Experimental support for MSC3266 Room Summary API. (#10394)

1 files changed, 1171 insertions, 0 deletions

diff --git a/synapse/handlers/room_summary.py b/synapse/handlers/room_summary.py
new file mode 100644
index 0000000000..ac6cfc0da9
--- /dev/null
+++ b/synapse/handlers/room_summary.py
@@ -0,0 +1,1171 @@
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import itertools
+import logging
+import re
+from collections import deque
+from typing import TYPE_CHECKING, Dict, Iterable, List, Optional, Sequence, Set, Tuple
+
+import attr
+
+from synapse.api.constants import (
+    EventContentFields,
+    EventTypes,
+    HistoryVisibility,
+    JoinRules,
+    Membership,
+    RoomTypes,
+)
+from synapse.api.errors import AuthError, Codes, NotFoundError, SynapseError
+from synapse.events import EventBase
+from synapse.events.utils import format_event_for_client_v2
+from synapse.types import JsonDict
+from synapse.util.caches.response_cache import ResponseCache
+from synapse.util.stringutils import random_string
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+# number of rooms to return. We'll stop once we hit this limit.
+MAX_ROOMS = 50
+
+# max number of events to return per room.
+MAX_ROOMS_PER_SPACE = 50
+
+# max number of federation servers to hit per room
+MAX_SERVERS_PER_SPACE = 3
+
+
+@attr.s(slots=True, frozen=True, auto_attribs=True)
+class _PaginationKey:
+    """The key used to find unique pagination session."""
+
+    # The first three entries match the request parameters (and cannot change
+    # during a pagination session).
+    room_id: str
+    suggested_only: bool
+    max_depth: Optional[int]
+    # The randomly generated token.
+    token: str
+
+
+@attr.s(slots=True, frozen=True, auto_attribs=True)
+class _PaginationSession:
+    """The information that is stored for pagination."""
+
+    # The time the pagination session was created, in milliseconds.
+    creation_time_ms: int
+    # The queue of rooms which are still to process.
+    room_queue: List["_RoomQueueEntry"]
+    # A set of rooms which have been processed.
+    processed_rooms: Set[str]
+
+
+class RoomSummaryHandler:
+    # The time a pagination session remains valid for.
+    _PAGINATION_SESSION_VALIDITY_PERIOD_MS = 5 * 60 * 1000
+
+    def __init__(self, hs: "HomeServer"):
+        self._clock = hs.get_clock()
+        self._event_auth_handler = hs.get_event_auth_handler()
+        self._store = hs.get_datastore()
+        self._event_serializer = hs.get_event_client_serializer()
+        self._server_name = hs.hostname
+        self._federation_client = hs.get_federation_client()
+
+        # A map of query information to the current pagination state.
+        #
+        # TODO Allow for multiple workers to share this data.
+        # TODO Expire pagination tokens.
+        self._pagination_sessions: Dict[_PaginationKey, _PaginationSession] = {}
+
+        # If a user tries to fetch the same page multiple times in quick succession,
+        # only process the first attempt and return its result to subsequent requests.
+        self._pagination_response_cache: ResponseCache[
+            Tuple[str, bool, Optional[int], Optional[int], Optional[str]]
+        ] = ResponseCache(
+            hs.get_clock(),
+            "get_room_hierarchy",
+        )
+
+    def _expire_pagination_sessions(self):
+        """Expire pagination session which are old."""
+        expire_before = (
+            self._clock.time_msec() - self._PAGINATION_SESSION_VALIDITY_PERIOD_MS
+        )
+        to_expire = []
+
+        for key, value in self._pagination_sessions.items():
+            if value.creation_time_ms < expire_before:
+                to_expire.append(key)
+
+        for key in to_expire:
+            logger.debug("Expiring pagination session id %s", key)
+            del self._pagination_sessions[key]
+
+    async def get_space_summary(
+        self,
+        requester: str,
+        room_id: str,
+        suggested_only: bool = False,
+        max_rooms_per_space: Optional[int] = None,
+    ) -> JsonDict:
+        """
+        Implementation of the space summary C-S API
+
+        Args:
+            requester:  user id of the user making this request
+
+            room_id: room id to start the summary at
+
+            suggested_only: whether we should only return children with the "suggested"
+                flag set.
+
+            max_rooms_per_space: an optional limit on the number of child rooms we will
+                return. This does not apply to the root room (ie, room_id), and
+                is overridden by MAX_ROOMS_PER_SPACE.
+
+        Returns:
+            summary dict to return
+        """
+        # First of all, check that the room is accessible.
