From 90a603efefe3629796d8bd20a5be214e96d2823c Mon Sep 17 00:00:00 2001
From: Timo Tijhof <krinklemail@gmail.com>
Date: Mon, 17 Jul 2017 21:36:25 -0500
Subject: [PATCH] resourceloader: Preload base modules request from startup
 module

This allows browsers that support W3C Preload <https://www.w3.org/TR/preload/>,
to start downloading and parsing the second load.php JavaScript request
as soon as the headers of the startup module arrive.

Before:
- HTML request, response start.
|\- Discover <script src async> for startup.js, request, response start.
| \ - Startup response end, JS parsing.
|  |
|__|- HTML parsing and rendering yields for async JS execution.
   \- JavaScript inserts <script>, Base-modules request starts. <<<

After:
- HTML request, response start.
|\- Discover <script src async> for startup.js, request, response start
  \ - Base-modules request starts. <<<
|  |- Startup response end, JS parsing.
|  |
|__|- HTML parsing and rendering yields for async JS execution.
   \- JavaScript inserts <script>. (Base-modules req on-going, or done)

On local testing with "Slow 3G" network throttling in Chrome,
this reduced page load time (window.onload, time to interaction for JS)
by about 1 second (from 14s to 13s).
See also https://phabricator.wikimedia.org/T164299#3572231

Bug: T164299
Change-Id: I7047f4ab881947cf3392256087cc5a0cb177dd3a
---
 .../resourceloader/ResourceLoaderStartUpModule.php    | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/includes/resourceloader/ResourceLoaderStartUpModule.php b/includes/resourceloader/ResourceLoaderStartUpModule.php
index 8973fe31c3..d535ffcd3a 100644
--- a/includes/resourceloader/ResourceLoaderStartUpModule.php
+++ b/includes/resourceloader/ResourceLoaderStartUpModule.php
@@ -296,6 +296,16 @@ class ResourceLoaderStartUpModule extends ResourceLoaderModule {
 		return true;
 	}
 
+	/**
+	 * @return array
+	 */
+	public function getPreloadLinks( ResourceLoaderContext $context ) {
+		$url = self::getStartupModulesUrl( $context );
+		return [
+			$url => [ 'as' => 'script' ]
+		];
+	}
+
 	/**
 	 * Base modules required for the base environment of ResourceLoader
 	 *
@@ -359,6 +369,7 @@ class ResourceLoaderStartUpModule extends ResourceLoaderModule {
 		}, [
 			'$VARS.wgLegacyJavaScriptGlobals' => $this->getConfig()->get( 'LegacyJavaScriptGlobals' ),
 			'$VARS.configuration' => $this->getConfigSettings( $context ),
+			// This url may be preloaded. See getPreloadLinks().
 			'$VARS.baseModulesUri' => self::getStartupModulesUrl( $context ),
 		] );
 		$pairs['$CODE.registrations()'] = str_replace(
-- 
2.20.1