path: root/toolkit/components/places/nsPlacesExpiration.js

/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*-
 * vim: sw=2 ts=2 sts=2 expandtab
 * 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/. */

/**
 * This component handles history and orphans expiration through asynchronous
 * Storage statements.
 * Expiration runs:
 * - At idle, but just once, we stop any other kind of expiration during idle
 *   to preserve batteries in portable devices.
 * - At shutdown, only if the database is dirty, we should still avoid to
 *   expire too heavily on shutdown.
 * - On ClearHistory we run a full expiration for privacy reasons.
 * - On a repeating timer we expire in small chunks.
 *
 * Expiration algorithm will adapt itself based on:
 * - Memory size of the device.
 * - Status of the database (clean or dirty).
 */

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

Cu.import("resource://gre/modules/XPCOMUtils.jsm");
Cu.import("resource://gre/modules/Services.jsm");
Cu.import("resource://gre/modules/Task.jsm");

XPCOMUtils.defineLazyModuleGetter(this, "PlacesUtils",
  "resource://gre/modules/PlacesUtils.jsm");

// Constants

// Last expiration step should run before the final sync.
const TOPIC_SHUTDOWN = "places-will-close-connection";
const TOPIC_PREF_CHANGED = "nsPref:changed";
const TOPIC_DEBUG_START_EXPIRATION = "places-debug-start-expiration";
const TOPIC_EXPIRATION_FINISHED = "places-expiration-finished";
const TOPIC_IDLE_BEGIN = "idle";
const TOPIC_IDLE_END = "active";
const TOPIC_IDLE_DAILY = "idle-daily";
const TOPIC_TESTING_MODE = "testing-mode";
const TOPIC_TEST_INTERVAL_CHANGED = "test-interval-changed";

// Branch for all expiration preferences.
const PREF_BRANCH = "places.history.expiration.";

// Max number of unique URIs to retain in history.
// Notice this is a lazy limit.  This means we will start to expire if we will
// go over it, but we won't ensure that we will stop exactly when we reach it,
// instead we will stop after the next expiration step that will bring us
// below it.
// If this preference does not exist or has a negative value, we will calculate
// a limit based on current hardware.
const PREF_MAX_URIS = "max_pages";
const PREF_MAX_URIS_NOTSET = -1; // Use our internally calculated limit.

// We save the current unique URIs limit to this pref, to make it available to
// other components without having to duplicate the full logic.
const PREF_READONLY_CALCULATED_MAX_URIS = "transient_current_max_pages";

// Seconds between each expiration step.
const PREF_INTERVAL_SECONDS = "interval_seconds";
const PREF_INTERVAL_SECONDS_NOTSET = 3 * 60;

// We calculate an optimal database size, based on hardware specs.
// This percentage of memory size is used to protect against calculating a too
// large database size on systems with small memory.
const DATABASE_TO_MEMORY_PERC = 4;
// This percentage of disk size is used to protect against calculating a too
// large database size on disks with tiny quota or available space.
const DATABASE_TO_DISK_PERC = 2;
// Maximum size of the optimal database.  High-end hardware has plenty of
// memory and disk space, but performances don't grow linearly.
const DATABASE_MAX_SIZE = 73400320; // 70MiB
// If the physical memory size is bogus, fallback to this.
const MEMSIZE_FALLBACK_BYTES = 268435456; // 256 MiB
// If the disk available space is bogus, fallback to this.
const DISKSIZE_FALLBACK_BYTES = 268435456; // 256 MiB

// Max number of entries to expire at each expiration step.
// This value is globally used for different kind of data we expire, can be
// tweaked based on data type.  See below in getBoundStatement.
const EXPIRE_LIMIT_PER_STEP = 6;
// When we run a large expiration step, the above limit is multiplied by this.
const EXPIRE_LIMIT_PER_LARGE_STEP_MULTIPLIER = 10;

// When history is clean or dirty enough we will adapt the expiration algorithm
// to be more lazy or more aggressive.
// This is done acting on the interval between expiration steps and the number
// of expirable items.
// 1. Clean history:
//   We expire at (default interval * EXPIRE_AGGRESSIVITY_MULTIPLIER) the
//   default number of entries.
// 2. Dirty history:
//   We expire at the default interval, but a greater number of entries
//   (default number of entries * EXPIRE_AGGRESSIVITY_MULTIPLIER).
const EXPIRE_AGGRESSIVITY_MULTIPLIER = 3;

// This is the average size in bytes of an URI entry in the database.
// Magic numbers are determined through analysis of the distribution of a ratio
// between number of unique URIs and database size among our users.
// Used as a fall back value when it's not possible to calculate the real value.
const URIENTRY_AVG_SIZE = 600;

// Seconds of idle time before starting a larger expiration step.
// Notice during idle we stop the expiration timer since we don't want to hurt
// stand-by or mobile devices batteries.
const IDLE_TIMEOUT_SECONDS = 5 * 60;

// If a clear history ran just before we shutdown, we will skip most of the
// expiration at shutdown.  This is maximum number of seconds from last
// clearHistory to decide to skip expiration at shutdown.
const SHUTDOWN_WITH_RECENT_CLEARHISTORY_TIMEOUT_SECONDS = 10;

// If the pages delta from the last ANALYZE is over this threashold, the tables
// should be analyzed again.
const ANALYZE_PAGES_THRESHOLD = 100;

