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diff --git a/browser/components/sessionstore/SessionSaver.sys.mjs b/browser/components/sessionstore/SessionSaver.sys.mjs
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+++ b/browser/components/sessionstore/SessionSaver.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/. */
+
+import {
+  cancelIdleCallback,
+  clearTimeout,
+  requestIdleCallback,
+  setTimeout,
+} from "resource://gre/modules/Timer.sys.mjs";
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  PrivacyFilter: "resource://gre/modules/sessionstore/PrivacyFilter.sys.mjs",
+  PrivateBrowsingUtils: "resource://gre/modules/PrivateBrowsingUtils.sys.mjs",
+  RunState: "resource:///modules/sessionstore/RunState.sys.mjs",
+  SessionFile: "resource:///modules/sessionstore/SessionFile.sys.mjs",
+  SessionStore: "resource:///modules/sessionstore/SessionStore.sys.mjs",
+});
+
+/*
+ * Minimal interval between two save operations (in milliseconds).
+ *
+ * To save system resources, we generally do not save changes immediately when
+ * a change is detected. Rather, we wait a little to see if this change is
+ * followed by other changes, in which case only the last write is necessary.
+ * This delay is defined by "browser.sessionstore.interval".
+ *
+ * Furthermore, when the user is not actively using the computer, webpages
+ * may still perform changes that require (re)writing to sessionstore, e.g.
+ * updating Session Cookies or DOM Session Storage, or refreshing, etc. We
+ * expect that these changes are much less critical to the user and do not
+ * need to be saved as often. In such cases, we increase the delay to
+ *  "browser.sessionstore.interval.idle".
+ *
+ * When the user returns to the computer, if a save is pending, we reschedule
+ * it to happen soon, with "browser.sessionstore.interval".
+ */
+const PREF_INTERVAL_ACTIVE = "browser.sessionstore.interval";
+const PREF_INTERVAL_IDLE = "browser.sessionstore.interval.idle";
+const PREF_IDLE_DELAY = "browser.sessionstore.idleDelay";
+
+// Notify observers about a given topic with a given subject.
+function notify(subject, topic) {
+  Services.obs.notifyObservers(subject, topic);
+}
+
+// TelemetryStopwatch helper functions.
+function stopWatch(method) {
+  return function(...histograms) {
+    for (let hist of histograms) {
+      TelemetryStopwatch[method]("FX_SESSION_RESTORE_" + hist);
+    }
+  };
+}
+
+var stopWatchStart = stopWatch("start");
+var stopWatchFinish = stopWatch("finish");
+
+/**
+ * The external API implemented by the SessionSaver module.
+ */
+export var SessionSaver = Object.freeze({
+  /**
+   * Immediately saves the current session to disk.
+   */
+  run() {
+    return SessionSaverInternal.run();
+  },
+
+  /**
+   * Saves the current session to disk delayed by a given amount of time. Should
+   * another delayed run be scheduled already, we will ignore the given delay
+   * and state saving may occur a little earlier.
+   */
+  runDelayed() {
+    SessionSaverInternal.runDelayed();
+  },
+
+  /**
+   * Sets the last save time to the current time. This will cause us to wait for
+   * at least the configured interval when runDelayed() is called next.
+   */
+  updateLastSaveTime() {
+    SessionSaverInternal.updateLastSaveTime();
+  },
+
+  /**
+   * Cancels all pending session saves.
+   */
+  cancel() {
+    SessionSaverInternal.cancel();
+  },
+});
+
+/**
+ * The internal API.
+ */
+var SessionSaverInternal = {
+  /**
+   * The timeout ID referencing an active timer for a delayed save. When no
+   * save is pending, this is null.
+   */
+  _timeoutID: null,
+
+  /**
+   * The idle callback ID referencing an active idle callback. When no idle
+   * callback is pending, this is null.
+   * */
+  _idleCallbackID: null,
+
+  /**
+   * A timestamp that keeps track of when we saved the session last. We will
+   * this to determine the correct interval between delayed saves to not deceed
+   * the configured session write interval.
+   */
+  _lastSaveTime: 0,
+
+  /**
+   * `true` if the user has been idle for at least
+   * `SessionSaverInternal._intervalWhileIdle` ms. Idleness is computed
+   * with `nsIUserIdleService`.
+   */
+  _isIdle: false,
+
+  /**
+   * `true` if the user was idle when we last scheduled a delayed save.
