browser/components/sessionstore/ContentSessionStore.jsm
author Jan Henning <jh+bugzilla@buttercookie.de>
Fri, 11 Jan 2019 19:50:09 +0000
changeset 453553 39207d39e5c2
parent 452560 5088f1dd3230
child 454282 67fa6cd74c6a
permissions -rw-r--r--
Bug 1498812 - Part 9: Switch session store/session history to use visual viewport for scroll position tracking. r=mikedeboer,snorp For simplicity's sake, for now we keep storing only one scroll position per history entry (bug 1499210), so if we have to choose between the layout and the visual viewport, the latter is a vastly better choice, as it more accurately represents the scroll position as perceived by the user, especially when the page has been pinch-zoomed. This also means that instead of the normal scroll events, the session store now has to listen for the corresponding events specific to the visual viewport. We also extend the scroll position test to check that the scroll position isn't just properly saved, but also actually properly restored in practice as well. We only add this test now instead of already adding it beforehand like we did with the rest of the test - to avoid having to temporarily extend the checkScroll() helper function to deal with todo()/todo_is etc. - because getting that part of the test to complete without timing out (which would be one of its natural failure modes, because the expected events would be missing) would require faking even more scroll events - because we already have the todo() tests that are telling us the we didn't *store* any scroll position in the first place, so there's no point in trying to actually restore anything For the GeckoView saveAndRestoreState test, we now spin the event loop once before setting the scroll position in order to give APZ opportunity to settle down after the initial page load. Differential Revision: https://phabricator.services.mozilla.com/D15690

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

"use strict";

var EXPORTED_SYMBOLS = ["ContentSessionStore"];

ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm", this);
ChromeUtils.import("resource://gre/modules/Timer.jsm", this);
ChromeUtils.import("resource://gre/modules/Services.jsm", this);

function debug(msg) {
  Services.console.logStringMessage("SessionStoreContent: " + msg);
}

ChromeUtils.defineModuleGetter(this, "FormData",
  "resource://gre/modules/FormData.jsm");

ChromeUtils.defineModuleGetter(this, "ContentRestore",
  "resource:///modules/sessionstore/ContentRestore.jsm");
ChromeUtils.defineModuleGetter(this, "SessionHistory",
  "resource://gre/modules/sessionstore/SessionHistory.jsm");
ChromeUtils.defineModuleGetter(this, "SessionStorage",
  "resource:///modules/sessionstore/SessionStorage.jsm");

ChromeUtils.defineModuleGetter(this, "Utils",
  "resource://gre/modules/sessionstore/Utils.jsm");
const ssu = Cc["@mozilla.org/browser/sessionstore/utils;1"]
              .getService(Ci.nsISessionStoreUtils);

// A bound to the size of data to store for DOM Storage.
const DOM_STORAGE_LIMIT_PREF = "browser.sessionstore.dom_storage_limit";

// This pref controls whether or not we send updates to the parent on a timeout
// or not, and should only be used for tests or debugging.
const TIMEOUT_DISABLED_PREF = "browser.sessionstore.debug.no_auto_updates";

const PREF_INTERVAL = "browser.sessionstore.interval";

const kNoIndex = Number.MAX_SAFE_INTEGER;
const kLastIndex = Number.MAX_SAFE_INTEGER - 1;

/**
 * A function that will recursively call |cb| to collect data for all
 * non-dynamic frames in the current frame/docShell tree.
 */
function mapFrameTree(mm, callback) {
  let [data] = Utils.mapFrameTree(mm.content, callback);
  return data;
}

class Handler {
  constructor(store) {
    this.store = store;
  }

  get contentRestore() {
    return this.store.contentRestore;
  }

  get contentRestoreInitialized() {
    return this.store.contentRestoreInitialized;
  }

  get mm() {
    return this.store.mm;
  }

  get messageQueue() {
    return this.store.messageQueue;
  }

  get stateChangeNotifier() {
    return this.store.stateChangeNotifier;
  }
}

/**
 * Listens for state change notifcations from webProgress and notifies each
 * registered observer for either the start of a page load, or its completion.
 */
class StateChangeNotifier extends Handler {
  constructor(store) {
    super(store);

    this._observers = new Set();
    let ifreq = this.mm.docShell.QueryInterface(Ci.nsIInterfaceRequestor);
    let webProgress = ifreq.getInterface(Ci.nsIWebProgress);
    webProgress.addProgressListener(this, Ci.nsIWebProgress.NOTIFY_STATE_DOCUMENT);
  }

