Make prev_state field optional

changelog.d/4837.bugfix

@@ -0,0 +1 @@
+Fix bug where synapse expected an un-specced `prev_state` field on state events.

synapse/events/__init__.py

@@ -141,7 +141,6 @@ def __init__(self, event_dict, signatures={}, unsigned={},
     origin = _event_dict_property("origin")
     origin_server_ts = _event_dict_property("origin_server_ts")
     prev_events = _event_dict_property("prev_events")
-    prev_state = _event_dict_property("prev_state")
     redacts = _event_dict_property("redacts")
     room_id = _event_dict_property("room_id")
     sender = _event_dict_property("sender")

synapse/storage/events.py

@@ -1407,21 +1407,6 @@ def _update_metadata_tables_txn(self, txn, events_and_contexts,
             values=state_values,
         )
 
-        self._simple_insert_many_txn(
-            txn,
-            table="event_edges",
-            values=[
-                {
-                    "event_id": event.event_id,
-                    "prev_event_id": prev_id,
-                    "room_id": event.room_id,
-                    "is_state": True,
-                }
-                for event, _ in state_events_and_contexts
-                for prev_id, _ in event.prev_state
-            ],
-        )
-
         # Prefill the event cache
         self._add_to_cache(txn, events_and_contexts)