+   * See `_isIdle` for details on idleness.
+   */
+  _wasIdle: false,
+
+  /**
+   * Minimal interval between two save operations (in ms), while the user
+   * is active.
+   */
+  _intervalWhileActive: null,
+
+  /**
+   * Minimal interval between two save operations (in ms), while the user
+   * is idle.
+   */
+  _intervalWhileIdle: null,
+
+  /**
+   * How long before we assume that the user is idle (ms).
+   */
+  _idleDelay: null,
+
+  /**
+   * Immediately saves the current session to disk.
+   */
+  run() {
+    return this._saveState(true /* force-update all windows */);
+  },
+
+  /**
+   * Saves the current session to disk delayed by a given amount of time. Should
+   * another delayed run be scheduled already, we will ignore the given delay
+   * and state saving may occur a little earlier.
+   *
+   * @param delay (optional)
+   *        The minimum delay in milliseconds to wait for until we collect and
+   *        save the current session.
+   */
+  runDelayed(delay = 2000) {
+    // Bail out if there's a pending run.
+    if (this._timeoutID) {
+      return;
+    }
+
+    // Interval until the next disk operation is allowed.
+    let interval = this._isIdle
+      ? this._intervalWhileIdle
+      : this._intervalWhileActive;
+    delay = Math.max(this._lastSaveTime + interval - Date.now(), delay, 0);
+
+    // Schedule a state save.
+    this._wasIdle = this._isIdle;
+    this._timeoutID = setTimeout(() => {
+      // Execute _saveStateAsync when we have idle time.
+      let saveStateAsyncWhenIdle = () => {
+        this._saveStateAsync();
+      };
+
+      this._idleCallbackID = requestIdleCallback(saveStateAsyncWhenIdle);
+    }, delay);
+  },
+
+  /**
+   * Sets the last save time to the current time. This will cause us to wait for
+   * at least the configured interval when runDelayed() is called next.
+   */
+  updateLastSaveTime() {
+    this._lastSaveTime = Date.now();
+  },
+
+  /**
+   * Cancels all pending session saves.
+   */
+  cancel() {
+    clearTimeout(this._timeoutID);
+    this._timeoutID = null;
+    cancelIdleCallback(this._idleCallbackID);
+    this._idleCallbackID = null;
+  },
+
+  /**
+   * Observe idle/ active notifications.
+   */
+  observe(subject, topic, data) {
+    switch (topic) {
+      case "idle":
+        this._isIdle = true;
+        break;
+      case "active":
+        this._isIdle = false;
+        if (this._timeoutID && this._wasIdle) {
+          // A state save has been scheduled while we were idle.
+          // Replace it by an active save.
+          clearTimeout(this._timeoutID);
+          this._timeoutID = null;
+          this.runDelayed();
+        }
+        break;
+      default:
+        throw new Error(`Unexpected change value ${topic}`);
+    }
+  },
+
+  /**
+   * Saves the current session state. Collects data and writes to disk.
+   *
+   * @param forceUpdateAllWindows (optional)
+   *        Forces us to recollect data for all windows and will bypass and
+   *        update the corresponding caches.
+   */
+  _saveState(forceUpdateAllWindows = false) {
+    // Cancel any pending timeouts.
+    this.cancel();
+
+    if (lazy.PrivateBrowsingUtils.permanentPrivateBrowsing) {
+      // Don't save (or even collect) anything in permanent private
+      // browsing mode
+
+      this.updateLastSaveTime();
+      return Promise.resolve();
+    }
+
+    stopWatchStart("COLLECT_DATA_MS");
+    let state = lazy.SessionStore.getCurrentState(forceUpdateAllWindows);
+    lazy.PrivacyFilter.filterPrivateWindowsAndTabs(state);
+
+    // Make sure we only write worth saving tabs to disk.
+    lazy.SessionStore.keepOnlyWorthSavingTabs(state);
+
+    // Make sure that we keep the previous session if we started with a single
+    // private window and no non-private windows have been opened, yet.
+    if (state.deferredInitialState) {
+      state.windows = state.deferredInitialState.windows || [];
+      delete state.deferredInitialState;
+    }
+
+    if (AppConstants.platform != "macosx") {
+      // We want to restore closed windows that are marked with _shouldRestore.