// If the number of pages over history limit is greater than this threshold,
// expiration will be more aggressive, to bring back history to a saner size.
const OVERLIMIT_PAGES_THRESHOLD = 1000;

const MSECS_PER_DAY = 86400000;
const ANNOS_EXPIRE_POLICIES = [
  { bind: "expire_days",
    type: Ci.nsIAnnotationService.EXPIRE_DAYS,
    time: 7 * 1000 * MSECS_PER_DAY },
  { bind: "expire_weeks",
    type: Ci.nsIAnnotationService.EXPIRE_WEEKS,
    time: 30 * 1000 * MSECS_PER_DAY },
  { bind: "expire_months",
    type: Ci.nsIAnnotationService.EXPIRE_MONTHS,
    time: 180 * 1000 * MSECS_PER_DAY },
];

// When we expire we can use these limits:
// - SMALL for usual partial expirations, will expire a small chunk.
// - LARGE for idle or shutdown expirations, will expire a large chunk.
// - UNLIMITED for clearHistory, will expire everything.
// - DEBUG will use a known limit, passed along with the debug notification.
const LIMIT = {
  SMALL: 0,
  LARGE: 1,
  UNLIMITED: 2,
  DEBUG: 3,
};

// Represents the status of history database.
const STATUS = {
  CLEAN: 0,
  DIRTY: 1,
  UNKNOWN: 2,
};

// Represents actions on which a query will run.
const ACTION = {
  TIMED:           1 << 0, // happens every this._interval
  TIMED_OVERLIMIT: 1 << 1, // like TIMED but only when history is over limits
  TIMED_ANALYZE:   1 << 2, // happens when ANALYZE statistics should be updated
  CLEAR_HISTORY:   1 << 3, // happens when history is cleared
  SHUTDOWN_DIRTY:  1 << 4, // happens at shutdown for DIRTY state
  IDLE_DIRTY:      1 << 5, // happens on idle for DIRTY state
  IDLE_DAILY:      1 << 6, // happens once a day on idle
  DEBUG:           1 << 7, // happens on TOPIC_DEBUG_START_EXPIRATION
};

// The queries we use to expire.
const EXPIRATION_QUERIES = {

  // Some visits can be expired more often than others, cause they are less
  // useful to the user and can pollute awesomebar results:
  // 1. urls over 255 chars
  // 2. redirect sources and downloads
  // Note: due to the REPLACE option, this should be executed before
  // QUERY_FIND_VISITS_TO_EXPIRE, that has a more complete result.
  QUERY_FIND_EXOTIC_VISITS_TO_EXPIRE: {
    sql: `INSERT INTO expiration_notify (v_id, url, guid, visit_date, reason)
          SELECT v.id, h.url, h.guid, v.visit_date, "exotic"
          FROM moz_historyvisits v
          JOIN moz_places h ON h.id = v.place_id
          WHERE visit_date < strftime('%s','now','localtime','start of day','-60 days','utc') * 1000000
          AND ( LENGTH(h.url) > 255 OR v.visit_type = 7 )
          ORDER BY v.visit_date ASC
          LIMIT :limit_visits`,
    actions: ACTION.TIMED_OVERLIMIT | ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY |
             ACTION.DEBUG
  },

  // Finds visits to be expired when history is over the unique pages limit,
  // otherwise will return nothing.
  // This explicitly excludes any visits added in the last 7 days, to protect
  // users with thousands of bookmarks from constantly losing history.
  QUERY_FIND_VISITS_TO_EXPIRE: {
    sql: `INSERT INTO expiration_notify
            (v_id, url, guid, visit_date, expected_results)
          SELECT v.id, h.url, h.guid, v.visit_date, :limit_visits
          FROM moz_historyvisits v
          JOIN moz_places h ON h.id = v.place_id
          WHERE (SELECT COUNT(*) FROM moz_places) > :max_uris
          AND visit_date < strftime('%s','now','localtime','start of day','-7 days','utc') * 1000000
          ORDER BY v.visit_date ASC
          LIMIT :limit_visits`,
    actions: ACTION.TIMED_OVERLIMIT | ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY |
             ACTION.DEBUG
  },

  // Removes the previously found visits.
  QUERY_EXPIRE_VISITS: {
    sql: `DELETE FROM moz_historyvisits WHERE id IN (
            SELECT v_id FROM expiration_notify WHERE v_id NOTNULL
          )`,
    actions: ACTION.TIMED_OVERLIMIT | ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY |
             ACTION.DEBUG
  },

  // Finds orphan URIs in the database.
  // Notice we won't notify single removed URIs on History.clear(), so we don't
  // run this query in such a case, but just delete URIs.
  // This could run in the middle of adding a visit or bookmark to a new page.
  // In such a case since it is async, could end up expiring the orphan page
  // before it actually gets the new visit or bookmark.
  // Thus, since new pages get frecency -1, we filter on that.
  QUERY_FIND_URIS_TO_EXPIRE: {
    sql: `INSERT INTO expiration_notify (p_id, url, guid, visit_date)
          SELECT h.id, h.url, h.guid, h.last_visit_date
          FROM moz_places h
          LEFT JOIN moz_historyvisits v ON h.id = v.place_id
          WHERE h.last_visit_date IS NULL
            AND h.foreign_count = 0
            AND v.id IS NULL
            AND frecency <> -1
          LIMIT :limit_uris`,
    actions: ACTION.TIMED | ACTION.TIMED_OVERLIMIT | ACTION.SHUTDOWN_DIRTY |
             ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY | ACTION.DEBUG
  },

  // Expire found URIs from the database.
  QUERY_EXPIRE_URIS: {
    sql: `DELETE FROM moz_places WHERE id IN (
            SELECT p_id FROM expiration_notify WHERE p_id NOTNULL
          ) AND foreign_count = 0 AND last_visit_date ISNULL`,
    actions: ACTION.TIMED | ACTION.TIMED_OVERLIMIT | ACTION.SHUTDOWN_DIRTY |
             ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY | ACTION.DEBUG
  },

