matrix-js-sdk/src/sync.ts

Matrix Client-Server SDK for Javascript

/*
Copyright 2015 - 2021 The Matrix.org Foundation C.I.C.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

/*
 * TODO:
 * This class mainly serves to take all the syncing logic out of client.js and
 * into a separate file. It's all very fluid, and this class gut wrenches a lot
 * of MatrixClient props (e.g. http). Given we want to support WebSockets as
 * an alternative syncing API, we may want to have a proper syncing interface
 * for HTTP and WS at some point.
 */

import { User, UserEvent } from "./models/user";
import { NotificationCountType, Room, RoomEvent } from "./models/room";
import * as utils from "./utils";
import { IDeferred } from "./utils";
import { Filter } from "./filter";
import { EventTimeline } from "./models/event-timeline";
import { PushProcessor } from "./pushprocessor";
import { logger } from './logger';
import { InvalidStoreError } from './errors';
import { ClientEvent, IStoredClientOpts, MatrixClient, PendingEventOrdering } from "./client";
import {
    IEphemeral,
    IInvitedRoom,
    IInviteState,
    IJoinedRoom,
    ILeftRoom,
    IMinimalEvent,
    IRoomEvent,
    IStateEvent,
    IStrippedState,
    ISyncResponse,
    ITimeline,
} from "./sync-accumulator";
import { MatrixEvent } from "./models/event";
import { MatrixError, Method } from "./http-api";
import { ISavedSync } from "./store";
import { EventType } from "./@types/event";
import { IPushRules } from "./@types/PushRules";
import { RoomStateEvent } from "./models/room-state";
import { RoomMemberEvent } from "./models/room-member";
import { BeaconEvent } from "./models/beacon";
import { IEventsResponse } from "./@types/requests";
import { IAbortablePromise } from "./@types/partials";

const DEBUG = true;

// /sync requests allow you to set a timeout= but the request may continue
// beyond that and wedge forever, so we need to track how long we are willing
// to keep open the connection. This constant is *ADDED* to the timeout= value
// to determine the max time we're willing to wait.
const BUFFER_PERIOD_MS = 80 * 1000;

// Number of consecutive failed syncs that will lead to a syncState of ERROR as opposed
// to RECONNECTING. This is needed to inform the client of server issues when the
// keepAlive is successful but the server /sync fails.
const FAILED_SYNC_ERROR_THRESHOLD = 3;

export enum SyncState {
    Error = "ERROR",
    Prepared = "PREPARED",
    Stopped = "STOPPED",
    Syncing = "SYNCING",
    Catchup = "CATCHUP",
    Reconnecting = "RECONNECTING",
}

function getFilterName(userId: string, suffix?: string): string {
    // scope this on the user ID because people may login on many accounts
    // and they all need to be stored!
    return "FILTER_SYNC_" + userId + (suffix ? "_" + suffix : "");
}

function debuglog(...params) {
    if (!DEBUG) {
        return;
    }
    logger.log(...params);
}

interface ISyncOptions {
    filterId?: string;
    hasSyncedBefore?: boolean;
}

export interface ISyncStateData {
    error?: MatrixError;
    oldSyncToken?: string;
    nextSyncToken?: string;
    catchingUp?: boolean;
    fromCache?: boolean;
}

enum SetPresence {
    Offline = "offline",
    Online = "online",
    Unavailable = "unavailable",
}

interface ISyncParams {
    filter?: string;
    timeout: number;
    since?: string;
    // eslint-disable-next-line camelcase
    full_state?: boolean;
    // eslint-disable-next-line camelcase
    set_presence?: SetPresence;
    _cacheBuster?: string | number; // not part of the API itself
}

type WrappedRoom<T> = T & {
    room: Room;
    isBrandNewRoom: boolean;
};

/**
 * <b>Internal class - unstable.</b>
 * Construct an entity which is able to sync with a homeserver.
 * @constructor
 * @param {MatrixClient} client The matrix client instance to use.
 * @param {Object} opts Config options
 * @param {module:crypto=} opts.crypto Crypto manager
 * @param {Function=} opts.canResetEntireTimeline A function which is called
 * with a room ID and returns a boolean. It should return 'true' if the SDK can
 * SAFELY remove events from this room. It may not be safe to remove events if
 * there are other references to the timelines for this room.
 * Default: returns false.
 * @param {Boolean=} opts.disablePresence True to perform syncing without automatically
 * updating presence.
 */
export class SyncApi {
    private _peekRoom: Room = null;
    private currentSyncRequest: IAbortablePromise<ISyncResponse> = null;
    private syncState: SyncState = null;
    private syncStateData: ISyncStateData = null; // additional data (eg. error object for failed sync)
    private catchingUp = false;
    private running = false;
    private keepAliveTimer: ReturnType<typeof setTimeout> = null;
    private connectionReturnedDefer: IDeferred<boolean> = null;
    private notifEvents: MatrixEvent[] = []; // accumulator of sync events in the current sync response
    private failedSyncCount = 0; // Number of consecutive failed /sync requests
    private storeIsInvalid = false; // flag set if the store needs to be cleared before we can start