+      // We're doing this here because we want to control this only when saving
+      // the file.
+      while (state._closedWindows.length) {
+        let i = state._closedWindows.length - 1;
+
+        if (!state._closedWindows[i]._shouldRestore) {
+          // We only need to go until _shouldRestore
+          // is falsy since we're going in reverse.
+          break;
+        }
+
+        delete state._closedWindows[i]._shouldRestore;
+        state.windows.unshift(state._closedWindows.pop());
+      }
+    }
+
+    // Clear cookies and storage on clean shutdown.
+    this._maybeClearCookiesAndStorage(state);
+
+    stopWatchFinish("COLLECT_DATA_MS");
+    return this._writeState(state);
+  },
+
+  /**
+   * Purges cookies and DOMSessionStorage data from the session on clean
+   * shutdown, only if requested by the user's preferences.
+   */
+  _maybeClearCookiesAndStorage(state) {
+    // Only do this on shutdown.
+    if (!lazy.RunState.isClosing) {
+      return;
+    }
+
+    // Don't clear when restarting.
+    if (
+      Services.prefs.getBoolPref("browser.sessionstore.resume_session_once")
+    ) {
+      return;
+    }
+    let sanitizeCookies =
+      Services.prefs.getBoolPref("privacy.sanitize.sanitizeOnShutdown") &&
+      Services.prefs.getBoolPref("privacy.clearOnShutdown.cookies");
+
+    if (sanitizeCookies) {
+      // Remove cookies.
+      delete state.cookies;
+
+      // Remove DOMSessionStorage data.
+      for (let window of state.windows) {
+        for (let tab of window.tabs) {
+          delete tab.storage;
+        }
+      }
+    }
+  },
+
+  /**
+   * Saves the current session state. Collects data asynchronously and calls
+   * _saveState() to collect data again (with a cache hit rate of hopefully
+   * 100%) and write to disk afterwards.
+   */
+  _saveStateAsync() {
+    // Allow scheduling delayed saves again.
+    this._timeoutID = null;
+
+    // Write to disk.
+    this._saveState();
+  },
+
+  /**
+   * Write the given state object to disk.
+   */
+  _writeState(state) {
+    // We update the time stamp before writing so that we don't write again
+    // too soon, if saving is requested before the write completes. Without
+    // this update we may save repeatedly if actions cause a runDelayed
+    // before writing has completed. See Bug 902280
+    this.updateLastSaveTime();
+
+    // Write (atomically) to a session file, using a tmp file. Once the session
+    // file is successfully updated, save the time stamp of the last save and
+    // notify the observers.
+    return lazy.SessionFile.write(state).then(() => {
+      this.updateLastSaveTime();
+      notify(null, "sessionstore-state-write-complete");
+    }, console.error);
+  },
+};
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  SessionSaverInternal,
+  "_intervalWhileActive",
+  PREF_INTERVAL_ACTIVE,
+  15000 /* 15 seconds */,
+  () => {
+    // Cancel any pending runs and call runDelayed() with
+    // zero to apply the newly configured interval.
+    SessionSaverInternal.cancel();
+    SessionSaverInternal.runDelayed(0);
+  }
+);
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  SessionSaverInternal,
+  "_intervalWhileIdle",
+  PREF_INTERVAL_IDLE,
+  3600000 /* 1 h */
+);
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  SessionSaverInternal,
+  "_idleDelay",
+  PREF_IDLE_DELAY,
+  180000 /* 3 minutes */,
+  (key, previous, latest) => {
+    // Update the idle observer for the new `PREF_IDLE_DELAY` value. Here we need
+    // to re-fetch the service instead of the original one in use; This is for a
+    // case that the Mock service in the unit test needs to be fetched to
+    // replace the original one.
+    var idleService = Cc["@mozilla.org/widget/useridleservice;1"].getService(
+      Ci.nsIUserIdleService
+    );
+    if (previous != undefined) {
+      idleService.removeIdleObserver(SessionSaverInternal, previous);
+    }
+    if (latest != undefined) {
+      idleService.addIdleObserver(SessionSaverInternal, latest);
+    }
+  }
+);
+
+var idleService = Cc["@mozilla.org/widget/useridleservice;1"].getService(
+  Ci.nsIUserIdleService
+);
+idleService.addIdleObserver(
+  SessionSaverInternal,
+  SessionSaverInternal._idleDelay
+);