  // Expire orphan URIs from the database.
  QUERY_SILENT_EXPIRE_ORPHAN_URIS: {
    sql: `DELETE FROM moz_places WHERE id IN (
            SELECT h.id
            FROM moz_places h
            LEFT JOIN moz_historyvisits v ON h.id = v.place_id
            WHERE h.last_visit_date IS NULL
              AND h.foreign_count = 0
              AND v.id IS NULL
            LIMIT :limit_uris
          )`,
    actions: ACTION.CLEAR_HISTORY
  },

  // Hosts accumulated during the places delete are updated through a trigger
  // (see nsPlacesTriggers.h).
  QUERY_UPDATE_HOSTS: {
    sql: `DELETE FROM moz_updatehosts_temp`,
    actions: ACTION.CLEAR_HISTORY | ACTION.TIMED | ACTION.TIMED_OVERLIMIT |
             ACTION.SHUTDOWN_DIRTY | ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY |
             ACTION.DEBUG
  },

  // Expire orphan icons from the database.
  QUERY_EXPIRE_FAVICONS: {
    sql: `DELETE FROM moz_favicons WHERE id IN (
            SELECT f.id FROM moz_favicons f
            LEFT JOIN moz_places h ON f.id = h.favicon_id
            WHERE h.favicon_id IS NULL
            LIMIT :limit_favicons
          )`,
    actions: ACTION.TIMED | ACTION.TIMED_OVERLIMIT | ACTION.CLEAR_HISTORY |
             ACTION.SHUTDOWN_DIRTY | ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY |
             ACTION.DEBUG
  },

  // Expire orphan page annotations from the database.
  QUERY_EXPIRE_ANNOS: {
    sql: `DELETE FROM moz_annos WHERE id in (
            SELECT a.id FROM moz_annos a
            LEFT JOIN moz_places h ON a.place_id = h.id
            WHERE h.id IS NULL
            LIMIT :limit_annos
          )`,
    actions: ACTION.TIMED | ACTION.TIMED_OVERLIMIT | ACTION.CLEAR_HISTORY |
             ACTION.SHUTDOWN_DIRTY | ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY |
             ACTION.DEBUG
  },

  // Expire page annotations based on expiration policy.
  QUERY_EXPIRE_ANNOS_WITH_POLICY: {
    sql: `DELETE FROM moz_annos
          WHERE (expiration = :expire_days
            AND :expire_days_time > MAX(lastModified, dateAdded))
             OR (expiration = :expire_weeks
            AND :expire_weeks_time > MAX(lastModified, dateAdded))
             OR (expiration = :expire_months
            AND :expire_months_time > MAX(lastModified, dateAdded))`,
    actions: ACTION.TIMED | ACTION.TIMED_OVERLIMIT | ACTION.CLEAR_HISTORY |
             ACTION.SHUTDOWN_DIRTY | ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY |
             ACTION.DEBUG
  },

  // Expire items annotations based on expiration policy.
  QUERY_EXPIRE_ITEMS_ANNOS_WITH_POLICY: {
    sql: `DELETE FROM moz_items_annos
          WHERE (expiration = :expire_days
            AND :expire_days_time > MAX(lastModified, dateAdded))
             OR (expiration = :expire_weeks
            AND :expire_weeks_time > MAX(lastModified, dateAdded))
             OR (expiration = :expire_months
            AND :expire_months_time > MAX(lastModified, dateAdded))`,
    actions: ACTION.TIMED | ACTION.TIMED_OVERLIMIT | ACTION.CLEAR_HISTORY |
             ACTION.SHUTDOWN_DIRTY | ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY |
             ACTION.DEBUG
  },

  // Expire page annotations based on expiration policy.
  QUERY_EXPIRE_ANNOS_WITH_HISTORY: {
    sql: `DELETE FROM moz_annos
          WHERE expiration = :expire_with_history
            AND NOT EXISTS (SELECT id FROM moz_historyvisits
                            WHERE place_id = moz_annos.place_id LIMIT 1)`,
    actions: ACTION.TIMED | ACTION.TIMED_OVERLIMIT | ACTION.CLEAR_HISTORY |
             ACTION.SHUTDOWN_DIRTY | ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY |
             ACTION.DEBUG
  },

  // Expire item annos without a corresponding item id.
  QUERY_EXPIRE_ITEMS_ANNOS: {
    sql: `DELETE FROM moz_items_annos WHERE id IN (
            SELECT a.id FROM moz_items_annos a
            LEFT JOIN moz_bookmarks b ON a.item_id = b.id
            WHERE b.id IS NULL
            LIMIT :limit_annos
          )`,
    actions: ACTION.CLEAR_HISTORY | ACTION.IDLE_DAILY | ACTION.DEBUG
  },

  // Expire all annotation names without a corresponding annotation.
  QUERY_EXPIRE_ANNO_ATTRIBUTES: {
    sql: `DELETE FROM moz_anno_attributes WHERE id IN (
            SELECT n.id FROM moz_anno_attributes n
            LEFT JOIN moz_annos a ON n.id = a.anno_attribute_id
            LEFT JOIN moz_items_annos t ON n.id = t.anno_attribute_id
            WHERE a.anno_attribute_id IS NULL
              AND t.anno_attribute_id IS NULL
            LIMIT :limit_annos
          )`,
    actions: ACTION.CLEAR_HISTORY | ACTION.SHUTDOWN_DIRTY | ACTION.IDLE_DIRTY |
             ACTION.IDLE_DAILY | ACTION.DEBUG
  },

