Rip out auth-event reconciliation code

synapse/handlers/federation_event.py

@@ -1528,13 +1528,36 @@ async def _check_event_auth(
         # https://spec.matrix.org/v1.3/server-server-api/#checks-performed-on-receipt-of-a-pdu:
         #   5. Passes authorization rules based on the state before the event,
         #      otherwise it is rejected.
-        event_types = event_auth.auth_types_for_event(event.room_version, event)
-        prev_state_ids = await context.get_prev_state_ids(
-            StateFilter.from_types(event_types)
-        )
+        #
+        # ... however, if we only have partial state for the room, then there is a good
+        # chance that we'll be missing some of the state needed to auth the new event.
+        # So, we state-resolve the auth event that we are given against the state that
+        # we know about, which ensures things like bans are applied.
+        if context.partial_state:
+            room_version = await self._store.get_room_version_id(event.room_id)
+
+            local_state_id_map = await context.get_prev_state_ids()
+            claimed_auth_events_id_map = {
+                (ev.type, ev.state_key): ev.event_id for ev in claimed_auth_events
+            }
+
+            state_for_auth_id_map = (
+                await self._state_resolution_handler.resolve_events_with_store(
+                    event.room_id,
+                    room_version,
+                    [local_state_id_map, claimed_auth_events_id_map],
+                    event_map=None,
+                    state_res_store=StateResolutionStore(self._store),
+                )
+            )
+        else:
+            event_types = event_auth.auth_types_for_event(event.room_version, event)
+            state_for_auth_id_map = await context.get_prev_state_ids(
+                StateFilter.from_types(event_types)
+            )
 
         calculated_auth_event_ids = self._event_auth_handler.compute_auth_events(
-            event, prev_state_ids, for_verification=True
+            event, state_for_auth_id_map, for_verification=True
         )
 
         # if those are the same, we're done here.