key`
                    #
                    # Make a copy so we don't run into an error: `dictionary changed size
                    # during iteration`, when we remove items
                    for existing_state_type, existing_state_key_set in list(
                        self.required_state_map.items()
                    ):
                        # Make a copy so we don't run into an error: `Set changed size during
                        # iteration`, when we filter out and remove items
                        for existing_state_key in existing_state_key_set.copy():
                            if existing_state_key == state_key:
                                existing_state_key_set.remove(state_key)

                        # If we've the left the `set()` empty, remove it from the map
                        if existing_state_key_set == set():
                            self.required_state_map.pop(existing_state_type, None)

                # If we're getting a wildcard `state_key`, get rid of any other state_keys
                # for this `state_type` since the wildcard will cover it already.
                if state_key == StateValues.WILDCARD:
                    self.required_state_map[state_type] = {state_key}
                    break
                # Otherwise, just add it to the set
                else:
                    if self.required_state_map.get(state_type) is None:
                        self.required_state_map[state_type] = {state_key}
                    else:
                        self.required_state_map[state_type].add(state_key)

    def must_await_full_state(
        self,
        is_mine_id: Callable[[str], bool],
    ) -> bool:
        """
        Check if we have a we're only requesting `required_state` which is completely
        satisfied even with partial state, then we don't need to `await_full_state` before
        we can return it.

        Also see `StateFilter.must_await_full_state(...)` for comparison

        Partially-stated rooms should have all state events except for remote membership
        events so if we require a remote membership event anywhere, then we need to
        return `True` (requires full state).

        Args:
            is_mine_id: a callable which confirms if a given state_key matches a mxid
               of a local user
        """
        wildcard_state_keys = self.required_state_map.get(StateValues.WILDCARD)
        # Requesting *all* state in the room so we have to wait
        if (
            wildcard_state_keys is not None
            and StateValues.WILDCARD in wildcard_state_keys
        ):
            return True

        # If the wildcards don't refer to remote user IDs, then we don't need to wait
        # for full state.
        if wildcard_state_keys is not None:
            for possible_user_id in wildcard_state_keys:
                if not possible_user_id[0].startswith(UserID.SIGIL):
                    # Not a user ID
                    continue

                localpart_hostname = possible_user_id.split(":", 1)
                if len(localpart_hostname) < 2:
                    # Not a user ID
                    continue

                if not is_mine_id(possible_user_id):
                    return True

        membership_state_keys = self.required_state_map.get(EventTypes.Member)
        # We aren't requesting any membership events at all so the partial state will
        # cover us.
        if membership_state_keys is None:
            return False

        # If we're requesting entirely local users, the partial state will cover us.
        for user_id in membership_state_keys:
            if user_id == StateValues.ME:
                continue
            # We're lazy-loading membership so we can just return the state we have.
            # Lazy-loading means we include membership for any event `sender` in the
            # timeline but since we had to auth those timeline events, we will have the
            # membership state for them (including from remote senders).
            elif user_id == StateValues.LAZY:
                continue
            elif user_id == StateValues.WILDCARD:
                return False
            elif not is_mine_id(user_id):
                return True

        # Local users only so the partial state will cover us.
        return False


class StateValues:
    """
    Understood values of the (type, state_key) tuple in `required_state`.
    """

    # Include all state events of the given type
    WILDCARD: Final = "*"
    # Lazy-load room membership events (include room membership events for any event
    # `sender` in the timeline). We only give special meaning to this value when it's a
    # `state_key`.
    LAZY: Final = "$LAZY"
    # Subsitute with the requester's user ID. Typically used by clients to get
    # the user's membership.
    ME: Final = "$ME"


class SlidingSyncHandler:
    def __init__(self, hs: "HomeServer"):
        self.clock = hs.get_clock()
        self.store = hs.get_datastores().main
        self.storage_controllers = hs.get_storage_controllers()
        self.auth_blocking = hs.get_auth_blocking()
        self.notifier = hs.get_notifier()
        self.event_sources = hs.get_event_sources()
        self.relations_handler = hs.get_relations_handler()
        self.device_handler = hs.get_device_handler()
        self.push_rules_handler = hs.get_push_rules_handler()
        self.rooms_to_exclude_globally = hs.config.server.rooms_to_exclude_from_sync
        self.is_mine_id = hs.is_mine_id

        self.connection_store = SlidingSyncConnectionStore()

    async def wait_for_sync_for_user(
        self,
        requester: Requester,
        sync_config: SlidingSyncConfig,
        from_token: Optional[SlidingSyncStreamToken] = None,
        timeout_ms: int = 0,
    ) -> SlidingSyncResult:
        """
        Get the sync for a client if we have new data for it now. Otherwise
        wait for new data to arrive on the server. If the timeout expires, then
        return an empty sync result.

        Args:
            requester: The user making the request
            sync_config: Sync configuration
            from_token: The point in the stream to sync from. Token of the end of the
                previous batch. May be `None` if this is the initial sync request.
            timeout_ms: The time in milliseconds to wait for new data to arrive. If 0,
                we will immediately but there might not be any new data so we just return an
                empty response.
        """
        # If the user is not part of the mau group, then check that limits have
        # not been exceeded (if not part of the group by this point, almost certain
        # auth_blocking will occur)
        await self.auth_blocking.check_auth_blocking(requester=requester)

        # If we're working with a user-provided token, we need to make sure to wait for
        # this worker to catch up with the token so we don't skip past any incoming
        # events or future events if the user is nefariously, manually modifying the
        # token.
        if from_token is not None:
            # We need to make sure this worker has caught up with the token. If
            # this returns false, it means we timed out waiting, and we should
            # just return an empty response.
            before_wait_ts = self.clock.time_msec()
            if not await self.notifier.wait_for_stream_token(from_token.stream_token):
                logger.warning(
                    "Timed out waiting for worker to catch up. Returning empty response"
                )
                return SlidingSyncResult.empty(from_token)

            # If we've spent significant time waiting to catch up, take it off
            # the timeout.
            after_wait_ts = self.clock.time_msec()
            if after_wait_ts - before_wait_ts > 1_000:
                timeout_ms -= after_wait_ts - before_wait_ts
                timeout_ms = max(timeout_ms, 0)

        # We're going to respond immediately if the timeout is 0 or if this is an
        # initial sync (without a `from_token`) so we can avoid calling
        # `notifier.wait_for_events()`.
        if timeout_ms == 0 or from_token is None:
            now_token = self.event_sources.get_current_token()
            result = await self.current_sync_for_user(
                sync_config,
                from_token=from_token,
                to_token=now_token,
            )
        else:
            # Otherwise, we wait for something to happen and report it to the user.
            async def current_sync_callback(
                before_token: StreamToken, after_token: StreamToken
            ) -> SlidingSyncResult:
                return await self.current_sync_for_user(
                    sync_config,
                    from_token=from_token,
                    to_token=after_token,
                )

            result = await self.notifier.wait_for_events(
                sync_config.user.to_string(),
                timeout_ms,
                current_sync_callback,
                from_token=from_token.stream_token,
            )

        return result

    @trace
    async def current_sync_for_user(
        self,
        sync_config: SlidingSyncConfig,
        to_token: StreamToken,
        from_token: Optional[SlidingSyncStreamToken] = None,
    ) -> SlidingSyncResult:
        """
        Generates the response body of a Sliding Sync result, represented as a
        `SlidingSyncResult`.

        We fetch data according to the token range (> `from_token` and <= `to_token`).

        Args:
            sync_config: Sync configuration
            to_token: The point in the stream to sync up to.
            from_token: The point in the stream to sync from. Token of the end of the
                previous batch. May be `None` if this is the initial sync request.
        """
        start_time_s = self.clock.time()

        user_id = sync_config.user.to_string()
        app_service = self.store.get_app_service_by_user_id(user_id)
        if app_service:
            # We no longer support AS users using /sync directly.
            # See https://github.com/matrix-org/matrix-doc/issues/1144
            raise NotImplementedError()

        # Get the per-connection state (if any).
        #
        # Raises an exception if there is a `connection_position` that we don't
        # recognize. If we don't do this and the client asks for the full range
        # of rooms, we end up sending down all rooms and their state from
        # scratch (which can be very slow). By expiring the connection we allow
        # the client a chance to do an initial request with a smaller range of
        # rooms to get them some results sooner but will end up taking the same
        # amount of time (more with round-trips and re-processing) in the end to
        # get everything again.
        previous_connection_state = (
            await self.connection_store.get_per_connection_state(
                sync_config, from_token
            )
        )

        await self.connection_store.mark_token_seen(
            sync_config=sync_config,
            from_token=from_token,
        )

        # Get all of the room IDs that the user should be able to see in the sync
        # response
        has_lists = sync_config.lists is not None and len(sync_config.lists) > 0
        has_room_subscriptions = (
            sync_config.room_subscriptions is not None
            and len(sync_config.room_subscriptions) > 0
        )
        if has_lists or has_room_subscriptions:
            room_membership_for_user_map = (
                await self.get_room_membership_for_user_at_to_token(
                    user=sync_config.user,
                    to_token=to_token,
                    from_token=from_token.stream_token if from_token else None,
                )
            )

        # Assemble sliding window lists
        lists: Dict[str, SlidingSyncResult.SlidingWindowList] = {}
        # Keep track of the rooms that we can display and need to fetch more info about
        relevant_room_map: Dict[str, RoomSyncConfig] = {}
        # The set of room IDs of all rooms that could appear in any list. These
        # include rooms that are outside the list ranges.
        all_rooms: Set[str] = set()
        if has_lists and sync_config.lists is not None:
            with start_active_span("assemble_sliding_window_lists"):
                sync_room_map = await self.filter_rooms_relevant_for_sync(
                    user=sync_config.user,
                    room_membership_for_user_map=room_membership_for_user_map,
                )

                for list_key, list_config in sync_config.lists.items():
                    # Apply filters
                    filtered_sync_room_map = sync_room_map
                    if list_config.filters is not None:
                        filtered_sync_room_map = await self.filter_rooms(
                            sync_config.user,
                            sync_room_map,
                            list_config.filters,
                            to_token,
                        )

                    # Find which rooms are partially stated and may need to be filtered out
                    # depending on the `required_state` requested (see below).
                    partial_state_room_map = (
                        await self.store.is_partial_state_room_batched(
                            filtered_sync_room_map.keys()
                        )
                    )

                    # Since creating the `RoomSyncConfig` takes some work, let's just do it
                    # once and make a copy whenever we need it.
                    room_sync_config = RoomSyncConfig.from_room_config(list_config)

                    # Exclude partially-stated rooms if we must wait for the room to be
                    # fully-stated
                    if room_sync_config.must_await_full_state(self.is_mine_id):
                        filtered_sync_room_map = {
                            room_id: room
                            for room_id, room in filtered_sync_room_map.items()
                            if not partial_state_room_map.get(room_id)
                        }

                    all_rooms.update(filtered_sync_room_map)

                    # Sort the list
                    sorted_room_info = await self.sort_rooms(
                        filtered_sync_room_map, to_token
                    )

                    ops: List[SlidingSyncResult.SlidingWindowList.Operation] = []
                    if list_config.ranges:
                        for range in list_config.ranges:
                            room_ids_in_list: List[str] = []

                            # We're going to loop through the sorted list of rooms starting
                            # at the range start index and keep adding rooms until we fill
                            # up the range or run out of rooms.
                            #
                            # Both sides of range are inclusive so we `+ 1`
                            max_num_rooms = range[1] - range[0] + 1
                            for room_membership in sorted_room_info[range[0] :]:
                                room_id = room_membership.room_id

                                if len(room_ids_in_list) >= max_num_rooms:
                                    break

                                # Take the superset of the `RoomSyncConfig` for each room.
                                #
                                # Update our `relevant_room_map` with the room we're going
                                # to display and need to fetch more info about.
                                existing_room_sync_config = relevant_room_map.get(
                                    room_id
                                )
                                if existing_room_sync_config is not None:
                                    existing_room_sync_config.combine_room_sync_config(
                                        room_sync_config
                                    )
                                else:
                                    # Make a copy so if we modify it later, it doesn't
                                    # affect all references.
                                    relevant_room_map[room_id] = (
                                        room_sync_config.deep_copy()
                                    )

                                room_ids_in_list.append(room_id)

                            ops.append(
                                SlidingSyncResult.SlidingWindowList.Operation(
                                    op=OperationType.SYNC,
                                    range=range,
                                    room_ids=room_ids_in_list,
                                )
                            )

                    lists[list_key] = SlidingSyncResult.SlidingWindowList(
                        count=len(sorted_room_info),
                        ops=ops,
                    )

        # Handle room subscriptions
        if has_room_subscriptions and sync_config.room_subscriptions is not None:
            with start_active_span("assemble_room_subscriptions"):
                # Find which rooms are partially stated and may need to be filtered out
                # depending on the `required_state` requested (see below).
                partial_state_room_map = await self.store.is_partial_state_room_batched(
                    sync_config.room_subscriptions.keys()
                )

                for (
                    room_id,
                    room_subscription,
                ) in sync_config.room_subscriptions.items():
                    room_membership_for_user_at_to_token = (
                        await self.check_room_subscription_allowed_for_user(
                            room_id=room_id,
                            room_membership_for_user_map=room_membership_for_user_map,
                            to_token=to_token,
                        )
                    )

                    # Skip this room if the user isn't allowed to see it
                    if not room_membership_for_user_at_to_token:
                        continue

                    all_rooms.add(room_id)

                    room_membership_for_user_map[room_id] = (
                        room_membership_for_user_at_to_token
                    )

                    # Take the superset of the `RoomSyncConfig` for each room.
                    room_sync_config = RoomSyncConfig.from_room_config(
                        room_subscription
                    )

                    # Exclude partially-stated rooms if we must wait for the room to be
                    # fully-stated
                    if room_sync_config.must_await_full_state(self.is_mine_id):
                        if partial_state_room_map.get(room_id):
                            continue

                    all_rooms.add(room_id)

                    # Update our `relevant_room_map` with the room we're going to display
                    # and need to fetch more info about.
                    existing_room_sync_config = relevant_room_map.get(room_id)
                    if existing_room_sync_config is not None:
                        existing_room_sync_config.combine_room_sync_config(
                            room_sync_config
                        )
                    else:
                        relevant_room_map[room_id] = room_sync_config

