browser/components/sessionstore/SessionStore.jsm
author Mike Conley <mconley@mozilla.com>
Thu, 07 Jul 2016 15:04:52 -0400
changeset 304246 51efc2643b800dc65e759ba97e70f585392e413e
parent 303372 6cce38101acc9c26d99e31a6994a446f333ed973
child 305131 270f0c316a083f579519a00cc94e5be63e2fbfab
permissions -rw-r--r--
Bug 1284687 - Hide windows on shutdown while persisting session instead of closing them. r=billm We were closing the windows before to improve perceived shutdown performance, but we end up in a state where we're likely to miss out on the last ~2 seconds of session activity for most tabs per window. This is because we were removing the session update message listeners and resolving the flush Promises once the domwindowclosed notification fired for the window. Hiding the window allows us to wait for the messages properly. What's more, we weren't even collecting the window state after we had flushed, so we have _always_ been missing (in the worst case) about 2 seconds of session state per window. This addresses that. MozReview-Commit-ID: BEOIHV4EErf

/* 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";

this.EXPORTED_SYMBOLS = ["SessionStore"];

const Cu = Components.utils;
const Cc = Components.classes;
const Ci = Components.interfaces;
const Cr = Components.results;

// Current version of the format used by Session Restore.
const FORMAT_VERSION = 1;

const TAB_STATE_NEEDS_RESTORE = 1;
const TAB_STATE_RESTORING = 2;
const TAB_STATE_WILL_RESTORE = 3;

// A new window has just been restored. At this stage, tabs are generally
// not restored.
const NOTIFY_SINGLE_WINDOW_RESTORED = "sessionstore-single-window-restored";
const NOTIFY_WINDOWS_RESTORED = "sessionstore-windows-restored";
const NOTIFY_BROWSER_STATE_RESTORED = "sessionstore-browser-state-restored";
const NOTIFY_LAST_SESSION_CLEARED = "sessionstore-last-session-cleared";
const NOTIFY_RESTORING_ON_STARTUP = "sessionstore-restoring-on-startup";
const NOTIFY_INITIATING_MANUAL_RESTORE = "sessionstore-initiating-manual-restore";

const NOTIFY_TAB_RESTORED = "sessionstore-debug-tab-restored"; // WARNING: debug-only

// Maximum number of tabs to restore simultaneously. Previously controlled by
// the browser.sessionstore.max_concurrent_tabs pref.
const MAX_CONCURRENT_TAB_RESTORES = 3;

// Amount (in CSS px) by which we allow window edges to be off-screen
// when restoring a window, before we override the saved position to
// pull the window back within the available screen area.
const SCREEN_EDGE_SLOP = 8;

// global notifications observed
const OBSERVING = [
  "browser-window-before-show", "domwindowclosed",
  "quit-application-granted", "browser-lastwindow-close-granted",
  "quit-application", "browser:purge-session-history",
  "browser:purge-domain-data",
  "idle-daily",
];

// XUL Window properties to (re)store
// Restored in restoreDimensions()
const WINDOW_ATTRIBUTES = ["width", "height", "screenX", "screenY", "sizemode"];

// Hideable window features to (re)store
// Restored in restoreWindowFeatures()
const WINDOW_HIDEABLE_FEATURES = [
  "menubar", "toolbar", "locationbar", "personalbar", "statusbar", "scrollbars"
];

// Messages that will be received via the Frame Message Manager.
const MESSAGES = [
  // The content script sends us data that has been invalidated and needs to
  // be saved to disk.
  "SessionStore:update",

  // The restoreHistory code has run. This is a good time to run SSTabRestoring.
  "SessionStore:restoreHistoryComplete",

  // The load for the restoring tab has begun. We update the URL bar at this
  // time; if we did it before, the load would overwrite it.
  "SessionStore:restoreTabContentStarted",

  // All network loads for a restoring tab are done, so we should
  // consider restoring another tab in the queue. The document has
  // been restored, and forms have been filled. We trigger
  // SSTabRestored at this time.
  "SessionStore:restoreTabContentComplete",

  // A crashed tab was revived by navigating to a different page. Remove its
  // browser from the list of crashed browsers to stop ignoring its messages.
  "SessionStore:crashedTabRevived",

  // The content script encountered an error.
  "SessionStore:error",
];

// The list of messages we accept from <xul:browser>s that have no tab
// assigned, or whose windows have gone away. Those are for example the
// ones that preload about:newtab pages, or from browsers where the window
// has just been closed.
const NOTAB_MESSAGES = new Set([
  // For a description see above.
  "SessionStore:crashedTabRevived",

  // For a description see above.
  "SessionStore:update",

  // For a description see above.
  "SessionStore:error",
]);

// The list of messages we accept without an "epoch" parameter.
// See getCurrentEpoch() and friends to find out what an "epoch" is.
const NOEPOCH_MESSAGES = new Set([
  // For a description see above.
  "SessionStore:crashedTabRevived",

  // For a description see above.
  "SessionStore:error",
]);

// The list of messages we want to receive even during the short period after a
// frame has been removed from the DOM and before its frame script has finished
// unloading.
const CLOSED_MESSAGES = new Set([
  // For a description see above.
  "SessionStore:crashedTabRevived",

  // For a description see above.
  "SessionStore:update",

  // For a description see above.
  "SessionStore:error",
]);

// These are tab events that we listen to.
const TAB_EVENTS = [
  "TabOpen", "TabClose", "TabSelect", "TabShow", "TabHide", "TabPinned",
  "TabUnpinned"
];

const NS_XUL = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";

Cu.import("resource://gre/modules/Services.jsm", this);
Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
Cu.import("resource://gre/modules/TelemetryTimestamps.jsm", this);
Cu.import("resource://gre/modules/TelemetryStopwatch.jsm", this);
Cu.import("resource://gre/modules/osfile.jsm", this);
Cu.import("resource://gre/modules/PrivateBrowsingUtils.jsm", this);
Cu.import("resource://gre/modules/Promise.jsm", this);
Cu.import("resource://gre/modules/Task.jsm", this);
Cu.import("resource://gre/modules/debug.js", this);

XPCOMUtils.defineLazyServiceGetter(this, "gSessionStartup",
  "@mozilla.org/browser/sessionstartup;1", "nsISessionStartup");
XPCOMUtils.defineLazyServiceGetter(this, "gScreenManager",
  "@mozilla.org/gfx/screenmanager;1", "nsIScreenManager");
XPCOMUtils.defineLazyServiceGetter(this, "Telemetry",
  "@mozilla.org/base/telemetry;1", "nsITelemetry");
XPCOMUtils.defineLazyModuleGetter(this, "console",
  "resource://gre/modules/Console.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "RecentWindow",
  "resource:///modules/RecentWindow.jsm");

XPCOMUtils.defineLazyModuleGetter(this, "AppConstants",
  "resource://gre/modules/AppConstants.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "GlobalState",
  "resource:///modules/sessionstore/GlobalState.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "PrivacyFilter",
  "resource:///modules/sessionstore/PrivacyFilter.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "RunState",
  "resource:///modules/sessionstore/RunState.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "ScratchpadManager",
  "resource://devtools/client/scratchpad/scratchpad-manager.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "SessionSaver",
  "resource:///modules/sessionstore/SessionSaver.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "SessionCookies",
  "resource:///modules/sessionstore/SessionCookies.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "SessionFile",
  "resource:///modules/sessionstore/SessionFile.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "TabAttributes",
  "resource:///modules/sessionstore/TabAttributes.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "TabState",
  "resource:///modules/sessionstore/TabState.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "TabStateCache",
  "resource:///modules/sessionstore/TabStateCache.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "TabStateFlusher",
  "resource:///modules/sessionstore/TabStateFlusher.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "Utils",
  "resource:///modules/sessionstore/Utils.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "ViewSourceBrowser",
  "resource://gre/modules/ViewSourceBrowser.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "AsyncShutdown",
  "resource://gre/modules/AsyncShutdown.jsm");

/**
 * |true| if we are in debug mode, |false| otherwise.
 * Debug mode is controlled by preference browser.sessionstore.debug
 */
var gDebuggingEnabled = false;
function debug(aMsg) {
  if (gDebuggingEnabled) {
    aMsg = ("SessionStore: " + aMsg).replace(/\S{80}/g, "$&\n");
    Services.console.logStringMessage(aMsg);
  }
}

this.SessionStore = {
  get promiseInitialized() {
    return SessionStoreInternal.promiseInitialized;
  },

  get canRestoreLastSession() {
    return SessionStoreInternal.canRestoreLastSession;
  },

  set canRestoreLastSession(val) {
    SessionStoreInternal.canRestoreLastSession = val;
  },

  init: function ss_init() {
    SessionStoreInternal.init();
  },

  getBrowserState: function ss_getBrowserState() {
    return SessionStoreInternal.getBrowserState();
  },

  setBrowserState: function ss_setBrowserState(aState) {
    SessionStoreInternal.setBrowserState(aState);
  },

  getWindowState: function ss_getWindowState(aWindow) {
    return SessionStoreInternal.getWindowState(aWindow);
  },

  setWindowState: function ss_setWindowState(aWindow, aState, aOverwrite) {
    SessionStoreInternal.setWindowState(aWindow, aState, aOverwrite);
  },

  getTabState: function ss_getTabState(aTab) {
    return SessionStoreInternal.getTabState(aTab);
  },

  setTabState: function ss_setTabState(aTab, aState) {
    SessionStoreInternal.setTabState(aTab, aState);
  },

  duplicateTab: function ss_duplicateTab(aWindow, aTab, aDelta = 0) {
    return SessionStoreInternal.duplicateTab(aWindow, aTab, aDelta);
  },

  getClosedTabCount: function ss_getClosedTabCount(aWindow) {
    return SessionStoreInternal.getClosedTabCount(aWindow);
  },

  getClosedTabData: function ss_getClosedTabDataAt(aWindow) {
    return SessionStoreInternal.getClosedTabData(aWindow);
  },

  undoCloseTab: function ss_undoCloseTab(aWindow, aIndex) {
    return SessionStoreInternal.undoCloseTab(aWindow, aIndex);
  },

  forgetClosedTab: function ss_forgetClosedTab(aWindow, aIndex) {
    return SessionStoreInternal.forgetClosedTab(aWindow, aIndex);
  },

  getClosedWindowCount: function ss_getClosedWindowCount() {
    return SessionStoreInternal.getClosedWindowCount();
  },

  getClosedWindowData: function ss_getClosedWindowData() {
    return SessionStoreInternal.getClosedWindowData();
  },

  undoCloseWindow: function ss_undoCloseWindow(aIndex) {
    return SessionStoreInternal.undoCloseWindow(aIndex);
  },

  forgetClosedWindow: function ss_forgetClosedWindow(aIndex) {
    return SessionStoreInternal.forgetClosedWindow(aIndex);
  },

  getWindowValue: function ss_getWindowValue(aWindow, aKey) {
    return SessionStoreInternal.getWindowValue(aWindow, aKey);
  },

  setWindowValue: function ss_setWindowValue(aWindow, aKey, aStringValue) {
    SessionStoreInternal.setWindowValue(aWindow, aKey, aStringValue);
  },

  deleteWindowValue: function ss_deleteWindowValue(aWindow, aKey) {
    SessionStoreInternal.deleteWindowValue(aWindow, aKey);
  },

  getTabValue: function ss_getTabValue(aTab, aKey) {
    return SessionStoreInternal.getTabValue(aTab, aKey);
  },

  setTabValue: function ss_setTabValue(aTab, aKey, aStringValue) {
    SessionStoreInternal.setTabValue(aTab, aKey, aStringValue);
  },

  deleteTabValue: function ss_deleteTabValue(aTab, aKey) {
    SessionStoreInternal.deleteTabValue(aTab, aKey);
  },

  getGlobalValue: function ss_getGlobalValue(aKey) {
    return SessionStoreInternal.getGlobalValue(aKey);
  },

  setGlobalValue: function ss_setGlobalValue(aKey, aStringValue) {
    SessionStoreInternal.setGlobalValue(aKey, aStringValue);
  },

  deleteGlobalValue: function ss_deleteGlobalValue(aKey) {
    SessionStoreInternal.deleteGlobalValue(aKey);
  },

  persistTabAttribute: function ss_persistTabAttribute(aName) {
    SessionStoreInternal.persistTabAttribute(aName);
  },

  restoreLastSession: function ss_restoreLastSession() {
    SessionStoreInternal.restoreLastSession();
  },

  getCurrentState: function (aUpdateAll) {
    return SessionStoreInternal.getCurrentState(aUpdateAll);
  },

  reviveCrashedTab(aTab) {
    return SessionStoreInternal.reviveCrashedTab(aTab);
  },

  reviveAllCrashedTabs() {
    return SessionStoreInternal.reviveAllCrashedTabs();
  },

  navigateAndRestore(tab, loadArguments, historyIndex) {
    return SessionStoreInternal.navigateAndRestore(tab, loadArguments, historyIndex);
  },

  getSessionHistory(tab, updatedCallback) {
    return SessionStoreInternal.getSessionHistory(tab, updatedCallback);
  },

  /**
   * Determines whether the passed version number is compatible with
   * the current version number of the SessionStore.
   *
   * @param version The format and version of the file, as an array, e.g.
   * ["sessionrestore", 1]
   */
  isFormatVersionCompatible(version) {
    if (!version) {
      return false;
    }
    if (!Array.isArray(version)) {
      // Improper format.
      return false;
    }
    if (version[0] != "sessionrestore") {
      // Not a Session Restore file.
      return false;
    }
    let number = Number.parseFloat(version[1]);
    if (Number.isNaN(number)) {
      return false;
    }
    return number <= FORMAT_VERSION;
  },
};

// Freeze the SessionStore object. We don't want anyone to modify it.
Object.freeze(SessionStore);

var SessionStoreInternal = {
  QueryInterface: XPCOMUtils.generateQI([
    Ci.nsIDOMEventListener,
    Ci.nsIObserver,
    Ci.nsISupportsWeakReference
  ]),

  _globalState: new GlobalState(),

  // During the initial restore and setBrowserState calls tracks the number of
  // windows yet to be restored
  _restoreCount: -1,

  // For each <browser> element, records the current epoch.
  _browserEpochs: new WeakMap(),

  // Any browsers that fires the oop-browser-crashed event gets stored in
  // here - that way we know which browsers to ignore messages from (until
  // they get restored).
  _crashedBrowsers: new WeakSet(),

  // A map (xul:browser -> nsIFrameLoader) that maps a browser to the last
  // associated frameLoader we heard about.
  _lastKnownFrameLoader: new WeakMap(),

  // A map (xul:browser -> object) that maps a browser associated with a
  // recently closed tab to all its necessary state information we need to
  // properly handle final update message.
  _closedTabs: new WeakMap(),

  // A map (xul:browser -> object) that maps a browser associated with a
  // recently closed tab due to a window closure to the tab state information
  // that is being stored in _closedWindows for that tab.
  _closedWindowTabs: new WeakMap(),

  // A set of window data that has the potential to be saved in the _closedWindows
  // array for the session. We will remove window data from this set whenever
  // forgetClosedWindow is called for the window, or when session history is
  // purged, so that we don't accidentally save that data after the flush has
  // completed. Closed tabs use a more complicated mechanism for this particular
  // problem. When forgetClosedTab is called, the browser is removed from the
  // _closedTabs map, so its data is not recorded. In the purge history case,
  // the closedTabs array per window is overwritten so that once the flush is
  // complete, the tab would only ever add itself to an array that SessionStore
  // no longer cares about. Bug 1230636 has been filed to make the tab case
  // work more like the window case, which is more explicit, and easier to
  // reason about.
  _saveableClosedWindowData: new WeakSet(),

  // A map (xul:browser -> object) that maps a browser that is switching
  // remoteness via navigateAndRestore, to the loadArguments that were
  // most recently passed when calling navigateAndRestore.
  _remotenessChangingBrowsers: new WeakMap(),

  // whether a setBrowserState call is in progress
  _browserSetState: false,

  // time in milliseconds when the session was started (saved across sessions),
  // defaults to now if no session was restored or timestamp doesn't exist
  _sessionStartTime: Date.now(),

  // states for all currently opened windows
  _windows: {},

  // counter for creating unique window IDs
  _nextWindowID: 0,

  // states for all recently closed windows
  _closedWindows: [],

  // collection of session states yet to be restored
  _statesToRestore: {},

  // counts the number of crashes since the last clean start
  _recentCrashes: 0,

  // whether the last window was closed and should be restored
  _restoreLastWindow: false,

  // number of tabs currently restoring
  _tabsRestoringCount: 0,

  // When starting Firefox with a single private window, this is the place
  // where we keep the session we actually wanted to restore in case the user
  // decides to later open a non-private window as well.
  _deferredInitialState: null,

  // A promise resolved once initialization is complete
  _deferredInitialized: (function () {
    let deferred = {};

    deferred.promise = new Promise((resolve, reject) => {
      deferred.resolve = resolve;
      deferred.reject = reject;
    });

    return deferred;
  })(),

  // Whether session has been initialized
  _sessionInitialized: false,

  // Promise that is resolved when we're ready to initialize
  // and restore the session.
  _promiseReadyForInitialization: null,

  // Keep busy state counters per window.
  _windowBusyStates: new WeakMap(),

  /**
   * A promise fulfilled once initialization is complete.
   */
  get promiseInitialized() {
    return this._deferredInitialized.promise;
  },

  get canRestoreLastSession() {
    return LastSession.canRestore;
  },

  set canRestoreLastSession(val) {
    // Cheat a bit; only allow false.
    if (!val) {
      LastSession.clear();
    }
  },

  /**
   * Initialize the sessionstore service.
   */
  init: function () {
    if (this._initialized) {
      throw new Error("SessionStore.init() must only be called once!");
    }

    TelemetryTimestamps.add("sessionRestoreInitialized");
    OBSERVING.forEach(function(aTopic) {
      Services.obs.addObserver(this, aTopic, true);
    }, this);

    this._initPrefs();
    this._initialized = true;
  },

  /**
   * Initialize the session using the state provided by SessionStartup
   */
  initSession: function () {
    TelemetryStopwatch.start("FX_SESSION_RESTORE_STARTUP_INIT_SESSION_MS");
    let state;
    let ss = gSessionStartup;

    if (ss.doRestore() ||
        ss.sessionType == Ci.nsISessionStartup.DEFER_SESSION) {
      state = ss.state;
    }

    if (state) {
      try {
        // If we're doing a DEFERRED session, then we want to pull pinned tabs
        // out so they can be restored.
        if (ss.sessionType == Ci.nsISessionStartup.DEFER_SESSION) {
          let [iniState, remainingState] = this._prepDataForDeferredRestore(state);
          // If we have a iniState with windows, that means that we have windows
          // with app tabs to restore.
          if (iniState.windows.length)
            state = iniState;
          else
            state = null;

          if (remainingState.windows.length) {
            LastSession.setState(remainingState);
          }
        }
        else {
          // Get the last deferred session in case the user still wants to
          // restore it
          LastSession.setState(state.lastSessionState);

          if (ss.previousSessionCrashed) {
            this._recentCrashes = (state.session &&
                                   state.session.recentCrashes || 0) + 1;

            if (this._needsRestorePage(state, this._recentCrashes)) {
              // replace the crashed session with a restore-page-only session
              let url = "about:sessionrestore";
              let formdata = {id: {sessionData: state}, url};
              state = { windows: [{ tabs: [{ entries: [{url}], formdata }] }] };
            } else if (this._hasSingleTabWithURL(state.windows,
                                                 "about:welcomeback")) {
              // On a single about:welcomeback URL that crashed, replace about:welcomeback
              // with about:sessionrestore, to make clear to the user that we crashed.
              state.windows[0].tabs[0].entries[0].url = "about:sessionrestore";
            }
          }