+        if not await self._is_local_room_accessible(room_id, requester):
+            raise AuthError(
+                403,
+                "User %s not in room %s, and room previews are disabled"
+                % (requester, room_id),
+            )
+
+        # the queue of rooms to process
+        room_queue = deque((_RoomQueueEntry(room_id, ()),))
+
+        # rooms we have already processed
+        processed_rooms: Set[str] = set()
+
+        # events we have already processed. We don't necessarily have their event ids,
+        # so instead we key on (room id, state key)
+        processed_events: Set[Tuple[str, str]] = set()
+
+        rooms_result: List[JsonDict] = []
+        events_result: List[JsonDict] = []
+
+        while room_queue and len(rooms_result) < MAX_ROOMS:
+            queue_entry = room_queue.popleft()
+            room_id = queue_entry.room_id
+            if room_id in processed_rooms:
+                # already done this room
+                continue
+
+            logger.debug("Processing room %s", room_id)
+
+            is_in_room = await self._store.is_host_joined(room_id, self._server_name)
+
+            # The client-specified max_rooms_per_space limit doesn't apply to the
+            # room_id specified in the request, so we ignore it if this is the
+            # first room we are processing.
+            max_children = max_rooms_per_space if processed_rooms else None
+
+            if is_in_room:
+                room_entry = await self._summarize_local_room(
+                    requester, None, room_id, suggested_only, max_children
+                )
+
+                events: Sequence[JsonDict] = []
+                if room_entry:
+                    rooms_result.append(room_entry.room)
+                    events = room_entry.children_state_events
+
+                logger.debug(
+                    "Query of local room %s returned events %s",
+                    room_id,
+                    ["%s->%s" % (ev["room_id"], ev["state_key"]) for ev in events],
+                )
+            else:
+                fed_rooms = await self._summarize_remote_room(
+                    queue_entry,
+                    suggested_only,
+                    max_children,
+                    exclude_rooms=processed_rooms,
+                )
+
+                # The results over federation might include rooms that the we,
+                # as the requesting server, are allowed to see, but the requesting
+                # user is not permitted see.
+                #
+                # Filter the returned results to only what is accessible to the user.
+                events = []
+                for room_entry in fed_rooms:
+                    room = room_entry.room
+                    fed_room_id = room_entry.room_id
+
+                    # The user can see the room, include it!
+                    if await self._is_remote_room_accessible(
+                        requester, fed_room_id, room
+                    ):
+                        # Before returning to the client, remove the allowed_room_ids
+                        # and allowed_spaces keys.
+                        room.pop("allowed_room_ids", None)
+                        room.pop("allowed_spaces", None)
+
+                        rooms_result.append(room)
+                        events.extend(room_entry.children_state_events)
+
+                    # All rooms returned don't need visiting again (even if the user
+                    # didn't have access to them).
+                    processed_rooms.add(fed_room_id)
+
+                logger.debug(
+                    "Query of %s returned rooms %s, events %s",
+                    room_id,
+                    [room_entry.room.get("room_id") for room_entry in fed_rooms],
+                    ["%s->%s" % (ev["room_id"], ev["state_key"]) for ev in events],
+                )
+
+            # the room we queried may or may not have been returned, but don't process
+            # it again, anyway.
+            processed_rooms.add(room_id)
+
+            # XXX: is it ok that we blindly iterate through any events returned by
+            #   a remote server, whether or not they actually link to any rooms in our
+            #   tree?
+            for ev in events:
+                # remote servers might return events we have already processed
+                # (eg, Dendrite returns inward pointers as well as outward ones), so
+                # we need to filter them out, to avoid returning duplicate links to the
+                # client.
+                ev_key = (ev["room_id"], ev["state_key"])
+                if ev_key in processed_events:
+                    continue
+                events_result.append(ev)
+
+                # add the child to the queue. we have already validated
+                # that the vias are a list of server names.
+                room_queue.append(
+                    _RoomQueueEntry(ev["state_key"], ev["content"]["via"])
+                )
+                processed_events.add(ev_key)
+
+        return {"rooms": rooms_result, "events": events_result}
+
+    async def get_room_hierarchy(
+        self,
+        requester: str,
+        requested_room_id: str,
+        suggested_only: bool = False,
+        max_depth: Optional[int] = None,
+        limit: Optional[int] = None,
+        from_token: Optional[str] = None,
+    ) -> JsonDict:
+        """
+        Implementation of the room hierarchy C-S API.
+
+        Args:
+            requester: The user ID of the user making this request.
+            requested_room_id: The room ID to start the hierarchy at (the "root" room).
+            suggested_only: Whether we should only return children with the "suggested"
+                flag set.
+            max_depth: The maximum depth in the tree to explore, must be a
+                non-negative integer.
+
+                0 would correspond to just the root room, 1 would include just
+                the root room's children, etc.
+            limit: An optional limit on the number of rooms to return per
+                page. Must be a positive integer.
+            from_token: An optional pagination token.
+
+        Returns:
+            The JSON hierarchy dictionary.