  /**
   * Adds a given observer |obs| to the set of observers that will be notified
   * when when a new document starts or finishes loading.
   *
   * @param obs (object)
   */
  addObserver(obs) {
    this._observers.add(obs);
  }

  /**
   * Notifies all observers that implement the given |method|.
   *
   * @param method (string)
   */
  notifyObservers(method) {
    for (let obs of this._observers) {
      if (typeof obs[method] == "function") {
        obs[method]();
      }
    }
  }

  /**
   * @see nsIWebProgressListener.onStateChange
   */
  onStateChange(webProgress, request, stateFlags, status) {
    // Ignore state changes for subframes because we're only interested in the
    // top-document starting or stopping its load.
    if (!webProgress.isTopLevel || webProgress.DOMWindow != this.mm.content) {
      return;
    }

    // onStateChange will be fired when loading the initial about:blank URI for
    // a browser, which we don't actually care about. This is particularly for
    // the case of unrestored background tabs, where the content has not yet
    // been restored: we don't want to accidentally send any updates to the
    // parent when the about:blank placeholder page has loaded.
    if (!this.mm.docShell.hasLoadedNonBlankURI) {
      return;
    }

    if (stateFlags & Ci.nsIWebProgressListener.STATE_START) {
      this.notifyObservers("onPageLoadStarted");
    } else if (stateFlags & Ci.nsIWebProgressListener.STATE_STOP) {
      this.notifyObservers("onPageLoadCompleted");
    }
  }
}
StateChangeNotifier.prototype.QueryInterface =
  ChromeUtils.generateQI([Ci.nsIWebProgressListener,
                          Ci.nsISupportsWeakReference]);

/**
 * Listens for and handles content events that we need for the
 * session store service to be notified of state changes in content.
 */
class EventListener extends Handler {
  constructor(store) {
    super(store);

    ssu.addDynamicFrameFilteredListener(this.mm, "load", this, true);
  }

  handleEvent(event) {
    let {content} = this.mm;

    // Ignore load events from subframes.
    if (event.target != content.document) {
      return;
    }

    if (content.document.documentURI.startsWith("about:reader")) {
      if (event.type == "load" &&
          !content.document.body.classList.contains("loaded")) {
        // Don't restore the scroll position of an about:reader page at this
        // point; listen for the custom event dispatched from AboutReader.jsm.
        content.addEventListener("AboutReaderContentReady", this);
        return;
      }

      content.removeEventListener("AboutReaderContentReady", this);
    }

    if (this.contentRestoreInitialized) {
      // Restore the form data and scroll position. If we're not currently
      // restoring a tab state then this call will simply be a noop.
      this.contentRestore.restoreDocument();
    }
  }
}

/**
 * Listens for changes to the session history. Whenever the user navigates
 * we will collect URLs and everything belonging to session history.
 *
 * Causes a SessionStore:update message to be sent that contains the current
 * session history.
 *
 * Example:
 *   {entries: [{url: "about:mozilla", ...}, ...], index: 1}
 */
class SessionHistoryListener extends Handler {
  constructor(store) {
    super(store);

    this._fromIdx = kNoIndex;


    // The state change observer is needed to handle initial subframe loads.
    // It will redundantly invalidate with the SHistoryListener in some cases
    // but these invalidations are very cheap.
    this.stateChangeNotifier.addObserver(this);

    // By adding the SHistoryListener immediately, we will unfortunately be
    // notified of every history entry as the tab is restored. We don't bother
    // waiting to add the listener later because these notifications are cheap.
    // We will likely only collect once since we are batching collection on
    // a delay.
    this.mm.docShell.QueryInterface(Ci.nsIWebNavigation)
      .sessionHistory.legacySHistory.addSHistoryListener(this);

