Avoid resetting state on rejected events

When we get a rejected event, give it the same state_group as its prev_event,
rather than no state_group at all.

This should fix https://github.com/matrix-org/synapse/issues/1935.

synapse/storage/events.py

@@ -645,6 +645,10 @@ class EventsStore(SQLBaseStore):
             events_and_contexts=events_and_contexts,
         )
 
+        # Insert into the state_groups, state_groups_state, and
+        # event_to_state_groups tables.
+        self._store_mult_state_groups_txn(txn, events_and_contexts)
+
         # _store_rejected_events_txn filters out any events which were
         # rejected, and returns the filtered list.
         events_and_contexts = self._store_rejected_events_txn(
@@ -1075,10 +1079,6 @@ class EventsStore(SQLBaseStore):
             ],
         )
 
-        # Insert into the state_groups, state_groups_state, and
-        # event_to_state_groups tables.
-        self._store_mult_state_groups_txn(txn, events_and_contexts)
-
         # Update the event_forward_extremities, event_backward_extremities and
         # event_edges tables.
         self._handle_mult_prev_events(

synapse/storage/state.py

@@ -136,6 +136,16 @@ class StateStore(SQLBaseStore):
                 continue
 
             if context.current_state_ids is None:
+                # AFAIK, this can never happen
+                logger.error(
+                    "Non-outlier event %s had current_state_ids==None",
+                    event.event_id)
+                continue
+
+            # if the event was rejected, just give it the same state as its
+            # predecessor.
+            if context.rejected:
+                state_groups[event.event_id] = context.prev_group
                 continue
 
             state_groups[event.event_id] = context.state_group