diff --git a/synapse/handlers/space_summary.py b/synapse/handlers/space_summary.py
new file mode 100644
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--- /dev/null
+++ b/synapse/handlers/space_summary.py
@@ -0,0 +1,199 @@
+# -*- coding: utf-8 -*-
+# 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
+from collections import deque
+from typing import TYPE_CHECKING, Iterable, List, Optional, Set
+
+from synapse.api.constants import EventContentFields, EventTypes, HistoryVisibility
+from synapse.api.errors import AuthError
+from synapse.events import EventBase
+from synapse.events.utils import format_event_for_client_v2
+from synapse.types import JsonDict
+
+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.
+# TODO: allow clients to reduce this with a request param.
+MAX_ROOMS = 50
+
+# max number of events to return per room.
+MAX_ROOMS_PER_SPACE = 50
+
+
+class SpaceSummaryHandler:
+ def __init__(self, hs: "HomeServer"):
+ self._clock = hs.get_clock()
+ self._auth = hs.get_auth()
+ self._room_list_handler = hs.get_room_list_handler()
+ self._state_handler = hs.get_state_handler()
+ self._store = hs.get_datastore()
+ self._event_serializer = hs.get_event_client_serializer()
+
+ 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 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 ROOMS_PER_SPACE_LIMIT.
+
+ Returns:
+ summary dict to return
+ """
+ # first of all, check that the user is in the room in question (or it's
+ # world-readable)
+ await self._auth.check_user_in_room_or_world_readable(room_id, requester)
+
+ # the queue of rooms to process
+ room_queue = deque((room_id,))
+
+ processed_rooms = set() # type: Set[str]
+
+ rooms_result = [] # type: List[JsonDict]
+ events_result = [] # type: List[JsonDict]
+
+ now = self._clock.time_msec()
+
+ while room_queue and len(rooms_result) < MAX_ROOMS:
+ room_id = room_queue.popleft()
+ logger.debug("Processing room %s", room_id)
+ processed_rooms.add(room_id)
+
+ try:
+ await self._auth.check_user_in_room_or_world_readable(
+ room_id, requester
+ )
+ except AuthError:
+ logger.info(
+ "user %s cannot view room %s, omitting from summary",
+ requester,
+ room_id,
+ )
+ continue
+
+ room_entry = await self._build_room_entry(room_id)
+ rooms_result.append(room_entry)
+
+ # 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)
+
+ # 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. Otherwise, apply any client-specified
+ # limit, capping to our built-in limit.
+ if max_rooms_per_space is not None and len(processed_rooms) > 1:
+ max_rooms = min(MAX_ROOMS_PER_SPACE, max_rooms_per_space)
+ else:
+ max_rooms = MAX_ROOMS_PER_SPACE
+
+ for edge_event in itertools.islice(child_events, max_rooms):
+ edge_room_id = edge_event.state_key
+
+ events_result.append(
+ await self._event_serializer.serialize_event(
+ edge_event,
+ time_now=now,
+ event_format=format_event_for_client_v2,
+ )
+ )
+
+ # if we haven't yet visited the target of this link, add it to the queue
+ if edge_room_id not in processed_rooms:
+ room_queue.append(edge_room_id)
+
+ return {"rooms": rooms_result, "events": events_result}
+
+ async def _build_room_entry(self, room_id: str) -> JsonDict:
+ """Generate en entry suitable for the 'rooms' list in the summary response"""
+ stats = await self._store.get_room_with_stats(room_id)
+
+ # currently this should be impossible because we call
+ # check_user_in_room_or_world_readable 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, "")]
+ )
+
+ # TODO: update once MSC1772 lands
+ room_type = create_event.content.get(EventContentFields.MSC1772_ROOM_TYPE)
+
+ 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"],
+ "world_readable": (
+ stats["history_visibility"] == HistoryVisibility.WORLD_READABLE
+ ),
+ "guest_can_join": stats["guest_access"] == "can_join",
+ "room_type": room_type,
+ }
+
+ # 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]:
+ # 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()
+ # TODO: update once MSC1772 lands
+ if key[0] == EventTypes.MSC1772_SPACE_CHILD
+ ]
+ )
+
+ # filter out any events without a "via" (which implies it has been redacted)
+ return (e for e in events if e.content.get("via"))
+
+
+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
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