    // Collect data if we start with a non-empty shistory.
    if (!SessionHistory.isEmpty(this.mm.docShell)) {
      this.collect();
      // When a tab is detached from the window, for the new window there is a
      // new SessionHistoryListener created. Normally it is empty at this point
      // but in a test env. the initial about:blank might have a children in which
      // case we fire off a history message here with about:blank in it. If we
      // don't do it ASAP then there is going to be a browser swap and the parent
      // will be all confused by that message.
      this.messageQueue.send();
    }

    // Listen for page title changes.
    this.mm.addEventListener("DOMTitleChanged", this);
  }

  uninit() {
    let sessionHistory = this.mm.docShell.QueryInterface(Ci.nsIWebNavigation).sessionHistory;
    if (sessionHistory) {
      sessionHistory.legacySHistory.removeSHistoryListener(this);
    }
  }

  collect() {
    // We want to send down a historychange even for full collects in case our
    // session history is a partial session history, in which case we don't have
    // enough information for a full update. collectFrom(-1) tells the collect
    // function to collect all data avaliable in this process.
    if (this.mm.docShell) {
      this.collectFrom(-1);
    }
  }

  // History can grow relatively big with the nested elements, so if we don't have to, we
  // don't want to send the entire history all the time. For a simple optimization
  // we keep track of the smallest index from after any change has occured and we just send
  // the elements from that index. If something more complicated happens we just clear it
  // and send the entire history. We always send the additional info like the current selected
  // index (so for going back and forth between history entries we set the index to kLastIndex
  // if nothing else changed send an empty array and the additonal info like the selected index)
  collectFrom(idx) {
    if (this._fromIdx <= idx) {
      // If we already know that we need to update history fromn index N we can ignore any changes
      // tha happened with an element with index larger than N.
      // Note: initially we use kNoIndex which is MAX_SAFE_INTEGER which means we don't ignore anything
      // here, and in case of navigation in the history back and forth we use kLastIndex which ignores
      // only the subsequent navigations, but not any new elements added.
      return;
    }

    this._fromIdx = idx;
    this.messageQueue.push("historychange", () => {
      if (this._fromIdx === kNoIndex) {
        return null;
      }

      let history = SessionHistory.collect(this.mm.docShell, this._fromIdx);
      this._fromIdx = kNoIndex;
      return history;
    });
  }

  handleEvent(event) {
    this.collect();
  }

  onPageLoadCompleted() {
    this.collect();
  }

  onPageLoadStarted() {
    this.collect();
  }

  OnHistoryNewEntry(newURI, oldIndex) {
    // We ought to collect the previously current entry as well, see bug 1350567.
    this.collectFrom(oldIndex);
  }

  OnHistoryGotoIndex(index, gotoURI) {
    // We ought to collect the previously current entry as well, see bug 1350567.
    this.collectFrom(kLastIndex);
  }

  OnHistoryPurge(numEntries) {
    this.collect();
  }

  OnHistoryReload(reloadURI, reloadFlags) {
    this.collect();
    return true;
  }

  OnHistoryReplaceEntry(index) {
    this.collect();
  }
}
SessionHistoryListener.prototype.QueryInterface =
  ChromeUtils.generateQI([Ci.nsISHistoryListener,
                          Ci.nsISupportsWeakReference]);

/**
 * Listens for scroll position changes. Whenever the user scrolls the top-most
 * frame we update the scroll position and will restore it when requested.
 *
 * Causes a SessionStore:update message to be sent that contains the current
 * scroll positions as a tree of strings. If no frame of the whole frame tree
 * is scrolled this will return null so that we don't tack a property onto
 * the tabData object in the parent process.
 *
 * Example:
 *   {scroll: "100,100", children: [null, null, {scroll: "200,200"}]}
 */
class ScrollPositionListener extends Handler {
  constructor(store) {
    super(store);

    ssu.addDynamicFrameFilteredListener(this.mm, "mozvisualscroll", this,
                                        /* capture */ false, /* system group */ true);
    this.stateChangeNotifier.addObserver(this);
  }

  handleEvent() {
    this.messageQueue.push("scroll", () => this.collect());
  }

  onPageLoadCompleted() {
    this.messageQueue.push("scroll", () => this.collect());
  }

  onPageLoadStarted() {
    this.messageQueue.push("scroll", () => null);
  }

  collect() {
    return mapFrameTree(this.mm, ssu.collectScrollPosition.bind(ssu));
  }
}

