/* -*- 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.
 * 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 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).
 */

import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";

const lazy = {};

ChromeUtils.defineESModuleGetters(lazy, {
  PlacesUtils: "resource://gre/modules/PlacesUtils.sys.mjs",
});

// Last expiration step should run before the final sync.
const TOPIC_DEBUG_START_EXPIRATION = "places-debug-start-expiration";
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";

// This value determines which systems we consider to have limited memory.
// This is used to protect against large database sizes on those systems.
const DATABASE_MEMORY_CONSTRAINED_THRESHOLD = 2147483648; // 2 GiB

// This value determines which systems we consider to have limited disk space.
// This is used to protect against large database sizes on those systems.
const DATABASE_DISK_CONSTRAINED_THRESHOLD = 5368709120; // 5 GiB

// 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 = 78643200; // 75 MiB
// 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 getQuery.
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 = 700;

// 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 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;

// Milliseconds in a day.
const MSECS_PER_DAY = 86400000;

// 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 will expire all the orphans.
// - 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.intervalSeconds
  TIMED_OVERLIMIT: 1 << 1, // like TIMED but only when history is over limits
  SHUTDOWN_DIRTY: 1 << 2, // happens at shutdown for DIRTY state
  IDLE_DIRTY: 1 << 3, // happens on idle for DIRTY state
  IDLE_DAILY: 1 << 4, // happens once a day on idle
  DEBUG: 1 << 5, // 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 having only one visit
  // 2. downloads
  // 3. non-typed hidden single-visit urls
  // We never expire redirect targets, because they are currently necessary to
  // recognize redirect sources (see Bug 468710 for better options).
  QUERY_FIND_EXOTIC_VISITS_TO_EXPIRE: {
    sql: `INSERT INTO expiration_notify (v_id, url, guid, visit_date, reason)
      WITH visits AS (
        SELECT v.id, url, guid, visit_type, visit_date, visit_count, hidden, typed
        FROM moz_historyvisits v
        JOIN moz_places h ON h.id = v.place_id
        WHERE visit_date < strftime('%s','now','localtime','start of day','-90 days','utc') * 1000000
        AND visit_type NOT IN (5,6)
      )
      SELECT id, url, guid, visit_date, "exotic"
      FROM visits
      WHERE (hidden = 1 AND typed = 0 AND visit_count <= 1) OR visit_type = 7
      UNION ALL
      SELECT id, url, guid, visit_date, "exotic"
      FROM visits
      WHERE visit_count = 1 AND LENGTH(url) > 255
      ORDER BY 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,
  },

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

  // Expire from favicons any page that has only relations older than 180 days,
  // if the page is not bookmarked, and we have a root icon that can be used
  // as a placeholder until the page is visited again.
  // The moz_pages_to_icons entries are removed by the table's FOREIGN KEY,
  // while orphan icons are removed by the following queries.
  QUERY_EXPIRE_OLD_FAVICONS: {
    sql: `
    DELETE FROM moz_pages_w_icons WHERE id IN (
      WITH pages_with_old_relations (page_id, page_url_hash) AS (
        SELECT page_id, page_url_hash
        FROM moz_icons_to_pages ip
        JOIN moz_pages_w_icons p ON p.id = page_id
        GROUP BY page_id
        HAVING max(expire_ms) < strftime('%s','now','localtime','start of day','-180 days','utc') * 1000
      )
      SELECT page_id
      FROM pages_with_old_relations
      JOIN moz_places h ON h.url_hash = page_url_hash
      JOIN moz_origins o ON h.origin_id = o.id
      WHERE foreign_count = 0
      AND EXISTS (
        SELECT 1 FROM moz_icons
        WHERE root = 1
          AND fixed_icon_url_hash = hash(fixup_url(o.host) || '/favicon.ico')
      )
      LIMIT 100
    )`,
    actions: ACTION.IDLE_DIRTY | ACTION.IDLE_DAILY | ACTION.DEBUG,
  },