          // Update the session start time using the restored session state.
          this._updateSessionStartTime(state);

          // make sure that at least the first window doesn't have anything hidden
          delete state.windows[0].hidden;
          // Since nothing is hidden in the first window, it cannot be a popup
          delete state.windows[0].isPopup;
          // We don't want to minimize and then open a window at startup.
          if (state.windows[0].sizemode == "minimized")
            state.windows[0].sizemode = "normal";
          // clear any lastSessionWindowID attributes since those don't matter
          // during normal restore
          state.windows.forEach(function(aWindow) {
            delete aWindow.__lastSessionWindowID;
          });
        }
      }
      catch (ex) { debug("The session file is invalid: " + ex); }
    }

    // at this point, we've as good as resumed the session, so we can
    // clear the resume_session_once flag, if it's set
    if (!RunState.isQuitting &&
        this._prefBranch.getBoolPref("sessionstore.resume_session_once"))
      this._prefBranch.setBoolPref("sessionstore.resume_session_once", false);

    TelemetryStopwatch.finish("FX_SESSION_RESTORE_STARTUP_INIT_SESSION_MS");
    return state;
  },

  _initPrefs : function() {
    this._prefBranch = Services.prefs.getBranch("browser.");

    gDebuggingEnabled = this._prefBranch.getBoolPref("sessionstore.debug");

    Services.prefs.addObserver("browser.sessionstore.debug", () => {
      gDebuggingEnabled = this._prefBranch.getBoolPref("sessionstore.debug");
    }, false);

    this._max_tabs_undo = this._prefBranch.getIntPref("sessionstore.max_tabs_undo");
    this._prefBranch.addObserver("sessionstore.max_tabs_undo", this, true);

    this._max_windows_undo = this._prefBranch.getIntPref("sessionstore.max_windows_undo");
    this._prefBranch.addObserver("sessionstore.max_windows_undo", this, true);
  },

  /**
   * Called on application shutdown, after notifications:
   * quit-application-granted, quit-application
   */
  _uninit: function ssi_uninit() {
    if (!this._initialized) {
      throw new Error("SessionStore is not initialized.");
    }

    // Prepare to close the session file and write the last state.
    RunState.setClosing();

    // save all data for session resuming
    if (this._sessionInitialized) {
      SessionSaver.run();
    }

    // clear out priority queue in case it's still holding refs
    TabRestoreQueue.reset();

    // Make sure to cancel pending saves.
    SessionSaver.cancel();
  },

  /**
   * Handle notifications
   */
  observe: function ssi_observe(aSubject, aTopic, aData) {
    switch (aTopic) {
      case "browser-window-before-show": // catch new windows
        this.onBeforeBrowserWindowShown(aSubject);
        break;
      case "domwindowclosed": // catch closed windows
        this.onClose(aSubject);
        break;
      case "quit-application-granted":
        let syncShutdown = aData == "syncShutdown";
        this.onQuitApplicationGranted(syncShutdown);
        break;
      case "browser-lastwindow-close-granted":
        this.onLastWindowCloseGranted();
        break;
      case "quit-application":
        this.onQuitApplication(aData);
        break;
      case "browser:purge-session-history": // catch sanitization
        this.onPurgeSessionHistory();
        break;
      case "browser:purge-domain-data":
        this.onPurgeDomainData(aData);
        break;
      case "nsPref:changed": // catch pref changes
        this.onPrefChange(aData);
        break;
      case "idle-daily":
        this.onIdleDaily();
        break;
    }
  },

  /**
   * This method handles incoming messages sent by the session store content
   * script via the Frame Message Manager or Parent Process Message Manager,
   * and thus enables communication with OOP tabs.
   */
  receiveMessage(aMessage) {
    // If we got here, that means we're dealing with a frame message
    // manager message, so the target will be a <xul:browser>.
    var browser = aMessage.target;
    let win = browser.ownerDocument.defaultView;
    let tab = win ? win.gBrowser.getTabForBrowser(browser) : null;

    // Ensure we receive only specific messages from <xul:browser>s that
    // have no tab or window assigned, e.g. the ones that preload
    // about:newtab pages, or windows that have closed.
    if (!tab && !NOTAB_MESSAGES.has(aMessage.name)) {
      throw new Error(`received unexpected message '${aMessage.name}' ` +
                      `from a browser that has no tab or window`);
    }

    let data = aMessage.data || {};
    let hasEpoch = data.hasOwnProperty("epoch");

    // Most messages sent by frame scripts require to pass an epoch.
    if (!hasEpoch && !NOEPOCH_MESSAGES.has(aMessage.name)) {
      throw new Error(`received message '${aMessage.name}' without an epoch`);
    }

    // Ignore messages from previous epochs.
    if (hasEpoch && !this.isCurrentEpoch(browser, data.epoch)) {
      return;
    }

    switch (aMessage.name) {
      case "SessionStore:update":
        // |browser.frameLoader| might be empty if the browser was already
        // destroyed and its tab removed. In that case we still have the last
        // frameLoader we know about to compare.
        let frameLoader = browser.frameLoader ||
                          this._lastKnownFrameLoader.get(browser.permanentKey);

        // If the message isn't targeting the latest frameLoader discard it.
        if (frameLoader != aMessage.targetFrameLoader) {
          return;
        }

        if (aMessage.data.isFinal) {
          // If this the final message we need to resolve all pending flush
          // requests for the given browser as they might have been sent too
          // late and will never respond. If they have been sent shortly after
          // switching a browser's remoteness there isn't too much data to skip.
          TabStateFlusher.resolveAll(browser);
        } else if (aMessage.data.flushID) {
          // This is an update kicked off by an async flush request. Notify the
          // TabStateFlusher so that it can finish the request and notify its
          // consumer that's waiting for the flush to be done.
          TabStateFlusher.resolve(browser, aMessage.data.flushID);
        }

        // Ignore messages from <browser> elements that have crashed
        // and not yet been revived.
        if (this._crashedBrowsers.has(browser.permanentKey)) {
          return;
        }

        // Record telemetry measurements done in the child and update the tab's
        // cached state. Mark the window as dirty and trigger a delayed write.
        this.recordTelemetry(aMessage.data.telemetry);
        TabState.update(browser, aMessage.data);
        this.saveStateDelayed(win);

        // Handle any updates sent by the child after the tab was closed. This
        // might be the final update as sent by the "unload" handler but also
        // any async update message that was sent before the child unloaded.
        if (this._closedTabs.has(browser.permanentKey)) {
          let {closedTabs, tabData} = this._closedTabs.get(browser.permanentKey);

          // Update the closed tab's state. This will be reflected in its
          // window's list of closed tabs as that refers to the same object.
          TabState.copyFromCache(browser, tabData.state);

          // Is this the tab's final message?
          if (aMessage.data.isFinal) {
            // We expect no further updates.
            this._closedTabs.delete(browser.permanentKey);
            // The tab state no longer needs this reference.
            delete tabData.permanentKey;

            // Determine whether the tab state is worth saving.
            let shouldSave = this._shouldSaveTabState(tabData.state);
            let index = closedTabs.indexOf(tabData);

            if (shouldSave && index == -1) {
              // If the tab state is worth saving and we didn't push it onto
              // the list of closed tabs when it was closed (because we deemed
              // the state not worth saving) then add it to the window's list
              // of closed tabs now.
              this.saveClosedTabData(closedTabs, tabData);
            } else if (!shouldSave && index > -1) {
              // Remove from the list of closed tabs. The update messages sent
              // after the tab was closed changed enough state so that we no
              // longer consider its data interesting enough to keep around.
              this.removeClosedTabData(closedTabs, index);
            }
          }
        }
        break;
      case "SessionStore:restoreHistoryComplete":
        // Notify the tabbrowser that the tab chrome has been restored.
        let tabData = TabState.collect(tab);

        // wall-paper fix for bug 439675: make sure that the URL to be loaded
        // is always visible in the address bar if no other value is present
        let activePageData = tabData.entries[tabData.index - 1] || null;
        let uri = activePageData ? activePageData.url || null : null;
        // NB: we won't set initial URIs (about:home, about:newtab, etc.) here
        // because their load will not normally trigger a location bar clearing
        // when they finish loading (to avoid race conditions where we then
        // clear user input instead), so we shouldn't set them here either.
        // They also don't fall under the issues in bug 439675 where user input
        // needs to be preserved if the load doesn't succeed.
        // We also don't do this for remoteness updates, where it should not
        // be necessary.
        if (!browser.userTypedValue && uri && !data.isRemotenessUpdate &&
            !win.gInitialPages.includes(uri)) {
          browser.userTypedValue = uri;
        }

        // If the page has a title, set it.
        if (activePageData) {
          if (activePageData.title) {
            tab.label = activePageData.title;
            tab.crop = "end";
          } else if (activePageData.url != "about:blank") {
            tab.label = activePageData.url;
            tab.crop = "center";
          }
        } else if (tab.hasAttribute("customizemode")) {
          win.gCustomizeMode.setTab(tab);
        }

        // Restore the tab icon.
        if ("image" in tabData) {
          // Using null as the loadingPrincipal because serializing
          // the principal would be overkill. Within SetIcon we
          // default to the systemPrincipal if aLoadingPrincipal is
          // null which will allow the favicon to load.
          win.gBrowser.setIcon(tab, tabData.image, null);
          TabStateCache.update(browser, {image: null});
        }

        let event = win.document.createEvent("Events");
        event.initEvent("SSTabRestoring", true, false);
        tab.dispatchEvent(event);
        break;
      case "SessionStore:restoreTabContentStarted":
        if (browser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE) {
          // If a load not initiated by sessionstore was started in a
          // previously pending tab. Mark the tab as no longer pending.
          this.markTabAsRestoring(tab);
        } else if (!data.isRemotenessUpdate) {
          // If the user was typing into the URL bar when we crashed, but hadn't hit
          // enter yet, then we just need to write that value to the URL bar without
          // loading anything. This must happen after the load, as the load will clear
          // userTypedValue.
          let tabData = TabState.collect(tab);
          if (tabData.userTypedValue && !tabData.userTypedClear && !browser.userTypedValue) {
            browser.userTypedValue = tabData.userTypedValue;
            win.URLBarSetURI();
          }

          // Remove state we don't need any longer.
          TabStateCache.update(browser, {
            userTypedValue: null, userTypedClear: null
          });
        }
        break;
      case "SessionStore:restoreTabContentComplete":
        // This callback is used exclusively by tests that want to
        // monitor the progress of network loads.
        if (gDebuggingEnabled) {
          Services.obs.notifyObservers(browser, NOTIFY_TAB_RESTORED, null);
        }

        SessionStoreInternal._resetLocalTabRestoringState(tab);
        SessionStoreInternal.restoreNextTab();

        this._sendTabRestoredNotification(tab, data.isRemotenessUpdate);
        break;
      case "SessionStore:crashedTabRevived":
        // The browser was revived by navigating to a different page
        // manually, so we remove it from the ignored browser set.
        this._crashedBrowsers.delete(browser.permanentKey);
        break;
      case "SessionStore:error":
        this.reportInternalError(data);
        TabStateFlusher.resolveAll(browser, false, "Received error from the content process");
        break;
      default:
        throw new Error(`received unknown message '${aMessage.name}'`);
        break;
    }
  },

  /**
   * Record telemetry measurements stored in an object.
   * @param telemetry
   *        {histogramID: value, ...} An object mapping histogramIDs to the
   *        value to be recorded for that ID,
   */
  recordTelemetry: function (telemetry) {
    for (let histogramId in telemetry){
      Telemetry.getHistogramById(histogramId).add(telemetry[histogramId]);
    }
  },

  /* ........ Window Event Handlers .............. */

  /**
   * Implement nsIDOMEventListener for handling various window and tab events
   */
  handleEvent: function ssi_handleEvent(aEvent) {
    let win = aEvent.currentTarget.ownerDocument.defaultView;
    let target = aEvent.originalTarget;
    switch (aEvent.type) {
      case "TabOpen":
        this.onTabAdd(win, target);
        break;
      case "TabClose":
        // `adoptedBy` will be set if the tab was closed because it is being
        // moved to a new window.
        if (!aEvent.detail.adoptedBy)
          this.onTabClose(win, target);
        this.onTabRemove(win, target);
        break;
      case "TabSelect":
        this.onTabSelect(win);
        break;
      case "TabShow":
        this.onTabShow(win, target);
        break;
      case "TabHide":
        this.onTabHide(win, target);
        break;
      case "TabPinned":
      case "TabUnpinned":
      case "SwapDocShells":
        this.saveStateDelayed(win);
        break;
      case "oop-browser-crashed":
        this.onBrowserCrashed(win, target);
        break;
      case "XULFrameLoaderCreated":
        if (target.namespaceURI == NS_XUL &&
            target.localName == "browser" &&
            target.frameLoader &&
            target.permanentKey) {
          this._lastKnownFrameLoader.set(target.permanentKey, target.frameLoader);
          this.resetEpoch(target);
        }
        break;
      default:
        throw new Error(`unhandled event ${aEvent.type}?`);
    }
    this._clearRestoringWindows();
  },

  /**
   * Generate a unique window identifier
   * @return string
   *         A unique string to identify a window
   */
  _generateWindowID: function ssi_generateWindowID() {
    return "window" + (this._nextWindowID++);
  },

  /**
   * Registers and tracks a given window.
   *
   * @param aWindow
   *        Window reference
   */
  onLoad(aWindow) {
    // return if window has already been initialized
    if (aWindow && aWindow.__SSi && this._windows[aWindow.__SSi])
      return;

    // ignore windows opened while shutting down
    if (RunState.isQuitting)
      return;

    // Assign the window a unique identifier we can use to reference
    // internal data about the window.
    aWindow.__SSi = this._generateWindowID();

    let mm = aWindow.getGroupMessageManager("browsers");
    MESSAGES.forEach(msg => {
      let listenWhenClosed = CLOSED_MESSAGES.has(msg);
      mm.addMessageListener(msg, this, listenWhenClosed);
    });

    // Load the frame script after registering listeners.
    mm.loadFrameScript("chrome://browser/content/content-sessionStore.js", true);

    // and create its data object
    this._windows[aWindow.__SSi] = { tabs: [], selected: 0, _closedTabs: [], busy: false };

    if (PrivateBrowsingUtils.isWindowPrivate(aWindow))
      this._windows[aWindow.__SSi].isPrivate = true;
    if (!this._isWindowLoaded(aWindow))
      this._windows[aWindow.__SSi]._restoring = true;
    if (!aWindow.toolbar.visible)
      this._windows[aWindow.__SSi].isPopup = true;

    let tabbrowser = aWindow.gBrowser;

    // add tab change listeners to all already existing tabs
    for (let i = 0; i < tabbrowser.tabs.length; i++) {
      this.onTabAdd(aWindow, tabbrowser.tabs[i], true);
    }
    // notification of tab add/remove/selection/show/hide
    TAB_EVENTS.forEach(function(aEvent) {
      tabbrowser.tabContainer.addEventListener(aEvent, this, true);
    }, this);

    // Keep track of a browser's latest frameLoader.
    aWindow.gBrowser.addEventListener("XULFrameLoaderCreated", this);
  },

  /**
   * Initializes a given window.
   *
   * Windows are registered as soon as they are created but we need to wait for
   * the session file to load, and the initial window's delayed startup to
   * finish before initializing a window, i.e. restoring data into it.
   *
   * @param aWindow
   *        Window reference
   * @param aInitialState
   *        The initial state to be loaded after startup (optional)
   */
  initializeWindow(aWindow, aInitialState = null) {
    let isPrivateWindow = PrivateBrowsingUtils.isWindowPrivate(aWindow);

    // perform additional initialization when the first window is loading
    if (RunState.isStopped) {
      RunState.setRunning();

      // restore a crashed session resp. resume the last session if requested
      if (aInitialState) {
        // Don't write to disk right after startup. Set the last time we wrote
        // to disk to NOW() to enforce a full interval before the next write.
        SessionSaver.updateLastSaveTime();

        if (isPrivateWindow) {
          // We're starting with a single private window. Save the state we
          // actually wanted to restore so that we can do it later in case
          // the user opens another, non-private window.
          this._deferredInitialState = gSessionStartup.state;

          // Nothing to restore now, notify observers things are complete.
          Services.obs.notifyObservers(null, NOTIFY_WINDOWS_RESTORED, "");
        } else {
          TelemetryTimestamps.add("sessionRestoreRestoring");
          this._restoreCount = aInitialState.windows ? aInitialState.windows.length : 0;

          // global data must be restored before restoreWindow is called so that
          // it happens before observers are notified
          this._globalState.setFromState(aInitialState);

          let overwrite = this._isCmdLineEmpty(aWindow, aInitialState);
          let options = {firstWindow: true, overwriteTabs: overwrite};
          this.restoreWindows(aWindow, aInitialState, options);
        }
      }
      else {
        // Nothing to restore, notify observers things are complete.
        Services.obs.notifyObservers(null, NOTIFY_WINDOWS_RESTORED, "");
      }
    }
    // this window was opened by _openWindowWithState
    else if (!this._isWindowLoaded(aWindow)) {
      let state = this._statesToRestore[aWindow.__SS_restoreID];
      let options = {overwriteTabs: true, isFollowUp: state.windows.length == 1};
      this.restoreWindow(aWindow, state.windows[0], options);
    }
    // The user opened another, non-private window after starting up with
    // a single private one. Let's restore the session we actually wanted to
    // restore at startup.
    else if (this._deferredInitialState && !isPrivateWindow &&
             aWindow.toolbar.visible) {

      // global data must be restored before restoreWindow is called so that
      // it happens before observers are notified
      this._globalState.setFromState(this._deferredInitialState);

      this._restoreCount = this._deferredInitialState.windows ?
        this._deferredInitialState.windows.length : 0;
      this.restoreWindows(aWindow, this._deferredInitialState, {firstWindow: true});
      this._deferredInitialState = null;
    }
    else if (this._restoreLastWindow && aWindow.toolbar.visible &&
             this._closedWindows.length && !isPrivateWindow) {