        # Fetch room data
        rooms: Dict[str, SlidingSyncResult.RoomResult] = {}

        # Filter out rooms that haven't received updates and we've sent down
        # previously.
        # Keep track of the rooms that we're going to display and need to fetch more info about
        relevant_rooms_to_send_map = relevant_room_map
        with start_active_span("filter_relevant_rooms_to_send"):
            if from_token:
                rooms_should_send = set()

                # First we check if there are rooms that match a list/room
                # subscription and have updates we need to send (i.e. either because
                # we haven't sent the room down, or we have but there are missing
                # updates).
                for room_id, room_config in relevant_room_map.items():
                    prev_room_sync_config = previous_connection_state.room_configs.get(
                        room_id
                    )
                    if prev_room_sync_config is not None:
                        # Always include rooms whose timeline limit has increased.
                        # (see the "XXX: Odd behavior" described below)
                        if (
                            prev_room_sync_config.timeline_limit
                            < room_config.timeline_limit
                        ):
                            rooms_should_send.add(room_id)
                            continue

                    status = previous_connection_state.rooms.have_sent_room(room_id)
                    if (
                        # The room was never sent down before so the client needs to know
                        # about it regardless of any updates.
                        status.status == HaveSentRoomFlag.NEVER
                        # `PREVIOUSLY` literally means the "room was sent down before *AND*
                        # there are updates we haven't sent down" so we already know this
                        # room has updates.
                        or status.status == HaveSentRoomFlag.PREVIOUSLY
                    ):
                        rooms_should_send.add(room_id)
                    elif status.status == HaveSentRoomFlag.LIVE:
                        # We know that we've sent all updates up until `from_token`,
                        # so we just need to check if there have been updates since
                        # then.
                        pass
                    else:
                        assert_never(status.status)

                # We only need to check for new events since any state changes
                # will also come down as new events.
                rooms_that_have_updates = self.store.get_rooms_that_might_have_updates(
                    relevant_room_map.keys(), from_token.stream_token.room_key
                )
                rooms_should_send.update(rooms_that_have_updates)
                relevant_rooms_to_send_map = {
                    room_id: room_sync_config
                    for room_id, room_sync_config in relevant_room_map.items()
                    if room_id in rooms_should_send
                }

        new_connection_state = previous_connection_state.get_mutable()

        @trace
        @tag_args
        async def handle_room(room_id: str) -> None:
            room_sync_result = await self.get_room_sync_data(
                sync_config=sync_config,
                previous_connection_state=previous_connection_state,
                new_connection_state=new_connection_state,
                room_id=room_id,
                room_sync_config=relevant_rooms_to_send_map[room_id],
                room_membership_for_user_at_to_token=room_membership_for_user_map[
                    room_id
                ],
                from_token=from_token,
                to_token=to_token,
            )

            # Filter out empty room results during incremental sync
            if room_sync_result or not from_token:
                rooms[room_id] = room_sync_result

        if relevant_rooms_to_send_map:
            with start_active_span("sliding_sync.generate_room_entries"):
                await concurrently_execute(handle_room, relevant_rooms_to_send_map, 10)

        extensions = await self.get_extensions_response(
            sync_config=sync_config,
            actual_lists=lists,
            previous_connection_state=previous_connection_state,
            new_connection_state=new_connection_state,
            # We're purposely using `relevant_room_map` instead of
            # `relevant_rooms_to_send_map` here. This needs to be all room_ids we could
            # send regardless of whether they have an event update or not. The
            # extensions care about more than just normal events in the rooms (like
            # account data, read receipts, typing indicators, to-device messages, etc).
            actual_room_ids=set(relevant_room_map.keys()),
            actual_room_response_map=rooms,
            from_token=from_token,
            to_token=to_token,
        )

        if has_lists or has_room_subscriptions:
            # We now calculate if any rooms outside the range have had updates,
            # which we are not sending down.
            #
            # We *must* record rooms that have had updates, but it is also fine
            # to record rooms as having updates even if there might not actually
            # be anything new for the user (e.g. due to event filters, events
            # having happened after the user left, etc).
            unsent_room_ids = []
            if from_token:
                # The set of rooms that the client (may) care about, but aren't
                # in any list range (or subscribed to).
                missing_rooms = all_rooms - relevant_room_map.keys()

                # We now just go and try fetching any events in the above rooms
                # to see if anything has happened since the `from_token`.
                #
                # TODO: Replace this with something faster. When we land the
                # sliding sync tables that record the most recent event
                # positions we can use that.
                missing_event_map_by_room = (
                    await self.store.get_room_events_stream_for_rooms(
                        room_ids=missing_rooms,
                        from_key=to_token.room_key,
                        to_key=from_token.stream_token.room_key,
                        limit=1,
                    )
                )
                unsent_room_ids = list(missing_event_map_by_room)

                new_connection_state.rooms.record_unsent_rooms(
                    unsent_room_ids, from_token.stream_token.room_key
                )

            new_connection_state.rooms.record_sent_rooms(
                relevant_rooms_to_send_map.keys()
            )

            connection_position = await self.connection_store.record_new_state(
                sync_config=sync_config,
                from_token=from_token,
                new_connection_state=new_connection_state,
            )
        elif from_token:
            connection_position = from_token.connection_position
        else:
            # Initial sync without a `from_token` starts at `0`
            connection_position = 0

        sliding_sync_result = SlidingSyncResult(
            next_pos=SlidingSyncStreamToken(to_token, connection_position),
            lists=lists,
            rooms=rooms,
            extensions=extensions,
        )

        # Make it easy to find traces for syncs that aren't empty
        set_tag(SynapseTags.RESULT_PREFIX + "result", bool(sliding_sync_result))
        set_tag(SynapseTags.FUNC_ARG_PREFIX + "sync_config.user", user_id)

        end_time_s = self.clock.time()
        sync_processing_time.labels(from_token is not None).observe(
            end_time_s - start_time_s
        )

        return sliding_sync_result

    @trace
    async def get_room_membership_for_user_at_to_token(
        self,
        user: UserID,
        to_token: StreamToken,
        from_token: Optional[StreamToken],
    ) -> Dict[str, _RoomMembershipForUser]:
        """
        Fetch room IDs that the user has had membership in (the full room list including
        long-lost left rooms that will be filtered, sorted, and sliced).

        We're looking for rooms where the user has had any sort of membership in the
        token range (> `from_token` and <= `to_token`)

        In order for bans/kicks to not show up, you need to `/forget` those rooms. This
        doesn't modify the event itself though and only adds the `forgotten` flag to the
        `room_memberships` table in Synapse. There isn't a way to tell when a room was
        forgotten at the moment so we can't factor it into the token range.

        Args:
            user: User to fetch rooms for
            to_token: The token to fetch rooms up to.
            from_token: The point in the stream to sync from.

        Returns:
            A dictionary of room IDs that the user has had membership in along with
            membership information in that room at the time of `to_token`.
        """
        user_id = user.to_string()

        # First grab a current snapshot rooms for the user
        # (also handles forgotten rooms)
        room_for_user_list = await self.store.get_rooms_for_local_user_where_membership_is(
            user_id=user_id,
            # We want to fetch any kind of membership (joined and left rooms) in order
            # to get the `event_pos` of the latest room membership event for the
            # user.
            membership_list=Membership.LIST,
            excluded_rooms=self.rooms_to_exclude_globally,
        )

        # If the user has never joined any rooms before, we can just return an empty list
        if not room_for_user_list:
            return {}

        # Our working list of rooms that can show up in the sync response
        sync_room_id_set = {
            # Note: The `room_for_user` we're assigning here will need to be fixed up
            # (below) because they are potentially from the current snapshot time
            # instead from the time of the `to_token`.
            room_for_user.room_id: _RoomMembershipForUser(
                room_id=room_for_user.room_id,
                event_id=room_for_user.event_id,
                event_pos=room_for_user.event_pos,
                membership=room_for_user.membership,
                sender=room_for_user.sender,
                # We will update these fields below to be accurate
                newly_joined=False,
                newly_left=False,
                is_dm=False,
            )
            for room_for_user in room_for_user_list
        }

        # Get the `RoomStreamToken` that represents the spot we queried up to when we got
        # our membership snapshot from `get_rooms_for_local_user_where_membership_is()`.
        #
        # First, we need to get the max stream_ordering of each event persister instance
        # that we queried events from.
        instance_to_max_stream_ordering_map: Dict[str, int] = {}
        for room_for_user in room_for_user_list:
            instance_name = room_for_user.event_pos.instance_name
            stream_ordering = room_for_user.event_pos.stream

            current_instance_max_stream_ordering = (
                instance_to_max_stream_ordering_map.get(instance_name)
            )
            if (
                current_instance_max_stream_ordering is None
                or stream_ordering > current_instance_max_stream_ordering
            ):
                instance_to_max_stream_ordering_map[instance_name] = stream_ordering

        # Then assemble the `RoomStreamToken`
        min_stream_pos = min(instance_to_max_stream_ordering_map.values())
        membership_snapshot_token = RoomStreamToken(
            # Minimum position in the `instance_map`
            stream=min_stream_pos,
            instance_map=immutabledict(
                {
                    instance_name: stream_pos
                    for instance_name, stream_pos in instance_to_max_stream_ordering_map.items()
                    if stream_pos > min_stream_pos
                }
            ),
        )

        # Since we fetched the users room list at some point in time after the from/to
        # tokens, we need to revert/rewind some membership changes to match the point in
        # time of the `to_token`. In particular, we need to make these fixups:
        #
        # - 1a) Remove rooms that the user joined after the `to_token`
        # - 1b) Add back rooms that the user left after the `to_token`
        # - 1c) Update room membership events to the point in time of the `to_token`
        # - 2) Figure out which rooms are `newly_left` rooms (> `from_token` and <= `to_token`)
        # - 3) Figure out which rooms are `newly_joined` (> `from_token` and <= `to_token`)
        # - 4) Figure out which rooms are DM's

        # 1) Fetch membership changes that fall in the range from `to_token` up to
        # `membership_snapshot_token`
        #
        # If our `to_token` is already the same or ahead of the latest room membership
        # for the user, we don't need to do any "2)" fix-ups and can just straight-up
        # use the room list from the snapshot as a base (nothing has changed)
        current_state_delta_membership_changes_after_to_token = []
        if not membership_snapshot_token.is_before_or_eq(to_token.room_key):
            current_state_delta_membership_changes_after_to_token = (
                await self.store.get_current_state_delta_membership_changes_for_user(
                    user_id,
                    from_key=to_token.room_key,
                    to_key=membership_snapshot_token,
                    excluded_room_ids=self.rooms_to_exclude_globally,
                )
            )

        # 1) Assemble a list of the first membership event after the `to_token` so we can
        # step backward to the previous membership that would apply to the from/to
        # range.
        first_membership_change_by_room_id_after_to_token: Dict[
            str, CurrentStateDeltaMembership
        ] = {}
        for membership_change in current_state_delta_membership_changes_after_to_token:
            # Only set if we haven't already set it
            first_membership_change_by_room_id_after_to_token.setdefault(
                membership_change.room_id, membership_change
            )

        # 1) Fixup
        #
        # Since we fetched a snapshot of the users room list at some point in time after
        # the from/to tokens, we need to revert/rewind some membership changes to match
        # the point in time of the `to_token`.
        for (
            room_id,
            first_membership_change_after_to_token,
        ) in first_membership_change_by_room_id_after_to_token.items():
            # 1a) Remove rooms that the user joined after the `to_token`
            if first_membership_change_after_to_token.prev_event_id is None:
                sync_room_id_set.pop(room_id, None)
            # 1b) 1c) From the first membership event after the `to_token`, step backward to the
            # previous membership that would apply to the from/to range.
            else:
                # We don't expect these fields to be `None` if we have a `prev_event_id`
                # but we're being defensive since it's possible that the prev event was
                # culled from the database.
                if (
                    first_membership_change_after_to_token.prev_event_pos is not None
                    and first_membership_change_after_to_token.prev_membership
                    is not None
                ):
                    sync_room_id_set[room_id] = _RoomMembershipForUser(
                        room_id=room_id,
                        event_id=first_membership_change_after_to_token.prev_event_id,
                        event_pos=first_membership_change_after_to_token.prev_event_pos,
                        membership=first_membership_change_after_to_token.prev_membership,
                        sender=first_membership_change_after_to_token.prev_sender,
                        # We will update these fields below to be accurate
                        newly_joined=False,
                        newly_left=False,
                        is_dm=False,
                    )
                else:
                    # If we can't find the previous membership event, we shouldn't
                    # include the room in the sync response since we can't determine the
                    # exact membership state and shouldn't rely on the current snapshot.
                    sync_room_id_set.pop(room_id, None)

        # 2) Fetch membership changes that fall in the range from `from_token` up to `to_token`
        current_state_delta_membership_changes_in_from_to_range = []
        if from_token:
            current_state_delta_membership_changes_in_from_to_range = (
                await self.store.get_current_state_delta_membership_changes_for_user(
                    user_id,
                    from_key=from_token.room_key,
                    to_key=to_token.room_key,
                    excluded_room_ids=self.rooms_to_exclude_globally,
                )
            )

        # 2) Assemble a list of the last membership events in some given ranges. Someone
        # could have left and joined multiple times during the given range but we only
        # care about end-result so we grab the last one.
        last_membership_change_by_room_id_in_from_to_range: Dict[
            str, CurrentStateDeltaMembership
        ] = {}
        # We also want to assemble a list of the first membership events during the token
        # range so we can step backward to the previous membership that would apply to
        # before the token range to see if we have `newly_joined` the room.
        first_membership_change_by_room_id_in_from_to_range: Dict[
            str, CurrentStateDeltaMembership
        ] = {}
        # Keep track if the room has a non-join event in the token range so we can later
        # tell if it was a `newly_joined` room. If the last membership event in the
        # token range is a join and there is also some non-join in the range, we know
        # they `newly_joined`.
        has_non_join_event_by_room_id_in_from_to_range: Dict[str, bool] = {}
        for (
            membership_change
        ) in current_state_delta_membership_changes_in_from_to_range:
            room_id = membership_change.room_id

            last_membership_change_by_room_id_in_from_to_range[room_id] = (
                membership_change
            )
            # Only set if we haven't already set it
            first_membership_change_by_room_id_in_from_to_range.setdefault(
                room_id, membership_change
            )

            if membership_change.membership != Membership.JOIN:
                has_non_join_event_by_room_id_in_from_to_range[room_id] = True