    constructor(private readonly client: MatrixClient, private readonly opts: Partial<IStoredClientOpts> = {}) {
        this.opts.initialSyncLimit = this.opts.initialSyncLimit ?? 8;
        this.opts.resolveInvitesToProfiles = this.opts.resolveInvitesToProfiles || false;
        this.opts.pollTimeout = this.opts.pollTimeout || (30 * 1000);
        this.opts.pendingEventOrdering = this.opts.pendingEventOrdering || PendingEventOrdering.Chronological;
        this.opts.experimentalThreadSupport = this.opts.experimentalThreadSupport === true;

        if (!opts.canResetEntireTimeline) {
            opts.canResetEntireTimeline = (roomId: string) => {
                return false;
            };
        }

        if (client.getNotifTimelineSet()) {
            client.reEmitter.reEmit(client.getNotifTimelineSet(), [
                RoomEvent.Timeline,
                RoomEvent.TimelineReset,
            ]);
        }
    }

    /**
     * @param {string} roomId
     * @return {Room}
     */
    public createRoom(roomId: string): Room {
        const client = this.client;
        const {
            timelineSupport,
            unstableClientRelationAggregation,
        } = client;
        const room = new Room(roomId, client, client.getUserId(), {
            lazyLoadMembers: this.opts.lazyLoadMembers,
            pendingEventOrdering: this.opts.pendingEventOrdering,
            timelineSupport,
            unstableClientRelationAggregation,
        });
        client.reEmitter.reEmit(room, [
            RoomEvent.Name,
            RoomEvent.Redaction,
            RoomEvent.RedactionCancelled,
            RoomEvent.Receipt,
            RoomEvent.Tags,
            RoomEvent.LocalEchoUpdated,
            RoomEvent.AccountData,
            RoomEvent.MyMembership,
            RoomEvent.Timeline,
            RoomEvent.TimelineReset,
        ]);
        this.registerStateListeners(room);
        return room;
    }

    /**
     * @param {Room} room
     * @private
     */
    private registerStateListeners(room: Room): void {
        const client = this.client;
        // we need to also re-emit room state and room member events, so hook it up
        // to the client now. We need to add a listener for RoomState.members in
        // order to hook them correctly. (TODO: find a better way?)
        client.reEmitter.reEmit(room.currentState, [
            RoomStateEvent.Events,
            RoomStateEvent.Members,
            RoomStateEvent.NewMember,
            RoomStateEvent.Update,
            BeaconEvent.New,
            BeaconEvent.Update,
            BeaconEvent.Destroy,
            BeaconEvent.LivenessChange,
        ]);

        room.currentState.on(RoomStateEvent.NewMember, function(event, state, member) {
            member.user = client.getUser(member.userId);
            client.reEmitter.reEmit(member, [
                RoomMemberEvent.Name,
                RoomMemberEvent.Typing,
                RoomMemberEvent.PowerLevel,
                RoomMemberEvent.Membership,
            ]);
        });
    }

    /**
     * @param {Room} room
     * @private
     */
    private deregisterStateListeners(room: Room): void {
        // could do with a better way of achieving this.
        room.currentState.removeAllListeners(RoomStateEvent.Events);
        room.currentState.removeAllListeners(RoomStateEvent.Members);
        room.currentState.removeAllListeners(RoomStateEvent.NewMember);
    }

    /**
     * Sync rooms the user has left.
     * @return {Promise} Resolved when they've been added to the store.
     */
    public syncLeftRooms() {
        const client = this.client;

        // grab a filter with limit=1 and include_leave=true
        const filter = new Filter(this.client.credentials.userId);
        filter.setTimelineLimit(1);
        filter.setIncludeLeaveRooms(true);

        const localTimeoutMs = this.opts.pollTimeout + BUFFER_PERIOD_MS;
        const qps: ISyncParams = {
            timeout: 0, // don't want to block since this is a single isolated req
        };

        return client.getOrCreateFilter(
            getFilterName(client.credentials.userId, "LEFT_ROOMS"), filter,
        ).then(function(filterId) {
            qps.filter = filterId;
            return client.http.authedRequest<ISyncResponse>(
                undefined, Method.Get, "/sync", qps as any, undefined, localTimeoutMs,
            );
        }).then(async (data) => {
            let leaveRooms = [];
            if (data.rooms?.leave) {
                leaveRooms = this.mapSyncResponseToRoomArray(data.rooms.leave);
            }
            return Promise.all(leaveRooms.map(async (leaveObj) => {
                const room = leaveObj.room;
                if (!leaveObj.isBrandNewRoom) {
                    // the intention behind syncLeftRooms is to add in rooms which were
                    // *omitted* from the initial /sync. Rooms the user were joined to
                    // but then left whilst the app is running will appear in this list
                    // and we do not want to bother with them since they will have the
                    // current state already (and may get dupe messages if we add
                    // yet more timeline events!), so skip them.
                    // NB: When we persist rooms to localStorage this will be more
                    //     complicated...
                    return;
                }
                leaveObj.timeline = leaveObj.timeline || {};
                const events = this.mapSyncEventsFormat(leaveObj.timeline, room);

                const stateEvents = this.mapSyncEventsFormat(leaveObj.state, room);