      // default to the most-recently closed window
      // don't use popup windows
      let closedWindowState = null;
      let closedWindowIndex;
      for (let i = 0; i < this._closedWindows.length; i++) {
        // Take the first non-popup, point our object at it, and break out.
        if (!this._closedWindows[i].isPopup) {
          closedWindowState = this._closedWindows[i];
          closedWindowIndex = i;
          break;
        }
      }

      if (closedWindowState) {
        let newWindowState;
        if (AppConstants.platform == "macosx" || !this._doResumeSession()) {
          // We want to split the window up into pinned tabs and unpinned tabs.
          // Pinned tabs should be restored. If there are any remaining tabs,
          // they should be added back to _closedWindows.
          // We'll cheat a little bit and reuse _prepDataForDeferredRestore
          // even though it wasn't built exactly for this.
          let [appTabsState, normalTabsState] =
            this._prepDataForDeferredRestore({ windows: [closedWindowState] });

          // These are our pinned tabs, which we should restore
          if (appTabsState.windows.length) {
            newWindowState = appTabsState.windows[0];
            delete newWindowState.__lastSessionWindowID;
          }

          // In case there were no unpinned tabs, remove the window from _closedWindows
          if (!normalTabsState.windows.length) {
            this._closedWindows.splice(closedWindowIndex, 1);
          }
          // Or update _closedWindows with the modified state
          else {
            delete normalTabsState.windows[0].__lastSessionWindowID;
            this._closedWindows[closedWindowIndex] = normalTabsState.windows[0];
          }
        }
        else {
          // If we're just restoring the window, make sure it gets removed from
          // _closedWindows.
          this._closedWindows.splice(closedWindowIndex, 1);
          newWindowState = closedWindowState;
          delete newWindowState.hidden;
        }

        if (newWindowState) {
          // Ensure that the window state isn't hidden
          this._restoreCount = 1;
          let state = { windows: [newWindowState] };
          let options = {overwriteTabs: this._isCmdLineEmpty(aWindow, state)};
          this.restoreWindow(aWindow, newWindowState, options);
        }
      }
      // we actually restored the session just now.
      this._prefBranch.setBoolPref("sessionstore.resume_session_once", false);
    }
    if (this._restoreLastWindow && aWindow.toolbar.visible) {
      // always reset (if not a popup window)
      // we don't want to restore a window directly after, for example,
      // undoCloseWindow was executed.
      this._restoreLastWindow = false;
    }
  },

  /**
   * Called right before a new browser window is shown.
   * @param aWindow
   *        Window reference
   */
  onBeforeBrowserWindowShown: function (aWindow) {
    // Register the window.
    this.onLoad(aWindow);

    // Just call initializeWindow() directly if we're initialized already.
    if (this._sessionInitialized) {
      this.initializeWindow(aWindow);
      return;
    }

    // The very first window that is opened creates a promise that is then
    // re-used by all subsequent windows. The promise will be used to tell
    // when we're ready for initialization.
    if (!this._promiseReadyForInitialization) {
      // Wait for the given window's delayed startup to be finished.
      let promise = new Promise(resolve => {
        Services.obs.addObserver(function obs(subject, topic) {
          if (aWindow == subject) {
            Services.obs.removeObserver(obs, topic);
            resolve();
          }
        }, "browser-delayed-startup-finished", false);
      });

      // We are ready for initialization as soon as the session file has been
      // read from disk and the initial window's delayed startup has finished.
      this._promiseReadyForInitialization =
        Promise.all([promise, gSessionStartup.onceInitialized]);
    }

    // We can't call this.onLoad since initialization
    // hasn't completed, so we'll wait until it is done.
    // Even if additional windows are opened and wait
    // for initialization as well, the first opened
    // window should execute first, and this.onLoad
    // will be called with the initialState.
    this._promiseReadyForInitialization.then(() => {
      if (aWindow.closed) {
        return;
      }

      if (this._sessionInitialized) {
        this.initializeWindow(aWindow);
      } else {
        let initialState = this.initSession();
        this._sessionInitialized = true;

        if (initialState) {
          Services.obs.notifyObservers(null, NOTIFY_RESTORING_ON_STARTUP, "");
        }
        TelemetryStopwatch.start("FX_SESSION_RESTORE_STARTUP_ONLOAD_INITIAL_WINDOW_MS");
        this.initializeWindow(aWindow, initialState);
        TelemetryStopwatch.finish("FX_SESSION_RESTORE_STARTUP_ONLOAD_INITIAL_WINDOW_MS");

        // Let everyone know we're done.
        this._deferredInitialized.resolve();
      }
    }, console.error);
  },

  /**
   * On window close...
   * - remove event listeners from tabs
   * - save all window data
   * @param aWindow
   *        Window reference
   */
  onClose: function ssi_onClose(aWindow) {
    // this window was about to be restored - conserve its original data, if any
    let isFullyLoaded = this._isWindowLoaded(aWindow);
    if (!isFullyLoaded) {
      if (!aWindow.__SSi) {
        aWindow.__SSi = this._generateWindowID();
      }

      this._windows[aWindow.__SSi] = this._statesToRestore[aWindow.__SS_restoreID];
      delete this._statesToRestore[aWindow.__SS_restoreID];
      delete aWindow.__SS_restoreID;
    }

    // ignore windows not tracked by SessionStore
    if (!aWindow.__SSi || !this._windows[aWindow.__SSi]) {
      return;
    }

    // notify that the session store will stop tracking this window so that
    // extensions can store any data about this window in session store before
    // that's not possible anymore
    let event = aWindow.document.createEvent("Events");
    event.initEvent("SSWindowClosing", true, false);
    aWindow.dispatchEvent(event);

    if (this.windowToFocus && this.windowToFocus == aWindow) {
      delete this.windowToFocus;
    }

    var tabbrowser = aWindow.gBrowser;

    let browsers = Array.from(tabbrowser.browsers);

    TAB_EVENTS.forEach(function(aEvent) {
      tabbrowser.tabContainer.removeEventListener(aEvent, this, true);
    }, this);

    aWindow.gBrowser.removeEventListener("XULFrameLoaderCreated", this);

    let winData = this._windows[aWindow.__SSi];

    // Collect window data only when *not* closed during shutdown.
    if (RunState.isRunning) {
      // Grab the most recent window data. The tab data will be updated
      // once we finish flushing all of the messages from the tabs.
      let tabMap = this._collectWindowData(aWindow);

      for (let [tab, tabData] of tabMap) {
        let permanentKey = tab.linkedBrowser.permanentKey;
        this._closedWindowTabs.set(permanentKey, tabData);
      }

      if (isFullyLoaded) {
        winData.title = tabbrowser.selectedBrowser.contentTitle || tabbrowser.selectedTab.label;
        winData.title = this._replaceLoadingTitle(winData.title, tabbrowser,
                                                  tabbrowser.selectedTab);
        SessionCookies.update([winData]);
      }

      if (AppConstants.platform != "macosx") {
        // Until we decide otherwise elsewhere, this window is part of a series
        // of closing windows to quit.
        winData._shouldRestore = true;
      }

      // Store the window's close date to figure out when each individual tab
      // was closed. This timestamp should allow re-arranging data based on how
      // recently something was closed.
      winData.closedAt = Date.now();

      // we don't want to save the busy state
      delete winData.busy;

      // When closing windows one after the other until Firefox quits, we
      // will move those closed in series back to the "open windows" bucket
      // before writing to disk. If however there is only a single window
      // with tabs we deem not worth saving then we might end up with a
      // random closed or even a pop-up window re-opened. To prevent that
      // we explicitly allow saving an "empty" window state.
      let isLastWindow =
        Object.keys(this._windows).length == 1 &&
        !this._closedWindows.some(win => win._shouldRestore || false);

      // clear this window from the list, since it has definitely been closed.
      delete this._windows[aWindow.__SSi];

      // This window has the potential to be saved in the _closedWindows
      // array (maybeSaveClosedWindows gets the final call on that).
      this._saveableClosedWindowData.add(winData);

      // Now we have to figure out if this window is worth saving in the _closedWindows
      // Object.
      //
      // We're about to flush the tabs from this window, but it's possible that we
      // might never hear back from the content process(es) in time before the user
      // chooses to restore the closed window. So we do the following:
      //
      // 1) Use the tab state cache to determine synchronously if the window is
      //    worth stashing in _closedWindows.
      // 2) Flush the window.
      // 3) When the flush is complete, revisit our decision to store the window
      //    in _closedWindows, and add/remove as necessary.
      if (!winData.isPrivate) {
        // Remove any open private tabs the window may contain.
        PrivacyFilter.filterPrivateTabs(winData);
        this.maybeSaveClosedWindow(winData, isLastWindow);
      }

      TabStateFlusher.flushWindow(aWindow).then(() => {
        // At this point, aWindow is closed! You should probably not try to
        // access any DOM elements from aWindow within this callback unless
        // you're holding on to them in the closure.

        for (let browser of browsers) {
          if (this._closedWindowTabs.has(browser.permanentKey)) {
            let tabData = this._closedWindowTabs.get(browser.permanentKey);
            TabState.copyFromCache(browser, tabData);
            this._closedWindowTabs.delete(browser.permanentKey);
          }
        }

        // Save non-private windows if they have at
        // least one saveable tab or are the last window.
        if (!winData.isPrivate) {
          // It's possible that a tab switched its privacy state at some point
          // before our flush, so we need to filter again.
          PrivacyFilter.filterPrivateTabs(winData);
          this.maybeSaveClosedWindow(winData, isLastWindow);
        }

        // Update the tabs data now that we've got the most
        // recent information.
        this.cleanUpWindow(aWindow, winData, browsers);

        // save the state without this window to disk
        this.saveStateDelayed();
      });
    } else {
      this.cleanUpWindow(aWindow, winData, browsers);
    }

    for (let i = 0; i < tabbrowser.tabs.length; i++) {
      this.onTabRemove(aWindow, tabbrowser.tabs[i], true);
    }
  },

  /**
   * Clean up the message listeners on a window that has finally
   * gone away. Call this once you're sure you don't want to hear
   * from any of this windows tabs from here forward.
   *
   * @param aWindow
   *        The browser window we're cleaning up.
   * @param winData
   *        The data for the window that we should hold in the
   *        DyingWindowCache in case anybody is still holding a
   *        reference to it.
   */
  cleanUpWindow(aWindow, winData, browsers) {
    // Any leftover TabStateFlusher Promises need to be resolved now,
    // since we're about to remove the message listeners.
    for (let browser of browsers) {
      TabStateFlusher.resolveAll(browser);
    }

    // Cache the window state until it is completely gone.
    DyingWindowCache.set(aWindow, winData);

    let mm = aWindow.getGroupMessageManager("browsers");
    MESSAGES.forEach(msg => mm.removeMessageListener(msg, this));

    this._saveableClosedWindowData.delete(winData);
    delete aWindow.__SSi;
  },

  /**
   * Decides whether or not a closed window should be put into the
   * _closedWindows Object. This might be called multiple times per
   * window, and will do the right thing of moving the window data
   * in or out of _closedWindows if the winData indicates that our
   * need for saving it has changed.
   *
   * @param winData
   *        The data for the closed window that we might save.
   * @param isLastWindow
   *        Whether or not the window being closed is the last
   *        browser window. Callers of this function should pass
   *        in the value of SessionStoreInternal.atLastWindow for
   *        this argument, and pass in the same value if they happen
   *        to call this method again asynchronously (for example, after
   *        a window flush).
   */
  maybeSaveClosedWindow(winData, isLastWindow) {
    // Make sure SessionStore is still running, and make sure that we
    // haven't chosen to forget this window.
    if (RunState.isRunning && this._saveableClosedWindowData.has(winData)) {
      // Determine whether the window has any tabs worth saving.
      let hasSaveableTabs = winData.tabs.some(this._shouldSaveTabState);

      // Note that we might already have this window stored in
      // _closedWindows from a previous call to this function.
      let winIndex = this._closedWindows.indexOf(winData);
      let alreadyStored = (winIndex != -1);
      let shouldStore = (hasSaveableTabs || isLastWindow);

      if (shouldStore && !alreadyStored) {
        let index = this._closedWindows.findIndex(win => {
          return win.closedAt < winData.closedAt;
        });

        // If we found no tab closed before our
        // tab then just append it to the list.
        if (index == -1) {
          index = this._closedWindows.length;
        }

        // Insert tabData at the right position.
        this._closedWindows.splice(index, 0, winData);
        this._capClosedWindows();
      } else if (!shouldStore && alreadyStored) {
        this._closedWindows.splice(winIndex, 1);
      }
    }
  },

  /**
   * On quit application granted
   */
  onQuitApplicationGranted: function ssi_onQuitApplicationGranted(syncShutdown=false) {
    // Collect an initial snapshot of window data before we do the flush
    this._forEachBrowserWindow((win) => {
      this._collectWindowData(win);
    });

    // Now add an AsyncShutdown blocker that'll spin the event loop
    // until the windows have all been flushed.

    // This progress object will track the state of async window flushing
    // and will help us debug things that go wrong with our AsyncShutdown
    // blocker.
    let progress = { total: -1, current: -1 };

    // We're going down! Switch state so that we treat closing windows and
    // tabs correctly.
    RunState.setQuitting();

    if (!syncShutdown) {
      // We've got some time to shut down, so let's do this properly.
      AsyncShutdown.quitApplicationGranted.addBlocker(
        "SessionStore: flushing all windows",
        this.flushAllWindowsAsync(progress),
        () => progress);
    } else {
      // We have to shut down NOW, which means we only get to save whatever
      // we already had cached.
    }
  },

  /**
   * An async Task that iterates all open browser windows and flushes
   * any outstanding messages from their tabs. This will also close
   * all of the currently open windows while we wait for the flushes
   * to complete.
   *
   * @param progress (Object)
   *        Optional progress object that will be updated as async
   *        window flushing progresses. flushAllWindowsSync will
   *        write to the following properties:
   *
   *        total (int):
   *          The total number of windows to be flushed.
   *        current (int):
   *          The current window that we're waiting for a flush on.
   *
   * @return Promise
   */
  flushAllWindowsAsync: Task.async(function*(progress={}) {
    let windowPromises = new Map();
    // We collect flush promises and close each window immediately so that
    // the user can't start changing any window state while we're waiting
    // for the flushes to finish.
    this._forEachBrowserWindow((win) => {
      windowPromises.set(win, TabStateFlusher.flushWindow(win));

      // We have to wait for these messages to come up from
      // each window and each browser. In the meantime, hide
      // the windows to improve perceived shutdown speed.
      let baseWin = win.QueryInterface(Ci.nsIInterfaceRequestor)
                       .getInterface(Ci.nsIDocShell)
                       .QueryInterface(Ci.nsIDocShellTreeItem)
                       .treeOwner
                       .QueryInterface(Ci.nsIBaseWindow);
      baseWin.visibility = false;
    });

    progress.total = windowPromises.size;
    progress.current = 0;

    // We'll iterate through the Promise array, yielding each one, so as to
    // provide useful progress information to AsyncShutdown.
    for (let [win, promise] of windowPromises) {
      yield promise;
      this._collectWindowData(win);
      progress.current++;
    };

    // We must cache this because _getMostRecentBrowserWindow will always
    // return null by the time quit-application occurs.
    var activeWindow = this._getMostRecentBrowserWindow();
    if (activeWindow)
      this.activeWindowSSiCache = activeWindow.__SSi || "";
    DirtyWindows.clear();
  }),

  /**
   * On last browser window close
   */
  onLastWindowCloseGranted: function ssi_onLastWindowCloseGranted() {
    // last browser window is quitting.
    // remember to restore the last window when another browser window is opened
    // do not account for pref(resume_session_once) at this point, as it might be
    // set by another observer getting this notice after us
    this._restoreLastWindow = true;
  },

  /**
   * On quitting application
   * @param aData
   *        String type of quitting
   */
  onQuitApplication: function ssi_onQuitApplication(aData) {
    if (aData == "restart") {
      this._prefBranch.setBoolPref("sessionstore.resume_session_once", true);
      // The browser:purge-session-history notification fires after the
      // quit-application notification so unregister the
      // browser:purge-session-history notification to prevent clearing
      // session data on disk on a restart.  It is also unnecessary to
      // perform any other sanitization processing on a restart as the
      // browser is about to exit anyway.
      Services.obs.removeObserver(this, "browser:purge-session-history");
    }

    if (aData != "restart") {
      // Throw away the previous session on shutdown
      LastSession.clear();
    }

    this._uninit();
  },

  /**
   * On purge of session history
   */
  onPurgeSessionHistory: function ssi_onPurgeSessionHistory() {
    SessionFile.wipe();
    // If the browser is shutting down, simply return after clearing the
    // session data on disk as this notification fires after the
    // quit-application notification so the browser is about to exit.
    if (RunState.isQuitting)
      return;
    LastSession.clear();

    let openWindows = {};
    // Collect open windows.
    this._forEachBrowserWindow(({__SSi: id}) => openWindows[id] = true);

    // also clear all data about closed tabs and windows
    for (let ix in this._windows) {
      if (ix in openWindows) {
        this._windows[ix]._closedTabs = [];
      } else {
        delete this._windows[ix];
      }
    }
    // also clear all data about closed windows
    this._closedWindows = [];
    // give the tabbrowsers a chance to clear their histories first
    var win = this._getMostRecentBrowserWindow();
    if (win) {
      win.setTimeout(() => SessionSaver.run(), 0);
    } else if (RunState.isRunning) {
      SessionSaver.run();
    }

    this._clearRestoringWindows();
    this._saveableClosedWindowData = new WeakSet();
  },