        # 2) Fixup
        #
        # 3) We also want to assemble a list of possibly newly joined rooms. Someone
        # could have left and joined multiple times during the given range but we only
        # care about whether they are joined at the end of the token range so we are
        # working with the last membership even in the token range.
        possibly_newly_joined_room_ids = set()
        for (
            last_membership_change_in_from_to_range
        ) in last_membership_change_by_room_id_in_from_to_range.values():
            room_id = last_membership_change_in_from_to_range.room_id

            # 3)
            if last_membership_change_in_from_to_range.membership == Membership.JOIN:
                possibly_newly_joined_room_ids.add(room_id)

            # 2) Figure out newly_left rooms (> `from_token` and <= `to_token`).
            if last_membership_change_in_from_to_range.membership == Membership.LEAVE:
                # 2) Mark this room as `newly_left`

                # If we're seeing a membership change here, we should expect to already
                # have it in our snapshot but if a state reset happens, it wouldn't have
                # shown up in our snapshot but appear as a change here.
                existing_sync_entry = sync_room_id_set.get(room_id)
                if existing_sync_entry is not None:
                    # Normal expected case
                    sync_room_id_set[room_id] = existing_sync_entry.copy_and_replace(
                        newly_left=True
                    )
                else:
                    # State reset!
                    logger.warn(
                        "State reset detected for room_id %s with %s who is no longer in the room",
                        room_id,
                        user_id,
                    )
                    # Even though a state reset happened which removed the person from
                    # the room, we still add it the list so the user knows they left the
                    # room. Downstream code can check for a state reset by looking for
                    # `event_id=None and membership is not None`.
                    sync_room_id_set[room_id] = _RoomMembershipForUser(
                        room_id=room_id,
                        event_id=last_membership_change_in_from_to_range.event_id,
                        event_pos=last_membership_change_in_from_to_range.event_pos,
                        membership=last_membership_change_in_from_to_range.membership,
                        sender=last_membership_change_in_from_to_range.sender,
                        newly_joined=False,
                        newly_left=True,
                        is_dm=False,
                    )

        # 3) Figure out `newly_joined`
        for room_id in possibly_newly_joined_room_ids:
            has_non_join_in_from_to_range = (
                has_non_join_event_by_room_id_in_from_to_range.get(room_id, False)
            )
            # If the last membership event in the token range is a join and there is
            # also some non-join in the range, we know they `newly_joined`.
            if has_non_join_in_from_to_range:
                # We found a `newly_joined` room (we left and joined within the token range)
                sync_room_id_set[room_id] = sync_room_id_set[room_id].copy_and_replace(
                    newly_joined=True
                )
            else:
                prev_event_id = first_membership_change_by_room_id_in_from_to_range[
                    room_id
                ].prev_event_id
                prev_membership = first_membership_change_by_room_id_in_from_to_range[
                    room_id
                ].prev_membership

                if prev_event_id is None:
                    # We found a `newly_joined` room (we are joining the room for the
                    # first time within the token range)
                    sync_room_id_set[room_id] = sync_room_id_set[
                        room_id
                    ].copy_and_replace(newly_joined=True)
                # Last resort, we need to step back to the previous membership event
                # just before the token range to see if we're joined then or not.
                elif prev_membership != Membership.JOIN:
                    # We found a `newly_joined` room (we left before the token range
                    # and joined within the token range)
                    sync_room_id_set[room_id] = sync_room_id_set[
                        room_id
                    ].copy_and_replace(newly_joined=True)

        # 4) Figure out which rooms the user considers to be direct-message (DM) rooms
        #
        # We're using global account data (`m.direct`) instead of checking for
        # `is_direct` on membership events because that property only appears for
        # the invitee membership event (doesn't show up for the inviter).
        #
        # We're unable to take `to_token` into account for global account data since
        # we only keep track of the latest account data for the user.
        dm_map = await self.store.get_global_account_data_by_type_for_user(
            user_id, AccountDataTypes.DIRECT
        )

        # Flatten out the map. Account data is set by the client so it needs to be
        # scrutinized.
        dm_room_id_set = set()
        if isinstance(dm_map, dict):
            for room_ids in dm_map.values():
                # Account data should be a list of room IDs. Ignore anything else
                if isinstance(room_ids, list):
                    for room_id in room_ids:
                        if isinstance(room_id, str):
                            dm_room_id_set.add(room_id)

        # 4) Fixup
        for room_id in sync_room_id_set:
            sync_room_id_set[room_id] = sync_room_id_set[room_id].copy_and_replace(
                is_dm=room_id in dm_room_id_set
            )

        return sync_room_id_set

    @trace
    async def filter_rooms_relevant_for_sync(
        self,
        user: UserID,
        room_membership_for_user_map: Dict[str, _RoomMembershipForUser],
    ) -> Dict[str, _RoomMembershipForUser]:
        """
        Filter room IDs that should/can be listed for this user in the sync response (the
        full room list that will be further filtered, sorted, and sliced).

        We're looking for rooms where the user has the following state in the token
        range (> `from_token` and <= `to_token`):

        - `invite`, `join`, `knock`, `ban` membership events
        - Kicks (`leave` membership events where `sender` is different from the
          `user_id`/`state_key`)
        - `newly_left` (rooms that were left during the given token range)
        - In order for bans/kicks to not show up in sync, you need to `/forget` those
          rooms. This doesn't modify the event itself though and only adds the
          `forgotten` flag to the `room_memberships` table in Synapse. There isn't a way
          to tell when a room was forgotten at the moment so we can't factor it into the
          from/to range.

        Args:
            user: User that is syncing
            room_membership_for_user_map: Room membership for the user

        Returns:
            A dictionary of room IDs that should be listed in the sync response along
            with membership information in that room at the time of `to_token`.
        """
        user_id = user.to_string()

        # Filter rooms to only what we're interested to sync with
        filtered_sync_room_map = {
            room_id: room_membership_for_user
            for room_id, room_membership_for_user in room_membership_for_user_map.items()
            if filter_membership_for_sync(
                user_id=user_id,
                room_membership_for_user=room_membership_for_user,
            )
        }

        return filtered_sync_room_map

    async def check_room_subscription_allowed_for_user(
        self,
        room_id: str,
        room_membership_for_user_map: Dict[str, _RoomMembershipForUser],
        to_token: StreamToken,
    ) -> Optional[_RoomMembershipForUser]:
        """
        Check whether the user is allowed to see the room based on whether they have
        ever had membership in the room or if the room is `world_readable`.

        Similar to `check_user_in_room_or_world_readable(...)`

        Args:
            room_id: Room to check
            room_membership_for_user_map: Room membership for the user at the time of
                the `to_token` (<= `to_token`).
            to_token: The token to fetch rooms up to.

        Returns:
            The room membership for the user if they are allowed to subscribe to the
            room else `None`.
        """

        # We can first check if they are already allowed to see the room based
        # on our previous work to assemble the `room_membership_for_user_map`.
        #
        # If they have had any membership in the room over time (up to the `to_token`),
        # let them subscribe and see what they can.
        existing_membership_for_user = room_membership_for_user_map.get(room_id)
        if existing_membership_for_user is not None:
            return existing_membership_for_user

        # TODO: Handle `world_readable` rooms
        return None

        # If the room is `world_readable`, it doesn't matter whether they can join,
        # everyone can see the room.
        # not_in_room_membership_for_user = _RoomMembershipForUser(
        #     room_id=room_id,
        #     event_id=None,
        #     event_pos=None,
        #     membership=None,
        #     sender=None,
        #     newly_joined=False,
        #     newly_left=False,
        #     is_dm=False,
        # )
        # room_state = await self.get_current_state_at(
        #     room_id=room_id,
        #     room_membership_for_user_at_to_token=not_in_room_membership_for_user,
        #     state_filter=StateFilter.from_types(
        #         [(EventTypes.RoomHistoryVisibility, "")]
        #     ),
        #     to_token=to_token,
        # )

        # visibility_event = room_state.get((EventTypes.RoomHistoryVisibility, ""))
        # if (
        #     visibility_event is not None
        #     and visibility_event.content.get("history_visibility")
        #     == HistoryVisibility.WORLD_READABLE
        # ):
        #     return not_in_room_membership_for_user

        # return None

    @trace
    async def _bulk_get_stripped_state_for_rooms_from_sync_room_map(
        self,
        room_ids: StrCollection,
        sync_room_map: Dict[str, _RoomMembershipForUser],
    ) -> Dict[str, Optional[StateMap[StrippedStateEvent]]]:
        """
        Fetch stripped state for a list of room IDs. Stripped state is only
        applicable to invite/knock rooms. Other rooms will have `None` as their
        stripped state.

        For invite rooms, we pull from `unsigned.invite_room_state`.
        For knock rooms, we pull from `unsigned.knock_room_state`.

        Args:
            room_ids: Room IDs to fetch stripped state for
            sync_room_map: Dictionary of room IDs to sort along with membership
                information in the room at the time of `to_token`.

        Returns:
            Mapping from room_id to mapping of (type, state_key) to stripped state
            event.
        """
        room_id_to_stripped_state_map: Dict[
            str, Optional[StateMap[StrippedStateEvent]]
        ] = {}

        # Fetch what we haven't before
        room_ids_to_fetch = [
            room_id
            for room_id in room_ids
            if room_id not in room_id_to_stripped_state_map
        ]

        # Gather a list of event IDs we can grab stripped state from
        invite_or_knock_event_ids: List[str] = []
        for room_id in room_ids_to_fetch:
            if sync_room_map[room_id].membership in (
                Membership.INVITE,
                Membership.KNOCK,
            ):
                event_id = sync_room_map[room_id].event_id
                # If this is an invite/knock then there should be an event_id
                assert event_id is not None
                invite_or_knock_event_ids.append(event_id)
            else:
                room_id_to_stripped_state_map[room_id] = None

        invite_or_knock_events = await self.store.get_events(invite_or_knock_event_ids)
        for invite_or_knock_event in invite_or_knock_events.values():
            room_id = invite_or_knock_event.room_id
            membership = invite_or_knock_event.membership

            raw_stripped_state_events = None
            if membership == Membership.INVITE:
                invite_room_state = invite_or_knock_event.unsigned.get(
                    "invite_room_state"
                )
                raw_stripped_state_events = invite_room_state
            elif membership == Membership.KNOCK:
                knock_room_state = invite_or_knock_event.unsigned.get(
                    "knock_room_state"
                )
                raw_stripped_state_events = knock_room_state
            else:
                raise AssertionError(
                    f"Unexpected membership {membership} (this is a problem with Synapse itself)"
                )

            stripped_state_map: Optional[MutableStateMap[StrippedStateEvent]] = None
            # Scrutinize unsigned things. `raw_stripped_state_events` should be a list
            # of stripped events
            if raw_stripped_state_events is not None:
                stripped_state_map = {}
                if isinstance(raw_stripped_state_events, list):
                    for raw_stripped_event in raw_stripped_state_events:
                        stripped_state_event = parse_stripped_state_event(
                            raw_stripped_event
                        )
                        if stripped_state_event is not None:
                            stripped_state_map[
                                (
                                    stripped_state_event.type,
                                    stripped_state_event.state_key,
                                )
                            ] = stripped_state_event

            room_id_to_stripped_state_map[room_id] = stripped_state_map

        return room_id_to_stripped_state_map

    @trace
    async def _bulk_get_partial_current_state_content_for_rooms(
        self,
        content_type: Literal[
            # `content.type` from `EventTypes.Create``
            "room_type",
            # `content.algorithm` from `EventTypes.RoomEncryption`
            "room_encryption",
        ],
        room_ids: Set[str],
        sync_room_map: Dict[str, _RoomMembershipForUser],
        to_token: StreamToken,
        room_id_to_stripped_state_map: Dict[
            str, Optional[StateMap[StrippedStateEvent]]
        ],
    ) -> Mapping[str, Union[Optional[str], StateSentinel]]:
        """
        Get the given state event content for a list of rooms. First we check the
        current state of the room, then fallback to stripped state if available, then
        historical state.

        Args:
            content_type: Which content to grab
            room_ids: Room IDs to fetch the given content field for.
            sync_room_map: Dictionary of room IDs to sort along with membership
                information in the room at the time of `to_token`.
            to_token: We filter based on the state of the room at this token
            room_id_to_stripped_state_map: This does not need to be filled in before
                calling this function. Mapping from room_id to mapping of (type, state_key)
                to stripped state event. Modified in place when we fetch new rooms so we can
                save work next time this function is called.

