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diff --git a/services/sync/modules/SyncedTabs.jsm b/services/sync/modules/SyncedTabs.jsm
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+++ b/services/sync/modules/SyncedTabs.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";
+
+var EXPORTED_SYMBOLS = ["SyncedTabs"];
+
+const { XPCOMUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/XPCOMUtils.sys.mjs"
+);
+const { Log } = ChromeUtils.importESModule(
+  "resource://gre/modules/Log.sys.mjs"
+);
+const { Weave } = ChromeUtils.import("resource://services-sync/main.js");
+const { Preferences } = ChromeUtils.importESModule(
+  "resource://gre/modules/Preferences.sys.mjs"
+);
+
+const lazy = {};
+
+// The Sync XPCOM service
+XPCOMUtils.defineLazyGetter(lazy, "weaveXPCService", function() {
+  return Cc["@mozilla.org/weave/service;1"].getService(
+    Ci.nsISupports
+  ).wrappedJSObject;
+});
+
+// from MDN...
+function escapeRegExp(string) {
+  return string.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+}
+
+// A topic we fire whenever we have new tabs available. This might be due
+// to a request made by this module to refresh the tab list, or as the result
+// of a regularly scheduled sync. The intent is that consumers just listen
+// for this notification and update their UI in response.
+const TOPIC_TABS_CHANGED = "services.sync.tabs.changed";
+
+// The interval, in seconds, before which we consider the existing list
+// of tabs "fresh enough" and don't force a new sync.
+const TABS_FRESH_ENOUGH_INTERVAL_SECONDS = 30;
+
+XPCOMUtils.defineLazyGetter(lazy, "log", function() {
+  let log = Log.repository.getLogger("Sync.RemoteTabs");
+  log.manageLevelFromPref("services.sync.log.logger.tabs");
+  return log;
+});
+
+// A private singleton that does the work.
+let SyncedTabsInternal = {
+  /* Make a "tab" record. Returns a promise */
+  async _makeTab(client, tab, url, showRemoteIcons) {
+    let icon;
+    if (showRemoteIcons) {
+      icon = tab.icon;
+    }
+    if (!icon) {
+      // By not specifying a size the favicon service will pick the default,
+      // that is usually set through setDefaultIconURIPreferredSize by the
+      // first browser window. Commonly it's 16px at current dpi.
+      icon = "page-icon:" + url;
+    }
+    return {
+      type: "tab",
+      title: tab.title || url,
+      url,
+      icon,
+      client: client.id,
+      lastUsed: tab.lastUsed,
+    };
+  },
+
+  /* Make a "client" record. Returns a promise for consistency with _makeTab */
+  async _makeClient(client) {
+    return {
+      id: client.id,
+      type: "client",
+      name: Weave.Service.clientsEngine.getClientName(client.id),
+      clientType: Weave.Service.clientsEngine.getClientType(client.id),
+      lastModified: client.lastModified * 1000, // sec to ms
+      tabs: [],
+    };
+  },
+
+  _tabMatchesFilter(tab, filter) {
+    let reFilter = new RegExp(escapeRegExp(filter), "i");
+    return reFilter.test(tab.url) || reFilter.test(tab.title);
+  },
+
+  async getTabClients(filter) {
+    lazy.log.info("Generating tab list with filter", filter);
+    let result = [];
+
+    // If Sync isn't ready, don't try and get anything.
+    if (!lazy.weaveXPCService.ready) {
+      lazy.log.debug("Sync isn't yet ready, so returning an empty tab list");
+      return result;
+    }
+
+    // A boolean that controls whether we should show the icon from the remote tab.