+        """
+        # If a user tries to fetch the same page multiple times in quick succession,
+        # only process the first attempt and return its result to subsequent requests.
+        #
+        # This is due to the pagination process mutating internal state, attempting
+        # to process multiple requests for the same page will result in errors.
+        return await self._pagination_response_cache.wrap(
+            (requested_room_id, suggested_only, max_depth, limit, from_token),
+            self._get_room_hierarchy,
+            requester,
+            requested_room_id,
+            suggested_only,
+            max_depth,
+            limit,
+            from_token,
+        )
+
+    async def _get_room_hierarchy(
+        self,
+        requester: str,
+        requested_room_id: str,
+        suggested_only: bool = False,
+        max_depth: Optional[int] = None,
+        limit: Optional[int] = None,
+        from_token: Optional[str] = None,
+    ) -> JsonDict:
+        """See docstring for SpaceSummaryHandler.get_room_hierarchy."""
+
+        # First of all, check that the room is accessible.
+        if not await self._is_local_room_accessible(requested_room_id, requester):
+            raise AuthError(
+                403,
+                "User %s not in room %s, and room previews are disabled"
+                % (requester, requested_room_id),
+            )
+
+        # If this is continuing a previous session, pull the persisted data.
+        if from_token:
+            self._expire_pagination_sessions()
+
+            pagination_key = _PaginationKey(
+                requested_room_id, suggested_only, max_depth, from_token
+            )
+            if pagination_key not in self._pagination_sessions:
+                raise SynapseError(400, "Unknown pagination token", Codes.INVALID_PARAM)
+
+            # Load the previous state.
+            pagination_session = self._pagination_sessions[pagination_key]
+            room_queue = pagination_session.room_queue
+            processed_rooms = pagination_session.processed_rooms
+        else:
+            # The queue of rooms to process, the next room is last on the stack.
+            room_queue = [_RoomQueueEntry(requested_room_id, ())]
+
+            # Rooms we have already processed.
+            processed_rooms = set()
+
+        rooms_result: List[JsonDict] = []
+
+        # Cap the limit to a server-side maximum.
+        if limit is None:
+            limit = MAX_ROOMS
+        else:
+            limit = min(limit, MAX_ROOMS)
+
+        # Iterate through the queue until we reach the limit or run out of
+        # rooms to include.
+        while room_queue and len(rooms_result) < limit:
+            queue_entry = room_queue.pop()
+            room_id = queue_entry.room_id
+            current_depth = queue_entry.depth
+            if room_id in processed_rooms:
+                # already done this room
+                continue
+
+            logger.debug("Processing room %s", room_id)
+
+            # A map of summaries for children rooms that might be returned over
+            # federation. The rationale for caching these and *maybe* using them
+            # is to prefer any information local to the homeserver before trusting
+            # data received over federation.
+            children_room_entries: Dict[str, JsonDict] = {}
+            # A set of room IDs which are children that did not have information
+            # returned over federation and are known to be inaccessible to the
+            # current server. We should not reach out over federation to try to
+            # summarise these rooms.
+            inaccessible_children: Set[str] = set()
+
+            # If the room is known locally, summarise it!
+            is_in_room = await self._store.is_host_joined(room_id, self._server_name)
+            if is_in_room:
+                room_entry = await self._summarize_local_room(
+                    requester,
+                    None,
+                    room_id,
+                    suggested_only,
+                    # TODO Handle max children.
+                    max_children=None,
+                )
+
+            # Otherwise, attempt to use information for federation.
+            else:
+                # A previous call might have included information for this room.
+                # It can be used if either:
+                #
+                # 1. The room is not a space.
+                # 2. The maximum depth has been achieved (since no children
+                #    information is needed).
+                if queue_entry.remote_room and (
+                    queue_entry.remote_room.get("room_type") != RoomTypes.SPACE
+                    or (max_depth is not None and current_depth >= max_depth)
+                ):
+                    room_entry = _RoomEntry(
+                        queue_entry.room_id, queue_entry.remote_room
+                    )
+
+                # If the above isn't true, attempt to fetch the room
+                # information over federation.
+                else:
+                    (
+                        room_entry,
+                        children_room_entries,
+                        inaccessible_children,
+                    ) = await self._summarize_remote_room_hierarchy(
+                        queue_entry,
+                        suggested_only,
+                    )
+
+                # Ensure this room is accessible to the requester (and not just
+                # the homeserver).
+                if room_entry and not await self._is_remote_room_accessible(
+                    requester, queue_entry.room_id, room_entry.room
+                ):
+                    room_entry = None
+
+            # This room has been processed and should be ignored if it appears
+            # elsewhere in the hierarchy.
+            processed_rooms.add(room_id)
+
+            # There may or may not be a room entry based on whether it is
+            # inaccessible to the requesting user.
+            if room_entry:
+                # Add the room (including the stripped m.space.child events).
