Skip processing policy server events through policy server (#18605)

Co-authored-by: Andrew Morgan <andrew@amorgan.xyz>

Author: turt2live

changelog.d/18605.bugfix

@@ -0,0 +1 @@
+Ensure policy servers are not asked to scan policy server change events, allowing rooms to disable the use of a policy server while the policy server is down.

synapse/handlers/room_policy.py

@@ -54,6 +54,9 @@ async def is_event_allowed(self, event: EventBase) -> bool:
         Returns:
             bool: True if the event is allowed in the room, False otherwise.
         """
+        if event.type == "org.matrix.msc4284.policy" and event.state_key is not None:
+            return True  # always allow policy server change events
+
         policy_event = await self._storage_controllers.state.get_current_state_event(
             event.room_id, "org.matrix.msc4284.policy", ""
         )