  /**
   * On purge of domain data
   * @param aData
   *        String domain data
   */
  onPurgeDomainData: function ssi_onPurgeDomainData(aData) {
    // does a session history entry contain a url for the given domain?
    function containsDomain(aEntry) {
      if (Utils.hasRootDomain(aEntry.url, aData)) {
        return true;
      }
      return aEntry.children && aEntry.children.some(containsDomain, this);
    }
    // remove all closed tabs containing a reference to the given domain
    for (let ix in this._windows) {
      let closedTabs = this._windows[ix]._closedTabs;
      for (let i = closedTabs.length - 1; i >= 0; i--) {
        if (closedTabs[i].state.entries.some(containsDomain, this))
          closedTabs.splice(i, 1);
      }
    }
    // remove all open & closed tabs containing a reference to the given
    // domain in closed windows
    for (let ix = this._closedWindows.length - 1; ix >= 0; ix--) {
      let closedTabs = this._closedWindows[ix]._closedTabs;
      let openTabs = this._closedWindows[ix].tabs;
      let openTabCount = openTabs.length;
      for (let i = closedTabs.length - 1; i >= 0; i--)
        if (closedTabs[i].state.entries.some(containsDomain, this))
          closedTabs.splice(i, 1);
      for (let j = openTabs.length - 1; j >= 0; j--) {
        if (openTabs[j].entries.some(containsDomain, this)) {
          openTabs.splice(j, 1);
          if (this._closedWindows[ix].selected > j)
            this._closedWindows[ix].selected--;
        }
      }
      if (openTabs.length == 0) {
        this._closedWindows.splice(ix, 1);
      }
      else if (openTabs.length != openTabCount) {
        // Adjust the window's title if we removed an open tab
        let selectedTab = openTabs[this._closedWindows[ix].selected - 1];
        // some duplication from restoreHistory - make sure we get the correct title
        let activeIndex = (selectedTab.index || selectedTab.entries.length) - 1;
        if (activeIndex >= selectedTab.entries.length)
          activeIndex = selectedTab.entries.length - 1;
        this._closedWindows[ix].title = selectedTab.entries[activeIndex].title;
      }
    }

    if (RunState.isRunning) {
      SessionSaver.run();
    }

    this._clearRestoringWindows();
  },

  /**
   * On preference change
   * @param aData
   *        String preference changed
   */
  onPrefChange: function ssi_onPrefChange(aData) {
    switch (aData) {
      // if the user decreases the max number of closed tabs they want
      // preserved update our internal states to match that max
      case "sessionstore.max_tabs_undo":
        this._max_tabs_undo = this._prefBranch.getIntPref("sessionstore.max_tabs_undo");
        for (let ix in this._windows) {
          this._windows[ix]._closedTabs.splice(this._max_tabs_undo, this._windows[ix]._closedTabs.length);
        }
        break;
      case "sessionstore.max_windows_undo":
        this._max_windows_undo = this._prefBranch.getIntPref("sessionstore.max_windows_undo");
        this._capClosedWindows();
        break;
    }
  },

  /**
   * set up listeners for a new tab
   * @param aWindow
   *        Window reference
   * @param aTab
   *        Tab reference
   * @param aNoNotification
   *        bool Do not save state if we're updating an existing tab
   */
  onTabAdd: function ssi_onTabAdd(aWindow, aTab, aNoNotification) {
    let browser = aTab.linkedBrowser;
    browser.addEventListener("SwapDocShells", this);
    browser.addEventListener("oop-browser-crashed", this);

    if (browser.frameLoader) {
      this._lastKnownFrameLoader.set(browser.permanentKey, browser.frameLoader);
    }

    if (!aNoNotification) {
      this.saveStateDelayed(aWindow);
    }
  },

  /**
   * remove listeners for a tab
   * @param aWindow
   *        Window reference
   * @param aTab
   *        Tab reference
   * @param aNoNotification
   *        bool Do not save state if we're updating an existing tab
   */
  onTabRemove: function ssi_onTabRemove(aWindow, aTab, aNoNotification) {
    let browser = aTab.linkedBrowser;
    browser.removeEventListener("SwapDocShells", this);
    browser.removeEventListener("oop-browser-crashed", this);

    // If this tab was in the middle of restoring or still needs to be restored,
    // we need to reset that state. If the tab was restoring, we will attempt to
    // restore the next tab.
    let previousState = browser.__SS_restoreState;
    if (previousState) {
      this._resetTabRestoringState(aTab);
      if (previousState == TAB_STATE_RESTORING)
        this.restoreNextTab();
    }

    if (!aNoNotification) {
      this.saveStateDelayed(aWindow);
    }
  },

  /**
   * When a tab closes, collect its properties
   * @param aWindow
   *        Window reference
   * @param aTab
   *        Tab reference
   */
  onTabClose: function ssi_onTabClose(aWindow, aTab) {
    // notify the tabbrowser that the tab state will be retrieved for the last time
    // (so that extension authors can easily set data on soon-to-be-closed tabs)
    var event = aWindow.document.createEvent("Events");
    event.initEvent("SSTabClosing", true, false);
    aTab.dispatchEvent(event);

    // don't update our internal state if we don't have to
    if (this._max_tabs_undo == 0) {
      return;
    }

    // Get the latest data for this tab (generally, from the cache)
    let tabState = TabState.collect(aTab);

    // Don't save private tabs
    let isPrivateWindow = PrivateBrowsingUtils.isWindowPrivate(aWindow);
    if (!isPrivateWindow && tabState.isPrivate) {
      return;
    }

    // Store closed-tab data for undo.
    let tabbrowser = aWindow.gBrowser;
    let tabTitle = this._replaceLoadingTitle(aTab.label, tabbrowser, aTab);
    let {permanentKey} = aTab.linkedBrowser;

    let tabData = {
      permanentKey,
      state: tabState,
      title: tabTitle,
      image: tabbrowser.getIcon(aTab),
      pos: aTab._tPos,
      closedAt: Date.now()
    };

    let closedTabs = this._windows[aWindow.__SSi]._closedTabs;

    // Determine whether the tab contains any information worth saving. Note
    // that there might be pending state changes queued in the child that
    // didn't reach the parent yet. If a tab is emptied before closing then we
    // might still remove it from the list of closed tabs later.
    if (this._shouldSaveTabState(tabState)) {
      // Save the tab state, for now. We might push a valid tab out
      // of the list but those cases should be extremely rare and
      // do probably never occur when using the browser normally.
      // (Tests or add-ons might do weird things though.)
      this.saveClosedTabData(closedTabs, tabData);
    }

    // Remember the closed tab to properly handle any last updates included in
    // the final "update" message sent by the frame script's unload handler.
    this._closedTabs.set(permanentKey, {closedTabs, tabData});
  },

  /**
   * Insert a given |tabData| object into the list of |closedTabs|. We will
   * determine the right insertion point based on the .closedAt properties of
   * all tabs already in the list. The list will be truncated to contain a
   * maximum of |this._max_tabs_undo| entries.
   *
   * @param closedTabs (array)
   *        The list of closed tabs for a window.
   * @param tabData (object)
   *        The tabData to be inserted.
   */
  saveClosedTabData(closedTabs, tabData) {
    // Find the index of the first tab in the list
    // of closed tabs that was closed before our tab.
    let index = closedTabs.findIndex(tab => {
      return tab.closedAt < tabData.closedAt;
    });

    // If we found no tab closed before our
    // tab then just append it to the list.
    if (index == -1) {
      index = closedTabs.length;
    }

    // Insert tabData at the right position.
    closedTabs.splice(index, 0, tabData);

    // Truncate the list of closed tabs, if needed.
    if (closedTabs.length > this._max_tabs_undo) {
      closedTabs.splice(this._max_tabs_undo, closedTabs.length);
    }
  },

  /**
   * Remove the closed tab data at |index| from the list of |closedTabs|. If
   * the tab's final message is still pending we will simply discard it when
   * it arrives so that the tab doesn't reappear in the list.
   *
   * @param closedTabs (array)
   *        The list of closed tabs for a window.
   * @param index (uint)
   *        The index of the tab to remove.
   */
  removeClosedTabData(closedTabs, index) {
    // Remove the given index from the list.
    let [closedTab] = closedTabs.splice(index, 1);

    // If the closed tab's state still has a .permanentKey property then we
    // haven't seen its final update message yet. Remove it from the map of
    // closed tabs so that we will simply discard its last messages and will
    // not add it back to the list of closed tabs again.
    if (closedTab.permanentKey) {
      this._closedTabs.delete(closedTab.permanentKey);
      this._closedWindowTabs.delete(closedTab.permanentKey);
      delete closedTab.permanentKey;
    }

    return closedTab;
  },

  /**
   * When a tab is selected, save session data
   * @param aWindow
   *        Window reference
   */
  onTabSelect: function ssi_onTabSelect(aWindow) {
    if (RunState.isRunning) {
      this._windows[aWindow.__SSi].selected = aWindow.gBrowser.tabContainer.selectedIndex;

      let tab = aWindow.gBrowser.selectedTab;
      // If __SS_restoreState is still on the browser and it is
      // TAB_STATE_NEEDS_RESTORE, then then we haven't restored
      // this tab yet. Explicitly call restoreTabContent to kick off the restore.
      if (tab.linkedBrowser.__SS_restoreState &&
          tab.linkedBrowser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE)
        this.restoreTabContent(tab);
    }
  },

  onTabShow: function ssi_onTabShow(aWindow, aTab) {
    // If the tab hasn't been restored yet, move it into the right bucket
    if (aTab.linkedBrowser.__SS_restoreState &&
        aTab.linkedBrowser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE) {
      TabRestoreQueue.hiddenToVisible(aTab);

      // let's kick off tab restoration again to ensure this tab gets restored
      // with "restore_hidden_tabs" == false (now that it has become visible)
      this.restoreNextTab();
    }

    // Default delay of 2 seconds gives enough time to catch multiple TabShow
    // events. This used to be due to changing groups in 'tab groups'. We
    // might be able to get rid of this now?
    this.saveStateDelayed(aWindow);
  },

  onTabHide: function ssi_onTabHide(aWindow, aTab) {
    // If the tab hasn't been restored yet, move it into the right bucket
    if (aTab.linkedBrowser.__SS_restoreState &&
        aTab.linkedBrowser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE) {
      TabRestoreQueue.visibleToHidden(aTab);
    }

    // Default delay of 2 seconds gives enough time to catch multiple TabHide
    // events. This used to be due to changing groups in 'tab groups'. We
    // might be able to get rid of this now?
    this.saveStateDelayed(aWindow);
  },

  /**
   * Handler for the event that is fired when a <xul:browser> crashes.
   *
   * @param aWindow
   *        The window that the crashed browser belongs to.
   * @param aBrowser
   *        The <xul:browser> that is now in the crashed state.
   */
  onBrowserCrashed: function(aWindow, aBrowser) {
    NS_ASSERT(aBrowser.isRemoteBrowser,
              "Only remote browsers should be able to crash");
    this._crashedBrowsers.add(aBrowser.permanentKey);

    // If we hadn't yet restored, or were still in the midst of
    // restoring this browser at the time of the crash, we need
    // to reset its state so that we can try to restore it again
    // when the user revives the tab from the crash.
    if (aBrowser.__SS_restoreState) {
      let tab = aWindow.gBrowser.getTabForBrowser(aBrowser);
      this._resetLocalTabRestoringState(tab);
    }

    // The browser crashed so we might never receive flush responses.
    // Resolve all pending flush requests for the crashed browser.
    TabStateFlusher.resolveAll(aBrowser);
  },

  // Clean up data that has been closed a long time ago.
  // Do not reschedule a save. This will wait for the next regular
  // save.
  onIdleDaily: function() {
    // Remove old closed windows
    this._cleanupOldData([this._closedWindows]);

    // Remove closed tabs of closed windows
    this._cleanupOldData(this._closedWindows.map((winData) => winData._closedTabs));

    // Remove closed tabs of open windows
    this._cleanupOldData(Object.keys(this._windows).map((key) => this._windows[key]._closedTabs));
  },

  // Remove "old" data from an array
  _cleanupOldData: function(targets) {
    const TIME_TO_LIVE = this._prefBranch.getIntPref("sessionstore.cleanup.forget_closed_after");
    const now = Date.now();

    for (let array of targets) {
      for (let i = array.length - 1; i >= 0; --i)  {
        let data = array[i];
        // Make sure that we have a timestamp to tell us when the target
        // has been closed. If we don't have a timestamp, default to a
        // safe timestamp: just now.
        data.closedAt = data.closedAt || now;
        if (now - data.closedAt > TIME_TO_LIVE) {
          array.splice(i, 1);
        }
      }
    }
  },

  /* ........ nsISessionStore API .............. */

  getBrowserState: function ssi_getBrowserState() {
    let state = this.getCurrentState();

    // Don't include the last session state in getBrowserState().
    delete state.lastSessionState;

    // Don't include any deferred initial state.
    delete state.deferredInitialState;

    return JSON.stringify(state);
  },

  setBrowserState: function ssi_setBrowserState(aState) {
    this._handleClosedWindows();

    try {
      var state = JSON.parse(aState);
    }
    catch (ex) { /* invalid state object - don't restore anything */ }
    if (!state) {
      throw Components.Exception("Invalid state string: not JSON", Cr.NS_ERROR_INVALID_ARG);
    }
    if (!state.windows) {
      throw Components.Exception("No windows", Cr.NS_ERROR_INVALID_ARG);
    }

    this._browserSetState = true;

    // Make sure the priority queue is emptied out
    this._resetRestoringState();

    var window = this._getMostRecentBrowserWindow();
    if (!window) {
      this._restoreCount = 1;
      this._openWindowWithState(state);
      return;
    }

    // close all other browser windows
    this._forEachBrowserWindow(function(aWindow) {
      if (aWindow != window) {
        aWindow.close();
        this.onClose(aWindow);
      }
    });

    // make sure closed window data isn't kept
    this._closedWindows = [];

    // determine how many windows are meant to be restored
    this._restoreCount = state.windows ? state.windows.length : 0;

    // global data must be restored before restoreWindow is called so that
    // it happens before observers are notified
    this._globalState.setFromState(state);

    // restore to the given state
    this.restoreWindows(window, state, {overwriteTabs: true});
  },

  getWindowState: function ssi_getWindowState(aWindow) {
    if ("__SSi" in aWindow) {
      return JSON.stringify(this._getWindowState(aWindow));
    }

    if (DyingWindowCache.has(aWindow)) {
      let data = DyingWindowCache.get(aWindow);
      return JSON.stringify({ windows: [data] });
    }

    throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
  },

  setWindowState: function ssi_setWindowState(aWindow, aState, aOverwrite) {
    if (!aWindow.__SSi) {
      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
    }

    this.restoreWindows(aWindow, aState, {overwriteTabs: aOverwrite});
  },

  getTabState: function ssi_getTabState(aTab) {
    if (!aTab.ownerDocument.defaultView.__SSi) {
      throw Components.Exception("Default view is not tracked", Cr.NS_ERROR_INVALID_ARG);
    }

    let tabState = TabState.collect(aTab);

    return JSON.stringify(tabState);
  },

  setTabState(aTab, aState) {
    // Remove the tab state from the cache.
    // Note that we cannot simply replace the contents of the cache
    // as |aState| can be an incomplete state that will be completed
    // by |restoreTabs|.
    let tabState = JSON.parse(aState);
    if (!tabState) {
      throw Components.Exception("Invalid state string: not JSON", Cr.NS_ERROR_INVALID_ARG);
    }
    if (typeof tabState != "object") {
      throw Components.Exception("Not an object", Cr.NS_ERROR_INVALID_ARG);
    }
    if (!("entries" in tabState)) {
      throw Components.Exception("Invalid state object: no entries", Cr.NS_ERROR_INVALID_ARG);
    }

    let window = aTab.ownerDocument.defaultView;
    if (!("__SSi" in window)) {
      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
    }

    if (aTab.linkedBrowser.__SS_restoreState) {
      this._resetTabRestoringState(aTab);
    }

    this.restoreTab(aTab, tabState);
  },

  duplicateTab: function ssi_duplicateTab(aWindow, aTab, aDelta = 0) {
    if (!aTab.ownerDocument.defaultView.__SSi) {
      throw Components.Exception("Default view is not tracked", Cr.NS_ERROR_INVALID_ARG);
    }
    if (!aWindow.gBrowser) {
      throw Components.Exception("Invalid window object: no gBrowser", Cr.NS_ERROR_INVALID_ARG);
    }

    // Create a new tab.
    let userContextId = aTab.getAttribute("usercontextid");
    let newTab = aTab == aWindow.gBrowser.selectedTab ?
      aWindow.gBrowser.addTab(null, {relatedToCurrent: true, ownerTab: aTab, userContextId}) :
      aWindow.gBrowser.addTab(null, {userContextId});

    // Set tab title to "Connecting..." and start the throbber to pretend we're
    // doing something while actually waiting for data from the frame script.
    aWindow.gBrowser.setTabTitleLoading(newTab);
    newTab.setAttribute("busy", "true");

    // Collect state before flushing.
    let tabState = TabState.clone(aTab);

    // Flush to get the latest tab state to duplicate.
    let browser = aTab.linkedBrowser;
    TabStateFlusher.flush(browser).then(() => {
      // The new tab might have been closed in the meantime.
      if (newTab.closing || !newTab.linkedBrowser) {
        return;
      }

      let window = newTab.ownerDocument && newTab.ownerDocument.defaultView;

      // The tab or its window might be gone.
      if (!window || !window.__SSi) {
        return;
      }

      // Update state with flushed data. We can't use TabState.clone() here as
      // the tab to duplicate may have already been closed. In that case we
      // only have access to the <xul:browser>.
      let options = {includePrivateData: true};
      TabState.copyFromCache(browser, tabState, options);

      tabState.index += aDelta;
      tabState.index = Math.max(1, Math.min(tabState.index, tabState.entries.length));
      tabState.pinned = false;

      // Restore the state into the new tab.
      this.restoreTab(newTab, tabState, {
        restoreImmediately: true /* Load this tab right away. */
      });
    });

    return newTab;
  },

  getClosedTabCount: function ssi_getClosedTabCount(aWindow) {
    if ("__SSi" in aWindow) {
      return this._windows[aWindow.__SSi]._closedTabs.length;
    }

    if (!DyingWindowCache.has(aWindow)) {
      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
    }

    return DyingWindowCache.get(aWindow)._closedTabs.length;
  },

  getClosedTabData: function ssi_getClosedTabDataAt(aWindow) {
    if ("__SSi" in aWindow) {
      return JSON.stringify(this._windows[aWindow.__SSi]._closedTabs);
    }

    if (!DyingWindowCache.has(aWindow)) {
      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
    }

    let data = DyingWindowCache.get(aWindow);
    return JSON.stringify(data._closedTabs);
  },

  undoCloseTab: function ssi_undoCloseTab(aWindow, aIndex) {
    if (!aWindow.__SSi) {
      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
    }

    var closedTabs = this._windows[aWindow.__SSi]._closedTabs;

    // default to the most-recently closed tab
    aIndex = aIndex || 0;
    if (!(aIndex in closedTabs)) {
      throw Components.Exception("Invalid index: not in the closed tabs", Cr.NS_ERROR_INVALID_ARG);
    }

    // fetch the data of closed tab, while removing it from the array
    let {state, pos} = this.removeClosedTabData(closedTabs, aIndex);

    // create a new tab
    let tabbrowser = aWindow.gBrowser;
    let tab = tabbrowser.selectedTab = tabbrowser.addTab(null, state);

    // restore tab content
    this.restoreTab(tab, state);

    // restore the tab's position
    tabbrowser.moveTabTo(tab, pos);

    // focus the tab's content area (bug 342432)
    tab.linkedBrowser.focus();

    return tab;
  },

  forgetClosedTab: function ssi_forgetClosedTab(aWindow, aIndex) {
    if (!aWindow.__SSi) {
      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
    }

    var closedTabs = this._windows[aWindow.__SSi]._closedTabs;