+                rooms_result.append(room_entry.as_json())
+
+                # If this room is not at the max-depth, check if there are any
+                # children to process.
+                if max_depth is None or current_depth < max_depth:
+                    # The children get added in reverse order so that the next
+                    # room to process, according to the ordering, is the last
+                    # item in the list.
+                    room_queue.extend(
+                        _RoomQueueEntry(
+                            ev["state_key"],
+                            ev["content"]["via"],
+                            current_depth + 1,
+                            children_room_entries.get(ev["state_key"]),
+                        )
+                        for ev in reversed(room_entry.children_state_events)
+                        if ev["type"] == EventTypes.SpaceChild
+                        and ev["state_key"] not in inaccessible_children
+                    )
+
+        result: JsonDict = {"rooms": rooms_result}
+
+        # If there's additional data, generate a pagination token (and persist state).
+        if room_queue:
+            next_batch = random_string(24)
+            result["next_batch"] = next_batch
+            pagination_key = _PaginationKey(
+                requested_room_id, suggested_only, max_depth, next_batch
+            )
+            self._pagination_sessions[pagination_key] = _PaginationSession(
+                self._clock.time_msec(), room_queue, processed_rooms
+            )
+
+        return result
+
+    async def federation_space_summary(
+        self,
+        origin: str,
+        room_id: str,
+        suggested_only: bool,
+        max_rooms_per_space: Optional[int],
+        exclude_rooms: Iterable[str],
+    ) -> JsonDict:
+        """
+        Implementation of the space summary Federation API
+
+        Args:
+            origin: The server requesting the spaces summary.
+
+            room_id: room id to start the summary at
+
+            suggested_only: whether we should only return children with the "suggested"
+                flag set.
+
+            max_rooms_per_space: an optional limit on the number of child rooms we will
+                return. Unlike the C-S API, this applies to the root room (room_id).
+                It is clipped to MAX_ROOMS_PER_SPACE.
+
+            exclude_rooms: a list of rooms to skip over (presumably because the
+                calling server has already seen them).
+
+        Returns:
+            summary dict to return
+        """
+        # the queue of rooms to process
+        room_queue = deque((room_id,))
+
+        # the set of rooms that we should not walk further. Initialise it with the
+        # excluded-rooms list; we will add other rooms as we process them so that
+        # we do not loop.
+        processed_rooms: Set[str] = set(exclude_rooms)
+
+        rooms_result: List[JsonDict] = []
+        events_result: List[JsonDict] = []
+
+        while room_queue and len(rooms_result) < MAX_ROOMS:
+            room_id = room_queue.popleft()
+            if room_id in processed_rooms:
+                # already done this room
+                continue
+
+            room_entry = await self._summarize_local_room(
+                None, origin, room_id, suggested_only, max_rooms_per_space
+            )
+
+            processed_rooms.add(room_id)
+
+            if room_entry:
+                rooms_result.append(room_entry.room)
+                events_result.extend(room_entry.children_state_events)
+
+                # add any children to the queue
+                room_queue.extend(
+                    edge_event["state_key"]
+                    for edge_event in room_entry.children_state_events
+                )
+
+        return {"rooms": rooms_result, "events": events_result}
+
+    async def get_federation_hierarchy(
+        self,
+        origin: str,
+        requested_room_id: str,
+        suggested_only: bool,
+    ):
+        """
+        Implementation of the room hierarchy Federation API.
+
+        This is similar to get_room_hierarchy, but does not recurse into the space.
+        It also considers whether anyone on the server may be able to access the
+        room, as opposed to whether a specific user can.
+
+        Args:
+            origin: The server requesting the spaces summary.
+            requested_room_id: The room ID to start the hierarchy at (the "root" room).
+            suggested_only: whether we should only return children with the "suggested"
+                flag set.
+
+        Returns:
+            The JSON hierarchy dictionary.
+        """
+        root_room_entry = await self._summarize_local_room(
+            None, origin, requested_room_id, suggested_only, max_children=None
+        )
+        if root_room_entry is None:
+            # Room is inaccessible to the requesting server.
+            raise SynapseError(404, "Unknown room: %s" % (requested_room_id,))
+
+        children_rooms_result: List[JsonDict] = []
+        inaccessible_children: List[str] = []
+
+        # Iterate through each child and potentially add it, but not its children,
+        # to the response.
+        for child_room in root_room_entry.children_state_events:
+            room_id = child_room.get("state_key")
+            assert isinstance(room_id, str)
+            # If the room is unknown, skip it.
+            if not await self._store.is_host_joined(room_id, self._server_name):
+                continue
+
+            room_entry = await self._summarize_local_room(
+                None, origin, room_id, suggested_only, max_children=0
+            )
+            # If the room is accessible, include it in the results.
