Add forward extremities endpoint to rooms admin API

GET /_synapse/admin/v1/rooms/<identifier>/forward_extremities now gets forward extremities for a room, returning count and the list of extremities.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

synapse/rest/admin/rooms.py

@@ -499,3 +499,56 @@ class MakeRoomAdminRestServlet(RestServlet):
         )
 
         return 200, {}
+
+
+class ForwardExtremitiesRestServlet(RestServlet):
+    """Allows a server admin to get or clear forward extremities.
+
+    Clearing does not require restarting the server.
+
+        Clear forward extremities:
+        DELETE /_synapse/admin/v1/rooms/<room_id_or_alias>/forward_extremities
+
+        Get forward_extremities:
+        GET /_synapse/admin/v1/rooms/<room_id_or_alias>/forward_extremities
+    """
+
+    PATTERNS = admin_patterns("/rooms/(?P<room_identifier>[^/]*)/forward_extremities")
+
+    def __init__(self, hs: "HomeServer"):
+        self.hs = hs
+        self.auth = hs.get_auth()
+        self.room_member_handler = hs.get_room_member_handler()
+        self.store = hs.get_datastore()
+
+    async def resolve_room_id(self, room_identifier: str) -> str:
+        """Resolve to a room ID, if necessary."""
+        if RoomID.is_valid(room_identifier):
+            return room_identifier
+        elif RoomAlias.is_valid(room_identifier):
+            room_alias = RoomAlias.from_string(room_identifier)
+            room_id, _ = await self.room_member_handler.lookup_room_alias(room_alias)
+            return room_id.to_string()
+        raise SynapseError(
+            400, "%s was not legal room ID or room alias" % (room_identifier,)
+        )
+
+    async def on_DELETE(self, request, room_identifier):
+        requester = await self.auth.get_user_by_req(request)
+        await assert_user_is_admin(self.auth, requester.user)
+
+        room_id = await self.resolve_room_id(room_identifier)
+
+    async def on_GET(self, request, room_identifier):
+        requester = await self.auth.get_user_by_req(request)
+        await assert_user_is_admin(self.auth, requester.user)
+
+        room_id = await self.resolve_room_id(room_identifier)
+        if not room_id:
+            raise SynapseError(400, "Unknown room ID or room alias %s" % room_identifier)
+
+        extremities = await self.store.get_forward_extremities_for_room(room_id)
+        return 200, {
+            "count": len(extremities),
+            "results": extremities,
+        }