Preload the ua-update.json file earlier than the first http request.

  Since bug 1363421 the initialization of UserAgentOverrides was delayed
  to improve startup performance. Initially it was moved to nsHttpHandler::Init(),
  but then due to
    https://bugzilla.mozilla.org/show_bug.cgi?id=1363421#c22
  and
    https://bugzilla.mozilla.org/show_bug.cgi?id=1363421#c31
  it was delayed even more -- until the first nsHttpChannel is created.

  UserAgentOverrides.init() calls UserAgentUpdates.init(), which then reads
  ua-update.json file from the disk asynchronously. This can lead to a situation
  when UserAgentUpdates.init() remains unfinished at the time of the first
  http request.

  To avoid this, pre-run the file reading part of the code earlier, just where
  UserAgentOverrides.init() was placed previously (before the bug 1363421).
  This does not affect startup performance, but the file has time to be loaded
  by the time of the first http request.


diff -Nrup mozilla-OLD/comm/suite/components/nsSuiteGlue.js mozilla/comm/suite/components/nsSuiteGlue.js
--- mozilla-OLD/comm/suite/components/nsSuiteGlue.js	2021-10-26 19:51:22.000000000 +0300
+++ mozilla/comm/suite/components/nsSuiteGlue.js	2022-02-15 01:24:51.769270135 +0300
@@ -29,6 +29,7 @@ XPCOMUtils.defineLazyModuleGetters(this,
   Integration: "resource://gre/modules/Integration.jsm",
   PermissionUI: "resource:///modules/PermissionUI.jsm",
   AppConstants: "resource://gre/modules/AppConstants.jsm",
+  UserAgentUpdates: "resource://gre/modules/UserAgentUpdates.jsm",
 });
 
 XPCOMUtils.defineLazyGetter(this, "DebuggerServer", () => {
@@ -380,6 +381,8 @@ SuiteGlue.prototype = {
 
     Sanitizer.onStartup();
 
+    UserAgentUpdates.preload();
+
     var timer = Cc["@mozilla.org/timer;1"]
                   .createInstance(Ci.nsITimer);
     timer.init(this, 3000, timer.TYPE_ONE_SHOT);
diff -Nrup mozilla-OLD/netwerk/protocol/http/UserAgentUpdates.jsm mozilla/netwerk/protocol/http/UserAgentUpdates.jsm
--- mozilla-OLD/netwerk/protocol/http/UserAgentUpdates.jsm	2020-02-18 02:38:00.000000000 +0300
+++ mozilla/netwerk/protocol/http/UserAgentUpdates.jsm	2022-02-15 01:22:59.529029299 +0300
@@ -58,6 +58,7 @@ const PREF_APP_DISTRIBUTION = "distribut
 const PREF_APP_DISTRIBUTION_VERSION = "distribution.version";
 
 var gInitialized = false;
+var gPreloadData = null;
 
 function readChannel(url) {
   return new Promise((resolve, reject) => {
@@ -84,6 +85,16 @@ function readChannel(url) {
 }
 
 var UserAgentUpdates = {
+  _preload_callback(update) {
+    gPreloadData = update;
+  },
+
+  preload: function() {
+    this._callback = this._preload_callback;
+    this._lastUpdated = 0;
+    this._applySavedUpdate();
+  },
+
   init: function(callback) {
     if (gInitialized) {
       return;
@@ -92,7 +103,12 @@ this.UserAgentUpdates = {
 
     this._callback = callback;
     this._lastUpdated = 0;
-    this._applySavedUpdate();
+    if (gPreloadData) {
+      this._applyUpdate(gPreloadData);
+      gPreloadData = null;
+    } else {
+      this._applySavedUpdate();
+    }
 
     Services.prefs.addObserver(PREF_UPDATES, this);
   },