                // set the back-pagination token. Do this *before* adding any
                // events so that clients can start back-paginating.
                room.getLiveTimeline().setPaginationToken(leaveObj.timeline.prev_batch, EventTimeline.BACKWARDS);

                await this.processRoomEvents(room, stateEvents, events);

                room.recalculate();
                client.store.storeRoom(room);
                client.emit(ClientEvent.Room, room);

                this.processEventsForNotifs(room, events);
                return room;
            }));
        });
    }

    /**
     * Peek into a room. This will result in the room in question being synced so it
     * is accessible via getRooms(). Live updates for the room will be provided.
     * @param {string} roomId The room ID to peek into.
     * @return {Promise} A promise which resolves once the room has been added to the
     * store.
     */
    public peek(roomId: string): Promise<Room> {
        if (this._peekRoom && this._peekRoom.roomId === roomId) {
            return Promise.resolve(this._peekRoom);
        }

        const client = this.client;
        this._peekRoom = this.createRoom(roomId);
        return this.client.roomInitialSync(roomId, 20).then((response) => {
            // make sure things are init'd
            response.messages = response.messages || { chunk: [] };
            response.messages.chunk = response.messages.chunk || [];
            response.state = response.state || [];

            // FIXME: Mostly duplicated from processRoomEvents but not entirely
            // because "state" in this API is at the BEGINNING of the chunk
            const oldStateEvents = utils.deepCopy(response.state)
                .map(client.getEventMapper());
            const stateEvents = response.state.map(client.getEventMapper());
            const messages = response.messages.chunk.map(client.getEventMapper());

            // XXX: copypasted from /sync until we kill off this minging v1 API stuff)
            // handle presence events (User objects)
            if (response.presence && Array.isArray(response.presence)) {
                response.presence.map(client.getEventMapper()).forEach(
                    function(presenceEvent) {
                        let user = client.store.getUser(presenceEvent.getContent().user_id);
                        if (user) {
                            user.setPresenceEvent(presenceEvent);
                        } else {
                            user = createNewUser(client, presenceEvent.getContent().user_id);
                            user.setPresenceEvent(presenceEvent);
                            client.store.storeUser(user);
                        }
                        client.emit(ClientEvent.Event, presenceEvent);
                    });
            }

            // set the pagination token before adding the events in case people
            // fire off pagination requests in response to the Room.timeline
            // events.
            if (response.messages.start) {
                this._peekRoom.oldState.paginationToken = response.messages.start;
            }

            // set the state of the room to as it was after the timeline executes
            this._peekRoom.oldState.setStateEvents(oldStateEvents);
            this._peekRoom.currentState.setStateEvents(stateEvents);

            this.resolveInvites(this._peekRoom);
            this._peekRoom.recalculate();

            // roll backwards to diverge old state. addEventsToTimeline
            // will overwrite the pagination token, so make sure it overwrites
            // it with the right thing.
            this._peekRoom.addEventsToTimeline(messages.reverse(), true,
                this._peekRoom.getLiveTimeline(),
                response.messages.start);

            client.store.storeRoom(this._peekRoom);
            client.emit(ClientEvent.Room, this._peekRoom);

            this.peekPoll(this._peekRoom);
            return this._peekRoom;
        });
    }

    /**
     * Stop polling for updates in the peeked room. NOPs if there is no room being
     * peeked.
     */
    public stopPeeking(): void {
        this._peekRoom = null;
    }

    /**
     * Do a peek room poll.
     * @param {Room} peekRoom
     * @param {string?} token from= token
     */
    private peekPoll(peekRoom: Room, token?: string): void {
        if (this._peekRoom !== peekRoom) {
            debuglog("Stopped peeking in room %s", peekRoom.roomId);
            return;
        }

        // FIXME: gut wrenching; hard-coded timeout values
        this.client.http.authedRequest<IEventsResponse>(undefined, Method.Get, "/events", {
            room_id: peekRoom.roomId,
            timeout: String(30 * 1000),
            from: token,
        }, undefined, 50 * 1000).then((res) => {
            if (this._peekRoom !== peekRoom) {
                debuglog("Stopped peeking in room %s", peekRoom.roomId);
                return;
            }
            // We have a problem that we get presence both from /events and /sync
            // however, /sync only returns presence for users in rooms
            // you're actually joined to.
            // in order to be sure to get presence for all of the users in the
            // peeked room, we handle presence explicitly here. This may result
            // in duplicate presence events firing for some users, which is a
            // performance drain, but such is life.
            // XXX: copypasted from /sync until we can kill this minging v1 stuff.

            res.chunk.filter(function(e) {
                return e.type === "m.presence";
            }).map(this.client.getEventMapper()).forEach((presenceEvent) => {
                let user = this.client.store.getUser(presenceEvent.getContent().user_id);
                if (user) {
                    user.setPresenceEvent(presenceEvent);
                } else {
                    user = createNewUser(this.client, presenceEvent.getContent().user_id);
                    user.setPresenceEvent(presenceEvent);
                    this.client.store.storeUser(user);
                }
                this.client.emit(ClientEvent.Event, presenceEvent);
            });

            // strip out events which aren't for the given room_id (e.g presence)
            // and also ephemeral events (which we're assuming is anything without
            // and event ID because the /events API doesn't separate them).
            const events = res.chunk.filter(function(e) {
                return e.room_id === peekRoom.roomId && e.event_id;
            }).map(this.client.getEventMapper());

            peekRoom.addLiveEvents(events);
            this.peekPoll(peekRoom, res.end);
        }, (err) => {
            logger.error("[%s] Peek poll failed: %s", peekRoom.roomId, err);
            setTimeout(() => {
                this.peekPoll(peekRoom, token);
            }, 30 * 1000);
        });
    }