+            #
+            # Note that only the room summary (without information on children)
+            # is included in the summary.
+            if room_entry:
+                children_rooms_result.append(room_entry.room)
+            #  Otherwise, note that the requesting server shouldn't bother
+            #  trying to summarize this room - they do not have access to it.
+            else:
+                inaccessible_children.append(room_id)
+
+        return {
+            # Include the requested room (including the stripped children events).
+            "room": root_room_entry.as_json(),
+            "children": children_rooms_result,
+            "inaccessible_children": inaccessible_children,
+        }
+
+    async def _summarize_local_room(
+        self,
+        requester: Optional[str],
+        origin: Optional[str],
+        room_id: str,
+        suggested_only: bool,
+        max_children: Optional[int],
+    ) -> Optional["_RoomEntry"]:
+        """
+        Generate a room entry and a list of event entries for a given room.
+
+        Args:
+            requester:
+                The user requesting the summary, if it is a local request. None
+                if this is a federation request.
+            origin:
+                The server requesting the summary, if it is a federation request.
+                None if this is a local request.
+            room_id: The room ID to summarize.
+            suggested_only: True if only suggested children should be returned.
+                Otherwise, all children are returned.
+            max_children:
+                The maximum number of children rooms to include. This is capped
+                to a server-set limit.
+
+        Returns:
+            A room entry if the room should be returned. None, otherwise.
+        """
+        if not await self._is_local_room_accessible(room_id, requester, origin):
+            return None
+
+        room_entry = await self._build_room_entry(room_id, for_federation=bool(origin))
+
+        # If the room is not a space or the children don't matter, return just
+        # the room information.
+        if room_entry.get("room_type") != RoomTypes.SPACE or max_children == 0:
+            return _RoomEntry(room_id, room_entry)
+
+        # Otherwise, look for child rooms/spaces.
+        child_events = await self._get_child_events(room_id)
+
+        if suggested_only:
+            # we only care about suggested children
+            child_events = filter(_is_suggested_child_event, child_events)
+
+        if max_children is None or max_children > MAX_ROOMS_PER_SPACE:
+            max_children = MAX_ROOMS_PER_SPACE
+
+        now = self._clock.time_msec()
+        events_result: List[JsonDict] = []
+        for edge_event in itertools.islice(child_events, max_children):
+            events_result.append(
+                await self._event_serializer.serialize_event(
+                    edge_event,
+                    time_now=now,
+                    event_format=format_event_for_client_v2,
+                )
+            )
+
+        return _RoomEntry(room_id, room_entry, events_result)
+
+    async def _summarize_remote_room(
+        self,
+        room: "_RoomQueueEntry",
+        suggested_only: bool,
+        max_children: Optional[int],
+        exclude_rooms: Iterable[str],
+    ) -> Iterable["_RoomEntry"]:
+        """
+        Request room entries and a list of event entries for a given room by querying a remote server.
+
+        Args:
+            room: The room to summarize.
+            suggested_only: True if only suggested children should be returned.
+                Otherwise, all children are returned.
+            max_children:
+                The maximum number of children rooms to include. This is capped
+                to a server-set limit.
+            exclude_rooms:
+                Rooms IDs which do not need to be summarized.
+
+        Returns:
+            An iterable of room entries.
+        """
+        room_id = room.room_id
+        logger.info("Requesting summary for %s via %s", room_id, room.via)
+
+        # we need to make the exclusion list json-serialisable
+        exclude_rooms = list(exclude_rooms)
+
+        via = itertools.islice(room.via, MAX_SERVERS_PER_SPACE)
+        try:
+            res = await self._federation_client.get_space_summary(
+                via,
+                room_id,
+                suggested_only=suggested_only,
+                max_rooms_per_space=max_children,
+                exclude_rooms=exclude_rooms,
+            )
+        except Exception as e:
+            logger.warning(
+                "Unable to get summary of %s via federation: %s",
+                room_id,
+                e,
+                exc_info=logger.isEnabledFor(logging.DEBUG),
+            )
+            return ()
+
+        # Group the events by their room.
+        children_by_room: Dict[str, List[JsonDict]] = {}
+        for ev in res.events:
+            if ev.event_type == EventTypes.SpaceChild:
+                children_by_room.setdefault(ev.room_id, []).append(ev.data)
+
+        # Generate the final results.
+        results = []
+        for fed_room in res.rooms:
+            fed_room_id = fed_room.get("room_id")
+            if not fed_room_id or not isinstance(fed_room_id, str):
+                continue
+
+            results.append(
+                _RoomEntry(
+                    fed_room_id,
+                    fed_room,
+                    children_by_room.get(fed_room_id, []),
+                )
+            )
+
+        return results
+
+    async def _summarize_remote_room_hierarchy(
+        self, room: "_RoomQueueEntry", suggested_only: bool
+    ) -> Tuple[Optional["_RoomEntry"], Dict[str, JsonDict], Set[str]]:
+        """
+        Request room entries and a list of event entries for a given room by querying a remote server.
