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diff --git a/browser/components/sessionstore/ContentRestore.sys.mjs b/browser/components/sessionstore/ContentRestore.sys.mjs
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+++ b/browser/components/sessionstore/ContentRestore.sys.mjs
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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/. */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  SessionHistory: "resource://gre/modules/sessionstore/SessionHistory.sys.mjs",
+  Utils: "resource://gre/modules/sessionstore/Utils.sys.mjs",
+});
+
+/**
+ * This module implements the content side of session restoration. The chrome
+ * side is handled by SessionStore.sys.mjs. The functions in this module are called
+ * by content-sessionStore.js based on messages received from SessionStore.sys.mjs
+ * (or, in one case, based on a "load" event). Each tab has its own
+ * ContentRestore instance, constructed by content-sessionStore.js.
+ *
+ * In a typical restore, content-sessionStore.js will call the following based
+ * on messages and events it receives:
+ *
+ *   restoreHistory(tabData, loadArguments, callbacks)
+ *     Restores the tab's history and session cookies.
+ *   restoreTabContent(loadArguments, finishCallback)
+ *     Starts loading the data for the current page to restore.
+ *   restoreDocument()
+ *     Restore form and scroll data.
+ *
+ * When the page has been loaded from the network, we call finishCallback. It
+ * should send a message to SessionStore.sys.mjs, which may cause other tabs to be
+ * restored.
+ *
+ * When the page has finished loading, a "load" event will trigger in
+ * content-sessionStore.js, which will call restoreDocument. At that point,
+ * form data is restored and the restore is complete.
+ *
+ * At any time, SessionStore.sys.mjs can cancel the ongoing restore by sending a
+ * reset message, which causes resetRestore to be called. At that point it's
+ * legal to begin another restore.
+ */
+export function ContentRestore(chromeGlobal) {
+  let internal = new ContentRestoreInternal(chromeGlobal);
+  let external = {};
+
+  let EXPORTED_METHODS = [
+    "restoreHistory",
+    "restoreTabContent",
+    "restoreDocument",
+    "resetRestore",
+  ];
+
+  for (let method of EXPORTED_METHODS) {
+    external[method] = internal[method].bind(internal);
+  }
+
+  return Object.freeze(external);
+}
+
+function ContentRestoreInternal(chromeGlobal) {
+  this.chromeGlobal = chromeGlobal;
+
+  // The following fields are only valid during certain phases of the restore
+  // process.
+
+  // The tabData for the restore. Set in restoreHistory and removed in
+  // restoreTabContent.
+  this._tabData = null;
+
+  // Contains {entry, scrollPositions, formdata}, where entry is a
+  // single entry from the tabData.entries array. Set in
+  // restoreTabContent and removed in restoreDocument.
+  this._restoringDocument = null;
+
+  // This listener is used to detect reloads on restoring tabs. Set in
+  // restoreHistory and removed in restoreTabContent.
+  this._historyListener = null;
+
+  // This listener detects when a pending tab starts loading (when not
+  // initiated by sessionstore) and when a restoring tab has finished loading
+  // data from the network. Set in restoreHistory() and restoreTabContent(),
+  // removed in resetRestore().
+  this._progressListener = null;
+}
+
+/**
+ * The API for the ContentRestore module. Methods listed in EXPORTED_METHODS are
+ * public.
+ */
+ContentRestoreInternal.prototype = {
+  get docShell() {
+    return this.chromeGlobal.docShell;
+  },
+
+  /**
+   * Starts the process of restoring a tab. The tabData to be restored is passed
+   * in here and used throughout the restoration. The epoch (which must be
+   * non-zero) is passed through to all the callbacks. If a load in the tab
+   * is started while it is pending, the appropriate callbacks are called.
+   */
+  restoreHistory(tabData, loadArguments, callbacks) {
+    this._tabData = tabData;
+
+    // In case about:blank isn't done yet.