    /**
     * Returns the current state of this sync object
     * @see module:client~MatrixClient#event:"sync"
     * @return {?String}
     */
    public getSyncState(): SyncState {
        return this.syncState;
    }

    /**
     * Returns the additional data object associated with
     * the current sync state, or null if there is no
     * such data.
     * Sync errors, if available, are put in the 'error' key of
     * this object.
     * @return {?Object}
     */
    public getSyncStateData(): ISyncStateData {
        return this.syncStateData;
    }

    public async recoverFromSyncStartupError(savedSyncPromise: Promise<void>, err: MatrixError): Promise<void> {
        // Wait for the saved sync to complete - we send the pushrules and filter requests
        // before the saved sync has finished so they can run in parallel, but only process
        // the results after the saved sync is done. Equivalently, we wait for it to finish
        // before reporting failures from these functions.
        await savedSyncPromise;
        const keepaliveProm = this.startKeepAlives();
        this.updateSyncState(SyncState.Error, { error: err });
        await keepaliveProm;
    }

    /**
     * Is the lazy loading option different than in previous session?
     * @param {boolean} lazyLoadMembers current options for lazy loading
     * @return {boolean} whether or not the option has changed compared to the previous session */
    private async wasLazyLoadingToggled(lazyLoadMembers = false): Promise<boolean> {
        // assume it was turned off before
        // if we don't know any better
        let lazyLoadMembersBefore = false;
        const isStoreNewlyCreated = await this.client.store.isNewlyCreated();
        if (!isStoreNewlyCreated) {
            const prevClientOptions = await this.client.store.getClientOptions();
            if (prevClientOptions) {
                lazyLoadMembersBefore = !!prevClientOptions.lazyLoadMembers;
            }
            return lazyLoadMembersBefore !== lazyLoadMembers;
        }
        return false;
    }

    private shouldAbortSync(error: MatrixError): boolean {
        if (error.errcode === "M_UNKNOWN_TOKEN") {
            // The logout already happened, we just need to stop.
            logger.warn("Token no longer valid - assuming logout");
            this.stop();
            this.updateSyncState(SyncState.Error, { error });
            return true;
        }
        return false;
    }

    /**
     * Main entry point
     */
    public sync(): void {
        const client = this.client;

        this.running = true;

        if (global.window && global.window.addEventListener) {
            global.window.addEventListener("online", this.onOnline, false);
        }

        let savedSyncPromise = Promise.resolve();
        let savedSyncToken = null;

        // We need to do one-off checks before we can begin the /sync loop.
        // These are:
        //   1) We need to get push rules so we can check if events should bing as we get
        //      them from /sync.
        //   2) We need to get/create a filter which we can use for /sync.
        //   3) We need to check the lazy loading option matches what was used in the
        //       stored sync. If it doesn't, we can't use the stored sync.

        const getPushRules = async () => {
            try {
                debuglog("Getting push rules...");
                const result = await client.getPushRules();
                debuglog("Got push rules");

                client.pushRules = result;
            } catch (err) {
                logger.error("Getting push rules failed", err);
                if (this.shouldAbortSync(err)) return;
                // wait for saved sync to complete before doing anything else,
                // otherwise the sync state will end up being incorrect
                debuglog("Waiting for saved sync before retrying push rules...");
                await this.recoverFromSyncStartupError(savedSyncPromise, err);
                getPushRules();
                return;
            }
            checkLazyLoadStatus(); // advance to the next stage
        };

        const buildDefaultFilter = () => {
            const filter = new Filter(client.credentials.userId);
            filter.setTimelineLimit(this.opts.initialSyncLimit);
            return filter;
        };

        const checkLazyLoadStatus = async () => {
            debuglog("Checking lazy load status...");
            if (this.opts.lazyLoadMembers && client.isGuest()) {
                this.opts.lazyLoadMembers = false;
            }
            if (this.opts.lazyLoadMembers) {
                debuglog("Checking server lazy load support...");
                const supported = await client.doesServerSupportLazyLoading();
                if (supported) {
                    debuglog("Enabling lazy load on sync filter...");
                    if (!this.opts.filter) {
                        this.opts.filter = buildDefaultFilter();
                    }
                    this.opts.filter.setLazyLoadMembers(true);
                } else {
                    debuglog("LL: lazy loading requested but not supported " +
                        "by server, so disabling");
                    this.opts.lazyLoadMembers = false;
                }
            }
            // need to vape the store when enabling LL and wasn't enabled before
            debuglog("Checking whether lazy loading has changed in store...");
            const shouldClear = await this.wasLazyLoadingToggled(this.opts.lazyLoadMembers);
            if (shouldClear) {
                this.storeIsInvalid = true;
                const reason = InvalidStoreError.TOGGLED_LAZY_LOADING;
                const error = new InvalidStoreError(reason, !!this.opts.lazyLoadMembers);
                this.updateSyncState(SyncState.Error, { error });
                // bail out of the sync loop now: the app needs to respond to this error.
                // we leave the state as 'ERROR' which isn't great since this normally means
                // we're retrying. The client must be stopped before clearing the stores anyway
                // so the app should stop the client, clear the store and start it again.
                logger.warn("InvalidStoreError: store is not usable: stopping sync.");
                return;
            }
            if (this.opts.lazyLoadMembers && this.opts.crypto) {
                this.opts.crypto.enableLazyLoading();
            }
            try {
                debuglog("Storing client options...");
                await this.client.storeClientOptions();
                debuglog("Stored client options");
            } catch (err) {
                logger.error("Storing client options failed", err);
                throw err;
            }