+
+        Args:
+            room: The room to summarize.
+            suggested_only: True if only suggested children should be returned.
+                Otherwise, all children are returned.
+
+        Returns:
+            A tuple of:
+                The room entry.
+                Partial room data return over federation.
+                A set of inaccessible children room IDs.
+        """
+        room_id = room.room_id
+        logger.info("Requesting summary for %s via %s", room_id, room.via)
+
+        via = itertools.islice(room.via, MAX_SERVERS_PER_SPACE)
+        try:
+            (
+                room_response,
+                children,
+                inaccessible_children,
+            ) = await self._federation_client.get_room_hierarchy(
+                via,
+                room_id,
+                suggested_only=suggested_only,
+            )
+        except Exception as e:
+            logger.warning(
+                "Unable to get hierarchy of %s via federation: %s",
+                room_id,
+                e,
+                exc_info=logger.isEnabledFor(logging.DEBUG),
+            )
+            return None, {}, set()
+
+        # Map the children to their room ID.
+        children_by_room_id = {
+            c["room_id"]: c
+            for c in children
+            if "room_id" in c and isinstance(c["room_id"], str)
+        }
+
+        return (
+            _RoomEntry(room_id, room_response, room_response.pop("children_state", ())),
+            children_by_room_id,
+            set(inaccessible_children),
+        )
+
+    async def _is_local_room_accessible(
+        self, room_id: str, requester: Optional[str], origin: Optional[str] = None
+    ) -> bool:
+        """
+        Calculate whether the room should be shown to the requester.
+
+        It should return true if:
+
+        * The requester is joined or can join the room (per MSC3173).
+        * The origin server has any user that is joined or can join the room.
+        * The history visibility is set to world readable.
+
+        Args:
+            room_id: The room ID to check accessibility of.
+            requester:
+                The user making the request, if it is a local request.
+                None if this is a federation request.
+            origin:
+                The server making the request, if it is a federation request.
+                None if this is a local request.
+
+        Returns:
+             True if the room is accessible to the requesting user or server.
+        """
+        state_ids = await self._store.get_current_state_ids(room_id)
+
+        # If there's no state for the room, it isn't known.
+        if not state_ids:
+            # The user might have a pending invite for the room.
+            if requester and await self._store.get_invite_for_local_user_in_room(
+                requester, room_id
+            ):
+                return True
+
+            logger.info("room %s is unknown, omitting from summary", room_id)
+            return False
+
+        room_version = await self._store.get_room_version(room_id)
+
+        # Include the room if it has join rules of public or knock.
+        join_rules_event_id = state_ids.get((EventTypes.JoinRules, ""))
+        if join_rules_event_id:
+            join_rules_event = await self._store.get_event(join_rules_event_id)
+            join_rule = join_rules_event.content.get("join_rule")
+            if join_rule == JoinRules.PUBLIC or (
+                room_version.msc2403_knocking and join_rule == JoinRules.KNOCK
+            ):
+                return True
+
+        # Include the room if it is peekable.
+        hist_vis_event_id = state_ids.get((EventTypes.RoomHistoryVisibility, ""))
+        if hist_vis_event_id:
+            hist_vis_ev = await self._store.get_event(hist_vis_event_id)
+            hist_vis = hist_vis_ev.content.get("history_visibility")
+            if hist_vis == HistoryVisibility.WORLD_READABLE:
+                return True
+
+        # Otherwise we need to check information specific to the user or server.
+
+        # If we have an authenticated requesting user, check if they are a member
+        # of the room (or can join the room).
+        if requester:
+            member_event_id = state_ids.get((EventTypes.Member, requester), None)
+
+            # If they're in the room they can see info on it.
+            if member_event_id:
+                member_event = await self._store.get_event(member_event_id)
+                if member_event.membership in (Membership.JOIN, Membership.INVITE):
+                    return True
+
+            # Otherwise, check if they should be allowed access via membership in a space.
+            if await self._event_auth_handler.has_restricted_join_rules(
+                state_ids, room_version
+            ):
+                allowed_rooms = (
+                    await self._event_auth_handler.get_rooms_that_allow_join(state_ids)
+                )
+                if await self._event_auth_handler.is_user_in_rooms(
+                    allowed_rooms, requester
+                ):
+                    return True
+
+        # If this is a request over federation, check if the host is in the room or
+        # has a user who could join the room.
+        elif origin:
+            if await self._event_auth_handler.check_host_in_room(
+                room_id, origin
+            ) or await self._store.is_host_invited(room_id, origin):
+                return True
+
+            # Alternately, if the host has a user in any of the spaces specified
+            # for access, then the host can see this room (and should do filtering
+            # if the requester cannot see it).