  // Expire orphan inputhistory.
  QUERY_EXPIRE_INPUTHISTORY: {
    sql: `DELETE FROM moz_inputhistory WHERE place_id IN (
            SELECT i.place_id FROM moz_inputhistory i
            LEFT JOIN moz_places h ON h.id = i.place_id
            WHERE h.id IS NULL
            LIMIT :limit_inputhistory
          )`,
    actions: ACTION.TIMED | ACTION.TIMED_OVERLIMIT | ACTION.CLEAR_HISTORY |
             ACTION.SHUTDOWN_DIRTY | ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY |
             ACTION.DEBUG
  },

  // Expire all session annotations.  Should only be called at shutdown.
  QUERY_EXPIRE_ANNOS_SESSION: {
    sql: "DELETE FROM moz_annos WHERE expiration = :expire_session",
    actions: ACTION.CLEAR_HISTORY | ACTION.DEBUG
  },

  // Expire all session item annotations.  Should only be called at shutdown.
  QUERY_EXPIRE_ITEMS_ANNOS_SESSION: {
    sql: "DELETE FROM moz_items_annos WHERE expiration = :expire_session",
    actions: ACTION.CLEAR_HISTORY | ACTION.DEBUG
  },

  // Select entries for notifications.
  // If p_id is set whole_entry = 1, then we have expired the full page.
  // Either p_id or v_id are always set.
  QUERY_SELECT_NOTIFICATIONS: {
    sql: `SELECT url, guid, MAX(visit_date) AS visit_date,
                 MAX(IFNULL(MIN(p_id, 1), MIN(v_id, 0))) AS whole_entry,
                 MAX(expected_results) AS expected_results,
                 (SELECT MAX(visit_date) FROM expiration_notify
                  WHERE reason = "expired" AND url = n.url AND p_id ISNULL
                 ) AS most_recent_expired_visit
          FROM expiration_notify n
          GROUP BY url`,
    actions: ACTION.TIMED | ACTION.TIMED_OVERLIMIT | ACTION.SHUTDOWN_DIRTY |
             ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY | ACTION.DEBUG
  },

  // Empty the notifications table.
  QUERY_DELETE_NOTIFICATIONS: {
    sql: "DELETE FROM expiration_notify",
    actions: ACTION.TIMED | ACTION.TIMED_OVERLIMIT | ACTION.SHUTDOWN_DIRTY |
             ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY | ACTION.DEBUG
  },

  // The following queries are used to adjust the sqlite_stat1 table to help the
  // query planner create better queries.  These should always be run LAST, and
  // are therefore at the end of the object.
  // Since also nsNavHistory.cpp executes ANALYZE, the analyzed tables
  // must be the same in both components.  So ensure they are in sync.

  QUERY_ANALYZE_MOZ_PLACES: {
    sql: "ANALYZE moz_places",
    actions: ACTION.TIMED_OVERLIMIT | ACTION.TIMED_ANALYZE |
             ACTION.CLEAR_HISTORY | ACTION.IDLE_DAILY | ACTION.DEBUG
  },
  QUERY_ANALYZE_MOZ_BOOKMARKS: {
    sql: "ANALYZE moz_bookmarks",
    actions: ACTION.TIMED_ANALYZE | ACTION.IDLE_DAILY | ACTION.DEBUG
  },
  QUERY_ANALYZE_MOZ_HISTORYVISITS: {
    sql: "ANALYZE moz_historyvisits",
    actions: ACTION.TIMED_OVERLIMIT | ACTION.TIMED_ANALYZE |
             ACTION.CLEAR_HISTORY | ACTION.IDLE_DAILY | ACTION.DEBUG
  },
  QUERY_ANALYZE_MOZ_INPUTHISTORY: {
    sql: "ANALYZE moz_inputhistory",
    actions: ACTION.TIMED | ACTION.TIMED_OVERLIMIT | ACTION.TIMED_ANALYZE |
             ACTION.CLEAR_HISTORY | ACTION.IDLE_DAILY | ACTION.DEBUG
  },
};

/**
 * Sends a bookmarks notification through the given observers.
 *
 * @param observers
 *        array of nsINavBookmarkObserver objects.
 * @param notification
 *        the notification name.
 * @param args
 *        array of arguments to pass to the notification.
 */
function notify(observers, notification, args = []) {
  for (let observer of observers) {
    try {
      observer[notification](...args);
    } catch (ex) {}
  }
}

// nsPlacesExpiration definition

function nsPlacesExpiration()
{
  // Smart Getters

  XPCOMUtils.defineLazyGetter(this, "_db", function () {
    let db = Cc["@mozilla.org/browser/nav-history-service;1"].
             getService(Ci.nsPIPlacesDatabase).
             DBConnection;

    // Create the temporary notifications table.
    let stmt = db.createAsyncStatement(
      `CREATE TEMP TABLE expiration_notify (
         id INTEGER PRIMARY KEY
       , v_id INTEGER
       , p_id INTEGER
       , url TEXT NOT NULL
       , guid TEXT NOT NULL
       , visit_date INTEGER
       , expected_results INTEGER NOT NULL DEFAULT 0
       , reason TEXT NOT NULL DEFAULT "expired"
       )`);
    stmt.executeAsync();
    stmt.finalize();

    return db;
  });

  XPCOMUtils.defineLazyServiceGetter(this, "_idle",
                                     "@mozilla.org/widget/idleservice;1",
                                     "nsIIdleService");

  this._prefBranch = Cc["@mozilla.org/preferences-service;1"].
                     getService(Ci.nsIPrefService).
                     getBranch(PREF_BRANCH);

  this._loadPrefs().then(() => {
    // Observe our preferences branch for changes.
    this._prefBranch.addObserver("", this, true);

    // Create our expiration timer.
    this._newTimer();
  }, Cu.reportError);

