Fix fetching missing state (#2163)

* Check that we have a populated state snapshot when determining if we closed the gap

* Do the same in the query API

* Use HasState more opportunistically

* Try to avoid falling down the hole of using a trustworthy but empty state snapshot for non-create events

* Refactor missing state and make sure that we really solve the problem for the new event

* Comments

* Review comments

* Tweak that check again

* Tidy up that create check further

* Fix build hopefully

* Update sendOutliers to use OrderAuthAndStateEvents

* Don't go out of bounds on missingEvents

roomserver/internal/input/input_events.go

@@ -255,13 +255,32 @@ func (r *Inputer) processRoomEvent(
 				hadEvents:  map[string]bool{},
 				haveEvents: map[string]*gomatrixserverlib.HeaderedEvent{},
 			}
-			if err := missingState.processEventWithMissingState(ctx, event, headered.RoomVersion); err != nil {
+			if stateSnapshot, err := missingState.processEventWithMissingState(ctx, event, headered.RoomVersion); err != nil {
+				// Something went wrong with retrieving the missing state, so we can't
+				// really do anything with the event other than reject it at this point.
 				isRejected = true
 				rejectionErr = fmt.Errorf("missingState.processEventWithMissingState: %w", err)
+			} else if stateSnapshot != nil {
+				// We retrieved some state and we ended up having to call /state_ids for
+				// the new event in question (probably because closing the gap by using
+				// /get_missing_events didn't do what we hoped) so we'll instead overwrite
+				// the state snapshot with the newly resolved state.
+				missingPrev = false
+				input.HasState = true
+				input.StateEventIDs = make([]string, 0, len(stateSnapshot.StateEvents))
+				for _, e := range stateSnapshot.StateEvents {
+					input.StateEventIDs = append(input.StateEventIDs, e.EventID())
+				}
 			} else {
+				// We retrieved some state and it would appear that rolling forward the
+				// state did everything we needed it to do, so we can just resolve the
+				// state for the event in the normal way.
 				missingPrev = false
 			}
 		} else {
+			// We're missing prev events or state for the event, but for some reason
+			// we don't know any servers to ask. In this case we can't do anything but
+			// reject the event and hope that it gets unrejected later.
 			isRejected = true
 			rejectionErr = fmt.Errorf("missing prev events and no other servers to ask")
 		}
@@ -299,7 +318,7 @@ func (r *Inputer) processRoomEvent(
 		return rollbackTransaction, fmt.Errorf("updater.RoomInfo missing for room %s", event.RoomID())
 	}
 
-	if !missingPrev && stateAtEvent.BeforeStateSnapshotNID == 0 {
+	if input.HasState || (!missingPrev && stateAtEvent.BeforeStateSnapshotNID == 0) {
 		// We haven't calculated a state for this event yet.
 		// Lets calculate one.
 		err = r.calculateAndSetState(ctx, updater, input, roomInfo, &stateAtEvent, event, isRejected)

roomserver/internal/input/input_missing.go

@@ -40,9 +40,10 @@ type missingStateReq struct {
 
 // processEventWithMissingState is the entrypoint for a missingStateReq
 // request, as called from processRoomEvent.
+// nolint:gocyclo
 func (t *missingStateReq) processEventWithMissingState(
 	ctx context.Context, e *gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion,
-) error {
+) (*parsedRespState, error) {
 	// We are missing the previous events for this events.
 	// This means that there is a gap in our view of the history of the
 	// room. There two ways that we can handle such a gap:
@@ -68,15 +69,15 @@ func (t *missingStateReq) processEventWithMissingState(
 	// - fill in the gap completely then process event `e` returning no backwards extremity
 	// - fail to fill in the gap and tell us to terminate the transaction err=not nil
 	// - fail to fill in the gap and tell us to fetch state at the new backwards extremity, and to not terminate the transaction
-	newEvents, isGapFilled, err := t.getMissingEvents(ctx, e, roomVersion)
+	newEvents, isGapFilled, prevStatesKnown, err := t.getMissingEvents(ctx, e, roomVersion)
 	if err != nil {
-		return fmt.Errorf("t.getMissingEvents: %w", err)
+		return nil, fmt.Errorf("t.getMissingEvents: %w", err)
 	}
 	if len(newEvents) == 0 {
-		return fmt.Errorf("expected to find missing events but didn't")
+		return nil, fmt.Errorf("expected to find missing events but didn't")
 	}
 	if isGapFilled {
-		logger.Infof("gap filled by /get_missing_events, injecting %d new events", len(newEvents))
+		logger.Infof("Gap filled by /get_missing_events, injecting %d new events", len(newEvents))
 		// we can just inject all the newEvents as new as we may have only missed 1 or 2 events and have filled
 		// in the gap in the DAG
 		for _, newEvent := range newEvents {
@@ -88,82 +89,31 @@ func (t *missingStateReq) processEventWithMissingState(
 			})
 			if err != nil {
 				if _, ok := err.(types.RejectedError); !ok {
-					return fmt.Errorf("t.inputer.processRoomEvent (filling gap): %w", err)
+					return nil, fmt.Errorf("t.inputer.processRoomEvent (filling gap): %w", err)
 				}
 			}
 		}
-		return nil
 	}
 