  // Expire orphan pages from the icons database.
  QUERY_EXPIRE_FAVICONS_PAGES: {
    sql: `DELETE FROM moz_pages_w_icons
          WHERE page_url_hash NOT IN (
            SELECT url_hash FROM moz_places
          ) OR NOT EXISTS (
            SELECT 1 FROM moz_icons_to_pages WHERE page_id = moz_pages_w_icons.id
          )`,
    actions:
      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_icons WHERE id IN (
            SELECT id FROM moz_icons WHERE root = 0
            EXCEPT
            SELECT icon_id FROM moz_icons_to_pages
          )`,
    actions:
      ACTION.TIMED_OVERLIMIT |
      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.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 p_id FROM expiration_notify)
          AND 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.SHUTDOWN_DIRTY |
      ACTION.IDLE_DIRTY |
      ACTION.IDLE_DAILY |
      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: `/* do not warn (bug no): temp table has no index */
          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,
  },

  // Expire interactions older than N days.
  QUERY_EXPIRE_INTERACTIONS: {
    sql: `DELETE FROM moz_places_metadata
          WHERE id IN (
            SELECT id FROM moz_places_metadata
            WHERE updated_at < strftime('%s','now','localtime','-' || :days_interactions || ' day','start of day','utc') * 1000
            ORDER BY updated_at ASC
            LIMIT :limit_interactions
          )`,
    get disabled() {
      return !Services.prefs.getBoolPref(
        "browser.places.interactions.enabled",
        false
      );
    },
    actions:
      ACTION.TIMED_OVERLIMIT |
      ACTION.SHUTDOWN_DIRTY |
      ACTION.IDLE_DIRTY |
      ACTION.IDLE_DAILY |
      ACTION.DEBUG,
  },
};

export function nsPlacesExpiration() {
  // Allows other components to easily access getPagesLimit.
  this.wrappedJSObject = this;

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

  // 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.
  XPCOMUtils.defineLazyPreferenceGetter(
    this,
    "maxPages",
    "places.history.expiration.max_pages",
    -1,
    () => {
      // Clear the cache.
      dump("max_pages changing\n");
      this._pagesLimit = null;
    }
  );

  // Seconds between each expiration step.
  XPCOMUtils.defineLazyPreferenceGetter(
    this,
    "intervalSeconds",
    "places.history.expiration.interval_seconds",
    3 * 60, // 3 minutes
    () => {
      // Renew the timer with the new interval value.
      this._newTimer();
    },
    v => (v > 0 ? v : 3 * 60) // Accept only positive values.
  );

  this._dbInitializedPromise = lazy.PlacesUtils.withConnectionWrapper(
    "PlacesExpiration.jsm: setup",
    async db => {
      await db.execute(
        `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"
        )`
      );
    }
  )
    .then(() => {
      // Start the expiration timer.
      this._newTimer();
      // Expire daily on idle.
      Services.obs.addObserver(this, TOPIC_IDLE_DAILY, true);
    })
    .catch(console.error);

  // Block shutdown.
  let shutdownClient =
    lazy.PlacesUtils.history.connectionShutdownClient.jsclient;
  shutdownClient.addBlocker("Places Expiration: shutdown", () => {
    if (this._shuttingDown) {
      return;
    }
    this._shuttingDown = true;
    this.expireOnIdle = false;
    if (this._timer) {
      this._timer.cancel();
      this._timer = null;
    }
    // If the database is dirty, we want to expire some entries, to speed up
    // the expiration process.
    if (this.status == STATUS.DIRTY) {
      this._expire(ACTION.SHUTDOWN_DIRTY, LIMIT.LARGE).catch(console.error);
    }
  });
}

nsPlacesExpiration.prototype = {
  observe(aSubject, aTopic, aData) {
    if (this._shuttingDown) {
      return;
    }

    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._expire(ACTION.DEBUG, LIMIT.UNLIMITED).catch(console.error);
      } 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._expire(ACTION.DEBUG, LIMIT.DEBUG).catch(console.error);
      } 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._expire(ACTION.DEBUG, LIMIT.DEBUG).catch(console.error);
      }
    } 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._expire(ACTION.IDLE_DIRTY, LIMIT.LARGE).catch(console.error);
      }
    } else if (aTopic == TOPIC_IDLE_END) {
      // Restart the expiration timer.
      if (!this._timer) {
        this._newTimer();
      }
    } else if (aTopic == TOPIC_IDLE_DAILY) {
      this._expire(ACTION.IDLE_DAILY, LIMIT.LARGE).catch(console.error);
    } else if (aTopic == TOPIC_TESTING_MODE) {
      this._testingMode = true;
    } else if (aTopic == lazy.PlacesUtils.TOPIC_INIT_COMPLETE) {
      const placesObserver = new PlacesWeakCallbackWrapper(
        // History status is clean after a clear history.
        () => {
          this.status = STATUS.CLEAN;
        }
      );
      PlacesObservers.addListener(["history-cleared"], placesObserver);
    }
  },