  // Register topic observers.
  Services.obs.addObserver(this, TOPIC_SHUTDOWN, true);
  Services.obs.addObserver(this, TOPIC_DEBUG_START_EXPIRATION, true);
  Services.obs.addObserver(this, TOPIC_IDLE_DAILY, true);
}

nsPlacesExpiration.prototype = {

  // nsIObserver

  observe: function PEX_observe(aSubject, aTopic, aData)
  {
    if (this._shuttingDown) {
      return;
    }

    if (aTopic == TOPIC_SHUTDOWN) {
      this._shuttingDown = true;
      this.expireOnIdle = false;

      if (this._timer) {
        this._timer.cancel();
        this._timer = null;
      }

      // If we didn't ran a clearHistory recently and database is dirty, we
      // want to expire some entries, to speed up the expiration process.
      let hasRecentClearHistory =
        Date.now() - this._lastClearHistoryTime <
          SHUTDOWN_WITH_RECENT_CLEARHISTORY_TIMEOUT_SECONDS * 1000;
      if (!hasRecentClearHistory && this.status == STATUS.DIRTY) {
        this._expireWithActionAndLimit(ACTION.SHUTDOWN_DIRTY, LIMIT.LARGE);
      }

      this._finalizeInternalStatements();
    }
    else if (aTopic == TOPIC_PREF_CHANGED) {
      this._loadPrefs().then(() => {
        if (aData == PREF_INTERVAL_SECONDS) {
          // Renew the timer with the new interval value.
          this._newTimer();
        }
      }, Cu.reportError);
    }
    else if (aTopic == TOPIC_DEBUG_START_EXPIRATION) {
      // The passed-in limit is the maximum number of visits to expire when
      // history is over capacity.  Mind to correctly handle the NaN value.
      let limit = parseInt(aData);
      if (limit == -1) {
        // Everything should be expired without any limit.  If history is over
        // capacity then all existing visits will be expired.
        // Should only be used in tests, since may cause dataloss.
        this._expireWithActionAndLimit(ACTION.DEBUG, LIMIT.UNLIMITED);
      }
      else if (limit > 0) {
        // The number of expired visits is limited by this amount.  It may be
        // used for testing purposes, like checking that limited queries work.
        this._debugLimit = limit;
        this._expireWithActionAndLimit(ACTION.DEBUG, LIMIT.DEBUG);
      }
      else {
        // Any other value is intended as a 0 limit, that means no visits
        // will be expired.  Even if this doesn't touch visits, it will remove
        // any orphan pages, icons, annotations and similar from the database,
        // so it may be used for cleanup purposes.
        this._debugLimit = -1;
        this._expireWithActionAndLimit(ACTION.DEBUG, LIMIT.DEBUG);
      }
    }
    else if (aTopic == TOPIC_IDLE_BEGIN) {
      // Stop the expiration timer.  We don't want to keep up expiring on idle
      // to preserve batteries on mobile devices and avoid killing stand-by.
      if (this._timer) {
        this._timer.cancel();
        this._timer = null;
      }
      if (this.expireOnIdle)
        this._expireWithActionAndLimit(ACTION.IDLE_DIRTY, LIMIT.LARGE);
    }
    else if (aTopic == TOPIC_IDLE_END) {
      // Restart the expiration timer.
      if (!this._timer)
        this._newTimer();
    }
    else if (aTopic == TOPIC_IDLE_DAILY) {
      this._expireWithActionAndLimit(ACTION.IDLE_DAILY, LIMIT.LARGE);
    }
    else if (aTopic == TOPIC_TESTING_MODE) {
      this._testingMode = true;
    }
  },

  // nsINavHistoryObserver

  _inBatchMode: false,
  onBeginUpdateBatch: function PEX_onBeginUpdateBatch()
  {
    this._inBatchMode = true;

    // We do not want to expire while we are doing batch work.
    if (this._timer) {
      this._timer.cancel();
      this._timer = null;
    }
  },

  onEndUpdateBatch: function PEX_onEndUpdateBatch()
  {
    this._inBatchMode = false;

    // Restore timer.
    if (!this._timer)
      this._newTimer();
  },

  _lastClearHistoryTime: 0,
  onClearHistory: function PEX_onClearHistory() {
    this._lastClearHistoryTime = Date.now();
    // Expire orphans.  History status is clean after a clear history.
    this.status = STATUS.CLEAN;
    this._expireWithActionAndLimit(ACTION.CLEAR_HISTORY, LIMIT.UNLIMITED);
  },

  onVisit: function() {},
  onTitleChanged: function() {},
  onDeleteURI: function() {},
  onPageChanged: function() {},
  onDeleteVisits: function() {},

  // nsITimerCallback

  notify: function PEX_timerCallback()
  {
    // Check if we are over history capacity, if so visits must be expired.
    this._getPagesStats((function onPagesCount(aPagesCount, aStatsCount) {
      let overLimitPages = aPagesCount - this._urisLimit;
      this._overLimit = overLimitPages > 0;

      let action = this._overLimit ? ACTION.TIMED_OVERLIMIT : ACTION.TIMED;
      // If the number of pages changed significantly from the last ANALYZE
      // update SQLite statistics.
      if (Math.abs(aPagesCount - aStatsCount) >= ANALYZE_PAGES_THRESHOLD) {
        action = action | ACTION.TIMED_ANALYZE;
      }

