Allow admins to proactively block rooms (#11228)

Co-authored-by: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

changelog.d/11228.feature

@@ -0,0 +1 @@
+Allow the admin [Delete Room API](https://matrix-org.github.io/synapse/latest/admin_api/rooms.html#delete-room-api) to block a room without the need to join it.

docs/admin_api/rooms.md

@@ -396,13 +396,17 @@ The new room will be created with the user specified by the `new_room_user_id` p
 as room administrator and will contain a message explaining what happened. Users invited
 to the new room will have power level `-10` by default, and thus be unable to speak.
 
-If `block` is `True` it prevents new joins to the old room.
+If `block` is `true`, users will be prevented from joining the old room.
+This option can also be used to pre-emptively block a room, even if it's unknown
+to this homeserver. In this case, the room will be blocked, and no further action
+will be taken. If `block` is `false`, attempting to delete an unknown room is
+invalid and will be rejected as a bad request.
 
 This API will remove all trace of the old room from your database after removing
 all local users. If `purge` is `true` (the default), all traces of the old room will
 be removed from your database after removing all local users. If you do not want
 this to happen, set `purge` to `false`.
-Depending on the amount of history being purged a call to the API may take
+Depending on the amount of history being purged, a call to the API may take
 several minutes or longer.
 
 The local server will only have the power to move local user and room aliases to
@@ -464,8 +468,9 @@ The following JSON body parameters are available:
               `new_room_user_id` in the new room. Ideally this will clearly convey why the
                original room was shut down. Defaults to `Sharing illegal content on this server
                is not permitted and rooms in violation will be blocked.`
-* `block` - Optional. If set to `true`, this room will be added to a blocking list, preventing
-            future attempts to join the room. Defaults to `false`.
+* `block` - Optional. If set to `true`, this room will be added to a blocking list,
+            preventing future attempts to join the room. Rooms can be blocked
+            even if they're not yet known to the homeserver. Defaults to `false`.
 * `purge` - Optional. If set to `true`, it will remove all traces of the room from your database.
             Defaults to `true`.
 * `force_purge` - Optional, and ignored unless `purge` is `true`. If set to `true`, it
@@ -483,7 +488,8 @@ The following fields are returned in the JSON response body:
 * `failed_to_kick_users` - An array of users (`user_id`) that that were not kicked.
 * `local_aliases` - An array of strings representing the local aliases that were migrated from
                     the old room to the new.
-* `new_room_id` - A string representing the room ID of the new room.
+* `new_room_id` - A string representing the room ID of the new room, or `null` if
+                  no such room was created.
 
 
 ## Undoing room deletions

synapse/handlers/room.py

@@ -12,8 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-"""Contains functions for performing events on rooms."""
-
+"""Contains functions for performing actions on rooms."""
 import itertools
 import logging
 import math
@@ -31,6 +30,8 @@
     Tuple,
 )
 
+from typing_extensions import TypedDict
+
 from synapse.api.constants import (
     EventContentFields,
     EventTypes,
@@ -1277,6 +1278,13 @@ def get_current_key_for_room(self, room_id: str) -> Awaitable[str]:
         return self.store.get_room_events_max_id(room_id)
 
 
+class ShutdownRoomResponse(TypedDict):
+    kicked_users: List[str]
+    failed_to_kick_users: List[str]
+    local_aliases: List[str]
+    new_room_id: Optional[str]
+
+
 class RoomShutdownHandler:
 
     DEFAULT_MESSAGE = (
@@ -1302,7 +1310,7 @@ async def shutdown_room(
         new_room_name: Optional[str] = None,
         message: Optional[str] = None,
         block: bool = False,
-    ) -> dict:
+    ) -> ShutdownRoomResponse:
         """
         Shuts down a room. Moves all local users and room aliases automatically
         to a new room if `new_room_user_id` is set. Otherwise local users only
@@ -1336,8 +1344,13 @@ async def shutdown_room(
                 Defaults to `Sharing illegal content on this server is not
                 permitted and rooms in violation will be blocked.`
             block:
-                If set to `true`, this room will be added to a blocking list,
-                preventing future attempts to join the room. Defaults to `false`.
+                If set to `True`, users will be prevented from joining the old
+                room. This option can also be used to pre-emptively block a room,
+                even if it's unknown to this homeserver. In this case, the room
+                will be blocked, and no further action will be taken. If `False`,
+                attempting to delete an unknown room is invalid.
+
+                Defaults to `False`.
 