+	// If we filled the gap *and* we know the state before the prev events
+	// then there's nothing else to do, we have everything we need to deal
+	// with the new event.
+	if isGapFilled && prevStatesKnown {
+		logger.Infof("Gap filled and state found for all prev events")
+		return nil, nil
+	}
+
+	// Otherwise, if we've reached this point, it's possible that we've
+	// either not closed the gap, or we did but we still don't seem to
+	// know the events before the new event. Start by looking up the
+	// state at the event at the back of the gap and we'll try to roll
+	// forward the state first.
 	backwardsExtremity := newEvents[0]
 	newEvents = newEvents[1:]
 
-	type respState struct {
-		// A snapshot is considered trustworthy if it came from our own roomserver.
-		// That's because the state will have been through state resolution once
-		// already in QueryStateAfterEvent.
-		trustworthy bool
-		*parsedRespState
-	}
-
-	// at this point we know we're going to have a gap: we need to work out the room state at the new backwards extremity.
-	// Therefore, we cannot just query /state_ids with this event to get the state before. Instead, we need to query
-	// the state AFTER all the prev_events for this event, then apply state resolution to that to get the state before the event.
-	var states []*respState
-	for _, prevEventID := range backwardsExtremity.PrevEventIDs() {
-		// Look up what the state is after the backward extremity. This will either
-		// come from the roomserver, if we know all the required events, or it will
-		// come from a remote server via /state_ids if not.
-		prevState, trustworthy, lerr := t.lookupStateAfterEvent(ctx, roomVersion, backwardsExtremity.RoomID(), prevEventID)
-		if lerr != nil {
-			logger.WithError(lerr).Errorf("Failed to lookup state after prev_event: %s", prevEventID)
-			return lerr
-		}
-		// Append the state onto the collected state. We'll run this through the
-		// state resolution next.
-		states = append(states, &respState{trustworthy, prevState})
-	}
-
-	// Now that we have collected all of the state from the prev_events, we'll
-	// run the state through the appropriate state resolution algorithm for the
-	// room if needed. This does a couple of things:
-	// 1. Ensures that the state is deduplicated fully for each state-key tuple
-	// 2. Ensures that we pick the latest events from both sets, in the case that
-	//    one of the prev_events is quite a bit older than the others
-	resolvedState := &parsedRespState{}
-	switch len(states) {
-	case 0:
-		extremityIsCreate := backwardsExtremity.Type() == gomatrixserverlib.MRoomCreate && backwardsExtremity.StateKeyEquals("")
-		if !extremityIsCreate {
-			// There are no previous states and this isn't the beginning of the
-			// room - this is an error condition!
-			logger.Errorf("Failed to lookup any state after prev_events")
-			return fmt.Errorf("expected %d states but got %d", len(backwardsExtremity.PrevEventIDs()), len(states))
-		}
-	case 1:
-		// There's only one previous state - if it's trustworthy (came from a
-		// local state snapshot which will already have been through state res),
-		// use it as-is. There's no point in resolving it again.
-		if states[0].trustworthy {
-			resolvedState = states[0].parsedRespState
-			break
-		}
-		// Otherwise, if it isn't trustworthy (came from federation), run it through
-		// state resolution anyway for safety, in case there are duplicates.
-		fallthrough
-	default:
-		respStates := make([]*parsedRespState, len(states))
-		for i := range states {
-			respStates[i] = states[i].parsedRespState
-		}
-		// There's more than one previous state - run them all through state res
-		t.roomsMu.Lock(e.RoomID())
-		resolvedState, err = t.resolveStatesAndCheck(ctx, roomVersion, respStates, backwardsExtremity)
-		t.roomsMu.Unlock(e.RoomID())
-		if err != nil {
-			logger.WithError(err).Errorf("Failed to resolve state conflicts for event %s", backwardsExtremity.EventID())
-			return err
-		}
+	resolvedState, err := t.lookupResolvedStateBeforeEvent(ctx, backwardsExtremity, roomVersion)
+	if err != nil {
+		return nil, fmt.Errorf("t.lookupState (backwards extremity): %w", err)
 	}
 