            getFilter(); // Now get the filter and start syncing
        };

        const getFilter = async () => {
            debuglog("Getting filter...");
            let filter;
            if (this.opts.filter) {
                filter = this.opts.filter;
            } else {
                filter = buildDefaultFilter();
            }

            let filterId;
            try {
                filterId = await client.getOrCreateFilter(getFilterName(client.credentials.userId), filter);
            } catch (err) {
                logger.error("Getting filter failed", err);
                if (this.shouldAbortSync(err)) return;
                // wait for saved sync to complete before doing anything else,
                // otherwise the sync state will end up being incorrect
                debuglog("Waiting for saved sync before retrying filter...");
                await this.recoverFromSyncStartupError(savedSyncPromise, err);
                getFilter();
                return;
            }
            // reset the notifications timeline to prepare it to paginate from
            // the current point in time.
            // The right solution would be to tie /sync pagination tokens into
            // /notifications API somehow.
            client.resetNotifTimelineSet();

            if (this.currentSyncRequest === null) {
                // Send this first sync request here so we can then wait for the saved
                // sync data to finish processing before we process the results of this one.
                debuglog("Sending first sync request...");
                this.currentSyncRequest = this.doSyncRequest({ filterId }, savedSyncToken);
            }

            // Now wait for the saved sync to finish...
            debuglog("Waiting for saved sync before starting sync processing...");
            await savedSyncPromise;
            this.doSync({ filterId });
        };

        if (client.isGuest()) {
            // no push rules for guests, no access to POST filter for guests.
            this.doSync({});
        } else {
            // Pull the saved sync token out first, before the worker starts sending
            // all the sync data which could take a while. This will let us send our
            // first incremental sync request before we've processed our saved data.
            debuglog("Getting saved sync token...");
            savedSyncPromise = client.store.getSavedSyncToken().then((tok) => {
                debuglog("Got saved sync token");
                savedSyncToken = tok;
                debuglog("Getting saved sync...");
                return client.store.getSavedSync();
            }).then((savedSync) => {
                debuglog(`Got reply from saved sync, exists? ${!!savedSync}`);
                if (savedSync) {
                    return this.syncFromCache(savedSync);
                }
            }).catch(err => {
                logger.error("Getting saved sync failed", err);
            });
            // Now start the first incremental sync request: this can also
            // take a while so if we set it going now, we can wait for it
            // to finish while we process our saved sync data.
            getPushRules();
        }
    }

    /**
     * Stops the sync object from syncing.
     */
    public stop(): void {
        debuglog("SyncApi.stop");
        // It is necessary to check for the existance of
        // global.window AND global.window.removeEventListener.
        // Some platforms (e.g. React Native) register global.window,
        // but do not have global.window.removeEventListener.
        if (global.window && global.window.removeEventListener) {
            global.window.removeEventListener("online", this.onOnline, false);
        }
        this.running = false;
        this.currentSyncRequest?.abort();
        if (this.keepAliveTimer) {
            clearTimeout(this.keepAliveTimer);
            this.keepAliveTimer = null;
        }
    }

    /**
     * Retry a backed off syncing request immediately. This should only be used when
     * the user <b>explicitly</b> attempts to retry their lost connection.
     * @return {boolean} True if this resulted in a request being retried.
     */
    public retryImmediately(): boolean {
        if (!this.connectionReturnedDefer) {
            return false;
        }
        this.startKeepAlives(0);
        return true;
    }
    /**
     * Process a single set of cached sync data.
     * @param {Object} savedSync a saved sync that was persisted by a store. This
     * should have been acquired via client.store.getSavedSync().
     */
    private async syncFromCache(savedSync: ISavedSync): Promise<void> {
        debuglog("sync(): not doing HTTP hit, instead returning stored /sync data");

        const nextSyncToken = savedSync.nextBatch;

        // Set sync token for future incremental syncing
        this.client.store.setSyncToken(nextSyncToken);

        // No previous sync, set old token to null
        const syncEventData = {
            oldSyncToken: null,
            nextSyncToken,
            catchingUp: false,
            fromCache: true,
        };

        const data: ISyncResponse = {
            next_batch: nextSyncToken,
            rooms: savedSync.roomsData,
            account_data: {
                events: savedSync.accountData,
            },
        };

        try {
            await this.processSyncResponse(syncEventData, data);
        } catch (e) {
            logger.error("Error processing cached sync", e);
        }

        // Don't emit a prepared if we've bailed because the store is invalid:
        // in this case the client will not be usable until stopped & restarted
        // so this would be useless and misleading.
        if (!this.storeIsInvalid) {
            this.updateSyncState(SyncState.Prepared, syncEventData);
        }
    }