+    let webNavigation = this.docShell.QueryInterface(Ci.nsIWebNavigation);
+    webNavigation.stop(Ci.nsIWebNavigation.STOP_ALL);
+
+    // Make sure currentURI is set so that switch-to-tab works before the tab is
+    // restored. We'll reset this to about:blank when we try to restore the tab
+    // to ensure that docshell doeesn't get confused. Don't bother doing this if
+    // we're restoring immediately due to a process switch. It just causes the
+    // URL bar to be temporarily blank.
+    let activeIndex = tabData.index - 1;
+    let activePageData = tabData.entries[activeIndex] || {};
+    let uri = activePageData.url || null;
+    if (uri && !loadArguments) {
+      webNavigation.setCurrentURIForSessionStore(Services.io.newURI(uri));
+    }
+
+    lazy.SessionHistory.restore(this.docShell, tabData);
+
+    // Add a listener to watch for reloads.
+    let listener = new HistoryListener(this.docShell, () => {
+      // On reload, restore tab contents.
+      this.restoreTabContent(null, false, callbacks.onLoadFinished);
+    });
+
+    webNavigation.sessionHistory.legacySHistory.addSHistoryListener(listener);
+    this._historyListener = listener;
+
+    // Make sure to reset the capabilities and attributes in case this tab gets
+    // reused.
+    SessionStoreUtils.restoreDocShellCapabilities(
+      this.docShell,
+      tabData.disallow
+    );
+
+    // Add a progress listener to correctly handle browser.loadURI()
+    // calls from foreign code.
+    this._progressListener = new ProgressListener(this.docShell, {
+      onStartRequest: () => {
+        // Some code called browser.loadURI() on a pending tab. It's safe to
+        // assume we don't care about restoring scroll or form data.
+        this._tabData = null;
+
+        // Listen for the tab to finish loading.
+        this.restoreTabContentStarted(callbacks.onLoadFinished);
+
+        // Notify the parent.
+        callbacks.onLoadStarted();
+      },
+    });
+  },
+
+  /**
+   * Start loading the current page. When the data has finished loading from the
+   * network, finishCallback is called. Returns true if the load was successful.
+   */
+  restoreTabContent(loadArguments, isRemotenessUpdate, finishCallback) {
+    let tabData = this._tabData;
+    this._tabData = null;
+
+    let webNavigation = this.docShell.QueryInterface(Ci.nsIWebNavigation);
+
+    // Listen for the tab to finish loading.
+    this.restoreTabContentStarted(finishCallback);
+
+    // Reset the current URI to about:blank. We changed it above for
+    // switch-to-tab, but now it must go back to the correct value before the
+    // load happens. Don't bother doing this if we're restoring immediately
+    // due to a process switch.
+    if (!isRemotenessUpdate) {
+      webNavigation.setCurrentURIForSessionStore(
+        Services.io.newURI("about:blank")
+      );
+    }
+
+    try {
+      if (loadArguments) {
+        // If the load was started in another process, and the in-flight channel
+        // was redirected into this process, resume that load within our process.
+        //
+        // NOTE: In this case `isRemotenessUpdate` must be true.
+        webNavigation.resumeRedirectedLoad(
+          loadArguments.redirectLoadSwitchId,
+          loadArguments.redirectHistoryIndex
+        );
+      } else if (tabData.userTypedValue && tabData.userTypedClear) {
+        // If the user typed a URL into the URL bar and hit enter right before
+        // we crashed, we want to start loading that page again. A non-zero
+        // userTypedClear value means that the load had started.
+        // Load userTypedValue and fix up the URL if it's partial/broken.
+        let loadURIOptions = {
+          triggeringPrincipal:
+            Services.scriptSecurityManager.getSystemPrincipal(),
+          loadFlags: Ci.nsIWebNavigation.LOAD_FLAGS_ALLOW_THIRD_PARTY_FIXUP,
+        };
+        webNavigation.fixupAndLoadURIString(
+          tabData.userTypedValue,
+          loadURIOptions
+        );
+      } else if (tabData.entries.length) {
+        // Stash away the data we need for restoreDocument.