        Returns:
            A mapping from room ID to the state event content if the room has
            the given state event (event_type, ""), otherwise `None`. Rooms unknown to
            this server will return `ROOM_UNKNOWN_SENTINEL`.
        """
        room_id_to_content: Dict[str, Union[Optional[str], StateSentinel]] = {}

        # As a bulk shortcut, use the current state if the server is particpating in the
        # room (meaning we have current state). Ideally, for leave/ban rooms, we would
        # want the state at the time of the membership instead of current state to not
        # leak anything but we consider the create/encryption stripped state events to
        # not be a secret given they are often set at the start of the room and they are
        # normally handed out on invite/knock.
        #
        # Be mindful to only use this for non-sensitive details. For example, even
        # though the room name/avatar/topic are also stripped state, they seem a lot
        # more senstive to leak the current state value of.
        #
        # Since this function is cached, we need to make a mutable copy via
        # `dict(...)`.
        event_type = ""
        event_content_field = ""
        if content_type == "room_type":
            event_type = EventTypes.Create
            event_content_field = EventContentFields.ROOM_TYPE
            room_id_to_content = dict(await self.store.bulk_get_room_type(room_ids))
        elif content_type == "room_encryption":
            event_type = EventTypes.RoomEncryption
            event_content_field = EventContentFields.ENCRYPTION_ALGORITHM
            room_id_to_content = dict(
                await self.store.bulk_get_room_encryption(room_ids)
            )
        else:
            assert_never(content_type)

        room_ids_with_results = [
            room_id
            for room_id, content_field in room_id_to_content.items()
            if content_field is not ROOM_UNKNOWN_SENTINEL
        ]

        # We might not have current room state for remote invite/knocks if we are
        # the first person on our server to see the room. The best we can do is look
        # in the optional stripped state from the invite/knock event.
        room_ids_without_results = room_ids.difference(
            chain(
                room_ids_with_results,
                [
                    room_id
                    for room_id, stripped_state_map in room_id_to_stripped_state_map.items()
                    if stripped_state_map is not None
                ],
            )
        )
        room_id_to_stripped_state_map.update(
            await self._bulk_get_stripped_state_for_rooms_from_sync_room_map(
                room_ids_without_results, sync_room_map
            )
        )

        # Update our `room_id_to_content` map based on the stripped state
        # (applies to invite/knock rooms)
        rooms_ids_without_stripped_state: Set[str] = set()
        for room_id in room_ids_without_results:
            stripped_state_map = room_id_to_stripped_state_map.get(
                room_id, Sentinel.UNSET_SENTINEL
            )
            assert stripped_state_map is not Sentinel.UNSET_SENTINEL, (
                f"Stripped state left unset for room {room_id}. "
                + "Make sure you're calling `_bulk_get_stripped_state_for_rooms_from_sync_room_map(...)` "
                + "with that room_id. (this is a problem with Synapse itself)"
            )

            # If there is some stripped state, we assume the remote server passed *all*
            # of the potential stripped state events for the room.
            if stripped_state_map is not None:
                create_stripped_event = stripped_state_map.get((EventTypes.Create, ""))
                stripped_event = stripped_state_map.get((event_type, ""))
                # Sanity check that we at-least have the create event
                if create_stripped_event is not None:
                    if stripped_event is not None:
                        room_id_to_content[room_id] = stripped_event.content.get(
                            event_content_field
                        )
                    else:
                        # Didn't see the state event we're looking for in the stripped
                        # state so we can assume relevant content field is `None`.
                        room_id_to_content[room_id] = None
            else:
                rooms_ids_without_stripped_state.add(room_id)

        # Last resort, we might not have current room state for rooms that the
        # server has left (no one local is in the room) but we can look at the
        # historical state.
        #
        # Update our `room_id_to_content` map based on the state at the time of
        # the membership event.
        for room_id in rooms_ids_without_stripped_state:
            # TODO: It would be nice to look this up in a bulk way (N+1 queries)
            #
            # TODO: `get_state_at(...)` doesn't take into account the "current state".
            room_state = await self.storage_controllers.state.get_state_at(
                room_id=room_id,
                stream_position=to_token.copy_and_replace(
                    StreamKeyType.ROOM,
                    sync_room_map[room_id].event_pos.to_room_stream_token(),
                ),
                state_filter=StateFilter.from_types(
                    [
                        (EventTypes.Create, ""),
                        (event_type, ""),
                    ]
                ),
                # Partially-stated rooms should have all state events except for
                # remote membership events so we don't need to wait at all because
                # we only want the create event and some non-member event.
                await_full_state=False,
            )
            # We can use the create event as a canary to tell whether the server has
            # seen the room before
            create_event = room_state.get((EventTypes.Create, ""))
            state_event = room_state.get((event_type, ""))

            if create_event is None:
                # Skip for unknown rooms
                continue

            if state_event is not None:
                room_id_to_content[room_id] = state_event.content.get(
                    event_content_field
                )
            else:
                # Didn't see the state event we're looking for in the stripped
                # state so we can assume relevant content field is `None`.
                room_id_to_content[room_id] = None

        return room_id_to_content

    @trace
    async def filter_rooms(
        self,
        user: UserID,
        sync_room_map: Dict[str, _RoomMembershipForUser],
        filters: SlidingSyncConfig.SlidingSyncList.Filters,
        to_token: StreamToken,
    ) -> Dict[str, _RoomMembershipForUser]:
        """
        Filter rooms based on the sync request.

        Args:
            user: User to filter rooms for
            sync_room_map: Dictionary of room IDs to sort along with membership
                information in the room at the time of `to_token`.
            filters: Filters to apply
            to_token: We filter based on the state of the room at this token

        Returns:
            A filtered dictionary of room IDs along with membership information in the
            room at the time of `to_token`.
        """
        room_id_to_stripped_state_map: Dict[
            str, Optional[StateMap[StrippedStateEvent]]
        ] = {}

        filtered_room_id_set = set(sync_room_map.keys())

        # Filter for Direct-Message (DM) rooms
        if filters.is_dm is not None:
            with start_active_span("filters.is_dm"):
                if filters.is_dm:
                    # Only DM rooms please
                    filtered_room_id_set = {
                        room_id
                        for room_id in filtered_room_id_set
                        if sync_room_map[room_id].is_dm
                    }
                else:
                    # Only non-DM rooms please
                    filtered_room_id_set = {
                        room_id
                        for room_id in filtered_room_id_set
                        if not sync_room_map[room_id].is_dm
                    }

        if filters.spaces is not None:
            with start_active_span("filters.spaces"):
                raise NotImplementedError()

        # Filter for encrypted rooms
        if filters.is_encrypted is not None:
            with start_active_span("filters.is_encrypted"):
                room_id_to_encryption = (
                    await self._bulk_get_partial_current_state_content_for_rooms(
                        content_type="room_encryption",
                        room_ids=filtered_room_id_set,
                        to_token=to_token,
                        sync_room_map=sync_room_map,
                        room_id_to_stripped_state_map=room_id_to_stripped_state_map,
                    )
                )

                # Make a copy so we don't run into an error: `Set changed size during
                # iteration`, when we filter out and remove items
                for room_id in filtered_room_id_set.copy():
                    encryption = room_id_to_encryption.get(
                        room_id, ROOM_UNKNOWN_SENTINEL
                    )

                    # Just remove rooms if we can't determine their encryption status
                    if encryption is ROOM_UNKNOWN_SENTINEL:
                        filtered_room_id_set.remove(room_id)
                        continue

                    # If we're looking for encrypted rooms, filter out rooms that are not
                    # encrypted and vice versa
                    is_encrypted = encryption is not None
                    if (filters.is_encrypted and not is_encrypted) or (
                        not filters.is_encrypted and is_encrypted
                    ):
                        filtered_room_id_set.remove(room_id)

        # Filter for rooms that the user has been invited to
        if filters.is_invite is not None:
            with start_active_span("filters.is_invite"):
                # Make a copy so we don't run into an error: `Set changed size during
                # iteration`, when we filter out and remove items
                for room_id in filtered_room_id_set.copy():
                    room_for_user = sync_room_map[room_id]
                    # If we're looking for invite rooms, filter out rooms that the user is
                    # not invited to and vice versa
                    if (
                        filters.is_invite
                        and room_for_user.membership != Membership.INVITE
                    ) or (
                        not filters.is_invite
                        and room_for_user.membership == Membership.INVITE
                    ):
                        filtered_room_id_set.remove(room_id)

        # Filter by room type (space vs room, etc). A room must match one of the types
        # provided in the list. `None` is a valid type for rooms which do not have a
        # room type.
        if filters.room_types is not None or filters.not_room_types is not None:
            with start_active_span("filters.room_types"):
                room_id_to_type = (
                    await self._bulk_get_partial_current_state_content_for_rooms(
                        content_type="room_type",
                        room_ids=filtered_room_id_set,
                        to_token=to_token,
                        sync_room_map=sync_room_map,
                        room_id_to_stripped_state_map=room_id_to_stripped_state_map,
                    )
                )

                # Make a copy so we don't run into an error: `Set changed size during
                # iteration`, when we filter out and remove items
                for room_id in filtered_room_id_set.copy():
                    room_type = room_id_to_type.get(room_id, ROOM_UNKNOWN_SENTINEL)

                    # Just remove rooms if we can't determine their type
                    if room_type is ROOM_UNKNOWN_SENTINEL:
                        filtered_room_id_set.remove(room_id)
                        continue

                    if (
                        filters.room_types is not None
                        and room_type not in filters.room_types
                    ):
                        filtered_room_id_set.remove(room_id)

                    if (
                        filters.not_room_types is not None
                        and room_type in filters.not_room_types
                    ):
                        filtered_room_id_set.remove(room_id)

        if filters.room_name_like is not None:
            with start_active_span("filters.room_name_like"):
                # TODO: The room name is a bit more sensitive to leak than the
                # create/encryption event. Maybe we should consider a better way to fetch
                # historical state before implementing this.
                #
                # room_id_to_create_content = await self._bulk_get_partial_current_state_content_for_rooms(
                #     content_type="room_name",
                #     room_ids=filtered_room_id_set,
                #     to_token=to_token,
                #     sync_room_map=sync_room_map,
                #     room_id_to_stripped_state_map=room_id_to_stripped_state_map,
                # )
                raise NotImplementedError()

        if filters.tags is not None or filters.not_tags is not None:
            with start_active_span("filters.tags"):
                raise NotImplementedError()

        # Assemble a new sync room map but only with the `filtered_room_id_set`
        return {room_id: sync_room_map[room_id] for room_id in filtered_room_id_set}

    @trace
    async def sort_rooms(
        self,
        sync_room_map: Dict[str, _RoomMembershipForUser],
        to_token: StreamToken,
    ) -> List[_RoomMembershipForUser]:
        """
        Sort by `stream_ordering` of the last event that the user should see in the
        room. `stream_ordering` is unique so we get a stable sort.

        Args:
            sync_room_map: Dictionary of room IDs to sort along with membership
                information in the room at the time of `to_token`.
            to_token: We sort based on the events in the room at this token (<= `to_token`)

        Returns:
            A sorted list of room IDs by `stream_ordering` along with membership information.
        """

        # Assemble a map of room ID to the `stream_ordering` of the last activity that the
        # user should see in the room (<= `to_token`)
        last_activity_in_room_map: Dict[str, int] = {}

        for room_id, room_for_user in sync_room_map.items():
            if room_for_user.membership != Membership.JOIN:
                # If the user has left/been invited/knocked/been banned from a
                # room, they shouldn't see anything past that point.
                #
                # FIXME: It's possible that people should see beyond this point
                # in invited/knocked cases if for example the room has
                # `invite`/`world_readable` history visibility, see
                # https://github.com/matrix-org/matrix-spec-proposals/pull/3575#discussion_r1653045932
                last_activity_in_room_map[room_id] = room_for_user.event_pos.stream

        # For fully-joined rooms, we find the latest activity at/before the
        # `to_token`.
        joined_room_positions = (
            await self.store.bulk_get_last_event_pos_in_room_before_stream_ordering(
                [
                    room_id
                    for room_id, room_for_user in sync_room_map.items()
                    if room_for_user.membership == Membership.JOIN
                ],
                to_token.room_key,
            )
        )

        last_activity_in_room_map.update(joined_room_positions)

        return sorted(
            sync_room_map.values(),
            # Sort by the last activity (stream_ordering) in the room
            key=lambda room_info: last_activity_in_room_map[room_info.room_id],
            # We want descending order
            reverse=True,
        )

    @trace
    async def get_current_state_ids_at(
        self,
        room_id: str,
        room_membership_for_user_at_to_token: _RoomMembershipForUser,
        state_filter: StateFilter,
        to_token: StreamToken,
    ) -> StateMap[str]:
        """
        Get current state IDs for the user in the room according to their membership. This
        will be the current state at the time of their LEAVE/BAN, otherwise will be the
        current state <= to_token.

        Args:
            room_id: The room ID to fetch data for
            room_membership_for_user_at_token: Membership information for the user
                in the room at the time of `to_token`.
            to_token: The point in the stream to sync up to.
        """
        state_ids: StateMap[str]
        # People shouldn't see past their leave/ban event
        if room_membership_for_user_at_to_token.membership in (
            Membership.LEAVE,
            Membership.BAN,
        ):
            # TODO: `get_state_ids_at(...)` doesn't take into account the "current
            # state". Maybe we need to use
            # `get_forward_extremities_for_room_at_stream_ordering(...)` to "Fetch the
            # current state at the time."
            state_ids = await self.storage_controllers.state.get_state_ids_at(
                room_id,
                stream_position=to_token.copy_and_replace(
                    StreamKeyType.ROOM,
                    room_membership_for_user_at_to_token.event_pos.to_room_stream_token(),
                ),
                state_filter=state_filter,
                # Partially-stated rooms should have all state events except for
                # remote membership events. Since we've already excluded
                # partially-stated rooms unless `required_state` only has
                # `["m.room.member", "$LAZY"]` for membership, we should be able to
                # retrieve everything requested. When we're lazy-loading, if there
                # are some remote senders in the timeline, we should also have their
                # membership event because we had to auth that timeline event. Plus
                # we don't want to block the whole sync waiting for this one room.
                await_full_state=False,
            )
        # Otherwise, we can get the latest current state in the room
        else:
            state_ids = await self.storage_controllers.state.get_current_state_ids(
                room_id,
                state_filter,
                # Partially-stated rooms should have all state events except for
                # remote membership events. Since we've already excluded
                # partially-stated rooms unless `required_state` only has
                # `["m.room.member", "$LAZY"]` for membership, we should be able to
                # retrieve everything requested. When we're lazy-loading, if there
                # are some remote senders in the timeline, we should also have their
                # membership event because we had to auth that timeline event. Plus
                # we don't want to block the whole sync waiting for this one room.
                await_full_state=False,
            )
            # TODO: Query `current_state_delta_stream` and reverse/rewind back to the `to_token`

        return state_ids

    @trace
    async def get_current_state_at(
        self,
        room_id: str,
        room_membership_for_user_at_to_token: _RoomMembershipForUser,
        state_filter: StateFilter,
        to_token: StreamToken,
    ) -> StateMap[EventBase]:
        """
        Get current state for the user in the room according to their membership. This
        will be the current state at the time of their LEAVE/BAN, otherwise will be the
        current state <= to_token.

        Args:
            room_id: The room ID to fetch data for
            room_membership_for_user_at_token: Membership information for the user
                in the room at the time of `to_token`.
            to_token: The point in the stream to sync up to.
        """
        state_ids = await self.get_current_state_ids_at(
            room_id=room_id,
            room_membership_for_user_at_to_token=room_membership_for_user_at_to_token,
            state_filter=state_filter,
            to_token=to_token,
        )

        event_map = await self.store.get_events(list(state_ids.values()))

        state_map = {}
        for key, event_id in state_ids.items():
            event = event_map.get(event_id)
            if event:
                state_map[key] = event

        return state_map

    async def get_room_sync_data(
        self,
        sync_config: SlidingSyncConfig,
        previous_connection_state: "PerConnectionState",
        new_connection_state: "MutablePerConnectionState",
        room_id: str,
        room_sync_config: RoomSyncConfig,
        room_membership_for_user_at_to_token: _RoomMembershipForUser,
        from_token: Optional[SlidingSyncStreamToken],
        to_token: StreamToken,
    ) -> SlidingSyncResult.RoomResult:
        """
        Fetch room data for the sync response.