+            if await self._event_auth_handler.has_restricted_join_rules(
+                state_ids, room_version
+            ):
+                allowed_rooms = (
+                    await self._event_auth_handler.get_rooms_that_allow_join(state_ids)
+                )
+                for space_id in allowed_rooms:
+                    if await self._event_auth_handler.check_host_in_room(
+                        space_id, origin
+                    ):
+                        return True
+
+        logger.info(
+            "room %s is unpeekable and requester %s is not a member / not allowed to join, omitting from summary",
+            room_id,
+            requester or origin,
+        )
+        return False
+
+    async def _is_remote_room_accessible(
+        self, requester: str, room_id: str, room: JsonDict
+    ) -> bool:
+        """
+        Calculate whether the room received over federation should be shown to the requester.
+
+        It should return true if:
+
+        * The requester is joined or can join the room (per MSC3173).
+        * The history visibility is set to world readable.
+
+        Note that the local server is not in the requested room (which is why the
+        remote call was made in the first place), but the user could have access
+        due to an invite, etc.
+
+        Args:
+            requester: The user requesting the summary.
+            room_id: The room ID returned over federation.
+            room: The summary of the room returned over federation.
+
+        Returns:
+            True if the room is accessible to the requesting user.
+        """
+        # The API doesn't return the room version so assume that a
+        # join rule of knock is valid.
+        if (
+            room.get("join_rules") in (JoinRules.PUBLIC, JoinRules.KNOCK)
+            or room.get("world_readable") is True
+        ):
+            return True
+
+        # Check if the user is a member of any of the allowed spaces
+        # from the response.
+        allowed_rooms = room.get("allowed_room_ids") or room.get("allowed_spaces")
+        if allowed_rooms and isinstance(allowed_rooms, list):
+            if await self._event_auth_handler.is_user_in_rooms(
+                allowed_rooms, requester
+            ):
+                return True
+
+        # Finally, check locally if we can access the room. The user might
+        # already be in the room (if it was a child room), or there might be a
+        # pending invite, etc.
+        return await self._is_local_room_accessible(room_id, requester)
+
+    async def _build_room_entry(self, room_id: str, for_federation: bool) -> JsonDict:
+        """
+        Generate en entry summarising a single room.
+
+        Args:
+            room_id: The room ID to summarize.
+            for_federation: True if this is a summary requested over federation
+                (which includes additional fields).
+
+        Returns:
+            The JSON dictionary for the room.
+        """
+        stats = await self._store.get_room_with_stats(room_id)
+
+        # currently this should be impossible because we call
+        # _is_local_room_accessible on the room before we get here, so
+        # there should always be an entry
+        assert stats is not None, "unable to retrieve stats for %s" % (room_id,)
+
+        current_state_ids = await self._store.get_current_state_ids(room_id)
+        create_event = await self._store.get_event(
+            current_state_ids[(EventTypes.Create, "")]
+        )
+
+        entry = {
+            "room_id": stats["room_id"],
+            "name": stats["name"],
+            "topic": stats["topic"],
+            "canonical_alias": stats["canonical_alias"],
+            "num_joined_members": stats["joined_members"],
+            "avatar_url": stats["avatar"],
+            "join_rules": stats["join_rules"],
+            "world_readable": (
+                stats["history_visibility"] == HistoryVisibility.WORLD_READABLE
+            ),
+            "guest_can_join": stats["guest_access"] == "can_join",
+            "creation_ts": create_event.origin_server_ts,
+            "room_type": create_event.content.get(EventContentFields.ROOM_TYPE),
+        }
+
+        # Federation requests need to provide additional information so the
+        # requested server is able to filter the response appropriately.
+        if for_federation:
+            room_version = await self._store.get_room_version(room_id)
+            if await self._event_auth_handler.has_restricted_join_rules(
+                current_state_ids, room_version
+            ):
+                allowed_rooms = (
+                    await self._event_auth_handler.get_rooms_that_allow_join(
+                        current_state_ids
+                    )
+                )
+                if allowed_rooms:
+                    entry["allowed_room_ids"] = allowed_rooms
+                    # TODO Remove this key once the API is stable.
+                    entry["allowed_spaces"] = allowed_rooms
+
+        # Filter out Nones – rather omit the field altogether
+        room_entry = {k: v for k, v in entry.items() if v is not None}
+
+        return room_entry
+
+    async def _get_child_events(self, room_id: str) -> Iterable[EventBase]:
+        """
+        Get the child events for a given room.
+
+        The returned results are sorted for stability.
+
+        Args:
+            room_id: The room id to get the children of.
+
+        Returns:
+            An iterable of sorted child events.
+        """
+
+        # look for child rooms/spaces.
