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diff --git a/components/sessionstore/nsSessionStartup.js b/components/sessionstore/nsSessionStartup.js
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+/* 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/. */
+
+/**
+ * Session Storage and Restoration
+ *
+ * Overview
+ * This service reads user's session file at startup, and makes a determination
+ * as to whether the session should be restored. It will restore the session
+ * under the circumstances described below.  If the auto-start Private Browsing
+ * mode is active, however, the session is never restored.
+ *
+ * Crash Detection
+ * The session file stores a session.state property, that
+ * indicates whether the browser is currently running. When the browser shuts
+ * down, the field is changed to "stopped". At startup, this field is read, and
+ * if its value is "running", then it's assumed that the browser had previously
+ * crashed, or at the very least that something bad happened, and that we should
+ * restore the session.
+ *
+ * Forced Restarts
+ * In the event that a restart is required due to application update or extension
+ * installation, set the browser.sessionstore.resume_session_once pref to true,
+ * and the session will be restored the next time the browser starts.
+ *
+ * Always Resume
+ * This service will always resume the session if the integer pref
+ * browser.startup.page is set to 3.
+ */
+
+/* :::::::: Constants and Helpers ::::::::::::::: */
+
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cr = Components.results;
+const Cu = Components.utils;
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/PrivateBrowsingUtils.jsm");
+Cu.import("resource://gre/modules/Promise.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "_SessionFile",
+  "resource:///modules/sessionstore/_SessionFile.jsm");
+
+const STATE_RUNNING_STR = "running";
+
+function debug(aMsg) {
+  aMsg = ("SessionStartup: " + aMsg).replace(/\S{80}/g, "$&\n");
+  Services.console.logStringMessage(aMsg);
+}
+
+var gOnceInitializedDeferred = Promise.defer();
+
+/* :::::::: The Service ::::::::::::::: */
+
+function SessionStartup() {
+}
+
+SessionStartup.prototype = {
+
+  // the state to restore at startup
+  _initialState: null,
+  _sessionType: Ci.nsISessionStartup.NO_SESSION,
+  _initialized: false,
+
+/* ........ Global Event Handlers .............. */
+
+  /**
+   * Initialize the component
+   */
+  init: function sss_init() {
+    // do not need to initialize anything in auto-started private browsing sessions
+    if (PrivateBrowsingUtils.permanentPrivateBrowsing) {
+      this._initialized = true;
+      gOnceInitializedDeferred.resolve();
+      return;
+    }
+
+    if (Services.prefs.getBoolPref("browser.sessionstore.resume_session_once") ||
+        Services.prefs.getIntPref("browser.startup.page") == 3) {
+      this._ensureInitialized();
+    } else {
+      _SessionFile.read().then(
+        this._onSessionFileRead.bind(this)
+      );
+    }
+  },
+
+  // Wrap a string as a nsISupports
+  _createSupportsString: function ssfi_createSupportsString(aData) {
+    let string = Cc["@mozilla.org/supports-string;1"]
+                   .createInstance(Ci.nsISupportsString);
+    string.data = aData;
+    return string;
+  },
+
+  _onSessionFileRead: function sss_onSessionFileRead(aStateString) {
+    if (this._initialized) {
+      // Initialization is complete, nothing else to do
+      return;
+    }
+    try {
+      this._initialized = true;
+
+      // Let observers modify the state before it is used
+      let supportsStateString = this._createSupportsString(aStateString);
+      Services.obs.notifyObservers(supportsStateString, "sessionstore-state-read", "");
+      aStateString = supportsStateString.data;
+
+      // No valid session found.
+      if (!aStateString) {
+        this._sessionType = Ci.nsISessionStartup.NO_SESSION;
+        return;
+      }
+
+      // parse the session state into a JS object
+      // remove unneeded braces (added for compatibility with Firefox 2.0 and 3.0)
+      if (aStateString.charAt(0) == '(')
+        aStateString = aStateString.slice(1, -1);
+      let corruptFile = false;
+      try {
+        this._initialState = JSON.parse(aStateString);
+      }
+      catch (ex) {
+        debug("The session file contained un-parse-able JSON: " + ex);
+        // This is not valid JSON, but this might still be valid JavaScript,
+        // as used in FF2/FF3, so we need to eval.
+        // evalInSandbox will throw if aStateString is not parse-able.
+        try {
+          var s = new Cu.Sandbox("about:blank", {sandboxName: 'nsSessionStartup'});
+          this._initialState = Cu.evalInSandbox("(" + aStateString + ")", s);
+        } catch(ex) {
+          debug("The session file contained un-eval-able JSON: " + ex);
+          corruptFile = true;
+        }
+      }
+      let doResumeSessionOnce = Services.prefs.getBoolPref("browser.sessionstore.resume_session_once");
+      let doResumeSession = doResumeSessionOnce ||
+            Services.prefs.getIntPref("browser.startup.page") == 3;
+
+      // If this is a normal restore then throw away any previous session
+      if (!doResumeSessionOnce)
+        delete this._initialState.lastSessionState;
+
+      let resumeFromCrash = Services.prefs.getBoolPref("browser.sessionstore.resume_from_crash");
+      let lastSessionCrashed =
+        this._initialState && this._initialState.session &&
+        this._initialState.session.state &&
+        this._initialState.session.state == STATE_RUNNING_STR;
+
+      // set the startup type
+      if (lastSessionCrashed && resumeFromCrash)
+        this._sessionType = Ci.nsISessionStartup.RECOVER_SESSION;
+      else if (!lastSessionCrashed && doResumeSession)
+        this._sessionType = Ci.nsISessionStartup.RESUME_SESSION;
+      else if (this._initialState)
+        this._sessionType = Ci.nsISessionStartup.DEFER_SESSION;
+      else
+        this._initialState = null; // reset the state
+
+      Services.obs.addObserver(this, "sessionstore-windows-restored", true);
+
+      if (this._sessionType != Ci.nsISessionStartup.NO_SESSION)
+        Services.obs.addObserver(this, "browser:purge-session-history", true);
+
+    } finally {
+      // We're ready. Notify everyone else.
