Don't return end if there are not more messages (#2542)

* Be more spec compliant

* Move lazyLoadMembers to own method

syncapi/routing/messages.go

@@ -50,7 +50,7 @@ type messagesReq struct {
 type messagesResp struct {
 	Start       string                          `json:"start"`
 	StartStream string                          `json:"start_stream,omitempty"` // NOTSPEC: used by Cerulean, so clients can hit /messages then immediately /sync with a latest sync token
-	End         string                          `json:"end"`
+	End         string                          `json:"end,omitempty"`
 	Chunk       []gomatrixserverlib.ClientEvent `json:"chunk"`
 	State       []gomatrixserverlib.ClientEvent `json:"state"`
 }
@@ -200,30 +200,6 @@ func OnIncomingMessagesRequest(
 		return jsonerror.InternalServerError()
 	}
 
-	// at least fetch the membership events for the users returned in chunk if LazyLoadMembers is set
-	state := []gomatrixserverlib.ClientEvent{}
-	if filter.LazyLoadMembers {
-		membershipToUser := make(map[string]*gomatrixserverlib.HeaderedEvent)
-		for _, evt := range clientEvents {
-			// Don't add membership events the client should already know about
-			if _, cached := lazyLoadCache.IsLazyLoadedUserCached(device, roomID, evt.Sender); cached {
-				continue
-			}
-			membership, err := db.GetStateEvent(req.Context(), roomID, gomatrixserverlib.MRoomMember, evt.Sender)
-			if err != nil {
-				util.GetLogger(req.Context()).WithError(err).Error("failed to get membership event for user")
-				continue
-			}
-			if membership != nil {
-				membershipToUser[evt.Sender] = membership
-				lazyLoadCache.StoreLazyLoadedUser(device, roomID, evt.Sender, membership.EventID())
-			}
-		}
-		for _, evt := range membershipToUser {
-			state = append(state, gomatrixserverlib.HeaderedToClientEvent(evt, gomatrixserverlib.FormatSync))
-		}
-	}
-
 	util.GetLogger(req.Context()).WithFields(logrus.Fields{
 		"from":         from.String(),
 		"to":           to.String(),
@@ -237,7 +213,13 @@ func OnIncomingMessagesRequest(
 		Chunk: clientEvents,
 		Start: start.String(),
 		End:   end.String(),
-		State: state,
+	}
+	res.applyLazyLoadMembers(req.Context(), db, roomID, device, filter.LazyLoadMembers, lazyLoadCache)
+
+	// If we didn't return any events, set the end to an empty string, so it will be omitted
+	// in the response JSON.
+	if len(res.Chunk) == 0 {
+		res.End = ""
 	}
 	if fromStream != nil {
 		res.StartStream = fromStream.String()
@@ -250,6 +232,40 @@ func OnIncomingMessagesRequest(
 	}
 }
 
+// applyLazyLoadMembers loads membership events for users returned in Chunk, if the filter has
+// LazyLoadMembers enabled.
+func (m *messagesResp) applyLazyLoadMembers(
+	ctx context.Context,
+	db storage.Database,
+	roomID string,
+	device *userapi.Device,
+	lazyLoad bool,
+	lazyLoadCache caching.LazyLoadCache,
+) {
+	if !lazyLoad {
+		return
+	}
+	membershipToUser := make(map[string]*gomatrixserverlib.HeaderedEvent)
+	for _, evt := range m.Chunk {
+		// Don't add membership events the client should already know about
+		if _, cached := lazyLoadCache.IsLazyLoadedUserCached(device, roomID, evt.Sender); cached {
+			continue
+		}
+		membership, err := db.GetStateEvent(ctx, roomID, gomatrixserverlib.MRoomMember, evt.Sender)
+		if err != nil {
+			util.GetLogger(ctx).WithError(err).Error("failed to get membership event for user")
+			continue
+		}
+		if membership != nil {
+			membershipToUser[evt.Sender] = membership
+			lazyLoadCache.StoreLazyLoadedUser(device, roomID, evt.Sender, membership.EventID())
+		}
+	}
+	for _, evt := range membershipToUser {
+		m.State = append(m.State, gomatrixserverlib.HeaderedToClientEvent(evt, gomatrixserverlib.FormatSync))
+	}
+}
+
 func checkIsRoomForgotten(ctx context.Context, roomID, userID string, rsAPI api.SyncRoomserverAPI) (forgotten bool, exists bool, err error) {
 	req := api.QueryMembershipForUserRequest{
 		RoomID: roomID,