Split out rewriting state from updating state from state res

federationsender/internal/perform.go

@@ -198,7 +198,7 @@ func (r *FederationSenderInternalAPI) performJoinUsingServer(
 	// If we successfully performed a send_join above then the other
 	// server now thinks we're a part of the room. Send the newly
 	// returned state to the roomserver to update our local view.
-	if err = roomserverAPI.SendEventWithState(
+	if err = roomserverAPI.SendEventWithRewrite(
 		ctx, r.rsAPI,
 		respState,
 		event.Headered(respMakeJoin.RoomVersion),

roomserver/api/wrapper.go

@@ -40,10 +40,50 @@ func SendEvents(
 	return SendInputRoomEvents(ctx, rsAPI, ires)
 }
 
-// SendEventWithState writes an event with KindNew to the roomserver along
+// SendEventWithState writes an event with KindNew to the roomserver
+// with the state at the event as KindOutlier before it. Will not send any event that is
+// marked as `true` in haveEventIDs
+func SendEventWithState(
+	ctx context.Context, rsAPI RoomserverInternalAPI, state *gomatrixserverlib.RespState,
+	event gomatrixserverlib.HeaderedEvent, haveEventIDs map[string]bool,
+) error {
+	outliers, err := state.Events()
+	if err != nil {
+		return err
+	}
+
+	var ires []InputRoomEvent
+	for _, outlier := range outliers {
+		if haveEventIDs[outlier.EventID()] {
+			continue
+		}
+		ires = append(ires, InputRoomEvent{
+			Kind:         KindOutlier,
+			Event:        outlier.Headered(event.RoomVersion),
+			AuthEventIDs: outlier.AuthEventIDs(),
+		})
+	}
+
+	stateEventIDs := make([]string, len(state.StateEvents))
+	for i := range state.StateEvents {
+		stateEventIDs[i] = state.StateEvents[i].EventID()
+	}
+
+	ires = append(ires, InputRoomEvent{
+		Kind:          KindNew,
+		Event:         event,
+		AuthEventIDs:  event.AuthEventIDs(),
+		HasState:      true,
+		StateEventIDs: stateEventIDs,
+	})
+
+	return SendInputRoomEvents(ctx, rsAPI, ires)
+}
+
+// SendEventWithRewrite writes an event with KindNew to the roomserver along
 // with a number of rewrite and outlier events for state and auth events
 // respectively.
-func SendEventWithState(
+func SendEventWithRewrite(
 	ctx context.Context, rsAPI RoomserverInternalAPI, state *gomatrixserverlib.RespState,
 	event gomatrixserverlib.HeaderedEvent, haveEventIDs map[string]bool,
 ) error {
@@ -75,6 +115,9 @@ func SendEventWithState(
 		if haveEventIDs[authOrStateEvent.EventID()] {
 			continue
 		}
+		if event.StateKey() == nil {
+			continue
+		}
 
 		// We will handle an event as if it's an outlier if one of the
 		// following conditions is true:

roomserver/internal/input/input_latest_events.go

@@ -342,6 +342,11 @@ func (u *latestEventsUpdater) makeOutputNewRoomEvent() (*api.OutputEvent, error)
 			return nil, fmt.Errorf("failed to load add_state_events from db: %w", err)
 		}
 	}
+	// State is rewritten if the input room event HasState and we actually produced a delta on state events.
+	// Without this check, /get_missing_events which produce events with associated (but not complete) state
+	// will incorrectly purge the room and set it to no state. TODO: This is likely flakey, as if /gme produced
+	// a state conflict res which just so happens to include 2+ events we might purge the room state downstream.
+	ore.RewritesState = len(ore.AddsStateEventIDs) > 1
 
 	return &api.OutputEvent{
 		Type:         api.OutputTypeNewRoomEvent,