/**
 * Listens for changes to input elements. Whenever the value of an input
 * element changes we will re-collect data for the current frame tree and send
 * a message to the parent process.
 *
 * Causes a SessionStore:update message to be sent that contains the form data
 * for all reachable frames.
 *
 * Example:
 *   {
 *     formdata: {url: "http://mozilla.org/", id: {input_id: "input value"}},
 *     children: [
 *       null,
 *       {url: "http://sub.mozilla.org/", id: {input_id: "input value 2"}}
 *     ]
 *   }
 */
class FormDataListener extends Handler {
  constructor(store) {
    super(store);

    ssu.addDynamicFrameFilteredListener(this.mm, "input", this, true);
    this.stateChangeNotifier.addObserver(this);
  }

  handleEvent() {
    this.messageQueue.push("formdata", () => this.collect());
  }

  onPageLoadStarted() {
    this.messageQueue.push("formdata", () => null);
  }

  collect() {
    return mapFrameTree(this.mm, FormData.collect);
  }
}

/**
 * Listens for changes to docShell capabilities. Whenever a new load is started
 * we need to re-check the list of capabilities and send message when it has
 * changed.
 *
 * Causes a SessionStore:update message to be sent that contains the currently
 * disabled docShell capabilities (all nsIDocShell.allow* properties set to
 * false) as a string - i.e. capability names separate by commas.
 */
class DocShellCapabilitiesListener extends Handler {
  constructor(store) {
    super(store);

    /**
     * This field is used to compare the last docShell capabilities to the ones
     * that have just been collected. If nothing changed we won't send a message.
     */
    this._latestCapabilities = "";

    this.stateChangeNotifier.addObserver(this);
  }

  onPageLoadStarted() {
    let caps = ssu.collectDocShellCapabilities(this.mm.docShell);

    // Send new data only when the capability list changes.
    if (caps != this._latestCapabilities) {
      this._latestCapabilities = caps;
      this.messageQueue.push("disallow", () => caps || null);
    }
  }
}

/**
 * Listens for changes to the DOMSessionStorage. Whenever new keys are added,
 * existing ones removed or changed, or the storage is cleared we will send a
 * message to the parent process containing up-to-date sessionStorage data.
 *
 * Causes a SessionStore:update message to be sent that contains the current
 * DOMSessionStorage contents. The data is a nested object using host names
 * as keys and per-host DOMSessionStorage data as values.
 */
class SessionStorageListener extends Handler {
  constructor(store) {
    super(store);

    // We don't want to send all the session storage data for all the frames
    // for every change. So if only a few value changed we send them over as
    // a "storagechange" event. If however for some reason before we send these
    // changes we have to send over the entire sessions storage data, we just
    // reset these changes.
    this._changes = undefined;

    // The event listener waiting for MozSessionStorageChanged events.
    this._listener = null;

    Services.obs.addObserver(this, "browser:purge-domain-data");
    this.stateChangeNotifier.addObserver(this);
    this.resetEventListener();
  }

  uninit() {
    Services.obs.removeObserver(this, "browser:purge-domain-data");
  }

  observe() {
    // Collect data on the next tick so that any other observer
    // that needs to purge data can do its work first.
    setTimeoutWithTarget(() => this.collect(), 0, this.mm.tabEventTarget);
  }

  resetChanges() {
    this._changes = undefined;
  }

  resetEventListener() {
    if (!this._listener) {
      this._listener =
        ssu.addDynamicFrameFilteredListener(this.mm, "MozSessionStorageChanged",
                                            this, true);
    }
  }

  removeEventListener() {
    ssu.removeDynamicFrameFilteredListener(this.mm, "MozSessionStorageChanged",
                                           this._listener, true);
    this._listener = null;
  }

  handleEvent(event) {
    if (!this.mm.docShell) {
      return;
    }

    let {content} = this.mm;

    // How much data does DOMSessionStorage contain?
    let usage = content.windowUtils.getStorageUsage(event.storageArea);

