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diff --git a/toolkit/modules/SessionRecorder.jsm b/toolkit/modules/SessionRecorder.jsm
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+++ b/toolkit/modules/SessionRecorder.jsm
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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/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = [
+  "SessionRecorder",
+];
+
+const {classes: Cc, interfaces: Ci, utils: Cu} = Components;
+
+Cu.import("resource://gre/modules/Preferences.jsm");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/Log.jsm");
+Cu.import("resource://services-common/utils.js");
+
+// We automatically prune sessions older than this.
+const MAX_SESSION_AGE_MS = 7 * 24 * 60 * 60 * 1000; // 7 days.
+const STARTUP_RETRY_INTERVAL_MS = 5000;
+
+// Wait up to 5 minutes for startup measurements before giving up.
+const MAX_STARTUP_TRIES = 300000 / STARTUP_RETRY_INTERVAL_MS;
+
+const LOGGER_NAME = "Toolkit.Telemetry";
+const LOGGER_PREFIX = "SessionRecorder::";
+
+/**
+ * Records information about browser sessions.
+ *
+ * This serves as an interface to both current session information as
+ * well as a history of previous sessions.
+ *
+ * Typically only one instance of this will be installed in an
+ * application. It is typically managed by an XPCOM service. The
+ * instance is instantiated at application start; onStartup is called
+ * once the profile is installed; onShutdown is called during shutdown.
+ *
+ * We currently record state in preferences. However, this should be
+ * invisible to external consumers. We could easily swap in a different
+ * storage mechanism if desired.
+ *
+ * Please note the different semantics for storing times and dates in
+ * preferences. Full dates (notably the session start time) are stored
+ * as strings because preferences have a 32-bit limit on integer values
+ * and milliseconds since UNIX epoch would overflow. Many times are
+ * stored as integer offsets from the session start time because they
+ * should not overflow 32 bits.
+ *
+ * Since this records history of all sessions, there is a possibility
+ * for unbounded data aggregation. This is curtailed through:
+ *
+ *   1) An "idle-daily" observer which delete sessions older than
+ *      MAX_SESSION_AGE_MS.
+ *   2) The creator of this instance explicitly calling
+ *      `pruneOldSessions`.
+ *
+ * @param branch
+ *        (string) Preferences branch on which to record state.
+ */
+this.SessionRecorder = function (branch) {
+  if (!branch) {
+    throw new Error("branch argument must be defined.");
+  }
+
+  if (!branch.endsWith(".")) {
+    throw new Error("branch argument must end with '.': " + branch);
+  }
+
+  this._log = Log.repository.getLoggerWithMessagePrefix(LOGGER_NAME, LOGGER_PREFIX);
+
+  this._prefs = new Preferences(branch);
+  this._lastActivityWasInactive = false;
+  this._activeTicks = 0;
+  this.fineTotalTime = 0;
+  this._started = false;
+  this._timer = null;
+  this._startupFieldTries = 0;
+
+  this._os = Cc["@mozilla.org/observer-service;1"]
+               .getService(Ci.nsIObserverService);
+
+};
+
+SessionRecorder.prototype = Object.freeze({
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver]),
+
+  STARTUP_RETRY_INTERVAL_MS: STARTUP_RETRY_INTERVAL_MS,
+
+  get _currentIndex() {
+    return this._prefs.get("currentIndex", 0);
+  },
+
+  set _currentIndex(value) {
+    this._prefs.set("currentIndex", value);
+  },
+
+  get _prunedIndex() {
+    return this._prefs.get("prunedIndex", 0);
+  },
+
+  set _prunedIndex(value) {
+    this._prefs.set("prunedIndex", value);
+  },
+
+  get startDate() {
+    return CommonUtils.getDatePref(this._prefs, "current.startTime");
+  },
+
+  set _startDate(value) {
+    CommonUtils.setDatePref(this._prefs, "current.startTime", value);
+  },
+
+  get activeTicks() {
+    return this._prefs.get("current.activeTicks", 0);
+  },
+
+  incrementActiveTicks: function () {
+    this._prefs.set("current.activeTicks", ++this._activeTicks);
+  },
+
+  /**
+   * Total time of this session in integer seconds.