 	hadEvents := map[string]bool{}
@@ -173,30 +123,37 @@ func (t *missingStateReq) processEventWithMissingState(
 	}
 	t.hadEventsMutex.Unlock()
 
-	// Send outliers first so we can send the new backwards extremity without causing errors
-	outliers, err := gomatrixserverlib.OrderAuthAndStateEvents(resolvedState.AuthEvents, resolvedState.StateEvents, roomVersion)
-	if err != nil {
-		return err
-	}
-	var outlierRoomEvents []api.InputRoomEvent
-	for _, outlier := range outliers {
-		if hadEvents[outlier.EventID()] {
-			continue
+	sendOutliers := func(resolvedState *parsedRespState) error {
+		outliers, oerr := gomatrixserverlib.OrderAuthAndStateEvents(resolvedState.AuthEvents, resolvedState.StateEvents, roomVersion)
+		if oerr != nil {
+			return fmt.Errorf("gomatrixserverlib.OrderAuthAndStateEvents: %w", oerr)
 		}
-		outlierRoomEvents = append(outlierRoomEvents, api.InputRoomEvent{
-			Kind:   api.KindOutlier,
-			Event:  outlier.Headered(roomVersion),
-			Origin: t.origin,
-		})
-	}
-	// TODO: we could do this concurrently?
-	for _, ire := range outlierRoomEvents {
-		_, err = t.inputer.processRoomEvent(ctx, t.db, &ire)
-		if err != nil {
-			if _, ok := err.(types.RejectedError); !ok {
-				return fmt.Errorf("t.inputer.processRoomEvent (outlier): %w", err)
+		var outlierRoomEvents []api.InputRoomEvent
+		for _, outlier := range outliers {
+			if hadEvents[outlier.EventID()] {
+				continue
+			}
+			outlierRoomEvents = append(outlierRoomEvents, api.InputRoomEvent{
+				Kind:   api.KindOutlier,
+				Event:  outlier.Headered(roomVersion),
+				Origin: t.origin,
+			})
+		}
+		for _, ire := range outlierRoomEvents {
+			_, err = t.inputer.processRoomEvent(ctx, t.db, &ire)
+			if err != nil {
+				if _, ok := err.(types.RejectedError); !ok {
+					return fmt.Errorf("t.inputer.processRoomEvent (outlier): %w", err)
+				}
 			}
 		}
+		return nil
+	}
+
+	// Send outliers first so we can send the state along with the new backwards
+	// extremity without any missing auth events.
+	if err = sendOutliers(resolvedState); err != nil {
+		return nil, fmt.Errorf("sendOutliers: %w", err)
 	}
 
 	// Now send the backward extremity into the roomserver with the
@@ -217,7 +174,7 @@ func (t *missingStateReq) processEventWithMissingState(
 	})
 	if err != nil {
 		if _, ok := err.(types.RejectedError); !ok {
-			return fmt.Errorf("t.inputer.processRoomEvent (backward extremity): %w", err)
+			return nil, fmt.Errorf("t.inputer.processRoomEvent (backward extremity): %w", err)
 		}
 	}
 
@@ -234,12 +191,109 @@ func (t *missingStateReq) processEventWithMissingState(
 		})
 		if err != nil {
 			if _, ok := err.(types.RejectedError); !ok {
-				return fmt.Errorf("t.inputer.processRoomEvent (fast forward): %w", err)
+				return nil, fmt.Errorf("t.inputer.processRoomEvent (fast forward): %w", err)
 			}
 		}
 	}
 