+        this._restoringDocument = {
+          formdata: tabData.formdata || {},
+          scrollPositions: tabData.scroll || {},
+        };
+
+        // In order to work around certain issues in session history, we need to
+        // force session history to update its internal index and call reload
+        // instead of gotoIndex. See bug 597315.
+        let history = webNavigation.sessionHistory.legacySHistory;
+        history.reloadCurrentEntry();
+      } else {
+        // If there's nothing to restore, we should still blank the page.
+        let loadURIOptions = {
+          triggeringPrincipal:
+            Services.scriptSecurityManager.getSystemPrincipal(),
+          loadFlags: Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_HISTORY,
+          // Specify an override to force the load to finish in the current
+          // process, as tests rely on this behaviour for non-fission session
+          // restore.
+          remoteTypeOverride: Services.appinfo.remoteType,
+        };
+        webNavigation.loadURI(
+          Services.io.newURI("about:blank"),
+          loadURIOptions
+        );
+      }
+
+      return true;
+    } catch (ex) {
+      if (ex instanceof Ci.nsIException) {
+        // Ignore page load errors, but return false to signal that the load never
+        // happened.
+        return false;
+      }
+    }
+    return null;
+  },
+
+  /**
+   * To be called after restoreHistory(). Removes all listeners needed for
+   * pending tabs and makes sure to notify when the tab finished loading.
+   */
+  restoreTabContentStarted(finishCallback) {
+    // The reload listener is no longer needed.
+    this._historyListener.uninstall();
+    this._historyListener = null;
+
+    // Remove the old progress listener.
+    this._progressListener.uninstall();
+
+    // We're about to start a load. This listener will be called when the load
+    // has finished getting everything from the network.
+    this._progressListener = new ProgressListener(this.docShell, {
+      onStopRequest: () => {
+        // Call resetRestore() to reset the state back to normal. The data
+        // needed for restoreDocument() (which hasn't happened yet) will
+        // remain in _restoringDocument.
+        this.resetRestore();
+
+        finishCallback();
+      },
+    });
+  },
+
+  /**
+   * Finish restoring the tab by filling in form data and setting the scroll
+   * position. The restore is complete when this function exits. It should be
+   * called when the "load" event fires for the restoring tab.  Returns true
+   * if we're restoring a document.
+   */
+  restoreDocument() {
+    if (!this._restoringDocument) {
+      return;
+    }
+
+    let { formdata, scrollPositions } = this._restoringDocument;
+    this._restoringDocument = null;
+
+    let window = this.docShell.domWindow;
+
+    // Restore form data.
+    lazy.Utils.restoreFrameTreeData(window, formdata, (frame, data) => {
+      // restore() will return false, and thus abort restoration for the
+      // current |frame| and its descendants, if |data.url| is given but
+      // doesn't match the loaded document's URL.
+      return SessionStoreUtils.restoreFormData(frame.document, data);
+    });
+
+    // Restore scroll data.
+    lazy.Utils.restoreFrameTreeData(window, scrollPositions, (frame, data) => {
+      if (data.scroll) {
+        SessionStoreUtils.restoreScrollPosition(frame, data);
+      }
+    });
+  },
+
+  /**
+   * Cancel an ongoing restore. This function can be called any time between
+   * restoreHistory and restoreDocument.
+   *
+   * This function is called externally (if a restore is canceled) and
+   * internally (when the loads for a restore have finished). In the latter
+   * case, it's called before restoreDocument, so it cannot clear
+   * _restoringDocument.
+   */
+  resetRestore() {
+    this._tabData = null;
+
+    if (this._historyListener) {
+      this._historyListener.uninstall();
+    }
+    this._historyListener = null;
+
+    if (this._progressListener) {
+      this._progressListener.uninstall();
+    }
+    this._progressListener = null;
+  },
+};
+
+/*
+ * This listener detects when a page being restored is reloaded. It triggers a
+ * callback and cancels the reload. The callback will send a message to
+ * SessionStore.sys.mjs so that it can restore the content immediately.