    // default to the most-recently closed tab
    aIndex = aIndex || 0;
    if (!(aIndex in closedTabs)) {
      throw Components.Exception("Invalid index: not in the closed tabs", Cr.NS_ERROR_INVALID_ARG);
    }

    // remove closed tab from the array
    this.removeClosedTabData(closedTabs, aIndex);
  },

  getClosedWindowCount: function ssi_getClosedWindowCount() {
    return this._closedWindows.length;
  },

  getClosedWindowData: function ssi_getClosedWindowData() {
    return JSON.stringify(this._closedWindows);
  },

  undoCloseWindow: function ssi_undoCloseWindow(aIndex) {
    if (!(aIndex in this._closedWindows)) {
      throw Components.Exception("Invalid index: not in the closed windows", Cr.NS_ERROR_INVALID_ARG);
    }

    // reopen the window
    let state = { windows: this._closedWindows.splice(aIndex, 1) };
    delete state.windows[0].closedAt; // Window is now open.

    let window = this._openWindowWithState(state);
    this.windowToFocus = window;
    return window;
  },

  forgetClosedWindow: function ssi_forgetClosedWindow(aIndex) {
    // default to the most-recently closed window
    aIndex = aIndex || 0;
    if (!(aIndex in this._closedWindows)) {
      throw Components.Exception("Invalid index: not in the closed windows", Cr.NS_ERROR_INVALID_ARG);
    }

    // remove closed window from the array
    let winData = this._closedWindows[aIndex];
    this._closedWindows.splice(aIndex, 1);
    this._saveableClosedWindowData.delete(winData);
  },

  getWindowValue: function ssi_getWindowValue(aWindow, aKey) {
    if ("__SSi" in aWindow) {
      var data = this._windows[aWindow.__SSi].extData || {};
      return data[aKey] || "";
    }

    if (DyingWindowCache.has(aWindow)) {
      let data = DyingWindowCache.get(aWindow).extData || {};
      return data[aKey] || "";
    }

    throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
  },

  setWindowValue: function ssi_setWindowValue(aWindow, aKey, aStringValue) {
    if (typeof aStringValue != "string") {
      throw new TypeError("setWindowValue only accepts string values");
    }

    if (!("__SSi" in aWindow)) {
      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
    }
    if (!this._windows[aWindow.__SSi].extData) {
      this._windows[aWindow.__SSi].extData = {};
    }
    this._windows[aWindow.__SSi].extData[aKey] = aStringValue;
    this.saveStateDelayed(aWindow);
  },

  deleteWindowValue: function ssi_deleteWindowValue(aWindow, aKey) {
    if (aWindow.__SSi && this._windows[aWindow.__SSi].extData &&
        this._windows[aWindow.__SSi].extData[aKey])
      delete this._windows[aWindow.__SSi].extData[aKey];
    this.saveStateDelayed(aWindow);
  },

  getTabValue: function ssi_getTabValue(aTab, aKey) {
    return (aTab.__SS_extdata || {})[aKey] || "";
  },

  setTabValue: function ssi_setTabValue(aTab, aKey, aStringValue) {
    if (typeof aStringValue != "string") {
      throw new TypeError("setTabValue only accepts string values");
    }

    // If the tab hasn't been restored, then set the data there, otherwise we
    // could lose newly added data.
    if (!aTab.__SS_extdata) {
      aTab.__SS_extdata = {};
    }

    aTab.__SS_extdata[aKey] = aStringValue;
    this.saveStateDelayed(aTab.ownerDocument.defaultView);
  },

  deleteTabValue: function ssi_deleteTabValue(aTab, aKey) {
    if (aTab.__SS_extdata && aKey in aTab.__SS_extdata) {
      delete aTab.__SS_extdata[aKey];
      this.saveStateDelayed(aTab.ownerDocument.defaultView);
    }
  },

  getGlobalValue: function ssi_getGlobalValue(aKey) {
    return this._globalState.get(aKey);
  },

  setGlobalValue: function ssi_setGlobalValue(aKey, aStringValue) {
    if (typeof aStringValue != "string") {
      throw new TypeError("setGlobalValue only accepts string values");
    }

    this._globalState.set(aKey, aStringValue);
    this.saveStateDelayed();
  },

  deleteGlobalValue: function ssi_deleteGlobalValue(aKey) {
    this._globalState.delete(aKey);
    this.saveStateDelayed();
  },

  persistTabAttribute: function ssi_persistTabAttribute(aName) {
    if (TabAttributes.persist(aName)) {
      this.saveStateDelayed();
    }
  },

  /**
   * Restores the session state stored in LastSession. This will attempt
   * to merge data into the current session. If a window was opened at startup
   * with pinned tab(s), then the remaining data from the previous session for
   * that window will be opened into that window. Otherwise new windows will
   * be opened.
   */
  restoreLastSession: function ssi_restoreLastSession() {
    // Use the public getter since it also checks PB mode
    if (!this.canRestoreLastSession) {
      throw Components.Exception("Last session can not be restored");
    }

    Services.obs.notifyObservers(null, NOTIFY_INITIATING_MANUAL_RESTORE, "");

    // First collect each window with its id...
    let windows = {};
    this._forEachBrowserWindow(function(aWindow) {
      if (aWindow.__SS_lastSessionWindowID)
        windows[aWindow.__SS_lastSessionWindowID] = aWindow;
    });

    let lastSessionState = LastSession.getState();

    // This shouldn't ever be the case...
    if (!lastSessionState.windows.length) {
      throw Components.Exception("lastSessionState has no windows", Cr.NS_ERROR_UNEXPECTED);
    }

    // We're technically doing a restore, so set things up so we send the
    // notification when we're done. We want to send "sessionstore-browser-state-restored".
    this._restoreCount = lastSessionState.windows.length;
    this._browserSetState = true;

    // We want to re-use the last opened window instead of opening a new one in
    // the case where it's "empty" and not associated with a window in the session.
    // We will do more processing via _prepWindowToRestoreInto if we need to use
    // the lastWindow.
    let lastWindow = this._getMostRecentBrowserWindow();
    let canUseLastWindow = lastWindow &&
                           !lastWindow.__SS_lastSessionWindowID;

    // global data must be restored before restoreWindow is called so that
    // it happens before observers are notified
    this._globalState.setFromState(lastSessionState);

    // Restore into windows or open new ones as needed.
    for (let i = 0; i < lastSessionState.windows.length; i++) {
      let winState = lastSessionState.windows[i];
      let lastSessionWindowID = winState.__lastSessionWindowID;
      // delete lastSessionWindowID so we don't add that to the window again
      delete winState.__lastSessionWindowID;

      // See if we can use an open window. First try one that is associated with
      // the state we're trying to restore and then fallback to the last selected
      // window.
      let windowToUse = windows[lastSessionWindowID];
      if (!windowToUse && canUseLastWindow) {
        windowToUse = lastWindow;
        canUseLastWindow = false;
      }

      let [canUseWindow, canOverwriteTabs] = this._prepWindowToRestoreInto(windowToUse);

      // If there's a window already open that we can restore into, use that
      if (canUseWindow) {
        // Since we're not overwriting existing tabs, we want to merge _closedTabs,
        // putting existing ones first. Then make sure we're respecting the max pref.
        if (winState._closedTabs && winState._closedTabs.length) {
          let curWinState = this._windows[windowToUse.__SSi];
          curWinState._closedTabs = curWinState._closedTabs.concat(winState._closedTabs);
          curWinState._closedTabs.splice(this._max_tabs_undo, curWinState._closedTabs.length);
        }

        // Restore into that window - pretend it's a followup since we'll already
        // have a focused window.
        //XXXzpao This is going to merge extData together (taking what was in
        //        winState over what is in the window already.
        let options = {overwriteTabs: canOverwriteTabs, isFollowUp: true};
        this.restoreWindow(windowToUse, winState, options);
      }
      else {
        this._openWindowWithState({ windows: [winState] });
      }
    }

    // Merge closed windows from this session with ones from last session
    if (lastSessionState._closedWindows) {
      this._closedWindows = this._closedWindows.concat(lastSessionState._closedWindows);
      this._capClosedWindows();
    }

    if (lastSessionState.scratchpads) {
      ScratchpadManager.restoreSession(lastSessionState.scratchpads);
    }

    // Set data that persists between sessions
    this._recentCrashes = lastSessionState.session &&
                          lastSessionState.session.recentCrashes || 0;

    // Update the session start time using the restored session state.
    this._updateSessionStartTime(lastSessionState);

    LastSession.clear();
  },

  /**
   * Revive a crashed tab and restore its state from before it crashed.
   *
   * @param aTab
   *        A <xul:tab> linked to a crashed browser. This is a no-op if the
   *        browser hasn't actually crashed, or is not associated with a tab.
   *        This function will also throw if the browser happens to be remote.
   */
  reviveCrashedTab(aTab) {
    if (!aTab) {
      throw new Error("SessionStore.reviveCrashedTab expected a tab, but got null.");
    }

    let browser = aTab.linkedBrowser;
    if (!this._crashedBrowsers.has(browser.permanentKey)) {
      return;
    }

    // Sanity check - the browser to be revived should not be remote
    // at this point.
    if (browser.isRemoteBrowser) {
      throw new Error("SessionStore.reviveCrashedTab: " +
                      "Somehow a crashed browser is still remote.")
    }

    // We put the browser at about:blank in case the user is
    // restoring tabs on demand. This way, the user won't see
    // a flash of the about:tabcrashed page after selecting
    // the revived tab.
    aTab.removeAttribute("crashed");
    browser.loadURI("about:blank", null, null);

    let data = TabState.collect(aTab);
    this.restoreTab(aTab, data, {
      forceOnDemand: true,
    });
  },

  /**
   * Revive all crashed tabs and reset the crashed tabs count to 0.
   */
  reviveAllCrashedTabs() {
    let windowsEnum = Services.wm.getEnumerator("navigator:browser");
    while (windowsEnum.hasMoreElements()) {
      let window = windowsEnum.getNext();
      for (let tab of window.gBrowser.tabs) {
        this.reviveCrashedTab(tab);
      }
    }
  },

  /**
   * Navigate the given |tab| by first collecting its current state and then
   * either changing only the index of the currently shown history entry,
   * or restoring the exact same state again and passing the new URL to load
   * in |loadArguments|. Use this method to seamlessly switch between pages
   * loaded in the parent and pages loaded in the child process.
   *
   * This method might be called multiple times before it has finished
   * flushing the browser tab. If that occurs, the loadArguments from
   * the most recent call to navigateAndRestore will be used once the
   * flush has finished.
   */
  navigateAndRestore(tab, loadArguments, historyIndex) {
    let window = tab.ownerDocument.defaultView;
    NS_ASSERT(window.__SSi, "tab's window must be tracked");
    let browser = tab.linkedBrowser;

    // Were we already waiting for a flush from a previous call to
    // navigateAndRestore on this tab?
    let alreadyRestoring =
      this._remotenessChangingBrowsers.has(browser.permanentKey);

    // Stash the most recent loadArguments in this WeakMap so that
    // we know to use it when the TabStateFlusher.flush resolves.
    this._remotenessChangingBrowsers.set(browser.permanentKey, loadArguments);

    if (alreadyRestoring) {
      // This tab was already being restored to run in the
      // correct process. We're done here.
      return;
    }

    // Set tab title to "Connecting..." and start the throbber to pretend we're
    // doing something while actually waiting for data from the frame script.
    window.gBrowser.setTabTitleLoading(tab);
    tab.setAttribute("busy", "true");

    // Flush to get the latest tab state.
    TabStateFlusher.flush(browser).then(() => {
      // loadArguments might have been overwritten by multiple calls
      // to navigateAndRestore while we waited for the tab to flush,
      // so we use the most recently stored one.
      let recentLoadArguments =
        this._remotenessChangingBrowsers.get(browser.permanentKey);
      this._remotenessChangingBrowsers.delete(browser.permanentKey);

      // The tab might have been closed/gone in the meantime.
      if (tab.closing || !tab.linkedBrowser) {
        return;
      }

      let window = tab.ownerDocument && tab.ownerDocument.defaultView;

      // The tab or its window might be gone.
      if (!window || !window.__SSi || window.closed) {
        return;
      }

      let tabState = TabState.clone(tab);
      let options = {restoreImmediately: true};

      if (historyIndex >= 0) {
        tabState.index = historyIndex + 1;
        tabState.index = Math.max(1, Math.min(tabState.index, tabState.entries.length));
      } else {
        options.loadArguments = recentLoadArguments;
      }

      // Need to reset restoring tabs.
      if (tab.linkedBrowser.__SS_restoreState) {
        this._resetLocalTabRestoringState(tab);
      }

      // Restore the state into the tab.
      this.restoreTab(tab, tabState, options);
    });

    tab.linkedBrowser.__SS_restoreState = TAB_STATE_WILL_RESTORE;
  },

  /**
   * Retrieves the latest session history information for a tab. The cached data
   * is returned immediately, but a callback may be provided that supplies
   * up-to-date data when or if it is available. The callback is passed a single
   * argument with data in the same format as the return value.
   *
   * @param tab tab to retrieve the session history for
   * @param updatedCallback function to call with updated data as the single argument
   * @returns a object containing 'index' specifying the current index, and an
   * array 'entries' containing an object for each history item.
   */
  getSessionHistory(tab, updatedCallback) {
    if (updatedCallback) {
      TabStateFlusher.flush(tab.linkedBrowser).then(() => {
        let sessionHistory = this.getSessionHistory(tab);
        if (sessionHistory) {
          updatedCallback(sessionHistory);
        }
      });
    }

    // Don't continue if the tab was closed before TabStateFlusher.flush resolves.
    if (tab.linkedBrowser) {
      let tabState = TabState.collect(tab);
      return { index: tabState.index - 1, entries: tabState.entries }
    }
  },

  /**
   * See if aWindow is usable for use when restoring a previous session via
   * restoreLastSession. If usable, prepare it for use.
   *
   * @param aWindow
   *        the window to inspect & prepare
   * @returns [canUseWindow, canOverwriteTabs]
   *          canUseWindow: can the window be used to restore into
   *          canOverwriteTabs: all of the current tabs are home pages and we
   *                            can overwrite them
   */
  _prepWindowToRestoreInto: function ssi_prepWindowToRestoreInto(aWindow) {
    if (!aWindow)
      return [false, false];

    // We might be able to overwrite the existing tabs instead of just adding
    // the previous session's tabs to the end. This will be set if possible.
    let canOverwriteTabs = false;

    // Look at the open tabs in comparison to home pages. If all the tabs are
    // home pages then we'll end up overwriting all of them. Otherwise we'll
    // just close the tabs that match home pages. Tabs with the about:blank
    // URI will always be overwritten.
    let homePages = ["about:blank"];
    let removableTabs = [];
    let tabbrowser = aWindow.gBrowser;
    let normalTabsLen = tabbrowser.tabs.length - tabbrowser._numPinnedTabs;
    let startupPref = this._prefBranch.getIntPref("startup.page");
    if (startupPref == 1)
      homePages = homePages.concat(aWindow.gHomeButton.getHomePage().split("|"));

    for (let i = tabbrowser._numPinnedTabs; i < tabbrowser.tabs.length; i++) {
      let tab = tabbrowser.tabs[i];
      if (homePages.indexOf(tab.linkedBrowser.currentURI.spec) != -1) {
        removableTabs.push(tab);
      }
    }

    if (tabbrowser.tabs.length == removableTabs.length) {
      canOverwriteTabs = true;
    }
    else {
      // If we're not overwriting all of the tabs, then close the home tabs.
      for (let i = removableTabs.length - 1; i >= 0; i--) {
        tabbrowser.removeTab(removableTabs.pop(), { animate: false });
      }
    }

    return [true, canOverwriteTabs];
  },

  /* ........ Saving Functionality .............. */

  /**
   * Store window dimensions, visibility, sidebar
   * @param aWindow
   *        Window reference
   */
  _updateWindowFeatures: function ssi_updateWindowFeatures(aWindow) {
    var winData = this._windows[aWindow.__SSi];

    WINDOW_ATTRIBUTES.forEach(function(aAttr) {
      winData[aAttr] = this._getWindowDimension(aWindow, aAttr);
    }, this);

    var hidden = WINDOW_HIDEABLE_FEATURES.filter(function(aItem) {
      return aWindow[aItem] && !aWindow[aItem].visible;
    });
    if (hidden.length != 0)
      winData.hidden = hidden.join(",");
    else if (winData.hidden)
      delete winData.hidden;

    var sidebar = aWindow.document.getElementById("sidebar-box").getAttribute("sidebarcommand");
    if (sidebar)
      winData.sidebar = sidebar;
    else if (winData.sidebar)
      delete winData.sidebar;
  },

  /**
   * gather session data as object
   * @param aUpdateAll
   *        Bool update all windows
   * @returns object
   */
  getCurrentState: function (aUpdateAll) {
    this._handleClosedWindows();

    var activeWindow = this._getMostRecentBrowserWindow();

    TelemetryStopwatch.start("FX_SESSION_RESTORE_COLLECT_ALL_WINDOWS_DATA_MS");
    if (RunState.isRunning) {
      // update the data for all windows with activities since the last save operation
      this._forEachBrowserWindow(function(aWindow) {
        if (!this._isWindowLoaded(aWindow)) // window data is still in _statesToRestore
          return;
        if (aUpdateAll || DirtyWindows.has(aWindow) || aWindow == activeWindow) {
          this._collectWindowData(aWindow);
        }
        else { // always update the window features (whose change alone never triggers a save operation)
          this._updateWindowFeatures(aWindow);
        }
      });
      DirtyWindows.clear();
    }
    TelemetryStopwatch.finish("FX_SESSION_RESTORE_COLLECT_ALL_WINDOWS_DATA_MS");

    // An array that at the end will hold all current window data.
    var total = [];
    // The ids of all windows contained in 'total' in the same order.
    var ids = [];
    // The number of window that are _not_ popups.
    var nonPopupCount = 0;
    var ix;

    // collect the data for all windows
    for (ix in this._windows) {
      if (this._windows[ix]._restoring) // window data is still in _statesToRestore
        continue;
      total.push(this._windows[ix]);
      ids.push(ix);
      if (!this._windows[ix].isPopup)
        nonPopupCount++;
    }

    TelemetryStopwatch.start("FX_SESSION_RESTORE_COLLECT_COOKIES_MS");
    SessionCookies.update(total);
    TelemetryStopwatch.finish("FX_SESSION_RESTORE_COLLECT_COOKIES_MS");

    // collect the data for all windows yet to be restored
    for (ix in this._statesToRestore) {
      for (let winData of this._statesToRestore[ix].windows) {
        total.push(winData);
        if (!winData.isPopup)
          nonPopupCount++;
      }
    }