    /**
     * Invoke me to do /sync calls
     * @param {Object} syncOptions
     * @param {string} syncOptions.filterId
     * @param {boolean} syncOptions.hasSyncedBefore
     */
    private async doSync(syncOptions: ISyncOptions): Promise<void> {
        const client = this.client;

        if (!this.running) {
            debuglog("Sync no longer running: exiting.");
            if (this.connectionReturnedDefer) {
                this.connectionReturnedDefer.reject();
                this.connectionReturnedDefer = null;
            }
            this.updateSyncState(SyncState.Stopped);
            return;
        }

        const syncToken = client.store.getSyncToken();

        let data;
        try {
            //debuglog('Starting sync since=' + syncToken);
            if (this.currentSyncRequest === null) {
                this.currentSyncRequest = this.doSyncRequest(syncOptions, syncToken);
            }
            data = await this.currentSyncRequest;
        } catch (e) {
            this.onSyncError(e, syncOptions);
            return;
        } finally {
            this.currentSyncRequest = null;
        }

        //debuglog('Completed sync, next_batch=' + data.next_batch);

        // set the sync token NOW *before* processing the events. We do this so
        // if something barfs on an event we can skip it rather than constantly
        // polling with the same token.
        client.store.setSyncToken(data.next_batch);

        // Reset after a successful sync
        this.failedSyncCount = 0;

        await client.store.setSyncData(data);

        const syncEventData = {
            oldSyncToken: syncToken,
            nextSyncToken: data.next_batch,
            catchingUp: this.catchingUp,
        };

        if (this.opts.crypto) {
            // tell the crypto module we're about to process a sync
            // response
            await this.opts.crypto.onSyncWillProcess(syncEventData);
        }

        try {
            await this.processSyncResponse(syncEventData, data);
        } catch (e) {
            // log the exception with stack if we have it, else fall back
            // to the plain description
            logger.error("Caught /sync error", e);

            // Emit the exception for client handling
            this.client.emit(ClientEvent.SyncUnexpectedError, e);
        }

        // update this as it may have changed
        syncEventData.catchingUp = this.catchingUp;

        // emit synced events
        if (!syncOptions.hasSyncedBefore) {
            this.updateSyncState(SyncState.Prepared, syncEventData);
            syncOptions.hasSyncedBefore = true;
        }

        // tell the crypto module to do its processing. It may block (to do a
        // /keys/changes request).
        if (this.opts.crypto) {
            await this.opts.crypto.onSyncCompleted(syncEventData);
        }

        // keep emitting SYNCING -> SYNCING for clients who want to do bulk updates
        this.updateSyncState(SyncState.Syncing, syncEventData);

        if (client.store.wantsSave()) {
            // We always save the device list (if it's dirty) before saving the sync data:
            // this means we know the saved device list data is at least as fresh as the
            // stored sync data which means we don't have to worry that we may have missed
            // device changes. We can also skip the delay since we're not calling this very
            // frequently (and we don't really want to delay the sync for it).
            if (this.opts.crypto) {
                await this.opts.crypto.saveDeviceList(0);
            }

            // tell databases that everything is now in a consistent state and can be saved.
            client.store.save();
        }

        // Begin next sync
        this.doSync(syncOptions);
    }

    private doSyncRequest(syncOptions: ISyncOptions, syncToken: string): IAbortablePromise<ISyncResponse> {
        const qps = this.getSyncParams(syncOptions, syncToken);
        return this.client.http.authedRequest<ISyncResponse>(
            undefined, Method.Get, "/sync", qps as any, undefined,
            qps.timeout + BUFFER_PERIOD_MS,
        );
    }

    private getSyncParams(syncOptions: ISyncOptions, syncToken: string): ISyncParams {
        let pollTimeout = this.opts.pollTimeout;

        if (this.getSyncState() !== 'SYNCING' || this.catchingUp) {
            // unless we are happily syncing already, we want the server to return
            // as quickly as possible, even if there are no events queued. This
            // serves two purposes:
            //
            // * When the connection dies, we want to know asap when it comes back,
            //   so that we can hide the error from the user. (We don't want to
            //   have to wait for an event or a timeout).
            //
            // * We want to know if the server has any to_device messages queued up
            //   for us. We do that by calling it with a zero timeout until it
            //   doesn't give us any more to_device messages.
            this.catchingUp = true;
            pollTimeout = 0;
        }

        let filterId = syncOptions.filterId;
        if (this.client.isGuest() && !filterId) {
            filterId = this.getGuestFilter();
        }

        const qps: ISyncParams = {
            filter: filterId,
            timeout: pollTimeout,
        };

        if (this.opts.disablePresence) {
            qps.set_presence = SetPresence.Offline;
        }

        if (syncToken) {
            qps.since = syncToken;
        } else {
            // use a cachebuster for initialsyncs, to make sure that
            // we don't get a stale sync
            // (https://github.com/vector-im/vector-web/issues/1354)
            qps._cacheBuster = Date.now();
        }

        if (this.getSyncState() == 'ERROR' || this.getSyncState() == 'RECONNECTING') {
            // we think the connection is dead. If it comes back up, we won't know
            // about it till /sync returns. If the timeout= is high, this could
            // be a long time. Set it to 0 when doing retries so we don't have to wait
            // for an event or a timeout before emiting the SYNCING event.
            qps.timeout = 0;
        }

