From e7d3bce00f418b1f2bc7732aa946b7c9d7c563d0 Mon Sep 17 00:00:00 2001
From: Timo Tijhof <krinklemail@gmail.com>
Date: Thu, 9 Feb 2017 21:46:37 +0000
Subject: [PATCH] resourceloader: Use perf.now() for mediaWikiLoadStart in
 startup.js

Currently we're using 'new Date' which is less accurate for high accuracy
performance measures. On top of that, we are actually using performance.now()
in Navigation Timing to measure mediaWikiLoadEnd, and subsequently
relating it to mediaWikiLoadStart to produce mediaWikiLoadComplete.

Mixing Date and performance.now produces inaccurate results since the
two are usually not in sync. See T153819 for further details.

Solve this by moving the polyfil to startup.js instead.

Also add a basic unit test for mw.now().

Bug: T153819
Change-Id: Ib44538155aa9ba432ec4c58b09ead5333a3a942d
---
 .eslintrc.json                                   |  1 -
 resources/src/mediawiki/mediawiki.js             | 11 ++++-------
 resources/src/startup.js                         | 16 ++++++++++++----
 .../suites/resources/mediawiki/mediawiki.test.js |  9 +++++++++
 4 files changed, 25 insertions(+), 12 deletions(-)

diff --git a/.eslintrc.json b/.eslintrc.json
index 044dd7202d..98d0f10fa9 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -9,7 +9,6 @@
 		"require": false,
 		"module": false,
 		"mediaWiki": false,
-		"mwPerformance": false,
 		"OO": false
 	},
 	"rules": {
diff --git a/resources/src/mediawiki/mediawiki.js b/resources/src/mediawiki/mediawiki.js
index fceeb64c4b..4df2df7985 100644
--- a/resources/src/mediawiki/mediawiki.js
+++ b/resources/src/mediawiki/mediawiki.js
@@ -8,6 +8,7 @@
  * @singleton
  */
 
+/* global mwNow */
 /* eslint-disable no-use-before-define */
 
 ( function ( $ ) {
@@ -524,13 +525,8 @@
 		 *
 		 * @return {number} Current time
 		 */
-		now: ( function () {
-			var perf = window.performance,
-				navStart = perf && perf.timing && perf.timing.navigationStart;
-			return navStart && typeof perf.now === 'function' ?
-				function () { return navStart + perf.now(); } :
-				function () { return +new Date(); };
-		}() ),
+		now: mwNow,
+		// mwNow is defined in startup.js
 
 		/**
 		 * Format a string. Replace $1, $2 ... $N with positional arguments.
@@ -2785,6 +2781,7 @@
 			return $.when.apply( $, all );
 		} );
 		loading.then( function () {
+			/* global mwPerformance */
 			mwPerformance.mark( 'mwLoadEnd' );
 			mw.hook( 'resourceloader.loadEnd' ).fire();
 		} );
diff --git a/resources/src/startup.js b/resources/src/startup.js
index 20818d2656..deb280a8f1 100644
--- a/resources/src/startup.js
+++ b/resources/src/startup.js
@@ -6,11 +6,19 @@
 
 /* global mw, $VARS, $CODE */
 
-// eslint-disable-next-line no-unused-vars
-var mediaWikiLoadStart = ( new Date() ).getTime(),
-	mwPerformance = ( window.performance && performance.mark ) ? performance : {
+var mwPerformance = ( window.performance && performance.mark ) ? performance : {
 		mark: function () {}
-	};
+	},
+	// Define now() here to ensure valid comparison with mediaWikiLoadEnd (T153819).
+	mwNow = ( function () {
+		var perf = window.performance,
+			navStart = perf && perf.timing && perf.timing.navigationStart;
+		return navStart && typeof perf.now === 'function' ?
+			function () { return navStart + perf.now(); } :
+			function () { return +new Date(); };
+	}() ),
+	// eslint-disable-next-line no-unused-vars
+	mediaWikiLoadStart = mwNow();
 
 mwPerformance.mark( 'mwLoadStart' );
 
diff --git a/tests/qunit/suites/resources/mediawiki/mediawiki.test.js b/tests/qunit/suites/resources/mediawiki/mediawiki.test.js
index bac8274fef..65b726312c 100644
--- a/tests/qunit/suites/resources/mediawiki/mediawiki.test.js
+++ b/tests/qunit/suites/resources/mediawiki/mediawiki.test.js
@@ -64,6 +64,15 @@
 		);
 	} );
 
+	QUnit.test( 'mw.now', function ( assert ) {
+		assert.equal( typeof mw.now(), 'number', 'Return a number' );
+		assert.equal(
+			String( Math.round( mw.now() ) ).length,
+			String( +new Date() ).length,
+			'Match size of current timestamp'
+		);
+	} );
+
 	QUnit.test( 'mw.Map', function ( assert ) {
 		var arry, conf, funky, globalConf, nummy, someValues;
 
-- 
2.20.1