  // nsINamed

  name: "nsPlacesExpiration",

  // nsITimerCallback

  notify() {
    // Run at the first idle, or after 5 minutes, whatever comes first.
    Services.tm.idleDispatchToMainThread(async () => {
      let db = await lazy.PlacesUtils.promiseDBConnection();
      let pagesCount = (
        await db.executeCached("SELECT count(*) AS count FROM moz_places")
      )[0].getResultByName("count");
      let pagesLimit = await this.getPagesLimit();
      // Check if we are over history capacity, if so visits must be expired.
      let overLimitPages = pagesCount - pagesLimit;
      let action = overLimitPages > 0 ? ACTION.TIMED_OVERLIMIT : ACTION.TIMED;
      // Adapt expiration aggressivity to the number of pages over the limit.
      let limit =
        overLimitPages > OVERLIMIT_PAGES_THRESHOLD ? LIMIT.LARGE : LIMIT.SMALL;
      this._expire(action, limit).catch(console.error);
    }, 300000);
  },

  _handleQueryResultAndAddNotification(row, notifications) {
    // We don't want to notify after shutdown.
    if (this._shuttingDown) {
      return;
    }

    // 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"));
    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"
    );

    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.
    const isRemovedFromStore = !!wholeEntry;
    notifications.push(
      new PlacesVisitRemoved({
        url: uri.spec,
        pageGuid: guid,
        reason: PlacesVisitRemoved.REASON_EXPIRED,
        isRemovedFromStore,
        isPartialVisistsRemoval: !isRemovedFromStore && visitDate > 0,
      })
    );
  },

  _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;
    }
  },
  get status() {
    return this._status;
  },

  async getPagesLimit() {
    if (this._pagesLimit != null) {
      return this._pagesLimit;
    }
    if (this.maxPages >= 0) {
      return (this._pagesLimit = this.maxPages);
    }

    // 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) {
      memSizeBytes = MEMSIZE_FALLBACK_BYTES;
    }

    let diskAvailableBytes = DISKSIZE_FALLBACK_BYTES;
    try {
      // Protect against a full disk or tiny quota.
      diskAvailableBytes =
        lazy.PlacesUtils.history.DBConnection.databaseFile.QueryInterface(
          Ci.nsIFile
        ).diskSpaceAvailable;
    } catch (ex) {}
    if (diskAvailableBytes <= 0) {
      diskAvailableBytes = DISKSIZE_FALLBACK_BYTES;
    }

    const isMemoryConstrained =
      memSizeBytes < DATABASE_MEMORY_CONSTRAINED_THRESHOLD;
    const isDiskConstrained =
      diskAvailableBytes < DATABASE_DISK_CONSTRAINED_THRESHOLD;

    let optimalDatabaseSize = DATABASE_MAX_SIZE;
    if (isMemoryConstrained || isDiskConstrained) {
      // This size is used to protect against a large database size
      // on disks with limited space or on systems with small memory
      optimalDatabaseSize /= 2;
    }

    // Calculate avg size of a URI in the database.
    let db;
    try {
      db = await lazy.PlacesUtils.promiseDBConnection();
      if (db) {
        let row = (
          await db.execute(`SELECT * FROM pragma_page_size(),
                                              pragma_page_count(),
                                              pragma_freelist_count(),
                                              (SELECT count(*) FROM moz_places)`)
        )[0];
        let pageSize = row.getResultByIndex(0);
        let pageCount = row.getResultByIndex(1);
        let freelistCount = row.getResultByIndex(2);
        let uriCount = row.getResultByIndex(3);
        let dbSize = (pageCount - freelistCount) * pageSize;
        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;
        }
        return (this._pagesLimit = Math.ceil(optimalDatabaseSize / avgURISize));
      }
    } catch (ex) {
      // We may have been initialized late in the shutdown process, maybe
      // by a call to clear history on shutdown.
      // If we're unable to get a connection clone, we'll just proceed with a
      // large default value, it should not be critical at this point in the
      // application life-cycle.
    }
    return (this._pagesLimit = 100000);
  },