+    const showRemoteIcons = Preferences.get(
+      "services.sync.syncedTabs.showRemoteIcons",
+      true
+    );
+
+    let engine = Weave.Service.engineManager.get("tabs");
+
+    let ntabs = 0;
+    let clientTabList = await engine.getAllClients();
+    for (let client of clientTabList) {
+      if (!Weave.Service.clientsEngine.remoteClientExists(client.id)) {
+        continue;
+      }
+      let clientRepr = await this._makeClient(client);
+      lazy.log.debug("Processing client", clientRepr);
+
+      for (let tab of client.tabs) {
+        let url = tab.urlHistory[0];
+        lazy.log.trace("remote tab", url);
+
+        if (!url) {
+          continue;
+        }
+        let tabRepr = await this._makeTab(client, tab, url, showRemoteIcons);
+        if (filter && !this._tabMatchesFilter(tabRepr, filter)) {
+          continue;
+        }
+        clientRepr.tabs.push(tabRepr);
+      }
+      // We return all clients, even those without tabs - the consumer should
+      // filter it if they care.
+      ntabs += clientRepr.tabs.length;
+      result.push(clientRepr);
+    }
+    lazy.log.info(
+      `Final tab list has ${result.length} clients with ${ntabs} tabs.`
+    );
+    return result;
+  },
+
+  async syncTabs(force) {
+    if (!force) {
+      // Don't bother refetching tabs if we already did so recently
+      let lastFetch = Preferences.get("services.sync.lastTabFetch", 0);
+      let now = Math.floor(Date.now() / 1000);
+      if (now - lastFetch < TABS_FRESH_ENOUGH_INTERVAL_SECONDS) {
+        lazy.log.info("_refetchTabs was done recently, do not doing it again");
+        return false;
+      }
+    }
+
+    // If Sync isn't configured don't try and sync, else we will get reports
+    // of a login failure.
+    if (Weave.Status.checkSetup() === Weave.CLIENT_NOT_CONFIGURED) {
+      lazy.log.info(
+        "Sync client is not configured, so not attempting a tab sync"
+      );
+      return false;
+    }
+    // If Sync can't log in we also don't try.
+    if (
+      !(
+        Weave.Status.login === Weave.STATUS_OK ||
+        Weave.Status.login === Weave.LOGIN_SUCCEEDED
+      )
+    ) {
+      lazy.log.info(
+        "Can't sync tabs due to the login status",
+        Weave.Status.login
+      );
+      return false;
+    }
+    // Ask Sync to just do the tabs engine if it can.
+    try {
+      lazy.log.info("Doing a tab sync.");
+      await Weave.Service.sync({ why: "tabs", engines: ["tabs"] });
+      return true;
+    } catch (ex) {
+      lazy.log.error("Sync failed", ex);
+      throw ex;
+    }
+  },
+
+  observe(subject, topic, data) {
+    lazy.log.trace(`observed topic=${topic}, data=${data}, subject=${subject}`);
+    switch (topic) {
+      case "weave:engine:sync:finish":
+        if (data != "tabs") {
+          return;
+        }
+        // The tabs engine just finished syncing
+        // Set our lastTabFetch pref here so it tracks both explicit sync calls
+        // and normally scheduled ones.
+        Preferences.set(
+          "services.sync.lastTabFetch",
+          Math.floor(Date.now() / 1000)
+        );
+        Services.obs.notifyObservers(null, TOPIC_TABS_CHANGED);
+        break;
+      case "weave:service:start-over":
+        // start-over needs to notify so consumers find no tabs.
+        Preferences.reset("services.sync.lastTabFetch");
+        Services.obs.notifyObservers(null, TOPIC_TABS_CHANGED);
+        break;
+      case "nsPref:changed":
+        Services.obs.notifyObservers(null, TOPIC_TABS_CHANGED);
+        break;
+      default:
+        break;
+    }
+  },
+
+  // Returns true if Sync is configured to Sync tabs, false otherwise
+  get isConfiguredToSyncTabs() {
+    if (!lazy.weaveXPCService.ready) {
+      lazy.log.debug("Sync isn't yet ready; assuming tab engine is enabled");
+      return true;
+    }
+
+    let engine = Weave.Service.engineManager.get("tabs");
+    return engine && engine.enabled;
+  },
+
+  get hasSyncedThisSession() {
+    let engine = Weave.Service.engineManager.get("tabs");
+    return engine && engine.hasSyncedThisSession;
+  },
+};
+
+Services.obs.addObserver(SyncedTabsInternal, "weave:engine:sync:finish");
+Services.obs.addObserver(SyncedTabsInternal, "weave:service:start-over");
+// Observe the pref the indicates the state of the tabs engine has changed.