    // shallow copy this._closedWindows to preserve current state
    let lastClosedWindowsCopy = this._closedWindows.slice();

    if (AppConstants.platform != "macosx") {
      // If no non-popup browser window remains open, return the state of the last
      // closed window(s). We only want to do this when we're actually "ending"
      // the session.
      //XXXzpao We should do this for _restoreLastWindow == true, but that has
      //        its own check for popups. c.f. bug 597619
      if (nonPopupCount == 0 && lastClosedWindowsCopy.length > 0 &&
          RunState.isQuitting) {
        // prepend the last non-popup browser window, so that if the user loads more tabs
        // at startup we don't accidentally add them to a popup window
        do {
          total.unshift(lastClosedWindowsCopy.shift())
        } while (total[0].isPopup && lastClosedWindowsCopy.length > 0)
      }
    }

    if (activeWindow) {
      this.activeWindowSSiCache = activeWindow.__SSi || "";
    }
    ix = ids.indexOf(this.activeWindowSSiCache);
    // We don't want to restore focus to a minimized window or a window which had all its
    // tabs stripped out (doesn't exist).
    if (ix != -1 && total[ix] && total[ix].sizemode == "minimized")
      ix = -1;

    let session = {
      lastUpdate: Date.now(),
      startTime: this._sessionStartTime,
      recentCrashes: this._recentCrashes
    };

    let state = {
      version: ["sessionrestore", FORMAT_VERSION],
      windows: total,
      selectedWindow: ix + 1,
      _closedWindows: lastClosedWindowsCopy,
      session: session,
      global: this._globalState.getState()
    };

    if (Cu.isModuleLoaded("resource://devtools/client/scratchpad/scratchpad-manager.jsm")) {
      // get open Scratchpad window states too
      let scratchpads = ScratchpadManager.getSessionState();
      if (scratchpads && scratchpads.length) {
        state.scratchpads = scratchpads;
      }
    }

    // Persist the last session if we deferred restoring it
    if (LastSession.canRestore) {
      state.lastSessionState = LastSession.getState();
    }

    // If we were called by the SessionSaver and started with only a private
    // window we want to pass the deferred initial state to not lose the
    // previous session.
    if (this._deferredInitialState) {
      state.deferredInitialState = this._deferredInitialState;
    }

    return state;
  },

  /**
   * serialize session data for a window
   * @param aWindow
   *        Window reference
   * @returns string
   */
  _getWindowState: function ssi_getWindowState(aWindow) {
    if (!this._isWindowLoaded(aWindow))
      return this._statesToRestore[aWindow.__SS_restoreID];

    if (RunState.isRunning) {
      this._collectWindowData(aWindow);
    }

    let windows = [this._windows[aWindow.__SSi]];
    SessionCookies.update(windows);

    return { windows: windows };
  },

  /**
   * Gathers data about a window and its tabs, and updates its
   * entry in this._windows.
   *
   * @param aWindow
   *        Window references.
   * @returns a Map mapping the browser tabs from aWindow to the tab
   *          entry that was put into the window data in this._windows.
   */
  _collectWindowData: function ssi_collectWindowData(aWindow) {
    let tabMap = new Map();

    if (!this._isWindowLoaded(aWindow))
      return tabMap;

    let tabbrowser = aWindow.gBrowser;
    let tabs = tabbrowser.tabs;
    let winData = this._windows[aWindow.__SSi];
    let tabsData = winData.tabs = [];

    // update the internal state data for this window
    for (let tab of tabs) {
      let tabData = TabState.collect(tab);
      tabMap.set(tab, tabData);
      tabsData.push(tabData);
    }
    winData.selected = tabbrowser.mTabBox.selectedIndex + 1;

    this._updateWindowFeatures(aWindow);

    // Make sure we keep __SS_lastSessionWindowID around for cases like entering
    // or leaving PB mode.
    if (aWindow.__SS_lastSessionWindowID)
      this._windows[aWindow.__SSi].__lastSessionWindowID =
        aWindow.__SS_lastSessionWindowID;

    DirtyWindows.remove(aWindow);
    return tabMap;
  },

  /* ........ Restoring Functionality .............. */

  /**
   * restore features to a single window
   * @param aWindow
   *        Window reference to the window to use for restoration
   * @param winData
   *        JS object
   * @param aOptions
   *        {overwriteTabs: true} to overwrite existing tabs w/ new ones
   *        {isFollowUp: true} if this is not the restoration of the 1st window
   *        {firstWindow: true} if this is the first non-private window we're
   *                            restoring in this session, that might open an
   *                            external link as well
   */
  restoreWindow: function ssi_restoreWindow(aWindow, winData, aOptions = {}) {
    let overwriteTabs = aOptions && aOptions.overwriteTabs;
    let isFollowUp = aOptions && aOptions.isFollowUp;
    let firstWindow = aOptions && aOptions.firstWindow;

    if (isFollowUp) {
      this.windowToFocus = aWindow;
    }

    // initialize window if necessary
    if (aWindow && (!aWindow.__SSi || !this._windows[aWindow.__SSi]))
      this.onLoad(aWindow);

    TelemetryStopwatch.start("FX_SESSION_RESTORE_RESTORE_WINDOW_MS");

    // We're not returning from this before we end up calling restoreTabs
    // for this window, so make sure we send the SSWindowStateBusy event.
    this._setWindowStateBusy(aWindow);

    if (!winData.tabs) {
      winData.tabs = [];
    }

    // don't restore a single blank tab when we've had an external
    // URL passed in for loading at startup (cf. bug 357419)
    else if (firstWindow && !overwriteTabs && winData.tabs.length == 1 &&
             (!winData.tabs[0].entries || winData.tabs[0].entries.length == 0)) {
      winData.tabs = [];
    }

    var tabbrowser = aWindow.gBrowser;
    var openTabCount = overwriteTabs ? tabbrowser.browsers.length : -1;
    var newTabCount = winData.tabs.length;
    var tabs = [];

    // disable smooth scrolling while adding, moving, removing and selecting tabs
    var tabstrip = tabbrowser.tabContainer.mTabstrip;
    var smoothScroll = tabstrip.smoothScroll;
    tabstrip.smoothScroll = false;

    // unpin all tabs to ensure they are not reordered in the next loop
    if (overwriteTabs) {
      for (let t = tabbrowser._numPinnedTabs - 1; t > -1; t--)
        tabbrowser.unpinTab(tabbrowser.tabs[t]);
    }

    // We need to keep track of the initially open tabs so that they
    // can be moved to the end of the restored tabs.
    let initialTabs = [];
    if (!overwriteTabs && firstWindow) {
      initialTabs = Array.slice(tabbrowser.tabs);
    }

    // make sure that the selected tab won't be closed in order to
    // prevent unnecessary flickering
    if (overwriteTabs && tabbrowser.selectedTab._tPos >= newTabCount)
      tabbrowser.moveTabTo(tabbrowser.selectedTab, newTabCount - 1);

    let numVisibleTabs = 0;

    for (var t = 0; t < newTabCount; t++) {
      // When trying to restore into existing tab, we also take the userContextId
      // into account if present.
      let userContextId = winData.tabs[t].userContextId;
      let reuseExisting = t < openTabCount &&
                          (tabbrowser.tabs[t].getAttribute("usercontextid") == (userContextId || ""));
      // If the tab is pinned, then we'll be loading it right away, and
      // there's no need to cause a remoteness flip by loading it initially
      // non-remote.
      let forceNotRemote = !winData.tabs[t].pinned;
      let tab = reuseExisting ? tabbrowser.tabs[t] :
                                tabbrowser.addTab("about:blank",
                                                  {skipAnimation: true,
                                                   forceNotRemote,
                                                   userContextId});

      // If we inserted a new tab because the userContextId didn't match with the
      // open tab, even though `t < openTabCount`, we need to remove that open tab
      // and put the newly added tab in its place.
      if (!reuseExisting && t < openTabCount) {
        tabbrowser.removeTab(tabbrowser.tabs[t]);
        tabbrowser.moveTabTo(tab, t);
      }

      tabs.push(tab);

      if (winData.tabs[t].pinned)
        tabbrowser.pinTab(tabs[t]);

      if (winData.tabs[t].hidden) {
        tabbrowser.hideTab(tabs[t]);
      }
      else {
        tabbrowser.showTab(tabs[t]);
        numVisibleTabs++;
      }

      if (!!winData.tabs[t].muted != tabs[t].linkedBrowser.audioMuted) {
        tabs[t].toggleMuteAudio(winData.tabs[t].muteReason);
      }
    }

    if (!overwriteTabs && firstWindow) {
      // Move the originally open tabs to the end
      let endPosition = tabbrowser.tabs.length - 1;
      for (let i = 0; i < initialTabs.length; i++) {
        tabbrowser.moveTabTo(initialTabs[i], endPosition);
      }
    }

    // if all tabs to be restored are hidden, make the first one visible
    if (!numVisibleTabs && winData.tabs.length) {
      winData.tabs[0].hidden = false;
      tabbrowser.showTab(tabs[0]);
    }

    // If overwriting tabs, we want to reset each tab's "restoring" state. Since
    // we're overwriting those tabs, they should no longer be restoring. The
    // tabs will be rebuilt and marked if they need to be restored after loading
    // state (in restoreTabs).
    if (overwriteTabs) {
      for (let i = 0; i < tabbrowser.tabs.length; i++) {
        let tab = tabbrowser.tabs[i];
        if (tabbrowser.browsers[i].__SS_restoreState)
          this._resetTabRestoringState(tab);
      }
    }

    // We want to correlate the window with data from the last session, so
    // assign another id if we have one. Otherwise clear so we don't do
    // anything with it.
    delete aWindow.__SS_lastSessionWindowID;
    if (winData.__lastSessionWindowID)
      aWindow.__SS_lastSessionWindowID = winData.__lastSessionWindowID;

    // when overwriting tabs, remove all superflous ones
    if (overwriteTabs && newTabCount < openTabCount) {
      Array.slice(tabbrowser.tabs, newTabCount, openTabCount)
           .forEach(tabbrowser.removeTab, tabbrowser);
    }

    if (overwriteTabs) {
      this.restoreWindowFeatures(aWindow, winData);
      delete this._windows[aWindow.__SSi].extData;
    }
    if (winData.cookies) {
      SessionCookies.restore(winData.cookies);
    }
    if (winData.extData) {
      if (!this._windows[aWindow.__SSi].extData) {
        this._windows[aWindow.__SSi].extData = {};
      }
      for (var key in winData.extData) {
        this._windows[aWindow.__SSi].extData[key] = winData.extData[key];
      }
    }

    let newClosedTabsData = winData._closedTabs || [];

    if (overwriteTabs || firstWindow) {
      // Overwrite existing closed tabs data when overwriteTabs=true
      // or we're the first window to be restored.
      this._windows[aWindow.__SSi]._closedTabs = newClosedTabsData;
    } else if (this._max_tabs_undo > 0) {
      // If we merge tabs, we also want to merge closed tabs data. We'll assume
      // the restored tabs were closed more recently and append the current list
      // of closed tabs to the new one...
      newClosedTabsData =
        newClosedTabsData.concat(this._windows[aWindow.__SSi]._closedTabs);

      // ... and make sure that we don't exceed the max number of closed tabs
      // we can restore.
      this._windows[aWindow.__SSi]._closedTabs =
        newClosedTabsData.slice(0, this._max_tabs_undo);
    }

    // Restore tabs, if any.
    if (winData.tabs.length) {
      this.restoreTabs(aWindow, tabs, winData.tabs,
        (overwriteTabs ? (parseInt(winData.selected || "1")) : 0));
    }

    // set smoothScroll back to the original value
    tabstrip.smoothScroll = smoothScroll;

    TelemetryStopwatch.finish("FX_SESSION_RESTORE_RESTORE_WINDOW_MS");

    this._setWindowStateReady(aWindow);

    Services.obs.notifyObservers(aWindow, NOTIFY_SINGLE_WINDOW_RESTORED, "");

    this._sendRestoreCompletedNotifications();
  },

  /**
   * Restore multiple windows using the provided state.
   * @param aWindow
   *        Window reference to the first window to use for restoration.
   *        Additionally required windows will be opened.
   * @param aState
   *        JS object or JSON string
   * @param aOptions
   *        {overwriteTabs: true} to overwrite existing tabs w/ new ones
   *        {isFollowUp: true} if this is not the restoration of the 1st window
   *        {firstWindow: true} if this is the first non-private window we're
   *                            restoring in this session, that might open an
   *                            external link as well
   */
  restoreWindows: function ssi_restoreWindows(aWindow, aState, aOptions = {}) {
    let isFollowUp = aOptions && aOptions.isFollowUp;

    if (isFollowUp) {
      this.windowToFocus = aWindow;
    }

    // initialize window if necessary
    if (aWindow && (!aWindow.__SSi || !this._windows[aWindow.__SSi]))
      this.onLoad(aWindow);

    let root;
    try {
      root = (typeof aState == "string") ? JSON.parse(aState) : aState;
    }
    catch (ex) { // invalid state object - don't restore anything
      debug(ex);
      this._sendRestoreCompletedNotifications();
      return;
    }

    // Restore closed windows if any.
    if (root._closedWindows) {
      this._closedWindows = root._closedWindows;
    }

    // We're done here if there are no windows.
    if (!root.windows || !root.windows.length) {
      this._sendRestoreCompletedNotifications();
      return;
    }

    if (!root.selectedWindow || root.selectedWindow > root.windows.length) {
      root.selectedWindow = 0;
    }

    // open new windows for all further window entries of a multi-window session
    // (unless they don't contain any tab data)
    let winData;
    for (var w = 1; w < root.windows.length; w++) {
      winData = root.windows[w];
      if (winData && winData.tabs && winData.tabs[0]) {
        var window = this._openWindowWithState({ windows: [winData] });
        if (w == root.selectedWindow - 1) {
          this.windowToFocus = window;
        }
      }
    }

    this.restoreWindow(aWindow, root.windows[0], aOptions);

    if (aState.scratchpads) {
      ScratchpadManager.restoreSession(aState.scratchpads);
    }
  },

  /**
   * Manage history restoration for a window
   * @param aWindow
   *        Window to restore the tabs into
   * @param aTabs
   *        Array of tab references
   * @param aTabData
   *        Array of tab data
   * @param aSelectTab
   *        Index of the tab to select. This is a 1-based index where "1"
   *        indicates the first tab should be selected, and "0" indicates that
   *        the currently selected tab will not be changed.
   */
  restoreTabs(aWindow, aTabs, aTabData, aSelectTab) {
    var tabbrowser = aWindow.gBrowser;

    if (!this._isWindowLoaded(aWindow)) {
      // from now on, the data will come from the actual window
      delete this._statesToRestore[aWindow.__SS_restoreID];
      delete aWindow.__SS_restoreID;
      delete this._windows[aWindow.__SSi]._restoring;
    }

    let numTabsToRestore = aTabs.length;
    let numTabsInWindow = tabbrowser.tabs.length;
    let tabsDataArray = this._windows[aWindow.__SSi].tabs;

    // Update the window state in case we shut down without being notified.
    // Individual tab states will be taken care of by restoreTab() below.
    if (numTabsInWindow == numTabsToRestore) {
      // Remove all previous tab data.
      tabsDataArray.length = 0;
    } else {
      // Remove all previous tab data except tabs that should not be overriden.
      tabsDataArray.splice(numTabsInWindow - numTabsToRestore);
    }

    // Let the tab data array have the right number of slots.
    tabsDataArray.length = numTabsInWindow;

    // If provided, set the selected tab.
    if (aSelectTab > 0 && aSelectTab <= aTabs.length) {
      tabbrowser.selectedTab = aTabs[aSelectTab - 1];

      // Update the window state in case we shut down without being notified.
      this._windows[aWindow.__SSi].selected = aSelectTab;
    }

    // Restore all tabs.
    for (let t = 0; t < aTabs.length; t++) {
      this.restoreTab(aTabs[t], aTabData[t]);
    }
  },

  // Restores the given tab state for a given tab.
  restoreTab(tab, tabData, options = {}) {
    NS_ASSERT(!tab.linkedBrowser.__SS_restoreState,
              "must reset tab before calling restoreTab()");

    let restoreImmediately = options.restoreImmediately;
    let loadArguments = options.loadArguments;
    let browser = tab.linkedBrowser;
    let window = tab.ownerDocument.defaultView;
    let tabbrowser = window.gBrowser;
    let forceOnDemand = options.forceOnDemand;

    // Increase the busy state counter before modifying the tab.
    this._setWindowStateBusy(window);

    // It's important to set the window state to dirty so that
    // we collect their data for the first time when saving state.
    DirtyWindows.add(window);

    // In case we didn't collect/receive data for any tabs yet we'll have to
    // fill the array with at least empty tabData objects until |_tPos| or
    // we'll end up with |null| entries.
    for (let otherTab of Array.slice(tabbrowser.tabs, 0, tab._tPos)) {
      let emptyState = {entries: [], lastAccessed: otherTab.lastAccessed};
      this._windows[window.__SSi].tabs.push(emptyState);
    }

    // Update the tab state in case we shut down without being notified.
    this._windows[window.__SSi].tabs[tab._tPos] = tabData;

    // Prepare the tab so that it can be properly restored. We'll pin/unpin
    // and show/hide tabs as necessary. We'll also attach a copy of the tab's
    // data in case we close it before it's been restored.
    if (tabData.pinned) {
      tabbrowser.pinTab(tab);
    } else {
      tabbrowser.unpinTab(tab);
    }

    if (tabData.hidden) {
      tabbrowser.hideTab(tab);
    } else {
      tabbrowser.showTab(tab);
    }

    if (!!tabData.muted != browser.audioMuted) {
      tab.toggleMuteAudio(tabData.muteReason);
    }

    if (tabData.lastAccessed) {
      tab.lastAccessed = tabData.lastAccessed;
    }

    if ("attributes" in tabData) {
      // Ensure that we persist tab attributes restored from previous sessions.
      Object.keys(tabData.attributes).forEach(a => TabAttributes.persist(a));
    }

    if (!tabData.entries) {
      tabData.entries = [];
    }
    if (tabData.extData) {
      tab.__SS_extdata = Cu.cloneInto(tabData.extData, {});
    } else {
      delete tab.__SS_extdata;
    }

    // Tab is now open.
    delete tabData.closedAt;

    // Ensure the index is in bounds.
    let activeIndex = (tabData.index || tabData.entries.length) - 1;
    activeIndex = Math.min(activeIndex, tabData.entries.length - 1);
    activeIndex = Math.max(activeIndex, 0);

    // Save the index in case we updated it above.
    tabData.index = activeIndex + 1;

    // Start a new epoch to discard all frame script messages relating to a
    // previous epoch. All async messages that are still on their way to chrome
    // will be ignored and don't override any tab data set when restoring.
    let epoch = this.startNextEpoch(browser);