        return qps;
    }

    private onSyncError(err: MatrixError, syncOptions: ISyncOptions): void {
        if (!this.running) {
            debuglog("Sync no longer running: exiting");
            if (this.connectionReturnedDefer) {
                this.connectionReturnedDefer.reject();
                this.connectionReturnedDefer = null;
            }
            this.updateSyncState(SyncState.Stopped);
            return;
        }

        logger.error("/sync error %s", err);
        logger.error(err);

        if (this.shouldAbortSync(err)) {
            return;
        }

        this.failedSyncCount++;
        logger.log('Number of consecutive failed sync requests:', this.failedSyncCount);

        debuglog("Starting keep-alive");
        // Note that we do *not* mark the sync connection as
        // lost yet: we only do this if a keepalive poke
        // fails, since long lived HTTP connections will
        // go away sometimes and we shouldn't treat this as
        // erroneous. We set the state to 'reconnecting'
        // instead, so that clients can observe this state
        // if they wish.
        this.startKeepAlives().then((connDidFail) => {
            // Only emit CATCHUP if we detected a connectivity error: if we didn't,
            // it's quite likely the sync will fail again for the same reason and we
            // want to stay in ERROR rather than keep flip-flopping between ERROR
            // and CATCHUP.
            if (connDidFail && this.getSyncState() === SyncState.Error) {
                this.updateSyncState(SyncState.Catchup, {
                    oldSyncToken: null,
                    nextSyncToken: null,
                    catchingUp: true,
                });
            }
            this.doSync(syncOptions);
        });

        this.currentSyncRequest = null;
        // Transition from RECONNECTING to ERROR after a given number of failed syncs
        this.updateSyncState(
            this.failedSyncCount >= FAILED_SYNC_ERROR_THRESHOLD ?
                SyncState.Error : SyncState.Reconnecting,
            { error: err },
        );
    }

    /**
     * Process data returned from a sync response and propagate it
     * into the model objects
     *
     * @param {Object} syncEventData Object containing sync tokens associated with this sync
     * @param {Object} data The response from /sync
     */
    private async processSyncResponse(syncEventData: ISyncStateData, data: ISyncResponse): Promise<void> {
        const client = this.client;

        // data looks like:
        // {
        //    next_batch: $token,
        //    presence: { events: [] },
        //    account_data: { events: [] },
        //    device_lists: { changed: ["@user:server", ... ]},
        //    to_device: { events: [] },
        //    device_one_time_keys_count: { signed_curve25519: 42 },
        //    rooms: {
        //      invite: {
        //        $roomid: {
        //          invite_state: { events: [] }
        //        }
        //      },
        //      join: {
        //        $roomid: {
        //          state: { events: [] },
        //          timeline: { events: [], prev_batch: $token, limited: true },
        //          ephemeral: { events: [] },
        //          summary: {
        //             m.heroes: [ $user_id ],
        //             m.joined_member_count: $count,
        //             m.invited_member_count: $count
        //          },
        //          account_data: { events: [] },
        //          unread_notifications: {
        //              highlight_count: 0,
        //              notification_count: 0,
        //          }
        //        }
        //      },
        //      leave: {
        //        $roomid: {
        //          state: { events: [] },
        //          timeline: { events: [], prev_batch: $token }
        //        }
        //      }
        //    }
        // }

        // TODO-arch:
        // - Each event we pass through needs to be emitted via 'event', can we
        //   do this in one place?
        // - The isBrandNewRoom boilerplate is boilerplatey.

        // handle presence events (User objects)
        if (data.presence && Array.isArray(data.presence.events)) {
            data.presence.events.map(client.getEventMapper()).forEach(
                function(presenceEvent) {
                    let user = client.store.getUser(presenceEvent.getSender());
                    if (user) {
                        user.setPresenceEvent(presenceEvent);
                    } else {
                        user = createNewUser(client, presenceEvent.getSender());
                        user.setPresenceEvent(presenceEvent);
                        client.store.storeUser(user);
                    }
                    client.emit(ClientEvent.Event, presenceEvent);
                });
        }

        // handle non-room account_data
        if (data.account_data && Array.isArray(data.account_data.events)) {
            const events = data.account_data.events.map(client.getEventMapper());
            const prevEventsMap = events.reduce((m, c) => {
                m[c.getId()] = client.store.getAccountData(c.getType());
                return m;
            }, {});
            client.store.storeAccountDataEvents(events);
            events.forEach(
                function(accountDataEvent) {
                    // Honour push rules that come down the sync stream but also
                    // honour push rules that were previously cached. Base rules
                    // will be updated when we receive push rules via getPushRules
                    // (see sync) before syncing over the network.
                    if (accountDataEvent.getType() === EventType.PushRules) {
                        const rules = accountDataEvent.getContent<IPushRules>();
                        client.pushRules = PushProcessor.rewriteDefaultRules(rules);
                    }
                    const prevEvent = prevEventsMap[accountDataEvent.getId()];
                    client.emit(ClientEvent.AccountData, accountDataEvent, prevEvent);
                    return accountDataEvent;
                },
            );
        }