        We fetch data according to the token range (> `from_token` and <= `to_token`).

        Args:
            user: User to fetch data for
            room_id: The room ID to fetch data for
            room_sync_config: Config for what data we should fetch for a room in the
                sync response.
            room_membership_for_user_at_to_token: Membership information for the user
                in the room at the time of `to_token`.
            from_token: The point in the stream to sync from.
            to_token: The point in the stream to sync up to.
        """
        user = sync_config.user

        set_tag(
            SynapseTags.FUNC_ARG_PREFIX + "membership",
            room_membership_for_user_at_to_token.membership,
        )
        set_tag(
            SynapseTags.FUNC_ARG_PREFIX + "timeline_limit",
            room_sync_config.timeline_limit,
        )

        # Determine whether we should limit the timeline to the token range.
        #
        # We should return historical messages (before token range) in the
        # following cases because we want clients to be able to show a basic
        # screen of information:
        #
        #  - Initial sync (because no `from_token` to limit us anyway)
        #  - When users `newly_joined`
        #  - For an incremental sync where we haven't sent it down this
        #    connection before
        #
        # Relevant spec issue:
        # https://github.com/matrix-org/matrix-spec/issues/1917
        #
        # XXX: Odd behavior - We also check if the `timeline_limit` has increased, if so
        # we ignore the from bound for the timeline to send down a larger chunk of
        # history and set `unstable_expanded_timeline` to true. This is only being added
        # to match the behavior of the Sliding Sync proxy as we expect the ElementX
        # client to feel a certain way and be able to trickle in a full page of timeline
        # messages to fill up the screen. This is a bit different to the behavior of the
        # Sliding Sync proxy (which sets initial=true, but then doesn't send down the
        # full state again), but existing apps, e.g. ElementX, just need `limited` set.
        # We don't explicitly set `limited` but this will be the case for any room that
        # has more history than we're trying to pull out. Using
        # `unstable_expanded_timeline` allows us to avoid contaminating what `initial`
        # or `limited` mean for clients that interpret them correctly. In future this
        # behavior is almost certainly going to change.
        #
        # TODO: Also handle changes to `required_state`
        from_bound = None
        initial = True
        ignore_timeline_bound = False
        if from_token and not room_membership_for_user_at_to_token.newly_joined:
            room_status = previous_connection_state.rooms.have_sent_room(room_id)
            if room_status.status == HaveSentRoomFlag.LIVE:
                from_bound = from_token.stream_token.room_key
                initial = False
            elif room_status.status == HaveSentRoomFlag.PREVIOUSLY:
                assert room_status.last_token is not None
                from_bound = room_status.last_token
                initial = False
            elif room_status.status == HaveSentRoomFlag.NEVER:
                from_bound = None
                initial = True
            else:
                assert_never(room_status.status)

            log_kv({"sliding_sync.room_status": room_status})

            prev_room_sync_config = previous_connection_state.room_configs.get(room_id)
            if prev_room_sync_config is not None:
                # Check if the timeline limit has increased, if so ignore the
                # timeline bound and record the change (see "XXX: Odd behavior"
                # above).
                if (
                    prev_room_sync_config.timeline_limit
                    < room_sync_config.timeline_limit
                ):
                    ignore_timeline_bound = True

                # TODO: Check for changes in `required_state``

        log_kv(
            {
                "sliding_sync.from_bound": from_bound,
                "sliding_sync.initial": initial,
                "sliding_sync.ignore_timeline_bound": ignore_timeline_bound,
            }
        )

        # Assemble the list of timeline events
        #
        # FIXME: It would be nice to make the `rooms` response more uniform regardless of
        # membership. Currently, we have to make all of these optional because
        # `invite`/`knock` rooms only have `stripped_state`. See
        # https://github.com/matrix-org/matrix-spec-proposals/pull/3575#discussion_r1653045932
        timeline_events: List[EventBase] = []
        bundled_aggregations: Optional[Dict[str, BundledAggregations]] = None
        limited: Optional[bool] = None
        prev_batch_token: Optional[StreamToken] = None
        num_live: Optional[int] = None
        if (
            room_sync_config.timeline_limit > 0
            # No timeline for invite/knock rooms (just `stripped_state`)
            and room_membership_for_user_at_to_token.membership
            not in (Membership.INVITE, Membership.KNOCK)
        ):
            limited = False
            # We want to start off using the `to_token` (vs `from_token`) because we look
            # backwards from the `to_token` up to the `timeline_limit` and we might not
            # reach the `from_token` before we hit the limit. We will update the room stream
            # position once we've fetched the events to point to the earliest event fetched.
            prev_batch_token = to_token

            # We're going to paginate backwards from the `to_token`
            to_bound = to_token.room_key
            # People shouldn't see past their leave/ban event
            if room_membership_for_user_at_to_token.membership in (
                Membership.LEAVE,
                Membership.BAN,
            ):
                to_bound = (
                    room_membership_for_user_at_to_token.event_pos.to_room_stream_token()
                )

            timeline_from_bound = from_bound
            if ignore_timeline_bound:
                timeline_from_bound = None

            # For initial `/sync` (and other historical scenarios mentioned above), we
            # want to view a historical section of the timeline; to fetch events by
            # `topological_ordering` (best representation of the room DAG as others were
            # seeing it at the time). This also aligns with the order that `/messages`
            # returns events in.
            #
            # For incremental `/sync`, we want to get all updates for rooms since
            # the last `/sync` (regardless if those updates arrived late or happened
            # a while ago in the past); to fetch events by `stream_ordering` (in the
            # order they were received by the server).
            #
            # Relevant spec issue: https://github.com/matrix-org/matrix-spec/issues/1917
            #
            # FIXME: Using workaround for mypy,
            # https://github.com/python/mypy/issues/10740#issuecomment-1997047277 and
            # https://github.com/python/mypy/issues/17479
            paginate_room_events_by_topological_ordering: PaginateFunction = (
                self.store.paginate_room_events_by_topological_ordering
            )
            paginate_room_events_by_stream_ordering: PaginateFunction = (
                self.store.paginate_room_events_by_stream_ordering
            )
            pagination_method: PaginateFunction = (
                # Use `topographical_ordering` for historical events
                paginate_room_events_by_topological_ordering
                if timeline_from_bound is None
                # Use `stream_ordering` for updates
                else paginate_room_events_by_stream_ordering
            )
            timeline_events, new_room_key = await pagination_method(
                room_id=room_id,
                # The bounds are reversed so we can paginate backwards
                # (from newer to older events) starting at to_bound.
                # This ensures we fill the `limit` with the newest events first,
                from_key=to_bound,
                to_key=timeline_from_bound,
                direction=Direction.BACKWARDS,
                # We add one so we can determine if there are enough events to saturate
                # the limit or not (see `limited`)
                limit=room_sync_config.timeline_limit + 1,
            )

            # We want to return the events in ascending order (the last event is the
            # most recent).
            timeline_events.reverse()

            # Determine our `limited` status based on the timeline. We do this before
            # filtering the events so we can accurately determine if there is more to
            # paginate even if we filter out some/all events.
            if len(timeline_events) > room_sync_config.timeline_limit:
                limited = True
                # Get rid of that extra "+ 1" event because we only used it to determine
                # if we hit the limit or not
                timeline_events = timeline_events[-room_sync_config.timeline_limit :]
                assert timeline_events[0].internal_metadata.stream_ordering
                new_room_key = RoomStreamToken(
                    stream=timeline_events[0].internal_metadata.stream_ordering - 1
                )

            # Make sure we don't expose any events that the client shouldn't see
            timeline_events = await filter_events_for_client(
                self.storage_controllers,
                user.to_string(),
                timeline_events,
                is_peeking=room_membership_for_user_at_to_token.membership
                != Membership.JOIN,
                filter_send_to_client=True,
            )
            # TODO: Filter out `EventTypes.CallInvite` in public rooms,
            # see https://github.com/element-hq/synapse/issues/17359

            # TODO: Handle timeline gaps (`get_timeline_gaps()`)

            # Determine how many "live" events we have (events within the given token range).
            #
            # This is mostly useful to determine whether a given @mention event should
            # make a noise or not. Clients cannot rely solely on the absence of
            # `initial: true` to determine live events because if a room not in the
            # sliding window bumps into the window because of an @mention it will have
            # `initial: true` yet contain a single live event (with potentially other
            # old events in the timeline)
            num_live = 0
            if from_token is not None:
                for timeline_event in reversed(timeline_events):
                    # This fields should be present for all persisted events
                    assert timeline_event.internal_metadata.stream_ordering is not None
                    assert timeline_event.internal_metadata.instance_name is not None

                    persisted_position = PersistedEventPosition(
                        instance_name=timeline_event.internal_metadata.instance_name,
                        stream=timeline_event.internal_metadata.stream_ordering,
                    )
                    if persisted_position.persisted_after(
                        from_token.stream_token.room_key
                    ):
                        num_live += 1
                    else:
                        # Since we're iterating over the timeline events in
                        # reverse-chronological order, we can break once we hit an event
                        # that's not live. In the future, we could potentially optimize
                        # this more with a binary search (bisect).
                        break

            # If the timeline is `limited=True`, the client does not have all events
            # necessary to calculate aggregations themselves.
            if limited:
                bundled_aggregations = (
                    await self.relations_handler.get_bundled_aggregations(
                        timeline_events, user.to_string()
                    )
                )

            # Update the `prev_batch_token` to point to the position that allows us to
            # keep paginating backwards from the oldest event we return in the timeline.
            prev_batch_token = prev_batch_token.copy_and_replace(
                StreamKeyType.ROOM, new_room_key
            )

        # Figure out any stripped state events for invite/knocks. This allows the
        # potential joiner to identify the room.
        stripped_state: List[JsonDict] = []
        if room_membership_for_user_at_to_token.membership in (
            Membership.INVITE,
            Membership.KNOCK,
        ):
            # This should never happen. If someone is invited/knocked on room, then
            # there should be an event for it.
            assert room_membership_for_user_at_to_token.event_id is not None

            invite_or_knock_event = await self.store.get_event(
                room_membership_for_user_at_to_token.event_id
            )

            stripped_state = []
            if invite_or_knock_event.membership == Membership.INVITE:
                stripped_state.extend(
                    invite_or_knock_event.unsigned.get("invite_room_state", [])
                )
            elif invite_or_knock_event.membership == Membership.KNOCK:
                stripped_state.extend(
                    invite_or_knock_event.unsigned.get("knock_room_state", [])
                )

            stripped_state.append(strip_event(invite_or_knock_event))

        # TODO: Handle state resets. For example, if we see
        # `room_membership_for_user_at_to_token.event_id=None and
        # room_membership_for_user_at_to_token.membership is not None`, we should
        # indicate to the client that a state reset happened. Perhaps we should indicate
        # this by setting `initial: True` and empty `required_state`.

        # Check whether the room has a name set
        name_state_ids = await self.get_current_state_ids_at(
            room_id=room_id,
            room_membership_for_user_at_to_token=room_membership_for_user_at_to_token,
            state_filter=StateFilter.from_types([(EventTypes.Name, "")]),
            to_token=to_token,
        )
        name_event_id = name_state_ids.get((EventTypes.Name, ""))

        room_membership_summary: Mapping[str, MemberSummary]
        empty_membership_summary = MemberSummary([], 0)
        if room_membership_for_user_at_to_token.membership in (
            Membership.LEAVE,
            Membership.BAN,
        ):
            # TODO: Figure out how to get the membership summary for left/banned rooms
            room_membership_summary = {}
        else:
            room_membership_summary = await self.store.get_room_summary(room_id)
            # TODO: Reverse/rewind back to the `to_token`

        # `heroes` are required if the room name is not set.
        #
        # Note: When you're the first one on your server to be invited to a new room
        # over federation, we only have access to some stripped state in
        # `event.unsigned.invite_room_state` which currently doesn't include `heroes`,
        # see https://github.com/matrix-org/matrix-spec/issues/380. This means that
        # clients won't be able to calculate the room name when necessary and just a
        # pitfall we have to deal with until that spec issue is resolved.
        hero_user_ids: List[str] = []
        # TODO: Should we also check for `EventTypes.CanonicalAlias`
        # (`m.room.canonical_alias`) as a fallback for the room name? see
        # https://github.com/matrix-org/matrix-spec-proposals/pull/3575#discussion_r1671260153
        if name_event_id is None:
            hero_user_ids = extract_heroes_from_room_summary(
                room_membership_summary, me=user.to_string()
            )

        # Fetch the `required_state` for the room
        #
        # No `required_state` for invite/knock rooms (just `stripped_state`)
        #
        # FIXME: It would be nice to make the `rooms` response more uniform regardless
        # of membership. Currently, we have to make this optional because
        # `invite`/`knock` rooms only have `stripped_state`. See
        # https://github.com/matrix-org/matrix-spec-proposals/pull/3575#discussion_r1653045932
        #
        # Calculate the `StateFilter` based on the `required_state` for the room
        required_state_filter = StateFilter.none()
        if room_membership_for_user_at_to_token.membership not in (
            Membership.INVITE,
            Membership.KNOCK,
        ):
            # If we have a double wildcard ("*", "*") in the `required_state`, we need
            # to fetch all state for the room
            #
            # Note: MSC3575 describes different behavior to how we're handling things
            # here but since it's not wrong to return more state than requested
            # (`required_state` is just the minimum requested), it doesn't matter if we
            # include more than client wanted. This complexity is also under scrutiny,
            # see
            # https://github.com/matrix-org/matrix-spec-proposals/pull/3575#discussion_r1185109050
            #
            # > One unique exception is when you request all state events via ["*", "*"]. When used,
            # > all state events are returned by default, and additional entries FILTER OUT the returned set
            # > of state events. These additional entries cannot use '*' themselves.
            # > For example, ["*", "*"], ["m.room.member", "@alice:example.com"] will _exclude_ every m.room.member
            # > event _except_ for @alice:example.com, and include every other state event.
            # > In addition, ["*", "*"], ["m.space.child", "*"] is an error, the m.space.child filter is not
            # > required as it would have been returned anyway.
            # >
            # > -- MSC3575 (https://github.com/matrix-org/matrix-spec-proposals/pull/3575)
            if StateValues.WILDCARD in room_sync_config.required_state_map.get(
                StateValues.WILDCARD, set()
            ):
                set_tag(
                    SynapseTags.FUNC_ARG_PREFIX + "required_state_wildcard",
                    True,
                )
                required_state_filter = StateFilter.all()
            # TODO: `StateFilter` currently doesn't support wildcard event types. We're
            # currently working around this by returning all state to the client but it
            # would be nice to fetch less from the database and return just what the
            # client wanted.
            elif (
                room_sync_config.required_state_map.get(StateValues.WILDCARD)
                is not None
            ):
                set_tag(
                    SynapseTags.FUNC_ARG_PREFIX + "required_state_wildcard_event_type",
                    True,
                )
                required_state_filter = StateFilter.all()
            else:
                required_state_types: List[Tuple[str, Optional[str]]] = []
                for (
                    state_type,
                    state_key_set,
                ) in room_sync_config.required_state_map.items():
                    num_wild_state_keys = 0
                    lazy_load_room_members = False
                    num_others = 0
                    for state_key in state_key_set:
                        if state_key == StateValues.WILDCARD:
                            num_wild_state_keys += 1
                            # `None` is a wildcard in the `StateFilter`
                            required_state_types.append((state_type, None))
                        # We need to fetch all relevant people when we're lazy-loading membership
                        elif (
                            state_type == EventTypes.Member
                            and state_key == StateValues.LAZY
                        ):
                            lazy_load_room_members = True
                            # Everyone in the timeline is relevant
                            timeline_membership: Set[str] = set()
                            if timeline_events is not None:
                                for timeline_event in timeline_events:
                                    timeline_membership.add(timeline_event.sender)

