Validation for events/rooms in fed requests

When we get a federation request which refers to an event id, make sure that
said event is in the room the caller claims it is in.

(patch supplied by @turt2live)

synapse/storage/event_federation.py

@@ -295,6 +295,35 @@ class EventFederationWorkerStore(EventsWorkerStore, SignatureWorkerStore,
             get_forward_extremeties_for_room_txn
         )
 
+    def get_room_ids_for_events(self, event_ids):
+        """Get a list of room IDs for which the given events belong.
+
+        Args:
+            event_ids (list): the events to look up the room of
+
+        Returns:
+            list, the room IDs for the events
+        """
+        return self.runInteraction(
+            "get_room_ids_for_events",
+            self._get_room_ids_for_events, event_ids
+        )
+
+    def _get_room_ids_for_events(self, txn, event_ids):
+        logger.debug("_get_room_ids_for_events: %s", repr(event_ids))
+
+        base_sql = (
+            "SELECT DISTINCT room_id FROM events"
+            " WHERE event_id IN (%s)"
+        )
+
+        txn.execute(
+            base_sql % (",".join(["?"] * len(event_ids)),),
+            event_ids
+        )
+
+        return [r[0] for r in txn]
+
     def get_backfill_events(self, room_id, event_list, limit):
         """Get a list of Events for a given topic that occurred before (and
         including) the events in event_list. Return a list of max size `limit`