        // handle to-device events
        if (Array.isArray(data.to_device?.events) && data.to_device.events.length > 0) {
            const cancelledKeyVerificationTxns = [];
            data.to_device.events
                .map(client.getEventMapper())
                .map((toDeviceEvent) => { // map is a cheap inline forEach
                    // We want to flag m.key.verification.start events as cancelled
                    // if there's an accompanying m.key.verification.cancel event, so
                    // we pull out the transaction IDs from the cancellation events
                    // so we can flag the verification events as cancelled in the loop
                    // below.
                    if (toDeviceEvent.getType() === "m.key.verification.cancel") {
                        const txnId = toDeviceEvent.getContent()['transaction_id'];
                        if (txnId) {
                            cancelledKeyVerificationTxns.push(txnId);
                        }
                    }

                    // as mentioned above, .map is a cheap inline forEach, so return
                    // the unmodified event.
                    return toDeviceEvent;
                })
                .forEach(
                    function(toDeviceEvent) {
                        const content = toDeviceEvent.getContent();
                        if (
                            toDeviceEvent.getType() == "m.room.message" &&
                            content.msgtype == "m.bad.encrypted"
                        ) {
                            // the mapper already logged a warning.
                            logger.log(
                                'Ignoring undecryptable to-device event from ' +
                                toDeviceEvent.getSender(),
                            );
                            return;
                        }

                        if (toDeviceEvent.getType() === "m.key.verification.start"
                            || toDeviceEvent.getType() === "m.key.verification.request") {
                            const txnId = content['transaction_id'];
                            if (cancelledKeyVerificationTxns.includes(txnId)) {
                                toDeviceEvent.flagCancelled();
                            }
                        }

                        client.emit(ClientEvent.ToDeviceEvent, toDeviceEvent);
                    },
                );
        } else {
            // no more to-device events: we can stop polling with a short timeout.
            this.catchingUp = false;
        }

        // the returned json structure is a bit crap, so make it into a
        // nicer form (array) after applying sanity to make sure we don't fail
        // on missing keys (on the off chance)
        let inviteRooms: WrappedRoom<IInvitedRoom>[] = [];
        let joinRooms: WrappedRoom<IJoinedRoom>[] = [];
        let leaveRooms: WrappedRoom<ILeftRoom>[] = [];

        if (data.rooms) {
            if (data.rooms.invite) {
                inviteRooms = this.mapSyncResponseToRoomArray(data.rooms.invite);
            }
            if (data.rooms.join) {
                joinRooms = this.mapSyncResponseToRoomArray(data.rooms.join);
            }
            if (data.rooms.leave) {
                leaveRooms = this.mapSyncResponseToRoomArray(data.rooms.leave);
            }
        }

        this.notifEvents = [];

        // Handle invites
        await utils.promiseMapSeries(inviteRooms, async (inviteObj) => {
            const room = inviteObj.room;
            const stateEvents = this.mapSyncEventsFormat(inviteObj.invite_state, room);

            await this.processRoomEvents(room, stateEvents);
            if (inviteObj.isBrandNewRoom) {
                room.recalculate();
                client.store.storeRoom(room);
                client.emit(ClientEvent.Room, room);
            } else {
                // Update room state for invite->reject->invite cycles
                room.recalculate();
            }
            stateEvents.forEach(function(e) {
                client.emit(ClientEvent.Event, e);
            });
        });

        // Handle joins
        await utils.promiseMapSeries(joinRooms, async (joinObj) => {
            const room = joinObj.room;
            const stateEvents = this.mapSyncEventsFormat(joinObj.state, room);
            // Prevent events from being decrypted ahead of time
            // this helps large account to speed up faster
            // room::decryptCriticalEvent is in charge of decrypting all the events
            // required for a client to function properly
            const events = this.mapSyncEventsFormat(joinObj.timeline, room, false);
            const ephemeralEvents = this.mapSyncEventsFormat(joinObj.ephemeral);
            const accountDataEvents = this.mapSyncEventsFormat(joinObj.account_data);

            const encrypted = client.isRoomEncrypted(room.roomId);
            // we do this first so it's correct when any of the events fire
            if (joinObj.unread_notifications) {
                room.setUnreadNotificationCount(
                    NotificationCountType.Total,
                    joinObj.unread_notifications.notification_count,
                );

                // We track unread notifications ourselves in encrypted rooms, so don't
                // bother setting it here. We trust our calculations better than the
                // server's for this case, and therefore will assume that our non-zero
                // count is accurate.
                if (!encrypted
                    || (encrypted && room.getUnreadNotificationCount(NotificationCountType.Highlight) <= 0)) {
                    room.setUnreadNotificationCount(
                        NotificationCountType.Highlight,
                        joinObj.unread_notifications.highlight_count,
                    );
                }
            }

            joinObj.timeline = joinObj.timeline || {} as ITimeline;

            if (joinObj.isBrandNewRoom) {
                // set the back-pagination token. Do this *before* adding any
                // events so that clients can start back-paginating.
                room.getLiveTimeline().setPaginationToken(
                    joinObj.timeline.prev_batch, EventTimeline.BACKWARDS);
            } else if (joinObj.timeline.limited) {
                let limited = true;

                // we've got a limited sync, so we *probably* have a gap in the
                // timeline, so should reset. But we mig