+// This will force consumers to re-evaluate the state of sync and update
+// accordingly.
+Services.prefs.addObserver("services.sync.engine.tabs", SyncedTabsInternal);
+
+// The public interface.
+var SyncedTabs = {
+  // A mock-point for tests.
+  _internal: SyncedTabsInternal,
+
+  // We make the topic for the observer notification public.
+  TOPIC_TABS_CHANGED,
+
+  // Expose the interval used to determine if synced tabs data needs a new sync
+  TABS_FRESH_ENOUGH_INTERVAL_SECONDS,
+
+  // Returns true if Sync is configured to Sync tabs, false otherwise
+  get isConfiguredToSyncTabs() {
+    return this._internal.isConfiguredToSyncTabs;
+  },
+
+  // Returns true if a tab sync has completed once this session. If this
+  // returns false, then getting back no clients/tabs possibly just means we
+  // are waiting for that first sync to complete.
+  get hasSyncedThisSession() {
+    return this._internal.hasSyncedThisSession;
+  },
+
+  // Return a promise that resolves with an array of client records, each with
+  // a .tabs array. Note that part of the contract for this module is that the
+  // returned objects are not shared between invocations, so callers are free
+  // to mutate the returned objects (eg, sort, truncate) however they see fit.
+  getTabClients(query) {
+    return this._internal.getTabClients(query);
+  },
+
+  // Starts a background request to start syncing tabs. Returns a promise that
+  // resolves when the sync is complete, but there's no resolved value -
+  // callers should be listening for TOPIC_TABS_CHANGED.
+  // If |force| is true we always sync. If false, we only sync if the most
+  // recent sync wasn't "recently".
+  syncTabs(force) {
+    return this._internal.syncTabs(force);
+  },
+
+  sortTabClientsByLastUsed(clients) {
+    // First sort the list of tabs for each client. Note that
+    // this module promises that the objects it returns are never
+    // shared, so we are free to mutate those objects directly.
+    for (let client of clients) {
+      let tabs = client.tabs;
+      tabs.sort((a, b) => b.lastUsed - a.lastUsed);
+    }
+    // Now sort the clients - the clients are sorted in the order of the
+    // most recent tab for that client (ie, it is important the tabs for
+    // each client are already sorted.)
+    clients.sort((a, b) => {
+      if (!a.tabs.length) {
+        return 1; // b comes first.
+      }
+      if (!b.tabs.length) {
+        return -1; // a comes first.
+      }
+      return b.tabs[0].lastUsed - a.tabs[0].lastUsed;
+    });
+  },
+
+  recordSyncedTabsTelemetry(object, tabEvent, extraOptions) {
+    Services.telemetry.setEventRecordingEnabled("synced_tabs", true);
+    Services.telemetry.recordEvent(
+      "synced_tabs",
+      tabEvent,
+      object,
+      null,
+      extraOptions
+    );
+  },
+
+  async getRecentTabs(maxCount) {
+    let tabs = [];
+    let clients = await this.getTabClients();
+
+    for (let client of clients) {
+      for (let tab of client.tabs) {
+        tab.device = client.name;
+        tab.deviceType = client.clientType;
+      }
+      tabs = [...tabs, ...client.tabs.reverse()];
+    }
+    tabs = tabs.sort((a, b) => b.lastUsed - a.lastUsed).slice(0, maxCount);
+    return tabs;
+  },
+};