    // Don't store any data if we exceed the limit. Wipe any data we previously
    // collected so that we don't confuse websites with partial state.
    if (usage > Services.prefs.getIntPref(DOM_STORAGE_LIMIT_PREF)) {
      this.messageQueue.push("storage", () => null);
      this.removeEventListener();
      this.resetChanges();
      return;
    }

    let {url, key, newValue} = event;
    let uri = Services.io.newURI(url);
    let domain = uri.prePath;
    if (!this._changes) {
      this._changes = {};
    }
    if (!this._changes[domain]) {
      this._changes[domain] = {};
    }

    // If the key isn't defined, then .clear() was called, and we send
    // up null for this domain to indicate that storage has been cleared
    // for it.
    if (!key) {
      this._changes[domain] = null;
    } else {
      this._changes[domain][key] = newValue;
    }

    this.messageQueue.push("storagechange", () => {
      let tmp = this._changes;
      // If there were multiple changes we send them merged.
      // First one will collect all the changes the rest of
      // these messages will be ignored.
      this.resetChanges();
      return tmp;
    });
  }

  collect() {
    if (!this.mm.docShell) {
      return;
    }

    let {content} = this.mm;

    // We need the entire session storage, let's reset the pending individual change
    // messages.
    this.resetChanges();

    this.messageQueue.push("storage", () => SessionStorage.collect(content));
  }

  onPageLoadCompleted() {
    this.collect();
  }

  onPageLoadStarted() {
    this.resetEventListener();
    this.collect();
  }
}

/**
 * Listen for changes to the privacy status of the tab.
 * By definition, tabs start in non-private mode.
 *
 * Causes a SessionStore:update message to be sent for
 * field "isPrivate". This message contains
 *  |true| if the tab is now private
 *  |null| if the tab is now public - the field is therefore
 *  not saved.
 */
class PrivacyListener extends Handler {
  constructor(store) {
    super(store);

    this.mm.docShell.addWeakPrivacyTransitionObserver(this);

    // Check that value at startup as it might have
    // been set before the frame script was loaded.
    if (this.mm.docShell.QueryInterface(Ci.nsILoadContext).usePrivateBrowsing) {
      this.messageQueue.push("isPrivate", () => true);
    }
  }

  // Ci.nsIPrivacyTransitionObserver
  privateModeChanged(enabled) {
    this.messageQueue.push("isPrivate", () => enabled || null);
  }
}
PrivacyListener.prototype.QueryInterface =
  ChromeUtils.generateQI([Ci.nsIPrivacyTransitionObserver,
                          Ci.nsISupportsWeakReference]);

/**
 * A message queue that takes collected data and will take care of sending it
 * to the chrome process. It allows flushing using synchronous messages and
 * takes care of any race conditions that might occur because of that. Changes
 * will be batched if they're pushed in quick succession to avoid a message
 * flood.
 */
class MessageQueue extends Handler {
  constructor(store) {
    super(store);

    /**
     * A map (string -> lazy fn) holding lazy closures of all queued data
     * collection routines. These functions will return data collected from the
     * docShell.
     */
    this._data = new Map();

    /**
     * The delay (in ms) used to delay sending changes after data has been
     * invalidated.
     */
    this.BATCH_DELAY_MS = 1000;

    /**
     * The minimum idle period (in ms) we need for sending data to chrome process.
     */
    this.NEEDED_IDLE_PERIOD_MS = 5;

    /**
     * Timeout for waiting an idle period to send data. We will set this from
     * the pref "browser.sessionstore.interval".
     */
    this._timeoutWaitIdlePeriodMs = null;

    /**
     * The current timeout ID, null if there is no queue data. We use timeouts
     * to damp a flood of data changes and send lots of changes as one batch.
     */
    this._timeout = null;

    /**
     * Whether or not sending batched messages on a timer is disabled. This should
     * only be used for debugging or testing. If you need to access this value,
     * you should probably use the timeoutDisabled getter.
     */
    this._timeoutDisabled = false;