      // Adapt expiration aggressivity to the number of pages over the limit.
      let limit = overLimitPages > OVERLIMIT_PAGES_THRESHOLD ? LIMIT.LARGE
                                                             : LIMIT.SMALL;

      this._expireWithActionAndLimit(action, limit);
    }).bind(this));
  },

  // mozIStorageStatementCallback

  handleResult: function PEX_handleResult(aResultSet)
  {
    // We don't want to notify after shutdown.
    if (this._shuttingDown)
      return;

    let row;
    while ((row = aResultSet.getNextRow())) {
      // expected_results is set to the number of expected visits by
      // QUERY_FIND_VISITS_TO_EXPIRE.  We decrease that counter for each found
      // visit and if it reaches zero we mark the database as dirty, since all
      // the expected visits were expired, so it's likely the next run will
      // find more.
      let expectedResults = row.getResultByName("expected_results");
      if (expectedResults > 0) {
        if (!("_expectedResultsCount" in this)) {
          this._expectedResultsCount = expectedResults;
        }
        if (this._expectedResultsCount > 0) {
          this._expectedResultsCount--;
        }
      }

      let uri = Services.io.newURI(row.getResultByName("url"), null, null);
      let guid = row.getResultByName("guid");
      let visitDate = row.getResultByName("visit_date");
      let wholeEntry = row.getResultByName("whole_entry");
      let mostRecentExpiredVisit = row.getResultByName("most_recent_expired_visit");
      let reason = Ci.nsINavHistoryObserver.REASON_EXPIRED;
      let observers = PlacesUtils.history.getObservers();

      if (mostRecentExpiredVisit) {
        let days = parseInt((Date.now() - (mostRecentExpiredVisit / 1000)) / MSECS_PER_DAY);
        if (!this._mostRecentExpiredVisitDays) {
          this._mostRecentExpiredVisitDays = days;
        }
        else if (days < this._mostRecentExpiredVisitDays) {
          this._mostRecentExpiredVisitDays = days;
        }
      }

      // Dispatch expiration notifications to history.
      if (wholeEntry) {
        notify(observers, "onDeleteURI", [uri, guid, reason]);
      } else {
        notify(observers, "onDeleteVisits", [uri, visitDate, guid, reason, 0]);
      }
    }
  },

  handleError: function PEX_handleError(aError)
  {
    Cu.reportError("Async statement execution returned with '" +
                   aError.result + "', '" + aError.message + "'");
  },

  // Number of expiration steps needed to reach a CLEAN status.
  _telemetrySteps: 1,
  handleCompletion: function PEX_handleCompletion(aReason)
  {
    if (aReason == Ci.mozIStorageStatementCallback.REASON_FINISHED) {

      if (this._mostRecentExpiredVisitDays) {
        try {
          Services.telemetry
                  .getHistogramById("PLACES_MOST_RECENT_EXPIRED_VISIT_DAYS")
                  .add(this._mostRecentExpiredVisitDays);
        } catch (ex) {
          Components.utils.reportError("Unable to report telemetry.");
        } finally {
          delete this._mostRecentExpiredVisitDays;
        }
      }

      if ("_expectedResultsCount" in this) {
        // Adapt the aggressivity of steps based on the status of history.
        // A dirty history will return all the entries we are expecting bringing
        // our countdown to zero, while a clean one will not.
        let oldStatus = this.status;
        this.status = this._expectedResultsCount == 0 ? STATUS.DIRTY
                                                      : STATUS.CLEAN;

        // Collect or send telemetry data.
        if (this.status == STATUS.DIRTY) {
          this._telemetrySteps++;
        }
        else {
          // Avoid reporting the common cases where the database is clean, or
          // a single step is needed.
          if (oldStatus == STATUS.DIRTY) {
            try {
              Services.telemetry
                      .getHistogramById("PLACES_EXPIRATION_STEPS_TO_CLEAN2")
                      .add(this._telemetrySteps);
            } catch (ex) {
              Components.utils.reportError("Unable to report telemetry.");
            }
          }
          this._telemetrySteps = 1;
        }

        delete this._expectedResultsCount;
      }

      // Dispatch a notification that expiration has finished.
      Services.obs.notifyObservers(null, TOPIC_EXPIRATION_FINISHED, null);
    }
  },

  // nsPlacesExpiration

  _urisLimit: PREF_MAX_URIS_NOTSET,
  _interval: PREF_INTERVAL_SECONDS_NOTSET,
  _shuttingDown: false,

  _status: STATUS.UNKNOWN,
  set status(aNewStatus) {
    if (aNewStatus != this._status) {
      // If status changes we should restart the timer.
      this._status = aNewStatus;
      this._newTimer();
      // If needed add/remove the cleanup step on idle.  We want to expire on
      // idle only if history is dirty, to preserve mobile devices batteries.
      this.expireOnIdle = aNewStatus == STATUS.DIRTY;
    }
    return aNewStatus;
  },
  get status() {
    return this._status;
  },

  _isIdleObserver: false,
  _expireOnIdle: false,
  set expireOnIdle(aExpireOnIdle) {
    // Observe idle regardless aExpireOnIdle, since we always want to stop
    // timed expiration on idle, to preserve mobile battery life.
    if (!this._isIdleObserver && !this._shuttingDown) {
      this._idle.addIdleObserver(this, IDLE_TIMEOUT_SECONDS);
      this._isIdleObserver = true;
    }
    else if (this._isIdleObserver && this._shuttingDown) {
      this._idle.removeIdleObserver(this, IDLE_TIMEOUT_SECONDS);
      this._isIdleObserver = false;
    }

    // If running a debug expiration we need full control of what happens
    // but idle cleanup could activate in the middle, since tinderboxes are
    // permanently idle.  That would cause unexpected oranges, so disable it.
    if (this._debugLimit !== undefined)
      this._expireOnIdle = false;
    else
      this._expireOnIdle = aExpireOnIdle;
    return this._expireOnIdle;
  },
  get expireOnIdle() {
    return this._expireOnIdle;
  },