                            for user_id in timeline_membership:
                                required_state_types.append(
                                    (EventTypes.Member, user_id)
                                )

                            # FIXME: We probably also care about invite, ban, kick, targets, etc
                            # but the spec only mentions "senders".
                        elif state_key == StateValues.ME:
                            num_others += 1
                            required_state_types.append((state_type, user.to_string()))
                        else:
                            num_others += 1
                            required_state_types.append((state_type, state_key))

                    set_tag(
                        SynapseTags.FUNC_ARG_PREFIX
                        + "required_state_wildcard_state_key_count",
                        num_wild_state_keys,
                    )
                    set_tag(
                        SynapseTags.FUNC_ARG_PREFIX + "required_state_lazy",
                        lazy_load_room_members,
                    )
                    set_tag(
                        SynapseTags.FUNC_ARG_PREFIX + "required_state_other_count",
                        num_others,
                    )

                required_state_filter = StateFilter.from_types(required_state_types)

        # We need this base set of info for the response so let's just fetch it along
        # with the `required_state` for the room
        meta_room_state = [(EventTypes.Name, ""), (EventTypes.RoomAvatar, "")] + [
            (EventTypes.Member, hero_user_id) for hero_user_id in hero_user_ids
        ]
        state_filter = StateFilter.all()
        if required_state_filter != StateFilter.all():
            state_filter = StateFilter(
                types=StateFilter.from_types(
                    chain(meta_room_state, required_state_filter.to_types())
                ).types,
                include_others=required_state_filter.include_others,
            )

        # We can return all of the state that was requested if this was the first
        # time we've sent the room down this connection.
        room_state: StateMap[EventBase] = {}
        if initial:
            room_state = await self.get_current_state_at(
                room_id=room_id,
                room_membership_for_user_at_to_token=room_membership_for_user_at_to_token,
                state_filter=state_filter,
                to_token=to_token,
            )
        else:
            assert from_bound is not None

            # TODO: Limit the number of state events we're about to send down
            # the room, if its too many we should change this to an
            # `initial=True`?
            deltas = await self.store.get_current_state_deltas_for_room(
                room_id=room_id,
                from_token=from_bound,
                to_token=to_token.room_key,
            )
            # TODO: Filter room state before fetching events
            # TODO: Handle state resets where event_id is None
            events = await self.store.get_events(
                [d.event_id for d in deltas if d.event_id]
            )
            room_state = {(s.type, s.state_key): s for s in events.values()}

        required_room_state: StateMap[EventBase] = {}
        if required_state_filter != StateFilter.none():
            required_room_state = required_state_filter.filter_state(room_state)

        # Find the room name and avatar from the state
        room_name: Optional[str] = None
        # TODO: Should we also check for `EventTypes.CanonicalAlias`
        # (`m.room.canonical_alias`) as a fallback for the room name? see
        # https://github.com/matrix-org/matrix-spec-proposals/pull/3575#discussion_r1671260153
        name_event = room_state.get((EventTypes.Name, ""))
        if name_event is not None:
            room_name = name_event.content.get("name")

        room_avatar: Optional[str] = None
        avatar_event = room_state.get((EventTypes.RoomAvatar, ""))
        if avatar_event is not None:
            room_avatar = avatar_event.content.get("url")

        # Assemble heroes: extract the info from the state we just fetched
        heroes: List[SlidingSyncResult.RoomResult.StrippedHero] = []
        for hero_user_id in hero_user_ids:
            member_event = room_state.get((EventTypes.Member, hero_user_id))
            if member_event is not None:
                heroes.append(
                    SlidingSyncResult.RoomResult.StrippedHero(
                        user_id=hero_user_id,
                        display_name=member_event.content.get("displayname"),
                        avatar_url=member_event.content.get("avatar_url"),
                    )
                )

        # Figure out the last bump event in the room
        last_bump_event_result = (
            await self.store.get_last_event_pos_in_room_before_stream_ordering(
                room_id, to_token.room_key, event_types=DEFAULT_BUMP_EVENT_TYPES
            )
        )

        # By default, just choose the membership event position
        bump_stamp = room_membership_for_user_at_to_token.event_pos.stream
        # But if we found a bump event, use that instead
        if last_bump_event_result is not None:
            _, new_bump_event_pos = last_bump_event_result

            # If we've just joined a remote room, then the last bump event may
            # have been backfilled (and so have a negative stream ordering).
            # These negative stream orderings can't sensibly be compared, so
            # instead we use the membership event position.
            if new_bump_event_pos.stream > 0:
                bump_stamp = new_bump_event_pos.stream

        unstable_expanded_timeline = False
        prev_room_sync_config = previous_connection_state.room_configs.get(room_id)
        # Record the `room_sync_config` if we're `ignore_timeline_bound` (which means
        # that the `timeline_limit` has increased)
        if ignore_timeline_bound:
            # FIXME: We signal the fact that we're sending down more events to
            # the client by setting `unstable_expanded_timeline` to true (see
            # "XXX: Odd behavior" above).
            unstable_expanded_timeline = True

            new_connection_state.room_configs[room_id] = RoomSyncConfig(
                timeline_limit=room_sync_config.timeline_limit,
                required_state_map=room_sync_config.required_state_map,
            )
        elif prev_room_sync_config is not None:
            # If the result is `limited` then we need to record that the
            # `timeline_limit` has been reduced, as when/if the client later requests
            # more timeline then we have more data to send.
            #
            # Otherwise (when not `limited`) we don't need to record that the
            # `timeline_limit` has been reduced, as the *effective* `timeline_limit`
            # (i.e. the amount of timeline we have previously sent to the client) is at
            # least the previous `timeline_limit`.
            #
            # This is to handle the case where the `timeline_limit` e.g. goes from 10 to
            # 5 to 10 again (without any timeline gaps), where there's no point sending
            # down the initial historical chunk events when the `timeline_limit` is
            # increased as the client already has the 10 previous events. However, if
            # client has a gap in the timeline (i.e. `limited` is True), then we *do*
            # need to record the reduced timeline.
            #
            # TODO: Handle timeline gaps (`get_timeline_gaps()`) - This is separate from
            # the gaps we might see on the client because a response was `limited` we're
            # talking about above.
            if (
                limited
                and prev_room_sync_config.timeline_limit
                > room_sync_config.timeline_limit
            ):
                new_connection_state.room_configs[room_id] = RoomSyncConfig(
                    timeline_limit=room_sync_config.timeline_limit,
                    required_state_map=room_sync_config.required_state_map,
                )

            # TODO: Record changes in required_state.

        else:
            new_connection_state.room_configs[room_id] = room_sync_config

        set_tag(SynapseTags.RESULT_PREFIX + "initial", initial)

        return SlidingSyncResult.RoomResult(
            name=room_name,
            avatar=room_avatar,
            heroes=heroes,
            is_dm=room_membership_for_user_at_to_token.is_dm,
            initial=initial,
            required_state=list(required_room_state.values()),
            timeline_events=timeline_events,
            bundled_aggregations=bundled_aggregations,
            stripped_state=stripped_state,
            prev_batch=prev_batch_token,
            limited=limited,
            unstable_expanded_timeline=unstable_expanded_timeline,
            num_live=num_live,
            bump_stamp=bump_stamp,
            joined_count=room_membership_summary.get(
                Membership.JOIN, empty_membership_summary
            ).count,
            invited_count=room_membership_summary.get(
                Membership.INVITE, empty_membership_summary
            ).count,
            # TODO: These are just dummy values. We could potentially just remove these
            # since notifications can only really be done correctly on the client anyway
            # (encrypted rooms).
            notification_count=0,
            highlight_count=0,
        )

    @trace
    async def get_extensions_response(
        self,
        sync_config: SlidingSyncConfig,
        previous_connection_state: "PerConnectionState",
        new_connection_state: "MutablePerConnectionState",
        actual_lists: Dict[str, SlidingSyncResult.SlidingWindowList],
        actual_room_ids: Set[str],
        actual_room_response_map: Dict[str, SlidingSyncResult.RoomResult],
        to_token: StreamToken,
        from_token: Optional[SlidingSyncStreamToken],
    ) -> SlidingSyncResult.Extensions:
        """Handle extension requests.

        Args:
            sync_config: Sync configuration
            new_connection_state: Snapshot of the current per-connection state
            new_per_connection_state: A mutable copy of the per-connection
                state, used to record updates to the state during this request.
            actual_lists: Sliding window API. A map of list key to list results in the
                Sliding Sync response.
            actual_room_ids: The actual room IDs in the the Sliding Sync response.
            actual_room_response_map: A map of room ID to room results in the the
                Sliding Sync response.
            to_token: The point in the stream to sync up to.
            from_token: The point in the stream to sync from.
        """

        if sync_config.extensions is None:
            return SlidingSyncResult.Extensions()

        to_device_response = None
        if sync_config.extensions.to_device is not None:
            to_device_response = await self.get_to_device_extension_response(
                sync_config=sync_config,
                to_device_request=sync_config.extensions.to_device,
                to_token=to_token,
            )

        e2ee_response = None
        if sync_config.extensions.e2ee is not None:
            e2ee_response = await self.get_e2ee_extension_response(
                sync_config=sync_config,
                e2ee_request=sync_config.extensions.e2ee,
                to_token=to_token,
                from_token=from_token,
            )

        account_data_response = None
        if sync_config.extensions.account_data is not None:
            account_data_response = await self.get_account_data_extension_response(
                sync_config=sync_config,
                actual_lists=actual_lists,
                actual_room_ids=actual_room_ids,
                account_data_request=sync_config.extensions.account_data,
                to_token=to_token,
                from_token=from_token,
            )

        receipts_response = None
        if sync_config.extensions.receipts is not None:
            receipts_response = await self.get_receipts_extension_response(
                sync_config=sync_config,
                previous_connection_state=previous_connection_state,
                new_connection_state=new_connection_state,
                actual_lists=actual_lists,
                actual_room_ids=actual_room_ids,
                actual_room_response_map=actual_room_response_map,
                receipts_request=sync_config.extensions.receipts,
                to_token=to_token,
                from_token=from_token,
            )

        typing_response = None
        if sync_config.extensions.typing is not None:
            typing_response = await self.get_typing_extension_response(
                sync_config=sync_config,
                actual_lists=actual_lists,
                actual_room_ids=actual_room_ids,
                actual_room_response_map=actual_room_response_map,
                typing_request=sync_config.extensions.typing,
                to_token=to_token,
                from_token=from_token,
            )

        return SlidingSyncResult.Extensions(
            to_device=to_device_response,
            e2ee=e2ee_response,
            account_data=account_data_response,
            receipts=receipts_response,
            typing=typing_response,
        )

    def find_relevant_room_ids_for_extension(
        self,
        requested_lists: Optional[List[str]],
        requested_room_ids: Optional[List[str]],
        actual_lists: Dict[str, SlidingSyncResult.SlidingWindowList],
        actual_room_ids: Set[str],
    ) -> Set[str]:
        """
        Handle the reserved `lists`/`rooms` keys for extensions. Extensions should only
        return results for rooms in the Sliding Sync response. This matches up the
        requested rooms/lists with the actual lists/rooms in the Sliding Sync response.

        {"lists": []}                    // Do not process any lists.
        {"lists": ["rooms", "dms"]}      // Process only a subset of lists.
        {"lists": ["*"]}                 // Process all lists defined in the Sliding Window API. (This is the default.)

        {"rooms": []}                    // Do not process any specific rooms.
        {"rooms": ["!a:b", "!c:d"]}      // Process only a subset of room subscriptions.
        {"rooms": ["*"]}                 // Process all room subscriptions defined in the Room Subscription API. (This is the default.)