+   *
+   * See also fineTotalTime for the time in milliseconds.
+   */
+  get totalTime() {
+    return this._prefs.get("current.totalTime", 0);
+  },
+
+  updateTotalTime: function () {
+    // We store millisecond precision internally to prevent drift from
+    // repeated rounding.
+    this.fineTotalTime = Date.now() - this.startDate;
+    this._prefs.set("current.totalTime", Math.floor(this.fineTotalTime / 1000));
+  },
+
+  get main() {
+    return this._prefs.get("current.main", -1);
+  },
+
+  set _main(value) {
+    if (!Number.isInteger(value)) {
+      throw new Error("main time must be an integer.");
+    }
+
+    this._prefs.set("current.main", value);
+  },
+
+  get firstPaint() {
+    return this._prefs.get("current.firstPaint", -1);
+  },
+
+  set _firstPaint(value) {
+    if (!Number.isInteger(value)) {
+      throw new Error("firstPaint must be an integer.");
+    }
+
+    this._prefs.set("current.firstPaint", value);
+  },
+
+  get sessionRestored() {
+    return this._prefs.get("current.sessionRestored", -1);
+  },
+
+  set _sessionRestored(value) {
+    if (!Number.isInteger(value)) {
+      throw new Error("sessionRestored must be an integer.");
+    }
+
+    this._prefs.set("current.sessionRestored", value);
+  },
+
+  getPreviousSessions: function () {
+    let result = {};
+
+    for (let i = this._prunedIndex; i < this._currentIndex; i++) {
+      let s = this.getPreviousSession(i);
+      if (!s) {
+        continue;
+      }
+
+      result[i] = s;
+    }
+
+    return result;
+  },
+
+  getPreviousSession: function (index) {
+    return this._deserialize(this._prefs.get("previous." + index));
+  },
+
+  /**
+   * Prunes old, completed sessions that started earlier than the
+   * specified date.
+   */
+  pruneOldSessions: function (date) {
+    for (let i = this._prunedIndex; i < this._currentIndex; i++) {
+      let s = this.getPreviousSession(i);
+      if (!s) {
+        continue;
+      }
+
+      if (s.startDate >= date) {
+        continue;
+      }
+
+      this._log.debug("Pruning session #" + i + ".");
+      this._prefs.reset("previous." + i);
+      this._prunedIndex = i;
+    }
+  },
+
+  recordStartupFields: function () {
+    let si = this._getStartupInfo();
+
+    if (!si.process) {
+      throw new Error("Startup info not available.");
+    }
+
+    let missing = false;
+
+    for (let field of ["main", "firstPaint", "sessionRestored"]) {
+      if (!(field in si)) {
+        this._log.debug("Missing startup field: " + field);
+        missing = true;
+        continue;
+      }
+
+      this["_" + field] = si[field].getTime() - si.process.getTime();
+    }
+
+    if (!missing || this._startupFieldTries > MAX_STARTUP_TRIES) {
+      this._clearStartupTimer();
+      return;
+    }
+
+    // If we have missing fields, install a timer and keep waiting for
+    // data.
+    this._startupFieldTries++;
+
+    if (!this._timer) {
+      this._timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+      this._timer.initWithCallback({
+        notify: this.recordStartupFields.bind(this),
+      }, this.STARTUP_RETRY_INTERVAL_MS, this._timer.TYPE_REPEATING_SLACK);
+    }
+  },
+
+  _clearStartupTimer: function () {
+    if (this._timer) {
+      this._timer.cancel();
+      delete this._timer;
+    }
+  },
+
+  /**
+   * Perform functionality on application startup.
+   *
+   * This is typically called in a "profile-do-change" handler.