  _loadPrefs: Task.async(function* () {
    // Get the user's limit, if it was set.
    try {
      // We want to silently fail since getIntPref throws if it does not exist,
      // and use a default to fallback to.
      this._urisLimit = this._prefBranch.getIntPref(PREF_MAX_URIS);
    } catch (ex) { /* User limit not set */ }

    if (this._urisLimit < 0) {
      // Some testing code expects a pref change to be synchronous, so
      // temporarily set this to a large value, while we asynchronously update
      // to the correct value.
      this._urisLimit = 300000;

      // The user didn't specify a custom limit, so we calculate the number of
      // unique places that may fit an optimal database size on this hardware.
      // Oldest pages over this threshold will be expired.
      let memSizeBytes = MEMSIZE_FALLBACK_BYTES;
      try {
        // Limit the size on systems with small memory.
         memSizeBytes = Services.sysinfo.getProperty("memsize");
      } catch (ex) {}
      if (memSizeBytes <= 0) {
        memsize = MEMSIZE_FALLBACK_BYTES;
      }

      let diskAvailableBytes = DISKSIZE_FALLBACK_BYTES;
      try {
        // Protect against a full disk or tiny quota.
        let dbFile = this._db.databaseFile;
        dbFile.QueryInterface(Ci.nsILocalFile);
        diskAvailableBytes = dbFile.diskSpaceAvailable;
      } catch (ex) {}
      if (diskAvailableBytes <= 0) {
        diskAvailableBytes = DISKSIZE_FALLBACK_BYTES;
      }

      let optimalDatabaseSize = Math.min(
        memSizeBytes * DATABASE_TO_MEMORY_PERC / 100,
        diskAvailableBytes * DATABASE_TO_DISK_PERC / 100,
        DATABASE_MAX_SIZE
      );

      // Calculate avg size of a URI in the database.
      let db = yield PlacesUtils.promiseDBConnection();
      let pageSize = (yield db.execute(`PRAGMA page_size`))[0].getResultByIndex(0);
      let pageCount = (yield db.execute(`PRAGMA page_count`))[0].getResultByIndex(0);
      let freelistCount = (yield db.execute(`PRAGMA freelist_count`))[0].getResultByIndex(0);
      let dbSize = (pageCount - freelistCount) * pageSize;
      let uriCount = (yield db.execute(`SELECT count(*) FROM moz_places`))[0].getResultByIndex(0);
      let avgURISize = Math.ceil(dbSize / uriCount);
      // For new profiles this value may be too large, due to the Sqlite header,
      // or Infinity when there are no pages.  Thus we must limit it.
      if (avgURISize > (URIENTRY_AVG_SIZE * 3)) {
        avgURISize = URIENTRY_AVG_SIZE;
      }
      this._urisLimit = Math.ceil(optimalDatabaseSize / avgURISize);
    }

    // Expose the calculated limit to other components.
    this._prefBranch.setIntPref(PREF_READONLY_CALCULATED_MAX_URIS,
                                this._urisLimit);

    // Get the expiration interval value.
    try {
      // We want to silently fail since getIntPref throws if it does not exist,
      // and use a default to fallback to.
      this._interval = this._prefBranch.getIntPref(PREF_INTERVAL_SECONDS);
    } catch (ex) { /* User interval not set */ }
    if (this._interval <= 0) {
      this._interval = PREF_INTERVAL_SECONDS_NOTSET;
    }
  }),

  /**
   * Evaluates the real number of pages in the database and the value currently
   * used by the SQLite query planner.
   *
   * @param aCallback
   *        invoked on success, function (aPagesCount, aStatsCount).
   */
  _getPagesStats: function PEX__getPagesStats(aCallback) {
    if (!this._cachedStatements["LIMIT_COUNT"]) {
      this._cachedStatements["LIMIT_COUNT"] = this._db.createAsyncStatement(
        `SELECT (SELECT COUNT(*) FROM moz_places),
                (SELECT SUBSTR(stat,1,LENGTH(stat)-2) FROM sqlite_stat1
                 WHERE idx = 'moz_places_url_uniqueindex')`
      );
    }
    this._cachedStatements["LIMIT_COUNT"].executeAsync({
      _pagesCount: 0,
      _statsCount: 0,
      handleResult: function(aResults) {
        let row = aResults.getNextRow();
        this._pagesCount = row.getResultByIndex(0);
        this._statsCount = row.getResultByIndex(1);
      },
      handleCompletion: function (aReason) {
        if (aReason == Ci.mozIStorageStatementCallback.REASON_FINISHED) {
          aCallback(this._pagesCount, this._statsCount);
        }
      },
      handleError: function(aError) {
        Cu.reportError("Async statement execution returned with '" +
                       aError.result + "', '" + aError.message + "'");
      }
    });
  },

  /**
   * Execute async statements to expire with the specified queries.
   *
   * @param aAction
   *        The ACTION we are expiring for.  See the ACTION const for values.
   * @param aLimit
   *        Whether to use small, large or no limits when expiring.  See the
   *        LIMIT const for values.
   */
  _expireWithActionAndLimit:
  function PEX__expireWithActionAndLimit(aAction, aLimit)
  {
    // Skip expiration during batch mode.
    if (this._inBatchMode)
      return;
    // Don't try to further expire after shutdown.
    if (this._shuttingDown && aAction != ACTION.SHUTDOWN_DIRTY) {
      return;
    }

    let boundStatements = [];
    for (let queryType in EXPIRATION_QUERIES) {
      if (EXPIRATION_QUERIES[queryType].actions & aAction)
        boundStatements.push(this._getBoundStatement(queryType, aLimit, aAction));
    }

    // Execute statements asynchronously in a transaction.
    this._db.executeAsync(boundStatements, boundStatements.length, this);
  },

