diff --git a/synapse/handlers/event_auth.py b/synapse/handlers/event_auth.py
new file mode 100644
index 0000000000..06da1a93d9
--- /dev/null
+++ b/synapse/handlers/event_auth.py
@@ -0,0 +1,82 @@
+# 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.
+from typing import TYPE_CHECKING
+
+from synapse.api.constants import EventTypes, JoinRules
+from synapse.api.room_versions import RoomVersion
+from synapse.types import StateMap
+
+if TYPE_CHECKING:
+ from synapse.server import HomeServer
+
+
+class EventAuthHandler:
+ def __init__(self, hs: "HomeServer"):
+ self._store = hs.get_datastore()
+
+ async def can_join_without_invite(
+ self, state_ids: StateMap[str], room_version: RoomVersion, user_id: str
+ ) -> bool:
+ """
+ Check whether a user can join a room without an invite.
+
+ When joining a room with restricted joined rules (as defined in MSC3083),
+ the membership of spaces must be checked during join.
+
+ Args:
+ state_ids: The state of the room as it currently is.
+ room_version: The room version of the room being joined.
+ user_id: The user joining the room.
+
+ Returns:
+ True if the user can join the room, false otherwise.
+ """
+ # This only applies to room versions which support the new join rule.
+ if not room_version.msc3083_join_rules:
+ return True
+
+ # If there's no join rule, then it defaults to public (so this doesn't apply).
+ join_rules_event_id = state_ids.get((EventTypes.JoinRules, ""), None)
+ if not join_rules_event_id:
+ return True
+
+ # If the join rule is not restricted, this doesn't apply.
+ join_rules_event = await self._store.get_event(join_rules_event_id)
+ if join_rules_event.content.get("join_rule") != JoinRules.MSC3083_RESTRICTED:
+ return True
+
+ # If allowed is of the wrong form, then only allow invited users.
+ allowed_spaces = join_rules_event.content.get("allow", [])
+ if not isinstance(allowed_spaces, list):
+ return False
+
+ # Get the list of joined rooms and see if there's an overlap.
+ joined_rooms = await self._store.get_rooms_for_user(user_id)
+
+ # Pull out the other room IDs, invalid data gets filtered.
+ for space in allowed_spaces:
+ if not isinstance(space, dict):
+ continue
+
+ space_id = space.get("space")
+ if not isinstance(space_id, str):
+ continue
+
+ # The user was joined to one of the spaces specified, they can join
+ # this room!
+ if space_id in joined_rooms:
+ return True
+
+ # The user was not in any of the required spaces.
+ return False
|