-	return nil
+	// Finally, check again if we know everything we need to know in order to
+	// make forward progress. If the prev state is known then we consider the
+	// rolled forward state to be sufficient — we now know all of the state
+	// before the prev events. If we don't then we need to look up the state
+	// before the new event as well, otherwise we will never make any progress.
+	if t.isPrevStateKnown(ctx, e) {
+		return nil, nil
+	}
+
+	// If we still haven't got the state for the prev events then we'll go and
+	// ask the federation for it if needed.
+	resolvedState, err = t.lookupResolvedStateBeforeEvent(ctx, e, roomVersion)
+	if err != nil {
+		return nil, fmt.Errorf("t.lookupState (new event): %w", err)
+	}
+
+	// Send the outliers for the retrieved state.
+	if err = sendOutliers(resolvedState); err != nil {
+		return nil, fmt.Errorf("sendOutliers: %w", err)
+	}
+
+	// Then return the resolved state, for which the caller can replace the
+	// HasState with the event IDs to create a new state snapshot when we
+	// process the new event.
+	return resolvedState, nil
+}
+
+func (t *missingStateReq) lookupResolvedStateBeforeEvent(ctx context.Context, e *gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion) (*parsedRespState, error) {
+	type respState struct {
+		// A snapshot is considered trustworthy if it came from our own roomserver.
+		// That's because the state will have been through state resolution once
+		// already in QueryStateAfterEvent.
+		trustworthy bool
+		*parsedRespState
+	}
+
+	// at this point we know we're going to have a gap: we need to work out the room state at the new backwards extremity.
+	// Therefore, we cannot just query /state_ids with this event to get the state before. Instead, we need to query
+	// the state AFTER all the prev_events for this event, then apply state resolution to that to get the state before the event.
+	var states []*respState
+	for _, prevEventID := range e.PrevEventIDs() {
+		// Look up what the state is after the backward extremity. This will either
+		// come from the roomserver, if we know all the required events, or it will
+		// come from a remote server via /state_ids if not.
+		prevState, trustworthy, err := t.lookupStateAfterEvent(ctx, roomVersion, e.RoomID(), prevEventID)
+		if err != nil {
+			return nil, fmt.Errorf("t.lookupStateAfterEvent: %w", err)
+		}
+		// Append the state onto the collected state. We'll run this through the
+		// state resolution next.
+		states = append(states, &respState{trustworthy, prevState})
+	}
+
+	// Now that we have collected all of the state from the prev_events, we'll
+	// run the state through the appropriate state resolution algorithm for the
+	// room if needed. This does a couple of things:
+	// 1. Ensures that the state is deduplicated fully for each state-key tuple
+	// 2. Ensures that we pick the latest events from both sets, in the case that
+	//    one of the prev_events is quite a bit older than the others
+	resolvedState := &parsedRespState{}
+	switch len(states) {
+	case 0:
+		extremityIsCreate := e.Type() == gomatrixserverlib.MRoomCreate && e.StateKeyEquals("")
+		if !extremityIsCreate {
+			// There are no previous states and this isn't the beginning of the
+			// room - this is an error condition!
+			return nil, fmt.Errorf("expected %d states but got %d", len(e.PrevEventIDs()), len(states))
+		}
+	case 1:
+		// There's only one previous state - if it's trustworthy (came from a
+		// local state snapshot which will already have been through state res),
+		// use it as-is. There's no point in resolving it again. Only trust a
+		// trustworthy state snapshot if it actually contains some state for all
+		// non-create events, otherwise we need to resolve what came from federation.
+		isCreate := e.Type() == gomatrixserverlib.MRoomCreate && e.StateKeyEquals("")
+		if states[0].trustworthy && (isCreate || len(states[0].StateEvents) > 0) {
+			resolvedState = states[0].parsedRespState
+			break
+		}
+		// Otherwise, if it isn't trustworthy (came from federation), run it through
+		// state resolution anyway for safety, in case there are duplicates.
+		fallthrough
+	default:
+		respStates := make([]*parsedRespState, len(states))
+		for i := range states {
+			respStates[i] = states[i].parsedRespState
+		}
+		// There's more than one previous state - run them all through state res
+		var err error
+		t.roomsMu.Lock(e.RoomID())
+		resolvedState, err = t.resolveStatesAndCheck(ctx, roomVersion, respStates, e)
+		t.roomsMu.Unlock(e.RoomID())
+		if err != nil {
+			return nil, fmt.Errorf("t.resolveStatesAndCheck: %w", err)
+		}
+	}
+
+	return resolvedState, nil
 }
 