  /**
   * Finalizes all of our mozIStorageStatements so we can properly close the
   * database.
   */
  _finalizeInternalStatements: function PEX__finalizeInternalStatements()
  {
    for (let queryType in this._cachedStatements) {
      let stmt = this._cachedStatements[queryType];
      stmt.finalize();
    }
  },

  /**
   * Generate the statement used for expiration.
   *
   * @param aQueryType
   *        Type of the query to build statement for.
   * @param aLimit
   *        Whether to use small, large or no limits when expiring.  See the
   *        LIMIT const for values.
   * @param aAction
   *        Current action causing the expiration.  See the ACTION const.
   */
  _cachedStatements: {},
  _getBoundStatement: function PEX__getBoundStatement(aQueryType, aLimit, aAction)
  {
    // Statements creation can be expensive, so we want to cache them.
    let stmt = this._cachedStatements[aQueryType];
    if (stmt === undefined) {
      stmt = this._cachedStatements[aQueryType] =
        this._db.createAsyncStatement(EXPIRATION_QUERIES[aQueryType].sql);
    }

    let baseLimit;
    switch (aLimit) {
      case LIMIT.UNLIMITED:
        baseLimit = -1;
        break;
      case LIMIT.SMALL:
        baseLimit = EXPIRE_LIMIT_PER_STEP;
        break;
      case LIMIT.LARGE:
        baseLimit = EXPIRE_LIMIT_PER_STEP * EXPIRE_LIMIT_PER_LARGE_STEP_MULTIPLIER;
        break;
      case LIMIT.DEBUG:
        baseLimit = this._debugLimit;
        break;
    }
    if (this.status == STATUS.DIRTY && aAction != ACTION.DEBUG &&
        baseLimit > 0) {
      baseLimit *= EXPIRE_AGGRESSIVITY_MULTIPLIER;
    }

    // Bind the appropriate parameters.
    let params = stmt.params;
    switch (aQueryType) {
      case "QUERY_FIND_EXOTIC_VISITS_TO_EXPIRE":
        // Avoid expiring all visits in case of an unlimited debug expiration,
        // just remove orphans instead.
        params.limit_visits =
          aLimit == LIMIT.DEBUG && baseLimit == -1 ? 0 : baseLimit;
        break;
      case "QUERY_FIND_VISITS_TO_EXPIRE":
        params.max_uris = this._urisLimit;
        // Avoid expiring all visits in case of an unlimited debug expiration,
        // just remove orphans instead.
        params.limit_visits =
          aLimit == LIMIT.DEBUG && baseLimit == -1 ? 0 : baseLimit;
        break;
      case "QUERY_FIND_URIS_TO_EXPIRE":
        params.limit_uris = baseLimit;
        break;
      case "QUERY_SILENT_EXPIRE_ORPHAN_URIS":
        params.limit_uris = baseLimit;
        break;
      case "QUERY_EXPIRE_FAVICONS":
        params.limit_favicons = baseLimit;
        break;
      case "QUERY_EXPIRE_ANNOS":
        // Each page may have multiple annos.
        params.limit_annos = baseLimit * EXPIRE_AGGRESSIVITY_MULTIPLIER;
        break;
      case "QUERY_EXPIRE_ANNOS_WITH_POLICY":
      case "QUERY_EXPIRE_ITEMS_ANNOS_WITH_POLICY":
        let microNow = Date.now() * 1000;
        ANNOS_EXPIRE_POLICIES.forEach(function(policy) {
          params[policy.bind] = policy.type;
          params[policy.bind + "_time"] = microNow - policy.time;
        });
        break;
      case "QUERY_EXPIRE_ANNOS_WITH_HISTORY":
        params.expire_with_history = Ci.nsIAnnotationService.EXPIRE_WITH_HISTORY;
        break;
      case "QUERY_EXPIRE_ITEMS_ANNOS":
        params.limit_annos = baseLimit;
        break;
      case "QUERY_EXPIRE_ANNO_ATTRIBUTES":
        params.limit_annos = baseLimit;
        break;
      case "QUERY_EXPIRE_INPUTHISTORY":
        params.limit_inputhistory = baseLimit;
        break;
      case "QUERY_EXPIRE_ANNOS_SESSION":
      case "QUERY_EXPIRE_ITEMS_ANNOS_SESSION":
        params.expire_session = Ci.nsIAnnotationService.EXPIRE_SESSION;
        break;
    }

    return stmt;
  },

  /**
   * Creates a new timer based on this._interval.
   *
   * @return a REPEATING_SLACK nsITimer that runs every this._interval.
   */
  _newTimer: function PEX__newTimer()
  {
    if (this._timer)
      this._timer.cancel();
    if (this._shuttingDown)
      return undefined;
    let interval = this.status != STATUS.DIRTY ?
      this._interval * EXPIRE_AGGRESSIVITY_MULTIPLIER : this._interval;

    let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
    timer.initWithCallback(this, interval * 1000,
                           Ci.nsITimer.TYPE_REPEATING_SLACK);
    if (this._testingMode) {
      Services.obs.notifyObservers(null, TOPIC_TEST_INTERVAL_CHANGED,
                                   interval);
    }
    return this._timer = timer;
  },

  // nsISupports

  classID: Components.ID("705a423f-2f69-42f3-b9fe-1517e0dee56f"),

  _xpcom_factory: XPCOMUtils.generateSingletonFactory(nsPlacesExpiration),

  QueryInterface: XPCOMUtils.generateQI([
    Ci.nsIObserver
  , Ci.nsINavHistoryObserver
  , Ci.nsITimerCallback
  , Ci.mozIStorageStatementCallback
  , Ci.nsISupportsWeakReference
  ])
};

// Module Registration

var components = [nsPlacesExpiration];
this.NSGetFactory = XPCOMUtils.generateNSGetFactory(components);