    // keep the data around to prevent dataloss in case
    // a tab gets closed before it's been properly restored
    browser.__SS_restoreState = TAB_STATE_NEEDS_RESTORE;
    browser.setAttribute("pending", "true");
    tab.setAttribute("pending", "true");

    // If we're restoring this tab, it certainly shouldn't be in
    // the ignored set anymore.
    this._crashedBrowsers.delete(browser.permanentKey);

    // Update the persistent tab state cache with |tabData| information.
    TabStateCache.update(browser, {
      history: {entries: tabData.entries, index: tabData.index},
      scroll: tabData.scroll || null,
      storage: tabData.storage || null,
      formdata: tabData.formdata || null,
      disallow: tabData.disallow || null,
      pageStyle: tabData.pageStyle || null,

      // This information is only needed until the tab has finished restoring.
      // When that's done it will be removed from the cache and we always
      // collect it in TabState._collectBaseTabData().
      image: tabData.image || "",
      userTypedValue: tabData.userTypedValue || "",
      userTypedClear: tabData.userTypedClear || 0
    });

    browser.messageManager.sendAsyncMessage("SessionStore:restoreHistory",
                                            {tabData: tabData, epoch: epoch, loadArguments});

    // Restore tab attributes.
    if ("attributes" in tabData) {
      TabAttributes.set(tab, tabData.attributes);
    }

    // This could cause us to ignore MAX_CONCURRENT_TAB_RESTORES a bit, but
    // it ensures each window will have its selected tab loaded.
    if (restoreImmediately || tabbrowser.selectedBrowser == browser || loadArguments) {
      this.restoreTabContent(tab, loadArguments);
    } else if (!forceOnDemand) {
      TabRestoreQueue.add(tab);
      this.restoreNextTab();
    }

    // Decrease the busy state counter after we're done.
    this._setWindowStateReady(window);
  },

  /**
   * Kicks off restoring the given tab.
   *
   * @param aTab
   *        the tab to restore
   * @param aLoadArguments
   *        optional load arguments used for loadURI()
   * @param aRemotenessSwitch
   *        true if we're restoring a tab's content because we flipped
   *        its remoteness (out-of-process) state.
   */
  restoreTabContent: function (aTab, aLoadArguments = null) {
    if (aTab.hasAttribute("customizemode")) {
      return;
    }

    let browser = aTab.linkedBrowser;
    let window = aTab.ownerDocument.defaultView;
    let tabbrowser = window.gBrowser;
    let tabData = TabState.clone(aTab);
    let activeIndex = tabData.index - 1;
    let activePageData = tabData.entries[activeIndex] || null;
    let uri = activePageData ? activePageData.url || null : null;
    if (aLoadArguments) {
      uri = aLoadArguments.uri;
    }

    // We have to mark this tab as restoring first, otherwise
    // the "pending" attribute will be applied to the linked
    // browser, which removes it from the display list. We cannot
    // flip the remoteness of any browser that is not being displayed.
    this.markTabAsRestoring(aTab);

    let isRemotenessUpdate = tabbrowser.updateBrowserRemotenessByURL(browser, uri);
    if (isRemotenessUpdate) {
      // We updated the remoteness, so we need to send the history down again.
      //
      // Start a new epoch to discard all frame script messages relating to a
      // previous epoch. All async messages that are still on their way to chrome
      // will be ignored and don't override any tab data set when restoring.
      let epoch = this.startNextEpoch(browser);

      browser.messageManager.sendAsyncMessage("SessionStore:restoreHistory", {
        tabData: tabData,
        epoch: epoch,
        loadArguments: aLoadArguments,
        isRemotenessUpdate,
      });

    }

    // If the restored browser wants to show view source content, start up a
    // view source browser that will load the required frame script.
    if (uri && ViewSourceBrowser.isViewSource(uri)) {
      new ViewSourceBrowser(browser);
    }

    browser.messageManager.sendAsyncMessage("SessionStore:restoreTabContent",
      {loadArguments: aLoadArguments, isRemotenessUpdate});
  },

  /**
   * Marks a given pending tab as restoring.
   *
   * @param aTab
   *        the pending tab to mark as restoring
   */
  markTabAsRestoring(aTab) {
    let browser = aTab.linkedBrowser;
    if (browser.__SS_restoreState != TAB_STATE_NEEDS_RESTORE) {
      throw new Error("Given tab is not pending.");
    }

    // Make sure that this tab is removed from the priority queue.
    TabRestoreQueue.remove(aTab);

    // Increase our internal count.
    this._tabsRestoringCount++;

    // Set this tab's state to restoring
    browser.__SS_restoreState = TAB_STATE_RESTORING;
    browser.removeAttribute("pending");
    aTab.removeAttribute("pending");
  },

  /**
   * This _attempts_ to restore the next available tab. If the restore fails,
   * then we will attempt the next one.
   * There are conditions where this won't do anything:
   *   if we're in the process of quitting
   *   if there are no tabs to restore
   *   if we have already reached the limit for number of tabs to restore
   */
  restoreNextTab: function ssi_restoreNextTab() {
    // If we call in here while quitting, we don't actually want to do anything
    if (RunState.isQuitting)
      return;

    // Don't exceed the maximum number of concurrent tab restores.
    if (this._tabsRestoringCount >= MAX_CONCURRENT_TAB_RESTORES)
      return;

    let tab = TabRestoreQueue.shift();
    if (tab) {
      this.restoreTabContent(tab);
    }
  },

  /**
   * Restore visibility and dimension features to a window
   * @param aWindow
   *        Window reference
   * @param aWinData
   *        Object containing session data for the window
   */
  restoreWindowFeatures: function ssi_restoreWindowFeatures(aWindow, aWinData) {
    var hidden = (aWinData.hidden)?aWinData.hidden.split(","):[];
    WINDOW_HIDEABLE_FEATURES.forEach(function(aItem) {
      aWindow[aItem].visible = hidden.indexOf(aItem) == -1;
    });

    if (aWinData.isPopup) {
      this._windows[aWindow.__SSi].isPopup = true;
      if (aWindow.gURLBar) {
        aWindow.gURLBar.readOnly = true;
        aWindow.gURLBar.setAttribute("enablehistory", "false");
      }
    }
    else {
      delete this._windows[aWindow.__SSi].isPopup;
      if (aWindow.gURLBar) {
        aWindow.gURLBar.readOnly = false;
        aWindow.gURLBar.setAttribute("enablehistory", "true");
      }
    }

    var _this = this;
    aWindow.setTimeout(function() {
      _this.restoreDimensions.apply(_this, [aWindow,
        +(aWinData.width || 0),
        +(aWinData.height || 0),
        "screenX" in aWinData ? +aWinData.screenX : NaN,
        "screenY" in aWinData ? +aWinData.screenY : NaN,
        aWinData.sizemode || "", aWinData.sidebar || ""]);
    }, 0);
  },

  /**
   * Restore a window's dimensions
   * @param aWidth
   *        Window width
   * @param aHeight
   *        Window height
   * @param aLeft
   *        Window left
   * @param aTop
   *        Window top
   * @param aSizeMode
   *        Window size mode (eg: maximized)
   * @param aSidebar
   *        Sidebar command
   */
  restoreDimensions: function ssi_restoreDimensions(aWindow, aWidth, aHeight, aLeft, aTop, aSizeMode, aSidebar) {
    var win = aWindow;
    var _this = this;
    function win_(aName) { return _this._getWindowDimension(win, aName); }

    // find available space on the screen where this window is being placed
    let screen = gScreenManager.screenForRect(aLeft, aTop, aWidth, aHeight);
    if (screen) {
      let screenLeft = {}, screenTop = {}, screenWidth = {}, screenHeight = {};
      screen.GetAvailRectDisplayPix(screenLeft, screenTop, screenWidth, screenHeight);
      // screenX/Y are based on the origin of the screen's desktop-pixel coordinate space
      let screenLeftCss = screenLeft.value;
      let screenTopCss = screenTop.value;
      // convert screen's device pixel dimensions to CSS px dimensions
      screen.GetAvailRect(screenLeft, screenTop, screenWidth, screenHeight);
      let cssToDevScale = screen.defaultCSSScaleFactor;
      let screenRightCss = screenLeftCss + screenWidth.value / cssToDevScale;
      let screenBottomCss = screenTopCss + screenHeight.value / cssToDevScale;

      // Pull the window within the screen's bounds (allowing a little slop
      // for windows that may be deliberately placed with their border off-screen
      // as when Win10 "snaps" a window to the left/right edge -- bug 1276516).
      // First, ensure the left edge is large enough...
      if (aLeft < screenLeftCss - SCREEN_EDGE_SLOP) {
        aLeft = screenLeftCss;
      }
      // Then check the resulting right edge, and reduce it if necessary.
      let right = aLeft + aWidth;
      if (right > screenRightCss + SCREEN_EDGE_SLOP) {
        right = screenRightCss;
        // See if we can move the left edge leftwards to maintain width.
        if (aLeft > screenLeftCss) {
          aLeft = Math.max(right - aWidth, screenLeftCss);
        }
      }
      // Finally, update aWidth to account for the adjusted left and right edges.
      aWidth = right - aLeft;

      // And do the same in the vertical dimension.
      if (aTop < screenTopCss - SCREEN_EDGE_SLOP) {
        aTop = screenTopCss;
      }
      let bottom = aTop + aHeight;
      if (bottom > screenBottomCss + SCREEN_EDGE_SLOP) {
        bottom = screenBottomCss;
        if (aTop > screenTopCss) {
          aTop = Math.max(bottom - aHeight, screenTopCss);
        }
      }
      aHeight = bottom - aTop;
    }

    // only modify those aspects which aren't correct yet
    if (!isNaN(aLeft) && !isNaN(aTop) && (aLeft != win_("screenX") || aTop != win_("screenY"))) {
      aWindow.moveTo(aLeft, aTop);
    }
    if (aWidth && aHeight && (aWidth != win_("width") || aHeight != win_("height"))) {
      // Don't resize the window if it's currently maximized and we would
      // maximize it again shortly after.
      if (aSizeMode != "maximized" || win_("sizemode") != "maximized") {
        aWindow.resizeTo(aWidth, aHeight);
      }
    }
    if (aSizeMode && win_("sizemode") != aSizeMode)
    {
      switch (aSizeMode)
      {
      case "maximized":
        aWindow.maximize();
        break;
      case "minimized":
        aWindow.minimize();
        break;
      case "normal":
        aWindow.restore();
        break;
      }
    }
    var sidebar = aWindow.document.getElementById("sidebar-box");
    if (sidebar.getAttribute("sidebarcommand") != aSidebar) {
      aWindow.SidebarUI.show(aSidebar);
    }
    // since resizing/moving a window brings it to the foreground,
    // we might want to re-focus the last focused window
    if (this.windowToFocus) {
      this.windowToFocus.focus();
    }
  },

  /* ........ Disk Access .............. */

  /**
   * Save the current session state to disk, after a delay.
   *
   * @param aWindow (optional)
   *        Will mark the given window as dirty so that we will recollect its
   *        data before we start writing.
   */
  saveStateDelayed: function (aWindow = null) {
    if (aWindow) {
      DirtyWindows.add(aWindow);
    }

    SessionSaver.runDelayed();
  },

  /* ........ Auxiliary Functions .............. */

  /**
   * Update the session start time and send a telemetry measurement
   * for the number of days elapsed since the session was started.
   *
   * @param state
   *        The session state.
   */
  _updateSessionStartTime: function ssi_updateSessionStartTime(state) {
    // Attempt to load the session start time from the session state
    if (state.session && state.session.startTime) {
      this._sessionStartTime = state.session.startTime;
    }
  },

  /**
   * call a callback for all currently opened browser windows
   * (might miss the most recent one)
   * @param aFunc
   *        Callback each window is passed to
   */
  _forEachBrowserWindow: function ssi_forEachBrowserWindow(aFunc) {
    var windowsEnum = Services.wm.getEnumerator("navigator:browser");

    while (windowsEnum.hasMoreElements()) {
      var window = windowsEnum.getNext();
      if (window.__SSi && !window.closed) {
        aFunc.call(this, window);
      }
    }
  },

  /**
   * Returns most recent window
   * @returns Window reference
   */
  _getMostRecentBrowserWindow: function ssi_getMostRecentBrowserWindow() {
    return RecentWindow.getMostRecentBrowserWindow({ allowPopups: true });
  },

  /**
   * Calls onClose for windows that are determined to be closed but aren't
   * destroyed yet, which would otherwise cause getBrowserState and
   * setBrowserState to treat them as open windows.
   */
  _handleClosedWindows: function ssi_handleClosedWindows() {
    var windowsEnum = Services.wm.getEnumerator("navigator:browser");

    while (windowsEnum.hasMoreElements()) {
      var window = windowsEnum.getNext();
      if (window.closed) {
        this.onClose(window);
      }
    }
  },

  /**
   * open a new browser window for a given session state
   * called when restoring a multi-window session
   * @param aState
   *        Object containing session data
   */
  _openWindowWithState: function ssi_openWindowWithState(aState) {
    var argString = Cc["@mozilla.org/supports-string;1"].
                    createInstance(Ci.nsISupportsString);
    argString.data = "";

    // Build feature string
    let features = "chrome,dialog=no,macsuppressanimation,all";
    let winState = aState.windows[0];
    WINDOW_ATTRIBUTES.forEach(function(aFeature) {
      // Use !isNaN as an easy way to ignore sizemode and check for numbers
      if (aFeature in winState && !isNaN(winState[aFeature]))
        features += "," + aFeature + "=" + winState[aFeature];
    });

    if (winState.isPrivate) {
      features += ",private";
    }

    var window =
      Services.ww.openWindow(null, this._prefBranch.getCharPref("chromeURL"),
                             "_blank", features, argString);

    do {
      var ID = "window" + Math.random();
    } while (ID in this._statesToRestore);
    this._statesToRestore[(window.__SS_restoreID = ID)] = aState;

    return window;
  },

  /**
   * Whether or not to resume session, if not recovering from a crash.
   * @returns bool
   */
  _doResumeSession: function ssi_doResumeSession() {
    return this._prefBranch.getIntPref("startup.page") == 3 ||
           this._prefBranch.getBoolPref("sessionstore.resume_session_once");
  },

  /**
   * whether the user wants to load any other page at startup
   * (except the homepage) - needed for determining whether to overwrite the current tabs
   * C.f.: nsBrowserContentHandler's defaultArgs implementation.
   * @returns bool
   */
  _isCmdLineEmpty: function ssi_isCmdLineEmpty(aWindow, aState) {
    var pinnedOnly = aState.windows &&
                     aState.windows.every(win =>
                       win.tabs.every(tab => tab.pinned));

    let hasFirstArgument = aWindow.arguments && aWindow.arguments[0];
    if (!pinnedOnly) {
      let defaultArgs = Cc["@mozilla.org/browser/clh;1"].
                        getService(Ci.nsIBrowserHandler).defaultArgs;
      if (aWindow.arguments &&
          aWindow.arguments[0] &&
          aWindow.arguments[0] == defaultArgs)
        hasFirstArgument = false;
    }

    return !hasFirstArgument;
  },

  /**
   * on popup windows, the XULWindow's attributes seem not to be set correctly
   * we use thus JSDOMWindow attributes for sizemode and normal window attributes
   * (and hope for reasonable values when maximized/minimized - since then
   * outerWidth/outerHeight aren't the dimensions of the restored window)
   * @param aWindow
   *        Window reference
   * @param aAttribute
   *        String sizemode | width | height | other window attribute
   * @returns string
   */
  _getWindowDimension: function ssi_getWindowDimension(aWindow, aAttribute) {
    if (aAttribute == "sizemode") {
      switch (aWindow.windowState) {
      case aWindow.STATE_FULLSCREEN:
      case aWindow.STATE_MAXIMIZED:
        return "maximized";
      case aWindow.STATE_MINIMIZED:
        return "minimized";
      default:
        return "normal";
      }
    }

    var dimension;
    switch (aAttribute) {
    case "width":
      dimension = aWindow.outerWidth;
      break;
    case "height":
      dimension = aWindow.outerHeight;
      break;
    default:
      dimension = aAttribute in aWindow ? aWindow[aAttribute] : "";
      break;
    }

    if (aWindow.windowState == aWindow.STATE_NORMAL) {
      return dimension;
    }
    return aWindow.document.documentElement.getAttribute(aAttribute) || dimension;
  },

  /**
   * @param aState is a session state
   * @param aRecentCrashes is the number of consecutive crashes
   * @returns whether a restore page will be needed for the session state
   */
  _needsRestorePage: function ssi_needsRestorePage(aState, aRecentCrashes) {
    const SIX_HOURS_IN_MS = 6 * 60 * 60 * 1000;

    // don't display the page when there's nothing to restore
    let winData = aState.windows || null;
    if (!winData || winData.length == 0)
      return false;

    // don't wrap a single about:sessionrestore page
    if (this._hasSingleTabWithURL(winData, "about:sessionrestore") ||
        this._hasSingleTabWithURL(winData, "about:welcomeback")) {
      return false;
    }

    // don't automatically restore in Safe Mode
    if (Services.appinfo.inSafeMode)
      return true;

    let max_resumed_crashes =
      this._prefBranch.getIntPref("sessionstore.max_resumed_crashes");
    let sessionAge = aState.session && aState.session.lastUpdate &&
                     (Date.now() - aState.session.lastUpdate);

    return max_resumed_crashes != -1 &&
           (aRecentCrashes > max_resumed_crashes ||
            sessionAge && sessionAge >= SIX_HOURS_IN_MS);
  },

  /**
   * @param aWinData is the set of windows in session state
   * @param aURL is the single URL we're looking for
   * @returns whether the window data contains only the single URL passed
   */
  _hasSingleTabWithURL: function(aWinData, aURL) {
    if (aWinData &&
        aWinData.length == 1 &&
        aWinData[0].tabs &&
        aWinData[0].tabs.length == 1 &&
        aWinData[0].tabs[0].entries &&
        aWinData[0].tabs[0].entries.length == 1) {
      return aURL == aWinData[0].tabs[0].entries[0].url;
    }
    return false;
  },

  /**
   * Determine if the tab state we're passed is something we should save. This
   * is used when closing a tab or closing a window with a single tab
   *
   * @param aTabState
   *        The current tab state
   * @returns boolean
   */
  _shouldSaveTabState: function ssi_shouldSaveTabState(aTabState) {
    // If the tab has only a transient about: history entry, no other
    // session history, and no userTypedValue, then we don't actually want to
    // store this tab's data.
    return aTabState.entries.length &&
           !(aTabState.entries.length == 1 &&
                (aTabState.entries[0].url == "about:blank" ||
                 aTabState.entries[0].url == "about:newtab" ||
                 aTabState.entries[0].url == "about:privatebrowsing") &&
                 !aTabState.userTypedValue);
  },