 // lookupStateAfterEvent returns the room state after `eventID`, which is the state before eventID with the state of `eventID` (if it's a state event)
@@ -408,7 +462,7 @@ retryAllowedState:
 
 // get missing events for `e`. If `isGapFilled`=true then `newEvents` contains all the events to inject,
 // without `e`. If `isGapFilled=false` then `newEvents` contains the response to /get_missing_events
-func (t *missingStateReq) getMissingEvents(ctx context.Context, e *gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion) (newEvents []*gomatrixserverlib.Event, isGapFilled bool, err error) {
+func (t *missingStateReq) getMissingEvents(ctx context.Context, e *gomatrixserverlib.Event, roomVersion gomatrixserverlib.RoomVersion) (newEvents []*gomatrixserverlib.Event, isGapFilled, prevStateKnown bool, err error) {
 	logger := util.GetLogger(ctx).WithField("event_id", e.EventID()).WithField("room_id", e.RoomID())
 
 	latest := t.db.LatestEvents()
@@ -435,7 +489,7 @@ func (t *missingStateReq) getMissingEvents(ctx context.Context, e *gomatrixserve
 			if errors.Is(err, context.DeadlineExceeded) {
 				select {
 				case <-ctx.Done(): // the parent request context timed out
-					return nil, false, context.DeadlineExceeded
+					return nil, false, false, context.DeadlineExceeded
 				default: // this request exceed its own timeout
 					continue
 				}
@@ -448,7 +502,7 @@ func (t *missingStateReq) getMissingEvents(ctx context.Context, e *gomatrixserve
 			"%s pushed us an event but %d server(s) couldn't give us details about prev_events via /get_missing_events - dropping this event until it can",
 			t.origin, len(t.servers),
 		)
-		return nil, false, missingPrevEventsError{
+		return nil, false, false, missingPrevEventsError{
 			eventID: e.EventID(),
 			err:     err,
 		}
@@ -457,17 +511,9 @@ func (t *missingStateReq) getMissingEvents(ctx context.Context, e *gomatrixserve
 	// Make sure events from the missingResp are using the cache - missing events
 	// will be added and duplicates will be removed.
 	logger.Debugf("get_missing_events returned %d events", len(missingResp.Events))
-	missingEvents := make([]*gomatrixserverlib.Event, len(missingResp.Events))
-	for i, evJSON := range missingResp.Events {
-		ev, err := gomatrixserverlib.NewEventFromUntrustedJSON(evJSON, roomVersion)
-		if err != nil {
-			logger.WithError(err).WithField("event", string(evJSON)).Warn("NewEventFromUntrustedJSON: failed")
-			return nil, false, missingPrevEventsError{
-				eventID: e.EventID(),
-				err:     err,
-			}
-		}
-		missingEvents[i] = t.cacheAndReturn(ev.Headered(roomVersion)).Unwrap()
+	missingEvents := make([]*gomatrixserverlib.Event, 0, len(missingResp.Events))
+	for _, ev := range missingResp.Events.UntrustedEvents(roomVersion) {
+		missingEvents = append(missingEvents, t.cacheAndReturn(ev.Headered(roomVersion)).Unwrap())
 	}
 
 	// topologically sort and sanity check that we are making forward progress
@@ -489,27 +535,51 @@ Event:
 			"%s pushed us an event but couldn't give us details about prev_events via /get_missing_events - dropping this event until it can",
 			t.origin,
 		)
-		return nil, false, missingPrevEventsError{
+		return nil, false, false, missingPrevEventsError{
 			eventID: e.EventID(),
 			err:     err,
 		}
 	}
 	if len(newEvents) == 0 {
-		return nil, false, nil // TODO: error instead?
+		return nil, false, false, nil // TODO: error instead?
 	}
 