+   */
+  onStartup: function () {
+    if (this._started) {
+      throw new Error("onStartup has already been called.");
+    }
+
+    let si = this._getStartupInfo();
+    if (!si.process) {
+      throw new Error("Process information not available. Misconfigured app?");
+    }
+
+    this._started = true;
+
+    this._os.addObserver(this, "profile-before-change", false);
+    this._os.addObserver(this, "user-interaction-active", false);
+    this._os.addObserver(this, "user-interaction-inactive", false);
+    this._os.addObserver(this, "idle-daily", false);
+
+    // This has the side-effect of clearing current session state.
+    this._moveCurrentToPrevious();
+
+    this._startDate = si.process;
+    this._prefs.set("current.activeTicks", 0);
+    this.updateTotalTime();
+
+    this.recordStartupFields();
+  },
+
+  /**
+   * Record application activity.
+   */
+  onActivity: function (active) {
+    let updateActive = active && !this._lastActivityWasInactive;
+    this._lastActivityWasInactive = !active;
+
+    this.updateTotalTime();
+
+    if (updateActive) {
+      this.incrementActiveTicks();
+    }
+  },
+
+  onShutdown: function () {
+    this._log.info("Recording clean session shutdown.");
+    this._prefs.set("current.clean", true);
+    this.updateTotalTime();
+    this._clearStartupTimer();
+
+    this._os.removeObserver(this, "profile-before-change");
+    this._os.removeObserver(this, "user-interaction-active");
+    this._os.removeObserver(this, "user-interaction-inactive");
+    this._os.removeObserver(this, "idle-daily");
+  },
+
+  _CURRENT_PREFS: [
+    "current.startTime",
+    "current.activeTicks",
+    "current.totalTime",
+    "current.main",
+    "current.firstPaint",
+    "current.sessionRestored",
+    "current.clean",
+  ],
+
+  // This is meant to be called only during onStartup().
+  _moveCurrentToPrevious: function () {
+    try {
+      if (!this.startDate.getTime()) {
+        this._log.info("No previous session. Is this first app run?");
+        return;
+      }
+
+      let clean = this._prefs.get("current.clean", false);
+
+      let count = this._currentIndex++;
+      let obj = {
+        s: this.startDate.getTime(),
+        a: this.activeTicks,
+        t: this.totalTime,
+        c: clean,
+        m: this.main,
+        fp: this.firstPaint,
+        sr: this.sessionRestored,
+      };
+
+      this._log.debug("Recording last sessions as #" + count + ".");
+      this._prefs.set("previous." + count, JSON.stringify(obj));
+    } catch (ex) {
+      this._log.warn("Exception when migrating last session", ex);
+    } finally {
+      this._log.debug("Resetting prefs from last session.");
+      for (let pref of this._CURRENT_PREFS) {
+        this._prefs.reset(pref);
+      }
+    }
+  },
+
+  _deserialize: function (s) {
+    let o;
+    try {
+      o = JSON.parse(s);
+    } catch (ex) {
+      return null;
+    }
+
+    return {
+      startDate: new Date(o.s),
+      activeTicks: o.a,
+      totalTime: o.t,
+      clean: !!o.c,
+      main: o.m,
+      firstPaint: o.fp,
+      sessionRestored: o.sr,
+    };
+  },
+
+  // Implemented as a function to allow for monkeypatching in tests.
+  _getStartupInfo: function () {
+    return Cc["@mozilla.org/toolkit/app-startup;1"]
+             .getService(Ci.nsIAppStartup)
+             .getStartupInfo();
+  },
+
+  observe: function (subject, topic, data) {
+    switch (topic) {
+      case "profile-before-change":
+        this.onShutdown();
+        break;
+
+      case "user-interaction-active":
+        this.onActivity(true);
+        break;
+
+      case "user-interaction-inactive":
+        this.onActivity(false);
+        break;
+
+      case "idle-daily":
+        this.pruneOldSessions(new Date(Date.now() - MAX_SESSION_AGE_MS));
+        break;
+    }
+  },
+});