  /**
   * This is going to take a state as provided at startup (via
   * nsISessionStartup.state) and split it into 2 parts. The first part
   * (defaultState) will be a state that should still be restored at startup,
   * while the second part (state) is a state that should be saved for later.
   * defaultState will be comprised of windows with only pinned tabs, extracted
   * from state. It will contain the cookies that go along with the history
   * entries in those tabs. It will also contain window position information.
   *
   * defaultState will be restored at startup. state will be passed into
   * LastSession and will be kept in case the user explicitly wants
   * to restore the previous session (publicly exposed as restoreLastSession).
   *
   * @param state
   *        The state, presumably from nsISessionStartup.state
   * @returns [defaultState, state]
   */
  _prepDataForDeferredRestore: function ssi_prepDataForDeferredRestore(state) {
    // Make sure that we don't modify the global state as provided by
    // nsSessionStartup.state.
    state = Cu.cloneInto(state, {});

    let defaultState = { windows: [], selectedWindow: 1 };

    state.selectedWindow = state.selectedWindow || 1;

    // Look at each window, remove pinned tabs, adjust selectedindex,
    // remove window if necessary.
    for (let wIndex = 0; wIndex < state.windows.length;) {
      let window = state.windows[wIndex];
      window.selected = window.selected || 1;
      // We're going to put the state of the window into this object
      let pinnedWindowState = { tabs: [], cookies: []};
      for (let tIndex = 0; tIndex < window.tabs.length;) {
        if (window.tabs[tIndex].pinned) {
          // Adjust window.selected
          if (tIndex + 1 < window.selected)
            window.selected -= 1;
          else if (tIndex + 1 == window.selected)
            pinnedWindowState.selected = pinnedWindowState.tabs.length + 2;
            // + 2 because the tab isn't actually in the array yet

          // Now add the pinned tab to our window
          pinnedWindowState.tabs =
            pinnedWindowState.tabs.concat(window.tabs.splice(tIndex, 1));
          // We don't want to increment tIndex here.
          continue;
        }
        tIndex++;
      }

      // At this point the window in the state object has been modified (or not)
      // We want to build the rest of this new window object if we have pinnedTabs.
      if (pinnedWindowState.tabs.length) {
        // First get the other attributes off the window
        WINDOW_ATTRIBUTES.forEach(function(attr) {
          if (attr in window) {
            pinnedWindowState[attr] = window[attr];
            delete window[attr];
          }
        });
        // We're just copying position data into the pinned window.
        // Not copying over:
        // - _closedTabs
        // - extData
        // - isPopup
        // - hidden

        // Assign a unique ID to correlate the window to be opened with the
        // remaining data
        window.__lastSessionWindowID = pinnedWindowState.__lastSessionWindowID
                                     = "" + Date.now() + Math.random();

        // Extract the cookies that belong with each pinned tab
        this._splitCookiesFromWindow(window, pinnedWindowState);

        // Actually add this window to our defaultState
        defaultState.windows.push(pinnedWindowState);
        // Remove the window from the state if it doesn't have any tabs
        if (!window.tabs.length) {
          if (wIndex + 1 <= state.selectedWindow)
            state.selectedWindow -= 1;
          else if (wIndex + 1 == state.selectedWindow)
            defaultState.selectedIndex = defaultState.windows.length + 1;

          state.windows.splice(wIndex, 1);
          // We don't want to increment wIndex here.
          continue;
        }


      }
      wIndex++;
    }

    return [defaultState, state];
  },

  /**
   * Splits out the cookies from aWinState into aTargetWinState based on the
   * tabs that are in aTargetWinState.
   * This alters the state of aWinState and aTargetWinState.
   */
  _splitCookiesFromWindow:
    function ssi_splitCookiesFromWindow(aWinState, aTargetWinState) {
    if (!aWinState.cookies || !aWinState.cookies.length)
      return;

    // Get the hosts for history entries in aTargetWinState
    let cookieHosts = SessionCookies.getHostsForWindow(aTargetWinState);

    // By creating a regex we reduce overhead and there is only one loop pass
    // through either array (cookieHosts and aWinState.cookies).
    let hosts = Object.keys(cookieHosts).join("|").replace(/\./g, "\\.");
    // If we don't actually have any hosts, then we don't want to do anything.
    if (!hosts.length)
      return;
    let cookieRegex = new RegExp(".*(" + hosts + ")");
    for (let cIndex = 0; cIndex < aWinState.cookies.length;) {
      if (cookieRegex.test(aWinState.cookies[cIndex].host)) {
        aTargetWinState.cookies =
          aTargetWinState.cookies.concat(aWinState.cookies.splice(cIndex, 1));
        continue;
      }
      cIndex++;
    }
  },

  _sendRestoreCompletedNotifications: function ssi_sendRestoreCompletedNotifications() {
    // not all windows restored, yet
    if (this._restoreCount > 1) {
      this._restoreCount--;
      return;
    }

    // observers were already notified
    if (this._restoreCount == -1)
      return;

    // This was the last window restored at startup, notify observers.
    Services.obs.notifyObservers(null,
      this._browserSetState ? NOTIFY_BROWSER_STATE_RESTORED : NOTIFY_WINDOWS_RESTORED,
      "");

    this._browserSetState = false;
    this._restoreCount = -1;
  },

   /**
   * Set the given window's busy state
   * @param aWindow the window
   * @param aValue the window's busy state
   */
  _setWindowStateBusyValue:
    function ssi_changeWindowStateBusyValue(aWindow, aValue) {

    this._windows[aWindow.__SSi].busy = aValue;

    // Keep the to-be-restored state in sync because that is returned by
    // getWindowState() as long as the window isn't loaded, yet.
    if (!this._isWindowLoaded(aWindow)) {
      let stateToRestore = this._statesToRestore[aWindow.__SS_restoreID].windows[0];
      stateToRestore.busy = aValue;
    }
  },

  /**
   * Set the given window's state to 'not busy'.
   * @param aWindow the window
   */
  _setWindowStateReady: function ssi_setWindowStateReady(aWindow) {
    let newCount = (this._windowBusyStates.get(aWindow) || 0) - 1;
    if (newCount < 0) {
      throw new Error("Invalid window busy state (less than zero).");
    }
    this._windowBusyStates.set(aWindow, newCount);

    if (newCount == 0) {
      this._setWindowStateBusyValue(aWindow, false);
      this._sendWindowStateEvent(aWindow, "Ready");
    }
  },

  /**
   * Set the given window's state to 'busy'.
   * @param aWindow the window
   */
  _setWindowStateBusy: function ssi_setWindowStateBusy(aWindow) {
    let newCount = (this._windowBusyStates.get(aWindow) || 0) + 1;
    this._windowBusyStates.set(aWindow, newCount);

    if (newCount == 1) {
      this._setWindowStateBusyValue(aWindow, true);
      this._sendWindowStateEvent(aWindow, "Busy");
    }
  },

  /**
   * Dispatch an SSWindowState_____ event for the given window.
   * @param aWindow the window
   * @param aType the type of event, SSWindowState will be prepended to this string
   */
  _sendWindowStateEvent: function ssi_sendWindowStateEvent(aWindow, aType) {
    let event = aWindow.document.createEvent("Events");
    event.initEvent("SSWindowState" + aType, true, false);
    aWindow.dispatchEvent(event);
  },

  /**
   * Dispatch the SSTabRestored event for the given tab.
   * @param aTab
   *        The tab which has been restored
   * @param aIsRemotenessUpdate
   *        True if this tab was restored due to flip from running from
   *        out-of-main-process to in-main-process or vice-versa.
   */
  _sendTabRestoredNotification(aTab, aIsRemotenessUpdate) {
    let event = aTab.ownerDocument.createEvent("CustomEvent");
    event.initCustomEvent("SSTabRestored", true, false, {
      isRemotenessUpdate: aIsRemotenessUpdate,
    });
    aTab.dispatchEvent(event);
  },

  /**
   * @param aWindow
   *        Window reference
   * @returns whether this window's data is still cached in _statesToRestore
   *          because it's not fully loaded yet
   */
  _isWindowLoaded: function ssi_isWindowLoaded(aWindow) {
    return !aWindow.__SS_restoreID;
  },

  /**
   * Replace "Loading..." with the tab label (with minimal side-effects)
   * @param aString is the string the title is stored in
   * @param aTabbrowser is a tabbrowser object, containing aTab
   * @param aTab is the tab whose title we're updating & using
   *
   * @returns aString that has been updated with the new title
   */
  _replaceLoadingTitle : function ssi_replaceLoadingTitle(aString, aTabbrowser, aTab) {
    if (aString == aTabbrowser.mStringBundle.getString("tabs.connecting")) {
      aTabbrowser.setTabTitle(aTab);
      [aString, aTab.label] = [aTab.label, aString];
    }
    return aString;
  },

  /**
   * Resize this._closedWindows to the value of the pref, except in the case
   * where we don't have any non-popup windows on Windows and Linux. Then we must
   * resize such that we have at least one non-popup window.
   */
  _capClosedWindows : function ssi_capClosedWindows() {
    if (this._closedWindows.length <= this._max_windows_undo)
      return;
    let spliceTo = this._max_windows_undo;
    if (AppConstants.platform != "macosx") {
      let normalWindowIndex = 0;
      // try to find a non-popup window in this._closedWindows
      while (normalWindowIndex < this._closedWindows.length &&
             !!this._closedWindows[normalWindowIndex].isPopup)
        normalWindowIndex++;
      if (normalWindowIndex >= this._max_windows_undo)
        spliceTo = normalWindowIndex + 1;
    }
    this._closedWindows.splice(spliceTo, this._closedWindows.length);
  },

  /**
   * Clears the set of windows that are "resurrected" before writing to disk to
   * make closing windows one after the other until shutdown work as expected.
   *
   * This function should only be called when we are sure that there has been
   * a user action that indicates the browser is actively being used and all
   * windows that have been closed before are not part of a series of closing
   * windows.
   */
  _clearRestoringWindows: function ssi_clearRestoringWindows() {
    for (let i = 0; i < this._closedWindows.length; i++) {
      delete this._closedWindows[i]._shouldRestore;
    }
  },

  /**
   * Reset state to prepare for a new session state to be restored.
   */
  _resetRestoringState: function ssi_initRestoringState() {
    TabRestoreQueue.reset();
    this._tabsRestoringCount = 0;
  },

  /**
   * Reset the restoring state for a particular tab. This will be called when
   * removing a tab or when a tab needs to be reset (it's being overwritten).
   *
   * @param aTab
   *        The tab that will be "reset"
   */
  _resetLocalTabRestoringState: function (aTab) {
    NS_ASSERT(aTab.linkedBrowser.__SS_restoreState,
              "given tab is not restoring");

    let window = aTab.ownerDocument.defaultView;
    let browser = aTab.linkedBrowser;

    // Keep the tab's previous state for later in this method
    let previousState = browser.__SS_restoreState;

    // The browser is no longer in any sort of restoring state.
    delete browser.__SS_restoreState;

    aTab.removeAttribute("pending");
    browser.removeAttribute("pending");

    if (previousState == TAB_STATE_RESTORING) {
      if (this._tabsRestoringCount)
        this._tabsRestoringCount--;
    } else if (previousState == TAB_STATE_NEEDS_RESTORE) {
      // Make sure that the tab is removed from the list of tabs to restore.
      // Again, this is normally done in restoreTabContent, but that isn't being called
      // for this tab.
      TabRestoreQueue.remove(aTab);
    }
  },

  _resetTabRestoringState: function (tab) {
    NS_ASSERT(tab.linkedBrowser.__SS_restoreState,
              "given tab is not restoring");

    let browser = tab.linkedBrowser;
    browser.messageManager.sendAsyncMessage("SessionStore:resetRestore", {});
    this._resetLocalTabRestoringState(tab);
  },

  /**
   * Each fresh tab starts out with epoch=0. This function can be used to
   * start a next epoch by incrementing the current value. It will enables us
   * to ignore stale messages sent from previous epochs. The function returns
   * the new epoch ID for the given |browser|.
   */
  startNextEpoch(browser) {
    let next = this.getCurrentEpoch(browser) + 1;
    this._browserEpochs.set(browser.permanentKey, next);
    return next;
  },

  /**
   * Returns the current epoch for the given <browser>. If we haven't assigned
   * a new epoch this will default to zero for new tabs.
   */
  getCurrentEpoch(browser) {
    return this._browserEpochs.get(browser.permanentKey) || 0;
  },

  /**
   * Each time a <browser> element is restored, we increment its "epoch". To
   * check if a message from content-sessionStore.js is out of date, we can
   * compare the epoch received with the message to the <browser> element's
   * epoch. This function does that, and returns true if |epoch| is up-to-date
   * with respect to |browser|.
   */
  isCurrentEpoch: function (browser, epoch) {
    return this.getCurrentEpoch(browser) == epoch;
  },

  /**
   * Resets the epoch for a given <browser>. We need to this every time we
   * receive a hint that a new docShell has been loaded into the browser as
   * the frame script starts out with epoch=0.
   */
  resetEpoch(browser) {
    this._browserEpochs.delete(browser.permanentKey);
  },

  /**
   * Handle an error report from a content process.
   */
  reportInternalError(data) {
    // For the moment, we only report errors through Telemetry.
    if (data.telemetry) {
      for (let key of Object.keys(data.telemetry)) {
        let histogram = Telemetry.getHistogramById(key);
        histogram.add(data.telemetry[key]);
      }
    }
  }
};

/**
 * Priority queue that keeps track of a list of tabs to restore and returns
 * the tab we should restore next, based on priority rules. We decide between
 * pinned, visible and hidden tabs in that and FIFO order. Hidden tabs are only
 * restored with restore_hidden_tabs=true.
 */
var TabRestoreQueue = {
  // The separate buckets used to store tabs.
  tabs: {priority: [], visible: [], hidden: []},

  // Preferences used by the TabRestoreQueue to determine which tabs
  // are restored automatically and which tabs will be on-demand.
  prefs: {
    // Lazy getter that returns whether tabs are restored on demand.
    get restoreOnDemand() {
      let updateValue = () => {
        let value = Services.prefs.getBoolPref(PREF);
        let definition = {value: value, configurable: true};
        Object.defineProperty(this, "restoreOnDemand", definition);
        return value;
      }

      const PREF = "browser.sessionstore.restore_on_demand";
      Services.prefs.addObserver(PREF, updateValue, false);
      return updateValue();
    },

    // Lazy getter that returns whether pinned tabs are restored on demand.
    get restorePinnedTabsOnDemand() {
      let updateValue = () => {
        let value = Services.prefs.getBoolPref(PREF);
        let definition = {value: value, configurable: true};
        Object.defineProperty(this, "restorePinnedTabsOnDemand", definition);
        return value;
      }

      const PREF = "browser.sessionstore.restore_pinned_tabs_on_demand";
      Services.prefs.addObserver(PREF, updateValue, false);
      return updateValue();
    },

    // Lazy getter that returns whether we should restore hidden tabs.
    get restoreHiddenTabs() {
      let updateValue = () => {
        let value = Services.prefs.getBoolPref(PREF);
        let definition = {value: value, configurable: true};
        Object.defineProperty(this, "restoreHiddenTabs", definition);
        return value;
      }

      const PREF = "browser.sessionstore.restore_hidden_tabs";
      Services.prefs.addObserver(PREF, updateValue, false);
      return updateValue();
    }
  },

  // Resets the queue and removes all tabs.
  reset: function () {
    this.tabs = {priority: [], visible: [], hidden: []};
  },

  // Adds a tab to the queue and determines its priority bucket.
  add: function (tab) {
    let {priority, hidden, visible} = this.tabs;

    if (tab.pinned) {
      priority.push(tab);
    } else if (tab.hidden) {
      hidden.push(tab);
    } else {
      visible.push(tab);
    }
  },

  // Removes a given tab from the queue, if it's in there.
  remove: function (tab) {
    let {priority, hidden, visible} = this.tabs;

    // We'll always check priority first since we don't
    // have an indicator if a tab will be there or not.
    let set = priority;
    let index = set.indexOf(tab);

    if (index == -1) {
      set = tab.hidden ? hidden : visible;
      index = set.indexOf(tab);
    }

    if (index > -1) {
      set.splice(index, 1);
    }
  },

  // Returns and removes the tab with the highest priority.
  shift: function () {
    let set;
    let {priority, hidden, visible} = this.tabs;

    let {restoreOnDemand, restorePinnedTabsOnDemand} = this.prefs;
    let restorePinned = !(restoreOnDemand && restorePinnedTabsOnDemand);
    if (restorePinned && priority.length) {
      set = priority;
    } else if (!restoreOnDemand) {
      if (visible.length) {
        set = visible;
      } else if (this.prefs.restoreHiddenTabs && hidden.length) {
        set = hidden;
      }
    }

    return set && set.shift();
  },

  // Moves a given tab from the 'hidden' to the 'visible' bucket.
  hiddenToVisible: function (tab) {
    let {hidden, visible} = this.tabs;
    let index = hidden.indexOf(tab);

    if (index > -1) {
      hidden.splice(index, 1);
      visible.push(tab);
    }
  },

  // Moves a given tab from the 'visible' to the 'hidden' bucket.
  visibleToHidden: function (tab) {
    let {visible, hidden} = this.tabs;
    let index = visible.indexOf(tab);

    if (index > -1) {
      visible.splice(index, 1);
      hidden.push(tab);
    }
  }
};

// A map storing a closed window's state data until it goes aways (is GC'ed).
// This ensures that API clients can still read (but not write) states of
// windows they still hold a reference to but we don't.
var DyingWindowCache = {
  _data: new WeakMap(),

  has: function (window) {
    return this._data.has(window);
  },

  get: function (window) {
    return this._data.get(window);
  },

  set: function (window, data) {
    this._data.set(window, data);
  },

  remove: function (window) {
    this._data.delete(window);
  }
};

// A weak set of dirty windows. We use it to determine which windows we need to
// recollect data for when getCurrentState() is called.
var DirtyWindows = {
  _data: new WeakMap(),

  has: function (window) {
    return this._data.has(window);
  },

  add: function (window) {
    return this._data.set(window, true);
  },

  remove: function (window) {
    this._data.delete(window);
  },

  clear: function (window) {
    this._data = new WeakMap();
  }
};

// The state from the previous session (after restoring pinned tabs). This
// state is persisted and passed through to the next session during an app
// restart to make the third party add-on warning not trash the deferred
// session
var LastSession = {
  _state: null,

  get canRestore() {
    return !!this._state;
  },

  getState: function () {
    return this._state;
  },

  setState: function (state) {
    this._state = state;
  },

  clear: function () {
    if (this._state) {
      this._state = null;
      Services.obs.notifyObservers(null, NOTIFY_LAST_SESSION_CLEARED, null);
    }
  }
};