-	// now check if we can fill the gap. Look to see if we have state snapshot IDs for the earliest event
 	earliestNewEvent := newEvents[0]
-	if state, err := t.db.StateAtEventIDs(ctx, []string{earliestNewEvent.EventID()}); err != nil || len(state) == 0 {
-		if earliestNewEvent.Type() == gomatrixserverlib.MRoomCreate && earliestNewEvent.StateKeyEquals("") {
-			// we got to the beginning of the room so there will be no state! It's all good we can process this
-			return newEvents, true, nil
-		}
-		// we don't have the state at this earliest event from /g_m_e so we won't have state for later events either
-		return newEvents, false, nil
+
+	// If we retrieved back to the beginning of the room then there's nothing else
+	// to do - we closed the gap.
+	if len(earliestNewEvent.PrevEventIDs()) == 0 && earliestNewEvent.Type() == gomatrixserverlib.MRoomCreate && earliestNewEvent.StateKeyEquals("") {
+		return newEvents, true, t.isPrevStateKnown(ctx, e), nil
 	}
-	// StateAtEventIDs returned some kind of state for the earliest event so we can fill in the gap!
-	return newEvents, true, nil
+
+	// If our backward extremity was not a known event to us then we obviously didn't
+	// close the gap.
+	if state, err := t.db.StateAtEventIDs(ctx, []string{earliestNewEvent.EventID()}); err != nil || len(state) == 0 && state[0].BeforeStateSnapshotNID == 0 {
+		return newEvents, false, false, nil
+	}
+
+	// At this point we are satisfied that we know the state both at the earliest
+	// retrieved event and at the prev events of the new event.
+	return newEvents, true, t.isPrevStateKnown(ctx, e), nil
+}
+
+func (t *missingStateReq) isPrevStateKnown(ctx context.Context, e *gomatrixserverlib.Event) bool {
+	expected := len(e.PrevEventIDs())
+	state, err := t.db.StateAtEventIDs(ctx, e.PrevEventIDs())
+	if err != nil || len(state) != expected {
+		// We didn't get as many state snapshots as we expected, or there was an error,
+		// so we haven't completely solved the problem for the new event.
+		return false
+	}
+	// Check to see if we have a populated state snapshot for all of the prev events.
+	for _, stateAtEvent := range state {
+		if stateAtEvent.BeforeStateSnapshotNID == 0 {
+			// One of the prev events still has unknown state, so we haven't really
+			// solved the problem.
+			return false
+		}
+	}
+	return true
 }
 
 func (t *missingStateReq) lookupMissingStateViaState(

roomserver/internal/query/query.go

@@ -150,7 +150,7 @@ func (r *Queryer) QueryMissingAuthPrevEvents(
 
 	for _, prevEventID := range request.PrevEventIDs {
 		state, err := r.DB.StateAtEventIDs(ctx, []string{prevEventID})
-		if err != nil || len(state) == 0 {
+		if err != nil || len(state) == 0 || (!state[0].IsCreate() && state[0].BeforeStateSnapshotNID == 0) {
 			response.MissingPrevEventIDs = append(response.MissingPrevEventIDs, prevEventID)
 		}
 	}

roomserver/storage/shared/room_updater.go

@@ -187,6 +187,12 @@ func (u *RoomUpdater) EventIDs(
 	return u.d.EventsTable.BulkSelectEventID(ctx, u.txn, eventNIDs)
 }
 
+func (u *RoomUpdater) EventNIDs(
+	ctx context.Context, eventIDs []string,
+) (map[string]types.EventNID, error) {
+	return u.d.eventNIDs(ctx, u.txn, eventIDs)
+}
+
 func (u *RoomUpdater) StateAtEventIDs(
 	ctx context.Context, eventIDs []string,
 ) ([]types.StateAtEvent, error) {

roomserver/storage/shared/storage.go

@@ -603,6 +603,8 @@ func (d *Database) storeEvent(
 			if err == sql.ErrNoRows {
 				// We've already inserted the event so select the numeric event ID
 				eventNID, stateNID, err = d.EventsTable.SelectEvent(ctx, txn, event.EventID())
+			} else if err != nil {
+				return fmt.Errorf("d.EventsTable.InsertEvent: %w", err)
 			}
 			if err != nil {
 				return fmt.Errorf("d.EventsTable.SelectEvent: %w", err)

roomserver/types/types.go

@@ -83,6 +83,10 @@ type StateKeyTuple struct {
 	EventStateKeyNID EventStateKeyNID
 }
 
+func (a StateKeyTuple) IsCreate() bool {
+	return a.EventTypeNID == MRoomCreateNID && a.EventStateKeyNID == EmptyStateKeyNID
+}
+
 // LessThan returns true if this state key is less than the other state key.
 // The ordering is arbitrary and is used to implement binary search and to efficiently deduplicate entries.
 func (a StateKeyTuple) LessThan(b StateKeyTuple) bool {