         Returns: a dict containing the following keys:
             kicked_users: An array of users (`user_id`) that were kicked.
@@ -1346,7 +1359,9 @@ async def shutdown_room(
             local_aliases:
                 An array of strings representing the local aliases that were
                 migrated from the old room to the new.
-            new_room_id: A string representing the room ID of the new room.
+            new_room_id:
+                A string representing the room ID of the new room, or None if
+                no such room was created.
         """
 
         if not new_room_name:
@@ -1357,14 +1372,28 @@ async def shutdown_room(
         if not RoomID.is_valid(room_id):
             raise SynapseError(400, "%s is not a legal room ID" % (room_id,))
 
-        if not await self.store.get_room(room_id):
-            raise NotFoundError("Unknown room id %s" % (room_id,))
-
-        # This will work even if the room is already blocked, but that is
-        # desirable in case the first attempt at blocking the room failed below.
+        # Action the block first (even if the room doesn't exist yet)
         if block:
+            # This will work even if the room is already blocked, but that is
+            # desirable in case the first attempt at blocking the room failed below.
             await self.store.block_room(room_id, requester_user_id)
 
+        if not await self.store.get_room(room_id):
+            if block:
+                # We allow you to block an unknown room.
+                return {
+                    "kicked_users": [],
+                    "failed_to_kick_users": [],
+                    "local_aliases": [],
+                    "new_room_id": None,
+                }
+            else:
+                # But if you don't want to preventatively block another room,
+                # this function can't do anything useful.
+                raise NotFoundError(
+                    "Cannot shut down room: unknown room id %s" % (room_id,)
+                )
+
         if new_room_user_id is not None:
             if not self.hs.is_mine_id(new_room_user_id):
                 raise SynapseError(

synapse/rest/admin/rooms.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 import logging
 from http import HTTPStatus
-from typing import TYPE_CHECKING, List, Optional, Tuple
+from typing import TYPE_CHECKING, List, Optional, Tuple, cast
 from urllib import parse as urlparse
 
 from synapse.api.constants import EventTypes, JoinRules, Membership
@@ -239,9 +239,22 @@ async def _delete_room(
 
         # Purge room
         if purge:
-            await pagination_handler.purge_room(room_id, force=force_purge)
-
-        return 200, ret
+            try:
+                await pagination_handler.purge_room(room_id, force=force_purge)
+            except NotFoundError:
+                if block:
+                    # We can block unknown rooms with this endpoint, in which case
+                    # a failed purge is expected.
+                    pass
+                else:
+                    # But otherwise, we expect this purge to have succeeded.
+                    raise
+
+        # Cast safety: cast away the knowledge that this is a TypedDict.
+        # See https://github.com/python/mypy/issues/4976#issuecomment-579883622
+        # for some discussion on why this is necessary. Either way,
+        # `ret` is an opaque dictionary blob as far as the rest of the app cares.
+        return 200, cast(JsonDict, ret)
 
 
 class RoomMembersRestServlet(RestServlet):

synapse/storage/databases/main/room.py

@@ -1751,7 +1751,12 @@ def _get_event_reports_paginate_txn(txn):
         )
 
     async def block_room(self, room_id: str, user_id: str) -> None:
-        """Marks the room as blocked. Can be called multiple times.
+        """Marks the room as blocked.
+
+        Can be called multiple times (though we'll only track the last user to
+        block this room).
+
+        Can be called on a room unknown to this homeserver.
 
         Args:
             room_id: Room to block

tests/rest/admin/test_room.py

@@ -14,9 +14,12 @@
 
 import json
 import urllib.parse
+from http import HTTPStatus
 from typing import List, Optional
 from unittest.mock import Mock
 
+from parameterized import parameterized
+
 import synapse.rest.admin
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import Codes
@@ -281,6 +284,31 @@ def test_block_room_and_not_purge(self):
         self._is_blocked(self.room_id, expect=True)
         self._has_no_members(self.room_id)
 
+    @parameterized.expand([(True,), (False,)])
+    def test_block_unknown_room(self, purge: bool) -> None:
+        """
+        We can block an unknown room. In this case, the `purge` argument
+        should be ignored.
+        """
+        room_id = "!unknown:test"
+
+        # The room isn't already in the blocked rooms table
+        self._is_blocked(room_id, expect=False)
+
+        # Request the room be blocked.
+        channel = self.make_request(
+            "DELETE",
+            f"/_synapse/admin/v1/rooms/{room_id}",
+            {"block": True, "purge": purge},
+            access_token=self.admin_user_tok,
+        )
+
+        # The room is now blocked.
+        self.assertEqual(
+            HTTPStatus.OK, int(channel.result["code"]), msg=channel.result["body"]
+        )
+        self._is_blocked(room_id)
+
     def test_shutdown_room_consent(self):
         """Test that we can shutdown rooms with local users who have not
         yet accepted the privacy policy. This used to fail when we tried to