Add an admin API to get the current room state (#9168)

This could arguably replace the existing admin API for `/members`, however that is out of scope of this change.

This sort of endpoint is ideal for moderation use cases as well as other applications, such as needing to retrieve various bits of information about a room to perform a task (like syncing power levels between two places). This endpoint exposes nothing more than an admin would be able to access with a `select *` query on their database.

synapse/rest/admin/rooms.py

@@ -292,6 +292,45 @@ class RoomMembersRestServlet(RestServlet):
         return 200, ret
 
 
+class RoomStateRestServlet(RestServlet):
+    """
+    Get full state within a room.
+    """
+
+    PATTERNS = admin_patterns("/rooms/(?P<room_id>[^/]+)/state")
+
+    def __init__(self, hs: "HomeServer"):
+        self.hs = hs
+        self.auth = hs.get_auth()
+        self.store = hs.get_datastore()
+        self.clock = hs.get_clock()
+        self._event_serializer = hs.get_event_client_serializer()
+
+    async def on_GET(
+        self, request: SynapseRequest, room_id: str
+    ) -> Tuple[int, JsonDict]:
+        requester = await self.auth.get_user_by_req(request)
+        await assert_user_is_admin(self.auth, requester.user)
+
+        ret = await self.store.get_room(room_id)
+        if not ret:
+            raise NotFoundError("Room not found")
+
+        event_ids = await self.store.get_current_state_ids(room_id)
+        events = await self.store.get_events(event_ids.values())
+        now = self.clock.time_msec()
+        room_state = await self._event_serializer.serialize_events(
+            events.values(),
+            now,
+            # We don't bother bundling aggregations in when asked for state
+            # events, as clients won't use them.
+            bundle_aggregations=False,
+        )
+        ret = {"state": room_state}
+
+        return 200, ret
+
+
 class JoinRoomAliasServlet(RestServlet):
 
     PATTERNS = admin_patterns("/join/(?P<room_identifier>[^/]*)")