+      Services.obs.notifyObservers(null, "sessionstore-state-finalized", "");
+      gOnceInitializedDeferred.resolve();
+    }
+  },
+
+  /**
+   * Handle notifications
+   */
+  observe: function sss_observe(aSubject, aTopic, aData) {
+    switch (aTopic) {
+    case "app-startup":
+      Services.obs.addObserver(this, "final-ui-startup", true);
+      Services.obs.addObserver(this, "quit-application", true);
+      break;
+    case "final-ui-startup":
+      Services.obs.removeObserver(this, "final-ui-startup");
+      Services.obs.removeObserver(this, "quit-application");
+      this.init();
+      break;
+    case "quit-application":
+      // no reason for initializing at this point (cf. bug 409115)
+      Services.obs.removeObserver(this, "final-ui-startup");
+      Services.obs.removeObserver(this, "quit-application");
+      if (this._sessionType != Ci.nsISessionStartup.NO_SESSION)
+        Services.obs.removeObserver(this, "browser:purge-session-history");
+      break;
+    case "sessionstore-windows-restored":
+      Services.obs.removeObserver(this, "sessionstore-windows-restored");
+      // free _initialState after nsSessionStore is done with it
+      this._initialState = null;
+      break;
+    case "browser:purge-session-history":
+      Services.obs.removeObserver(this, "browser:purge-session-history");
+      // reset all state on sanitization
+      this._sessionType = Ci.nsISessionStartup.NO_SESSION;
+      break;
+    }
+  },
+
+/* ........ Public API ................*/
+
+  get onceInitialized() {
+    return gOnceInitializedDeferred.promise;
+  },
+
+  /**
+   * Get the session state as a jsval
+   */
+  get state() {
+    this._ensureInitialized();
+    return this._initialState;
+  },
+
+  /**
+   * Determines whether there is a pending session restore and makes sure that
+   * we're initialized before returning. If we're not yet this will read the
+   * session file synchronously.
+   * @returns bool
+   */
+  doRestore: function sss_doRestore() {
+    this._ensureInitialized();
+    return this._willRestore();
+  },
+
+  /**
+   * Determines whether there is a pending session restore.
+   * @returns bool
+   */
+  _willRestore: function () {
+    return this._sessionType == Ci.nsISessionStartup.RECOVER_SESSION ||
+           this._sessionType == Ci.nsISessionStartup.RESUME_SESSION;
+  },
+
+  /**
+   * Returns whether we will restore a session that ends up replacing the
+   * homepage. The browser uses this to not start loading the homepage if
+   * we're going to stop its load anyway shortly after.
+   *
+   * This is meant to be an optimization for the average case that loading the
+   * session file finishes before we may want to start loading the default
+   * homepage. Should this be called before the session file has been read it
+   * will just return false.
+   *
+   * @returns bool
+   */
+  get willOverrideHomepage() {
+    if (this._initialState && this._willRestore()) {
+      let windows = this._initialState.windows || null;
+      // If there are valid windows with not only pinned tabs, signal that we
+      // will override the default homepage by restoring a session.
+      return windows && windows.some(w => w.tabs.some(t => !t.pinned));
+    }
+    return false;
+  },
+
+  /**
+   * Get the type of pending session store, if any.
+   */
+  get sessionType() {
+    this._ensureInitialized();
+    return this._sessionType;
+  },
+
+  // Ensure that initialization is complete.
+  // If initialization is not complete yet, fall back to a synchronous
+  // initialization and kill ongoing asynchronous initialization
+  _ensureInitialized: function sss__ensureInitialized() {
+    try {
+      if (this._initialized) {
+        // Initialization is complete, nothing else to do
+        return;
+      }
+      let contents = _SessionFile.syncRead();
+      this._onSessionFileRead(contents);
+    } catch(ex) {
+      debug("ensureInitialized: could not read session " + ex + ", " + ex.stack);
+      throw ex;
+    }
+  },
+
+  /* ........ QueryInterface .............. */
+  QueryInterface : XPCOMUtils.generateQI([Ci.nsIObserver,
+                                          Ci.nsISupportsWeakReference,
+                                          Ci.nsISessionStartup]),
+  classID:          Components.ID("{ec7a6c20-e081-11da-8ad9-0800200c9a66}")
+};
+
+this.NSGetFactory = XPCOMUtils.generateNSGetFactory([SessionStartup]);