  _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;
    }
  },
  get expireOnIdle() {
    return this._expireOnIdle;
  },

  // Number of expiration steps needed to reach a CLEAN status.
  _telemetrySteps: 1,

  /**
   * Expires visits and orphans.
   *
   * @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.
   */
  async _expire(aAction, aLimit) {
    // Don't try to further expire after shutdown.
    if (this._shuttingDown && aAction != ACTION.SHUTDOWN_DIRTY) {
      return;
    }
    await this._dbInitializedPromise;

    try {
      let notifications = [];
      await lazy.PlacesUtils.withConnectionWrapper(
        "PlacesExpiration.jsm: expire",
        async db => {
          await db.executeTransaction(async () => {
            for (let queryType in EXPIRATION_QUERIES) {
              let query = EXPIRATION_QUERIES[queryType];
              if (query.actions & aAction && !query.disabled) {
                let params = await this._getQueryParams(
                  queryType,
                  aLimit,
                  aAction
                );
                await db.executeCached(query.sql, params, row => {
                  this._handleQueryResultAndAddNotification(row, notifications);
                });
              }
            }
          });
        }
      );
      if (notifications.length) {
        PlacesObservers.notifyListeners(notifications);
      }
    } catch (ex) {
      console.error(ex);
      return;
    }

    if (this._mostRecentExpiredVisitDays) {
      try {
        Services.telemetry
          .getHistogramById("PLACES_MOST_RECENT_EXPIRED_VISIT_DAYS")
          .add(this._mostRecentExpiredVisitDays);
      } catch (ex) {
        console.error("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) {
            console.error("Unable to report telemetry.");
          }
        }
        this._telemetrySteps = 1;
      }

      delete this._expectedResultsCount;
    }

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

  /**
   * Generate a query used for expiration.
   *
   * @param aQueryType
   *        Type of the query.
   * @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.
   */
  async _getQueryParams(aQueryType, aLimit, aAction) {
    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;
    }

    switch (aQueryType) {
      case "QUERY_FIND_EXOTIC_VISITS_TO_EXPIRE":
        return {
          // Avoid expiring all visits in case of an unlimited debug expiration,
          // just remove orphans instead.
          limit_visits:
            aLimit == LIMIT.DEBUG && baseLimit == -1 ? 0 : baseLimit,
        };
      case "QUERY_FIND_VISITS_TO_EXPIRE":
        return {
          max_uris: await this.getPagesLimit(),
          // Avoid expiring all visits in case of an unlimited debug expiration,
          // just remove orphans instead.
          limit_visits:
            aLimit == LIMIT.DEBUG && baseLimit == -1 ? 0 : baseLimit,
        };
      case "QUERY_FIND_URIS_TO_EXPIRE":
        return {
          limit_uris: baseLimit,
        };
      case "QUERY_EXPIRE_ANNOS":
        return {
          // Each page may have multiple annos.
          limit_annos: baseLimit * EXPIRE_AGGRESSIVITY_MULTIPLIER,
        };
      case "QUERY_EXPIRE_INPUTHISTORY":
        return {
          limit_inputhistory: baseLimit,
        };
      case "QUERY_EXPIRE_INTERACTIONS":
        return {
          days_interactions: Services.prefs.getIntPref(
            "browser.places.interactions.expireDays",
            60
          ),
          limit_interactions:
            aLimit == LIMIT.DEBUG && baseLimit == -1 ? 0 : baseLimit,
        };
    }
    return undefined;
  },

  /**
   * Creates a new timer based on this.intervalSeconds.
   *
   * @return a REPEATING_SLACK nsITimer that runs every this.intervalSeconds.
   */
  _newTimer() {
    if (this._timer) {
      this._timer.cancel();
    }
    if (this._shuttingDown) {
      return undefined;
    }

    if (!this._isIdleObserver) {
      this._idle.addIdleObserver(this, IDLE_TIMEOUT_SECONDS);
      this._isIdleObserver = true;
    }

    let interval =
      this.status != STATUS.DIRTY
        ? this.intervalSeconds * EXPIRE_AGGRESSIVITY_MULTIPLIER
        : this.intervalSeconds;

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

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

  QueryInterface: ChromeUtils.generateQI([
    "nsINamed",
    "nsIObserver",
    "nsISupportsWeakReference",
    "nsITimerCallback",
  ]),
};