    /**
     * True if there is already a send pending idle dispatch, set to prevent
     * scheduling more than one. If false there may or may not be one scheduled.
     */
    this._idleScheduled = false;


    this.timeoutDisabled =
      Services.prefs.getBoolPref(TIMEOUT_DISABLED_PREF);
    this._timeoutWaitIdlePeriodMs =
      Services.prefs.getIntPref(PREF_INTERVAL);

    Services.prefs.addObserver(TIMEOUT_DISABLED_PREF, this);
    Services.prefs.addObserver(PREF_INTERVAL, this);
  }

  /**
   * True if batched messages are not being fired on a timer. This should only
   * ever be true when debugging or during tests.
   */
  get timeoutDisabled() {
    return this._timeoutDisabled;
  }

  /**
   * Disables sending batched messages on a timer. Also cancels any pending
   * timers.
   */
  set timeoutDisabled(val) {
    this._timeoutDisabled = val;

    if (val && this._timeout) {
      clearTimeout(this._timeout);
      this._timeout = null;
    }

    return val;
  }

  uninit() {
    Services.prefs.removeObserver(TIMEOUT_DISABLED_PREF, this);
    Services.prefs.removeObserver(PREF_INTERVAL, this);
    this.cleanupTimers();
  }

  /**
   * Cleanup pending idle callback and timer.
   */
  cleanupTimers() {
    this._idleScheduled = false;
    if (this._timeout) {
      clearTimeout(this._timeout);
      this._timeout = null;
    }
  }

  observe(subject, topic, data) {
    if (topic == "nsPref:changed") {
      switch (data) {
        case TIMEOUT_DISABLED_PREF:
          this.timeoutDisabled =
            Services.prefs.getBoolPref(TIMEOUT_DISABLED_PREF);
          break;
        case PREF_INTERVAL:
          this._timeoutWaitIdlePeriodMs =
            Services.prefs.getIntPref(PREF_INTERVAL);
          break;
        default:
          debug("received unknown message '" + data + "'");
          break;
      }
    }
  }

  /**
   * Pushes a given |value| onto the queue. The given |key| represents the type
   * of data that is stored and can override data that has been queued before
   * but has not been sent to the parent process, yet.
   *
   * @param key (string)
   *        A unique identifier specific to the type of data this is passed.
   * @param fn (function)
   *        A function that returns the value that will be sent to the parent
   *        process.
   */
  push(key, fn) {
    this._data.set(key, fn);

    if (!this._timeout && !this._timeoutDisabled) {
      // Wait a little before sending the message to batch multiple changes.
      this._timeout = setTimeoutWithTarget(
        () => this.sendWhenIdle(), this.BATCH_DELAY_MS, this.mm.tabEventTarget);
    }
  }

  /**
   * Sends queued data when the remaining idle time is enough or waiting too
   * long; otherwise, request an idle time again. If the |deadline| is not
   * given, this function is going to schedule the first request.
   *
   * @param deadline (object)
   *        An IdleDeadline object passed by idleDispatch().
   */
  sendWhenIdle(deadline) {
    if (!this.mm.content) {
      // The frameloader is being torn down. Nothing more to do.
      return;
    }

    if (deadline) {
      if (deadline.didTimeout || deadline.timeRemaining() > this.NEEDED_IDLE_PERIOD_MS) {
        this.send();
        return;
      }
    } else if (this._idleScheduled) {
      // Bail out if there's a pending run.
      return;
    }
    ChromeUtils.idleDispatch((deadline_) => this.sendWhenIdle(deadline_),
                             {timeout: this._timeoutWaitIdlePeriodMs});
    this._idleScheduled = true;
  }

  /**
   * Sends queued data to the chrome process.
   *
   * @param options (object)
   *        {flushID: 123} to specify that this is a flush
   *        {isFinal: true} to signal this is the final message sent on unload
   */
  send(options = {}) {
    // Looks like we have been called off a timeout after the tab has been
    // closed. The docShell is gone now and we can just return here as there
    // is nothing to do.
    if (!this.mm.docShell) {
      return;
    }

    this.cleanupTimers();

    let flushID = (options && options.flushID) || 0;
    let histID = "FX_SESSION_RESTORE_CONTENT_COLLECT_DATA_MS";