        Args:
            requested_lists: The `lists` from the extension request.
            requested_room_ids: The `rooms` from the extension request.
            actual_lists: The actual lists from the Sliding Sync response.
            actual_room_ids: The actual room subscriptions from the Sliding Sync request.
        """

        # We only want to include account data for rooms that are already in the sliding
        # sync response AND that were requested in the account data request.
        relevant_room_ids: Set[str] = set()

        # See what rooms from the room subscriptions we should get account data for
        if requested_room_ids is not None:
            for room_id in requested_room_ids:
                # A wildcard means we process all rooms from the room subscriptions
                if room_id == "*":
                    relevant_room_ids.update(actual_room_ids)
                    break

                if room_id in actual_room_ids:
                    relevant_room_ids.add(room_id)

        # See what rooms from the sliding window lists we should get account data for
        if requested_lists is not None:
            for list_key in requested_lists:
                # Just some typing because we share the variable name in multiple places
                actual_list: Optional[SlidingSyncResult.SlidingWindowList] = None

                # A wildcard means we process rooms from all lists
                if list_key == "*":
                    for actual_list in actual_lists.values():
                        # We only expect a single SYNC operation for any list
                        assert len(actual_list.ops) == 1
                        sync_op = actual_list.ops[0]
                        assert sync_op.op == OperationType.SYNC

                        relevant_room_ids.update(sync_op.room_ids)

                    break

                actual_list = actual_lists.get(list_key)
                if actual_list is not None:
                    # We only expect a single SYNC operation for any list
                    assert len(actual_list.ops) == 1
                    sync_op = actual_list.ops[0]
                    assert sync_op.op == OperationType.SYNC

                    relevant_room_ids.update(sync_op.room_ids)

        return relevant_room_ids

    @trace
    async def get_to_device_extension_response(
        self,
        sync_config: SlidingSyncConfig,
        to_device_request: SlidingSyncConfig.Extensions.ToDeviceExtension,
        to_token: StreamToken,
    ) -> Optional[SlidingSyncResult.Extensions.ToDeviceExtension]:
        """Handle to-device extension (MSC3885)

        Args:
            sync_config: Sync configuration
            to_device_request: The to-device extension from the request
            to_token: The point in the stream to sync up to.
        """
        user_id = sync_config.user.to_string()
        device_id = sync_config.requester.device_id

        # Skip if the extension is not enabled
        if not to_device_request.enabled:
            return None

        # Check that this request has a valid device ID (not all requests have
        # to belong to a device, and so device_id is None)
        if device_id is None:
            return SlidingSyncResult.Extensions.ToDeviceExtension(
                next_batch=f"{to_token.to_device_key}",
                events=[],
            )

        since_stream_id = 0
        if to_device_request.since is not None:
            # We've already validated this is an int.
            since_stream_id = int(to_device_request.since)

            if to_token.to_device_key < since_stream_id:
                # The since token is ahead of our current token, so we return an
                # empty response.
                logger.warning(
                    "Got to-device.since from the future. since token: %r is ahead of our current to_device stream position: %r",
                    since_stream_id,
                    to_token.to_device_key,
                )
                return SlidingSyncResult.Extensions.ToDeviceExtension(
                    next_batch=to_device_request.since,
                    events=[],
                )

            # Delete everything before the given since token, as we know the
            # device must have received them.
            deleted = await self.store.delete_messages_for_device(
                user_id=user_id,
                device_id=device_id,
                up_to_stream_id=since_stream_id,
            )

            logger.debug(
                "Deleted %d to-device messages up to %d for %s",
                deleted,
                since_stream_id,
                user_id,
            )

        messages, stream_id = await self.store.get_messages_for_device(
            user_id=user_id,
            device_id=device_id,
            from_stream_id=since_stream_id,
            to_stream_id=to_token.to_device_key,
            limit=min(to_device_request.limit, 100),  # Limit to at most 100 events
        )

        return SlidingSyncResult.Extensions.ToDeviceExtension(
            next_batch=f"{stream_id}",
            events=messages,
        )

    @trace
    async def get_e2ee_extension_response(
        self,
        sync_config: SlidingSyncConfig,
        e2ee_request: SlidingSyncConfig.Extensions.E2eeExtension,
        to_token: StreamToken,
        from_token: Optional[SlidingSyncStreamToken],
    ) -> Optional[SlidingSyncResult.Extensions.E2eeExtension]:
        """Handle E2EE device extension (MSC3884)

        Args:
            sync_config: Sync configuration
            e2ee_request: The e2ee extension from the request
            to_token: The point in the stream to sync up to.
            from_token: The point in the stream to sync from.
        """
        user_id = sync_config.user.to_string()
        device_id = sync_config.requester.device_id

        # Skip if the extension is not enabled
        if not e2ee_request.enabled:
            return None

        device_list_updates: Optional[DeviceListUpdates] = None
        if from_token is not None:
            # TODO: This should take into account the `from_token` and `to_token`
            device_list_updates = await self.device_handler.get_user_ids_changed(
                user_id=user_id,
                from_token=from_token.stream_token,
            )

        device_one_time_keys_count: Mapping[str, int] = {}
        device_unused_fallback_key_types: Sequence[str] = []
        if device_id:
            # TODO: We should have a way to let clients differentiate between the states of:
            #   * no change in OTK count since the provided since token
            #   * the server has zero OTKs left for this device
            #  Spec issue: https://github.com/matrix-org/matrix-doc/issues/3298
            device_one_time_keys_count = await self.store.count_e2e_one_time_keys(
                user_id, device_id
            )
            device_unused_fallback_key_types = (
                await self.store.get_e2e_unused_fallback_key_types(user_id, device_id)
            )

        return SlidingSyncResult.Extensions.E2eeExtension(
            device_list_updates=device_list_updates,
            device_one_time_keys_count=device_one_time_keys_count,
            device_unused_fallback_key_types=device_unused_fallback_key_types,
        )

    @trace
    async def get_account_data_extension_response(
        self,
        sync_config: SlidingSyncConfig,
        actual_lists: Dict[str, SlidingSyncResult.SlidingWindowList],
        actual_room_ids: Set[str],
        account_data_request: SlidingSyncConfig.Extensions.AccountDataExtension,
        to_token: StreamToken,
        from_token: Optional[SlidingSyncStreamToken],
    ) -> Optional[SlidingSyncResult.Extensions.AccountDataExtension]:
        """Handle Account Data extension (MSC3959)

        Args:
            sync_config: Sync configuration
            actual_lists: Sliding window API. A map of list key to list results in the
                Sliding Sync response.
            actual_room_ids: The actual room IDs in the the Sliding Sync response.
            account_data_request: The account_data extension from the request
            to_token: The point in the stream to sync up to.
            from_token: The point in the stream to sync from.
        """
        user_id = sync_config.user.to_string()

        # Skip if the extension is not enabled
        if not account_data_request.enabled:
            return None

        global_account_data_map: Mapping[str, JsonMapping] = {}
        if from_token is not None:
            # TODO: This should take into account the `from_token` and `to_token`
            global_account_data_map = (
                await self.store.get_updated_global_account_data_for_user(
                    user_id, from_token.stream_token.account_data_key
                )
            )

            have_push_rules_changed = await self.store.have_push_rules_changed_for_user(
                user_id, from_token.stream_token.push_rules_key
            )
            if have_push_rules_changed:
                global_account_data_map = dict(global_account_data_map)
                # TODO: This should take into account the `from_token` and `to_token`
                global_account_data_map[AccountDataTypes.PUSH_RULES] = (
                    await self.push_rules_handler.push_rules_for_user(sync_config.user)
                )
        else:
            # TODO: This should take into account the `to_token`
            all_global_account_data = await self.store.get_global_account_data_for_user(
                user_id
            )

            global_account_data_map = dict(all_global_account_data)
            # TODO: This should take into account the  `to_token`
            global_account_data_map[AccountDataTypes.PUSH_RULES] = (
                await self.push_rules_handler.push_rules_for_user(sync_config.user)
            )

        # Fetch room account data
        account_data_by_room_map: Mapping[str, Mapping[str, JsonMapping]] = {}
        relevant_room_ids = self.find_relevant_room_ids_for_extension(
            requested_lists=account_data_request.lists,
            requested_room_ids=account_data_request.rooms,
            actual_lists=actual_lists,
            actual_room_ids=actual_room_ids,
        )
        if len(relevant_room_ids) > 0:
            if from_token is not None:
                # TODO: This should take into account the `from_token` and `to_token`
                account_data_by_room_map = (
                    await self.store.get_updated_room_account_data_for_user(
                        user_id, from_token.stream_token.account_data_key
                    )
                )
            else:
                # TODO: This should take into account the `to_token`
                account_data_by_room_map = (
                    await self.store.get_room_account_data_for_user(user_id)
                )

        # Filter down to the relevant rooms
        account_data_by_room_map = {
            room_id: account_data_map
            for room_id, account_data_map in account_data_by_room_map.items()
            if room_id in relevant_room_ids
        }

        return SlidingSyncResult.Extensions.AccountDataExtension(
            global_account_data_map=global_account_data_map,
            account_data_by_room_map=account_data_by_room_map,
        )

    @trace
    async def get_receipts_extension_response(
        self,
        sync_config: SlidingSyncConfig,
        previous_connection_state: "PerConnectionState",
        new_connection_state: "MutablePerConnectionState",
        actual_lists: Dict[str, SlidingSyncResult.SlidingWindowList],
        actual_room_ids: Set[str],
        actual_room_response_map: Dict[str, SlidingSyncResult.RoomResult],
        receipts_request: SlidingSyncConfig.Extensions.ReceiptsExtension,
        to_token: StreamToken,
        from_token: Optional[SlidingSyncStreamToken],
    ) -> Optional[SlidingSyncResult.Extensions.ReceiptsExtension]:
        """Handle Receipts extension (MSC3960)

        Args:
            sync_config: Sync configuration
            previous_connection_state: The current per-connection state
            new_connection_state: A mutable copy of the per-connection
                state, used to record updates to the state.
            actual_lists: Sliding window API. A map of list key to list results in the
                Sliding Sync response.
            actual_room_ids: The actual room IDs in the the Sliding Sync response.
            actual_room_response_map: A map of room ID to room results in the the
                Sliding Sync response.
            account_data_request: The account_data extension from the request
            to_token: The point in the stream to sync up to.
            from_token: The point in the stream to sync from.
        """
        # Skip if the extension is not enabled
        if not receipts_request.enabled:
            return None

        relevant_room_ids = self.find_relevant_room_ids_for_extension(
            requested_lists=receipts_request.lists,
            requested_room_ids=receipts_request.rooms,
            actual_lists=actual_lists,
            actual_room_ids=actual_room_ids,
        )

        room_id_to_receipt_map: Dict[str, JsonMapping] = {}
        if len(relevant_room_ids) > 0:
            # We need to handle the different cases depending on if we have sent
            # down receipts previously or not, so we split the relevant rooms
            # up into different collections based on status.
            live_rooms = set()
            previously_rooms: Dict[str, MultiWriterStreamToken] = {}
            initial_rooms = set()

            for room_id in relevant_room_ids:
                if not from_token:
                    initial_rooms.add(room_id)
                    continue

                # If we're sending down the room from scratch again for some reason, we
                # should always resend the receipts as well (regardless of if
                # we've sent them down before). This is to mimic the behaviour
                # of what happens on initial sync, where you get a chunk of
                # timeline with all of the corresponding receipts for the events in the timeline.
                room_result = actual_room_response_map.get(room_id)
                if room_result is not None and room_result.initial:
                    initial_rooms.add(room_id)
                    continue

                room_status = previous_connection_state.receipts.have_sent_room(room_id)
                if room_status.status == HaveSentRoomFlag.LIVE:
                    live_rooms.add(room_id)
                elif room_status.status == HaveSentRoomFlag.PREVIOUSLY:
                    assert room_status.last_token is not None
                    previously_rooms[room_id] = room_status.last_token
                elif room_status.status == HaveSentRoomFlag.NEVER:
                    initial_rooms.add(room_id)
                else:
                    assert_never(room_status.status)

            # The set of receipts that we fetched. Private receipts need to be
            # filtered out before returning.
            fetched_receipts = []

            # For live rooms we just fetch all receipts in those rooms since the
            # `since` token.
            if live_rooms:
                assert from_token is not None
                receipts = await self.store.get_linearized_receipts_for_rooms(
                    room_ids=live_rooms,
                    from_key=from_token.stream_token.receipt_key,
                    to_key=to_token.receipt_key,
                )
                fetched_receipts.extend(receipts)

            # For rooms we've previously sent down, but aren't up to date, we
            # need to use the from token from the room status.
            if previously_rooms:
                for room_id, receipt_token in previously_rooms.items():
                    # TODO: Limit the number of receipts we're about to send down
                    # for the room, if its too many we should TODO
                    previously_receipts = (
                        await self.store.get_linearized_receipts_for_room(
                            room_id=room_id,
                            from_key=receipt_token,
                            to_key=to_token.receipt_key,
                        )
                    )
                    fetched_receipts.extend(previously_receipts)

            # For rooms we haven't previously sent down, we could send all receipts
            # from that room but we only want to include receipts for events
            # in the timeline to avoid bloating and blowing up the sync response
            # as the number of users in the room increases. (this behavior is part of the spec)
            initial_rooms_and_event_ids = [
                (room_id, event.event_id)
                for room_id in initial_rooms
                if room_id in actual_room_response_map
                for event in actual_room_response_map[room_id].timeline_events
            ]
            if initial_rooms_and_event_ids:
                initial_receipts = await self.store.get_linearized_receipts_for_events(
                    room_and_event_ids=initial_rooms_and_event_ids,
                )
                fetched_receipts.extend(initial_receipts)

            fetched_receipts = ReceiptEventSource.filter_out_private_receipts(
                fetched_receipts, sync_config.user.to_string()
            )

            for receipt in fetched_receipts:
                # These fields should exist for every receipt
                room_id = receipt["room_id"]
                type = receipt["type"]
                content = receipt["content"]

                room_id_to_receipt_map[room_id] = {"type": type, "content": content}

        # Now we update the per-connection state to track which receipts we have
        # and haven't sent down.
        new_connection_state.receipts.record_sent_rooms(relevant_room_ids)

        if from_token:
            # Now find the set of rooms that may have receipts that we're not sending
            # down. We only need to check rooms that we have previously returned
            # receipts for (in `previous_connection_state`) because we only care about
            # updating `LIVE` rooms to `PREVIOUSLY`. The `PREVIOUSLY` rooms will just
            # stay pointing at their previous position so we don't need to waste time
            # checking those and since we default to `NEVER`, rooms that were `NEVER`
            # sent before don't need to be recorded as we'll handle them correctly when
            # they come into range for the first time.
            rooms_no_receipts = [
                room_id
                for room_id, room_status in previous_connection_state.receipts._statuses.items()
                if room_status.status == HaveSentRoomFlag.LIVE
                and room_id not in relevant_room_ids
            ]
            changed_rooms = await self.store.get_rooms_with_receipts_between(
                rooms_no_receipts,
                from_key=from_token.stream_token.receipt_key,
                to_key=to_token.receipt_key,
            )
            new_connection_state.receipts.record_unsent_rooms(
                changed_rooms, from_token.stream_token.receipt_key
            )

        return SlidingSyncResult.Extensions.ReceiptsExtension(
            room_id_to_receipt_map=room_id_to_receipt_map,
        )

    async def get_typing_extension_response(
        self,
        sync_config: SlidingSyncConfig,
        actual_lists: Dict[str, SlidingSyncResult.SlidingWindowList],
        actual_room_ids: Set[str],
        actual_room_response_map: Dict[str, SlidingSyncResult.RoomResult],
        typing_request: SlidingSyncConfig.Extensions.TypingExtension,
        to_token: StreamToken,
        from_token: Optional[SlidingSyncStreamToken],
    ) -> Optional[SlidingSyncResult.Extensions.TypingExtension]:
        """Handle Typing Notification extension (MSC3961)

