Check for space membership during a remote join of a restricted room. (#9763)

When receiving a /send_join request for a room with join rules set to 'restricted',
check if the user is a member of the spaces defined in the 'allow' key of the join
rules.
    
This only applies to an experimental room version, as defined in MSC3083.

Author: clokep

changelog.d/9763.feature

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+Update experimental support for [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083): restricting room access via group membership.

changelog.d/9800.feature

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+Update experimental support for [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083): restricting room access via group membership.

synapse/handlers/event_auth.py

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+# 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