+ */
+function HistoryListener(docShell, callback) {
+  let webNavigation = docShell.QueryInterface(Ci.nsIWebNavigation);
+  webNavigation.sessionHistory.legacySHistory.addSHistoryListener(this);
+
+  this.webNavigation = webNavigation;
+  this.callback = callback;
+}
+HistoryListener.prototype = {
+  QueryInterface: ChromeUtils.generateQI([
+    "nsISHistoryListener",
+    "nsISupportsWeakReference",
+  ]),
+
+  uninstall() {
+    let shistory = this.webNavigation.sessionHistory.legacySHistory;
+    if (shistory) {
+      shistory.removeSHistoryListener(this);
+    }
+  },
+
+  OnHistoryGotoIndex() {},
+  OnHistoryPurge() {},
+  OnHistoryReplaceEntry() {},
+
+  // This will be called for a pending tab when loadURI(uri) is called where
+  // the given |uri| only differs in the fragment.
+  OnHistoryNewEntry(newURI) {
+    let currentURI = this.webNavigation.currentURI;
+
+    // Ignore new SHistory entries with the same URI as those do not indicate
+    // a navigation inside a document by changing the #hash part of the URL.
+    // We usually hit this when purging session history for browsers.
+    if (currentURI && currentURI.spec == newURI.spec) {
+      return;
+    }
+
+    // Reset the tab's URL to what it's actually showing. Without this loadURI()
+    // would use the current document and change the displayed URL only.
+    this.webNavigation.setCurrentURIForSessionStore(
+      Services.io.newURI("about:blank")
+    );
+
+    // Kick off a new load so that we navigate away from about:blank to the
+    // new URL that was passed to loadURI(). The new load will cause a
+    // STATE_START notification to be sent and the ProgressListener will then
+    // notify the parent and do the rest.
+    let loadFlags = Ci.nsIWebNavigation.LOAD_FLAGS_ALLOW_THIRD_PARTY_FIXUP;
+    let loadURIOptions = {
+      triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
+      loadFlags,
+    };
+    this.webNavigation.loadURI(newURI, loadURIOptions);
+  },
+
+  OnHistoryReload() {
+    this.callback();
+
+    // Cancel the load.
+    return false;
+  },
+};
+
+/**
+ * This class informs SessionStore.sys.mjs whenever the network requests for a
+ * restoring page have completely finished. We only restore three tabs
+ * simultaneously, so this is the signal for SessionStore.sys.mjs to kick off
+ * another restore (if there are more to do).
+ *
+ * The progress listener is also used to be notified when a load not initiated
+ * by sessionstore starts. Pending tabs will then need to be marked as no
+ * longer pending.
+ */
+function ProgressListener(docShell, callbacks) {
+  let webProgress = docShell
+    .QueryInterface(Ci.nsIInterfaceRequestor)
+    .getInterface(Ci.nsIWebProgress);
+  webProgress.addProgressListener(this, Ci.nsIWebProgress.NOTIFY_STATE_WINDOW);
+
+  this.webProgress = webProgress;
+  this.callbacks = callbacks;
+}
+
+ProgressListener.prototype = {
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIWebProgressListener",
+    "nsISupportsWeakReference",
+  ]),
+
+  uninstall() {
+    this.webProgress.removeProgressListener(this);
+  },
+
+  onStateChange(webProgress, request, stateFlags, status) {
+    let { STATE_IS_WINDOW, STATE_STOP, STATE_START } =
+      Ci.nsIWebProgressListener;
+    if (!webProgress.isTopLevel || !(stateFlags & STATE_IS_WINDOW)) {
+      return;
+    }
+
+    if (stateFlags & STATE_START && this.callbacks.onStartRequest) {
+      this.callbacks.onStartRequest();
+    }
+
+    if (stateFlags & STATE_STOP && this.callbacks.onStopRequest) {
+      this.callbacks.onStopRequest();
+    }
+  },
+};