        Args:
            sync_config: Sync configuration
            actual_lists: Sliding window API. A map of list key to list results in the
                Sliding Sync response.
            actual_room_ids: The actual room IDs in the the Sliding Sync response.
            actual_room_response_map: A map of room ID to room results in the the
                Sliding Sync response.
            account_data_request: The account_data extension from the request
            to_token: The point in the stream to sync up to.
            from_token: The point in the stream to sync from.
        """
        # Skip if the extension is not enabled
        if not typing_request.enabled:
            return None

        relevant_room_ids = self.find_relevant_room_ids_for_extension(
            requested_lists=typing_request.lists,
            requested_room_ids=typing_request.rooms,
            actual_lists=actual_lists,
            actual_room_ids=actual_room_ids,
        )

        room_id_to_typing_map: Dict[str, JsonMapping] = {}
        if len(relevant_room_ids) > 0:
            # Note: We don't need to take connection tracking into account for typing
            # notifications because they'll get anything still relevant and hasn't timed
            # out when the room comes into range. We consider the gap where the room
            # fell out of range, as long enough for any typing notifications to have
            # timed out (it's not worth the 30 seconds of data we may have missed).
            typing_source = self.event_sources.sources.typing
            typing_notifications, _ = await typing_source.get_new_events(
                user=sync_config.user,
                from_key=(from_token.stream_token.typing_key if from_token else 0),
                to_key=to_token.typing_key,
                # This is a dummy value and isn't used in the function
                limit=0,
                room_ids=relevant_room_ids,
                is_guest=False,
            )

            for typing_notification in typing_notifications:
                # These fields should exist for every typing notification
                room_id = typing_notification["room_id"]
                type = typing_notification["type"]
                content = typing_notification["content"]

                room_id_to_typing_map[room_id] = {"type": type, "content": content}

        return SlidingSyncResult.Extensions.TypingExtension(
            room_id_to_typing_map=room_id_to_typing_map,
        )


class HaveSentRoomFlag(Enum):
    """Flag for whether we have sent the room down a sliding sync connection.

    The valid state changes here are:
        NEVER -> LIVE
        LIVE -> PREVIOUSLY
        PREVIOUSLY -> LIVE
    """

    # The room has never been sent down (or we have forgotten we have sent it
    # down).
    NEVER = 1

    # We have previously sent the room down, but there are updates that we
    # haven't sent down.
    PREVIOUSLY = 2

    # We have sent the room down and the client has received all updates.
    LIVE = 3


T = TypeVar("T")


@attr.s(auto_attribs=True, slots=True, frozen=True)
class HaveSentRoom(Generic[T]):
    """Whether we have sent the room data down a sliding sync connection.

    We are generic over the type of token used, e.g. `RoomStreamToken` or
    `MultiWriterStreamToken`.

    Attributes:
        status: Flag of if we have or haven't sent down the room
        last_token: If the flag is `PREVIOUSLY` then this is non-null and
            contains the last stream token of the last updates we sent down
            the room, i.e. we still need to send everything since then to the
            client.
    """

    status: HaveSentRoomFlag
    last_token: Optional[T]

    @staticmethod
    def live() -> "HaveSentRoom[T]":
        return HaveSentRoom(HaveSentRoomFlag.LIVE, None)

    @staticmethod
    def previously(last_token: T) -> "HaveSentRoom[T]":
        """Constructor for `PREVIOUSLY` flag."""
        return HaveSentRoom(HaveSentRoomFlag.PREVIOUSLY, last_token)

    @staticmethod
    def never() -> "HaveSentRoom[T]":
        return HaveSentRoom(HaveSentRoomFlag.NEVER, None)


@attr.s(auto_attribs=True, slots=True, frozen=True)
class RoomStatusMap(Generic[T]):
    """For a given stream, e.g. events, records what we have or have not sent
    down for that stream in a given room."""

    # `room_id` -> `HaveSentRoom`
    _statuses: Mapping[str, HaveSentRoom[T]] = attr.Factory(dict)

    def have_sent_room(self, room_id: str) -> HaveSentRoom[T]:
        """Return whether we have previously sent the room down"""
        return self._statuses.get(room_id, HaveSentRoom.never())

    def get_mutable(self) -> "MutableRoomStatusMap[T]":
        """Get a mutable copy of this state."""
        return MutableRoomStatusMap(
            statuses=self._statuses,
        )

    def copy(self) -> "RoomStatusMap[T]":
        """Make a copy of the class. Useful for converting from a mutable to
        immutable version."""

        return RoomStatusMap(statuses=dict(self._statuses))


class MutableRoomStatusMap(RoomStatusMap[T]):
    """A mutable version of `RoomStatusMap`"""

    # We use a ChainMap here so that we can easily track what has been updated
    # and what hasn't. Note that when we persist the per connection state this
    # will get flattened to a normal dict (via calling `.copy()`)
    _statuses: typing.ChainMap[str, HaveSentRoom[T]]

    def __init__(
        self,
        statuses: Mapping[str, HaveSentRoom[T]],
    ) -> None:
        # ChainMap requires a mutable mapping, but we're not actually going to
        # mutate it.
        statuses = cast(MutableMapping, statuses)

        super().__init__(
            statuses=ChainMap({}, statuses),
        )

    def get_updates(self) -> Mapping[str, HaveSentRoom[T]]:
        """Return only the changes that were made"""
        return self._statuses.maps[0]

    def record_sent_rooms(self, room_ids: StrCollection) -> None:
        """Record that we have sent these rooms in the response"""
        for room_id in room_ids:
            current_status = self._statuses.get(room_id, HaveSentRoom.never())
            if current_status.status == HaveSentRoomFlag.LIVE:
                continue

            self._statuses[room_id] = HaveSentRoom.live()

    def record_unsent_rooms(self, room_ids: StrCollection, from_token: T) -> None:
        """Record that we have not sent these rooms in the response, but there
        have been updates.
        """
        # Whether we add/update the entries for unsent rooms depends on the
        # existing entry:
        #   - LIVE: We have previously sent down everything up to
        #     `last_room_token, so we update the entry to be `PREVIOUSLY` with
        #     `last_room_token`.
        #   - PREVIOUSLY: We have previously sent down everything up to *a*
        #     given token, so we don't need to update the entry.
        #   - NEVER: We have never previously sent down the room, and we haven't
        #     sent anything down this time either so we leave it as NEVER.

        for room_id in room_ids:
            current_status = self._statuses.get(room_id, HaveSentRoom.never())
            if current_status.status != HaveSentRoomFlag.LIVE:
                continue

            self._statuses[room_id] = HaveSentRoom.previously(from_token)


@attr.s(auto_attribs=True)
class PerConnectionState:
    """The per-connection state. A snapshot of what we've sent down the
    connection before.

    Currently, we track whether we've sent down various aspects of a given room
    before.

    We use the `rooms` field to store the position in the events stream for each
    room that we've previously sent to the client before. On the next request
    that includes the room, we can then send only what's changed since that
    recorded position.

    Same goes for the `receipts` field so we only need to send the new receipts
    since the last time you made a sync request.

    Attributes:
        rooms: The status of each room for the events stream.
        receipts: The status of each room for the receipts stream.
        room_configs: Map from room_id to the `RoomSyncConfig` of all
            rooms that we have previously sent down.
    """

    rooms: RoomStatusMap[RoomStreamToken] = attr.Factory(RoomStatusMap)
    receipts: RoomStatusMap[MultiWriterStreamToken] = attr.Factory(RoomStatusMap)

    room_configs: Mapping[str, RoomSyncConfig] = attr.Factory(dict)

    def get_mutable(self) -> "MutablePerConnectionState":
        """Get a mutable copy of this state."""
        room_configs = cast(MutableMapping[str, RoomSyncConfig], self.room_configs)

        return MutablePerConnectionState(
            rooms=self.rooms.get_mutable(),
            receipts=self.receipts.get_mutable(),
            room_configs=ChainMap({}, room_configs),
        )

    def copy(self) -> "PerConnectionState":
        return PerConnectionState(
            rooms=self.rooms.copy(),
            receipts=self.receipts.copy(),
            room_configs=dict(self.room_configs),
        )


@attr.s(auto_attribs=True)
class MutablePerConnectionState(PerConnectionState):
    """A mutable version of `PerConnectionState`"""

    rooms: MutableRoomStatusMap[RoomStreamToken]
    receipts: MutableRoomStatusMap[MultiWriterStreamToken]

    room_configs: typing.ChainMap[str, RoomSyncConfig]

    def has_updates(self) -> bool:
        return (
            bool(self.rooms.get_updates())
            or bool(self.receipts.get_updates())
            or bool(self.get_room_config_updates())
        )

    def get_room_config_updates(self) -> Mapping[str, RoomSyncConfig]:
        """Get updates to the room sync config"""
        return self.room_configs.maps[0]


@attr.s(auto_attribs=True)
class SlidingSyncConnectionStore:
    """In-memory store of per-connection state, including what rooms we have
    previously sent down a sliding sync connection.

    Note: This is NOT safe to run in a worker setup because connection positions will
    point to different sets of rooms on different workers. e.g. for the same connection,
    a connection position of 5 might have totally different states on worker A and
    worker B.

    One complication that we need to deal with here is needing to handle requests being
    resent, i.e. if we sent down a room in a response that the client received, we must
    consider the room *not* sent when we get the request again.

    This is handled by using an integer "token", which is returned to the client
    as part of the sync token. For each connection we store a mapping from
    tokens to the room states, and create a new entry when we send down new
    rooms.

    Note that for any given sliding sync connection we will only store a maximum
    of two different tokens: the previous token from the request and a new token
    sent in the response. When we receive a request with a given token, we then
    clear out all other entries with a different token.

    Attributes:
        _connections: Mapping from `(user_id, conn_id)` to mapping of `token`
            to mapping of room ID to `HaveSentRoom`.
    """

    # `(user_id, conn_id)` -> `connection_position` -> `PerConnectionState`
    _connections: Dict[Tuple[str, str], Dict[int, PerConnectionState]] = attr.Factory(
        dict
    )

    async def is_valid_token(
        self, sync_config: SlidingSyncConfig, connection_token: int
    ) -> bool:
        """Return whether the connection token is valid/recognized"""
        if connection_token == 0:
            return True

        conn_key = self._get_connection_key(sync_config)
        return connection_token in self._connections.get(conn_key, {})

    async def get_per_connection_state(
        self,
        sync_config: SlidingSyncConfig,
        from_token: Optional[SlidingSyncStreamToken],
    ) -> PerConnectionState:
        """Fetch the per-connection state for the token.

        Raises:
            SlidingSyncUnknownPosition if the connection_token is unknown
        """
        if from_token is None:
            return PerConnectionState()

        connection_position = from_token.connection_position
        if connection_position == 0:
            # Initial sync (request without a `from_token`) starts at `0` so
            # there is no existing per-connection state
            return PerConnectionState()

        conn_key = self._get_connection_key(sync_config)
        sync_statuses = self._connections.get(conn_key, {})
        connection_state = sync_statuses.get(connection_position)

        if connection_state is None:
            raise SlidingSyncUnknownPosition()

        return connection_state

    @trace
    async def record_new_state(
        self,
        sync_config: SlidingSyncConfig,
        from_token: Optional[SlidingSyncStreamToken],
        new_connection_state: MutablePerConnectionState,
    ) -> int:
        """Record updated per-connection state, returning the connection
        position associated with the new state.
        If there are no changes to the state this may return the same token as
        the existing per-connection state.
        """
        prev_connection_token = 0
        if from_token is not None:
            prev_connection_token = from_token.connection_position

        if not new_connection_state.has_updates():
            return prev_connection_token

        conn_key = self._get_connection_key(sync_config)
        sync_statuses = self._connections.setdefault(conn_key, {})

        # Generate a new token, removing any existing entries in that token
        # (which can happen if requests get resent).
        new_store_token = prev_connection_token + 1
        sync_statuses.pop(new_store_token, None)

        # We copy the `MutablePerConnectionState` so that the inner `ChainMap`s
        # don't grow forever.
        sync_statuses[new_store_token] = new_connection_state.copy()

        return new_store_token

    @trace
    async def mark_token_seen(
        self,
        sync_config: SlidingSyncConfig,
        from_token: Optional[SlidingSyncStreamToken],
    ) -> None:
        """We have received a request with the given token, so we can clear out
        any other tokens associated with the connection.

        If there is no from token then we have started afresh, and so we delete
        all tokens associated with the device.
        """
        # Clear out any tokens for the connection that doesn't match the one
        # from the request.

        conn_key = self._get_connection_key(sync_config)
        sync_statuses = self._connections.pop(conn_key, {})
        if from_token is None:
            return

        sync_statuses = {
            connection_token: room_statuses
            for connection_token, room_statuses in sync_statuses.items()
            if connection_token == from_token.connection_position
        }
        if sync_statuses:
            self._connections[conn_key] = sync_statuses

    @staticmethod
    def _get_connection_key(sync_config: SlidingSyncConfig) -> Tuple[str, str]:
        """Return a unique identifier for this connection.

        The first part is simply the user ID.

        The second part is generally a combination of device ID and conn_id.
        However, both these two are optional (e.g. puppet access tokens don't
        have device IDs), so this handles those edge cases.

        We use this over the raw `conn_id` to avoid clashes between different
        clients that use the same `conn_id`. Imagine a user uses a web client
        that uses `conn_id: main_sync_loop` and an Android client that also has
        a `conn_id: main_sync_loop`.
        """

        user_id = sync_config.user.to_string()

        # Only one sliding sync connection is allowed per given conn_id (empty
        # or not).
        conn_id = sync_config.conn_id or ""

        if sync_config.requester.device_id:
            return (user_id, f"D/{sync_config.requester.device_id}/{conn_id}")

        if sync_config.requester.access_token_id:
            # If we don't have a device, then the access token ID should be a
            # stable ID.
            return (user_id, f"A/{sync_config.requester.access_token_id}/{conn_id}")

        # If we have neither then its likely an AS or some weird token. Either
        # way we can just fail here.
        raise Exception("Cannot use sliding sync with access token type")