+        current_state_ids = await self._store.get_current_state_ids(room_id)
+
+        events = await self._store.get_events_as_list(
+            [
+                event_id
+                for key, event_id in current_state_ids.items()
+                if key[0] == EventTypes.SpaceChild
+            ]
+        )
+
+        # filter out any events without a "via" (which implies it has been redacted),
+        # and order to ensure we return stable results.
+        return sorted(filter(_has_valid_via, events), key=_child_events_comparison_key)
+
+    async def get_room_summary(
+        self,
+        requester: Optional[str],
+        room_id: str,
+        remote_room_hosts: Optional[List[str]] = None,
+    ) -> JsonDict:
+        """
+        Implementation of the room summary C-S API from MSC3266
+
+        Args:
+            requester:  user id of the user making this request, will be None
+                for unauthenticated requests
+
+            room_id: room id to summarise.
+
+            remote_room_hosts: a list of homeservers to try fetching data through
+                if we don't know it ourselves
+
+        Returns:
+            summary dict to return
+        """
+        is_in_room = await self._store.is_host_joined(room_id, self._server_name)
+
+        if is_in_room:
+            room_entry = await self._summarize_local_room(
+                requester,
+                None,
+                room_id,
+                # Suggested-only doesn't matter since no children are requested.
+                suggested_only=False,
+                max_children=0,
+            )
+
+            if not room_entry:
+                raise NotFoundError("Room not found or is not accessible")
+
+            room_summary = room_entry.room
+
+            # If there was a requester, add their membership.
+            if requester:
+                (
+                    membership,
+                    _,
+                ) = await self._store.get_local_current_membership_for_user_in_room(
+                    requester, room_id
+                )
+
+                room_summary["membership"] = membership or "leave"
+        else:
+            # TODO federation API, descoped from initial unstable implementation
+            #      as MSC needs more maturing on that side.
+            raise SynapseError(400, "Federation is not currently supported.")
+
+        return room_summary
+
+
+@attr.s(frozen=True, slots=True, auto_attribs=True)
+class _RoomQueueEntry:
+    # The room ID of this entry.
+    room_id: str
+    # The server to query if the room is not known locally.
+    via: Sequence[str]
+    # The minimum number of hops necessary to get to this room (compared to the
+    # originally requested room).
+    depth: int = 0
+    # The room summary for this room returned via federation. This will only be
+    # used if the room is not known locally (and is not a space).
+    remote_room: Optional[JsonDict] = None
+
+
+@attr.s(frozen=True, slots=True, auto_attribs=True)
+class _RoomEntry:
+    room_id: str
+    # The room summary for this room.
+    room: JsonDict
+    # An iterable of the sorted, stripped children events for children of this room.
+    #
+    # This may not include all children.
+    children_state_events: Sequence[JsonDict] = ()
+
+    def as_json(self) -> JsonDict:
+        """
+        Returns a JSON dictionary suitable for the room hierarchy endpoint.
+
+        It returns the room summary including the stripped m.space.child events
+        as a sub-key.
+        """
+        result = dict(self.room)
+        result["children_state"] = self.children_state_events
+        return result
+
+
+def _has_valid_via(e: EventBase) -> bool:
+    via = e.content.get("via")
+    if not via or not isinstance(via, Sequence):
+        return False
+    for v in via:
+        if not isinstance(v, str):
+            logger.debug("Ignoring edge event %s with invalid via entry", e.event_id)
+            return False
+    return True
+
+
+def _is_suggested_child_event(edge_event: EventBase) -> bool:
+    suggested = edge_event.content.get("suggested")
+    if isinstance(suggested, bool) and suggested:
+        return True
+    logger.debug("Ignorning not-suggested child %s", edge_event.state_key)
+    return False
+
+
+# Order may only contain characters in the range of \x20 (space) to \x7E (~) inclusive.
+_INVALID_ORDER_CHARS_RE = re.compile(r"[^\x20-\x7E]")
+
+
+def _child_events_comparison_key(child: EventBase) -> Tuple[bool, Optional[str], str]:
+    """
+    Generate a value for comparing two child events for ordering.
+
+    The rules for ordering are supposed to be:
+
+    1. The 'order' key, if it is valid.
+    2. The 'origin_server_ts' of the 'm.room.create' event.
+    3. The 'room_id'.
+
+    But we skip step 2 since we may not have any state from the room.
+
+    Args:
+        child: The event for generating a comparison key.
+
+    Returns:
+        The comparison key as a tuple of:
+            False if the ordering is valid.
+            The ordering field.
+            The room ID.
+    """
+    order = child.content.get("order")
+    # If order is not a string or doesn't meet the requirements, ignore it.
+    if not isinstance(order, str):
+        order = None
+    elif len(order) > 50 or _INVALID_ORDER_CHARS_RE.search(order):
+        order = None
+
+    # Items without an order come last.
+    return (order is None, order, child.room_id)