    let data = {};
    for (let [key, func] of this._data) {
      if (key != "isPrivate") {
        TelemetryStopwatch.startKeyed(histID, key);
      }

      let value = func();

      if (key != "isPrivate") {
        TelemetryStopwatch.finishKeyed(histID, key);
      }

      if (value || (key != "storagechange" && key != "historychange")) {
        data[key] = value;
      }
    }

    this._data.clear();

    try {
      // Send all data to the parent process.
      this.mm.sendAsyncMessage("SessionStore:update", {
        data, flushID,
        isFinal: options.isFinal || false,
        epoch: this.store.epoch,
      });
    } catch (ex) {
      if (ex && ex.result == Cr.NS_ERROR_OUT_OF_MEMORY) {
        Services.telemetry.getHistogramById("FX_SESSION_RESTORE_SEND_UPDATE_CAUSED_OOM").add(1);
        this.mm.sendAsyncMessage("SessionStore:error");
      }
    }
  }
}

/**
 * Listens for and handles messages sent by the session store service.
 */
const MESSAGES = [
  "SessionStore:restoreHistory",
  "SessionStore:restoreTabContent",
  "SessionStore:resetRestore",
  "SessionStore:flush",
  "SessionStore:becomeActiveProcess",
];

class ContentSessionStore {
  constructor(mm) {
    this.mm = mm;
    this.messageQueue = new MessageQueue(this);
    this.stateChangeNotifier = new StateChangeNotifier(this);

    this.epoch = 0;

    this.contentRestoreInitialized = false;

    XPCOMUtils.defineLazyGetter(this, "contentRestore",
                                () => {
                                  this.contentRestoreInitialized = true;
                                  return new ContentRestore(mm);
                                });

    this.handlers = [
      new EventListener(this),
      new FormDataListener(this),
      new SessionHistoryListener(this),
      new SessionStorageListener(this),
      new ScrollPositionListener(this),
      new DocShellCapabilitiesListener(this),
      new PrivacyListener(this),
      this.stateChangeNotifier,
      this.messageQueue,
    ];

    MESSAGES.forEach(m => mm.addMessageListener(m, this));

    // If we're browsing from the tab crashed UI to a blacklisted URI that keeps
    // this browser non-remote, we'll handle that in a pagehide event.
    mm.addEventListener("pagehide", this);
    mm.addEventListener("unload", this);
  }

  receiveMessage({name, data}) {
    // The docShell might be gone. Don't process messages,
    // that will just lead to errors anyway.
    if (!this.mm.docShell) {
      return;
    }

    // A fresh tab always starts with epoch=0. The parent has the ability to
    // override that to signal a new era in this tab's life. This enables it
    // to ignore async messages that were already sent but not yet received
    // and would otherwise confuse the internal tab state.
    if (data.epoch && data.epoch != this.epoch) {
      this.epoch = data.epoch;
    }

    switch (name) {
      case "SessionStore:restoreHistory":
        this.restoreHistory(data);
        break;
      case "SessionStore:restoreTabContent":
        if (data.isRemotenessUpdate) {
          let histogram = Services.telemetry.getKeyedHistogramById("FX_TAB_REMOTE_NAVIGATION_DELAY_MS");
          histogram.add("SessionStore:restoreTabContent",
                        Services.telemetry.msSystemNow() - data.requestTime);
        }
        this.restoreTabContent(data);
        break;
      case "SessionStore:resetRestore":
        this.contentRestore.resetRestore();
        break;
      case "SessionStore:flush":
        this.flush(data);
        break;
      case "SessionStore:becomeActiveProcess":
        SessionHistoryListener.collect();
        break;
      default:
        debug("received unknown message '" + name + "'");
        break;
    }
  }

  restoreHistory({epoch, tabData, loadArguments, isRemotenessUpdate}) {
    this.contentRestore.restoreHistory(tabData, loadArguments, {
      // Note: The callbacks passed here will only be used when a load starts
      // that was not initiated by sessionstore itself. This can happen when
      // some code calls browser.loadURI() or browser.reload() on a pending
      // browser/tab.

      onLoadStarted: () => {
        // Notify the parent that the tab is no longer pending.
        this.mm.sendAsyncMessage("SessionStore:restoreTabContentStarted", {epoch});
      },

      onLoadFinished: () => {
        // Tell SessionStore.jsm that it may want to restore some more tabs,
        // since it restores a max of MAX_CONCURRENT_TAB_RESTORES at a time.
        this.mm.sendAsyncMessage("SessionStore:restoreTabContentComplete", {epoch});
      },
    });

    if (Services.appinfo.processType == Services.appinfo.PROCESS_TYPE_DEFAULT) {
      // For non-remote tabs, when restoreHistory finishes, we send a synchronous
      // message to SessionStore.jsm so that it can run SSTabRestoring. Users of
      // SSTabRestoring seem to get confused if chrome and content are out of
      // sync about the state of the restore (particularly regarding
      // docShell.currentURI). Using a synchronous message is the easiest way
      // to temporarily synchronize them.
      //
      // For remote tabs, because all nsIWebProgress notifications are sent
      // asynchronously using messages, we get the same-order guarantees of the
      // message manager, and can use an async message.
      this.mm.sendSyncMessage("SessionStore:restoreHistoryComplete", {epoch, isRemotenessUpdate});
    } else {
      this.mm.sendAsyncMessage("SessionStore:restoreHistoryComplete", {epoch, isRemotenessUpdate});
    }
  }

  restoreTabContent({loadArguments, isRemotenessUpdate, reason}) {
    let epoch = this.epoch;

    // We need to pass the value of didStartLoad back to SessionStore.jsm.
    let didStartLoad = this.contentRestore.restoreTabContent(loadArguments, isRemotenessUpdate, () => {
      // Tell SessionStore.jsm that it may want to restore some more tabs,
      // since it restores a max of MAX_CONCURRENT_TAB_RESTORES at a time.
      this.mm.sendAsyncMessage("SessionStore:restoreTabContentComplete", {epoch, isRemotenessUpdate});
    });

    this.mm.sendAsyncMessage("SessionStore:restoreTabContentStarted", {
      epoch, isRemotenessUpdate, reason,
    });

    if (!didStartLoad) {
      // Pretend that the load succeeded so that event handlers fire correctly.
      this.mm.sendAsyncMessage("SessionStore:restoreTabContentComplete", {epoch, isRemotenessUpdate});
    }
  }

  flush({id}) {
    // Flush the message queue, send the latest updates.
    this.messageQueue.send({flushID: id});
  }

  handleEvent(event) {
    if (event.type == "pagehide") {
      this.handleRevivedTab();
    } else if (event.type == "unload") {
      this.onUnload();
    }
  }

  onUnload() {
    // Upon frameLoader destruction, send a final update message to
    // the parent and flush all data currently held in the child.
    this.messageQueue.send({isFinal: true});

    // If we're browsing from the tab crashed UI to a URI that causes the tab
    // to go remote again, we catch this in the unload event handler, because
    // swapping out the non-remote browser for a remote one in
    // tabbrowser.xml's updateBrowserRemoteness doesn't cause the pagehide
    // event to be fired.
    this.handleRevivedTab();

    for (let handler of this.handlers) {
      if (handler.uninit) {
        handler.uninit();
      }
    }

    if (this.contentRestoreInitialized) {
      // Remove progress listeners.
      this.contentRestore.resetRestore();
    }

    // We don't need to take care of any StateChangeNotifier observers as they
    // will die with the content script. The same goes for the privacy transition
    // observer that will die with the docShell when the tab is closed.
  }

  handleRevivedTab() {
    let {content} = this.mm;

    if (!content) {
      this.mm.removeEventListener("pagehide", this);
      return;
    }

    if (content.document.documentURI.startsWith("about:tabcrashed")) {
      if (Services.appinfo.processType != Services.appinfo.PROCESS_TYPE_DEFAULT) {
        // Sanity check - we'd better be loading this in a non-remote browser.
        throw new Error("We seem to be navigating away from about:tabcrashed in " +
                        "a non-remote browser. This should really never happen.");
      }

      this.mm.removeEventListener("pagehide", this);

      // Notify the parent.
      this.mm.sendAsyncMessage("SessionStore:crashedTabRevived");
    }
  }
}