Closes #3423 Add preconnect tag for CDN URLs (#3463)

* Add add_preconnect_cdn() method

* Add integration test draft and notes.

* Implement preconnect for non-crossorigin cdn-urls

* Add fixture, adjust integration test

* Revise forcorrect cdn urls, use crossorigin second

* Run phpcbf

* Update CDN preconnect testcases

* Use agnostic scheme when not provided

* Rerun phpcs

* Clean dns-prefetch out of testcases :)

* Add testcase/solution for `test/tests`

inc/Engine/CDN/Subscriber.php

@@ -28,7 +28,7 @@ class Subscriber implements Subscriber_Interface {
 	 * Constructor
 	 *
 	 * @param Options_Data $options WP Rocket Options instance.
-	 * @param CDN          $cdn CDN instance.
+	 * @param CDN          $cdn     CDN instance.
 	 */
 	public function __construct( Options_Data $options, CDN $cdn ) {
 		$this->options = $options;
@@ -55,6 +55,7 @@ public static function get_subscribed_events() {
 			'rocket_css_url'          => [ 'add_cdn_url', 10, 2 ],
 			'rocket_js_url'           => [ 'add_cdn_url', 10, 2 ],
 			'rocket_asset_url'        => [ 'maybe_replace_url', 10, 2 ],
+			'wp_resource_hints'       => [ 'add_preconnect_cdn', 10, 2 ],
 		];
 	}
 
@@ -64,6 +65,7 @@ public static function get_subscribed_events() {
 	 * @since 3.4
 	 *
 	 * @param string $html HTML content.
+	 *
 	 * @return string
 	 */
 	public function rewrite( $html ) {
@@ -80,6 +82,7 @@ public function rewrite( $html ) {
 	 * @since 3.4.0.4
 	 *
 	 * @param string $html HTML content.
+	 *
 	 * @return string
 	 */
 	public function rewrite_srcset( $html ) {
@@ -96,6 +99,7 @@ public function rewrite_srcset( $html ) {
 	 * @since 3.4
 	 *
 	 * @param string $content CSS content.
+	 *
 	 * @return string
 	 */
 	public function rewrite_css_properties( $content ) {
@@ -155,6 +159,7 @@ public function get_cdn_hosts( array $hosts = [], array $zones = [ 'all' ] ) {
 	 * @since 3.4
 	 *
 	 * @param array $domains Domain names to DNS prefetch.
+	 *
 	 * @return array
 	 */
 	public function add_dns_prefetch_cdn( $domains ) {
@@ -176,8 +181,9 @@ public function add_dns_prefetch_cdn( $domains ) {
 	 *
 	 * @since 3.4
 	 *
-	 * @param string $url URL to rewrite.
+	 * @param string $url          URL to rewrite.
 	 * @param string $original_url Original URL for this URL. Optional.
+	 *
 	 * @return string
 	 */
 	public function add_cdn_url( $url, $original_url = '' ) {
@@ -195,8 +201,9 @@ public function add_cdn_url( $url, $original_url = '' ) {
 	 *
 	 * @since 3.5.3
 	 *
-	 * @param string $url URL of the asset.
+	 * @param string $url   URL of the asset.
 	 * @param array  $zones Array of corresponding zones for the asset.
+	 *
 	 * @return string
 	 */
 	public function maybe_replace_url( $url, array $zones = [ 'all' ] ) {
@@ -241,6 +248,69 @@ public function maybe_replace_url( $url, array $zones = [ 'all' ] ) {
 		return $url;
 	}
 
+	/**
+	 * Add a preconnect tag for the CDN.
+	 *
+	 * @since 3.8.3
+	 *
+	 * @param array  $urls          The initial array of wp_resource_hint urls.
+	 * @param string $relation_type The relation type for the hint: eg., 'preconnect', 'prerender', etc.
+	 *
+	 * @return array The filtered urls.
+	 */
+	public function add_preconnect_cdn( array $urls, string $relation_type ): array {
+		if (
+			'preconnect' !== $relation_type
+			||
+			rocket_bypass()
+			||
+			! $this->is_allowed()
+			||
+			! $this->is_cdn_enabled()
+		) {
+			return $urls;
+		}
+
+		$cdn_urls = $this->cdn->get_cdn_urls( [ 'all', 'images', 'css_and_js', 'css', 'js' ] );
+
+		if ( empty( $cdn_urls ) ) {
+			return $urls;
+		}
+
+		foreach ( $cdn_urls as $url ) {
+			$url_parts = get_rocket_parse_url( $url );
+
+			if ( empty( $url_parts['scheme'] ) ) {
+				if ( preg_match( '/^(?![\/])(?=[^\.]+\/).+/i', $url ) ) {
+					continue;
+				}
+
+				$url       = '//' . $url;
+				$url_parts = get_rocket_parse_url( $url );
+			}
+
+			$domain = empty( $url_parts['scheme'] )
+				? '//' . $url_parts['host']
+				: $url_parts['scheme'] . '://' . $url_parts['host'];
+
+			// Note: As of 22 Feb, 2021 we cannot add more than one instance of a domain url
+			// on the wp_resource_hint() hook -- wp_resource_hint() will
+			// only actually print the first one.
+			// Ideally, we want both because CSS resources will use the crossorigin version,
+			// But JS resources will not.
+			// Jonathan has submitted a ticket to change this behavior:
+			// @see https://core.trac.wordpress.org/ticket/52465
+			// Until then, we order these to prefer/print the non-crossorigin version.
+			$urls[] = [ 'href' => $domain ];
+			$urls[] = [
+				'href'        => $domain,
+				'crossorigin' => 'anonymous',
+			];
+		}
+
+		return $urls;
+	}
+
 	/**
 	 * Checks if CDN can be applied
 	 *

tests/Fixtures/inc/Engine/CDN/Subscriber/add-preconnect-cdn.php

@@ -0,0 +1,23 @@
+<?php
+
+return [
+	'shouldAddPreconnectLinkForCdn' => [
+		'cdn-cnames'    => [
+			'123456.rocketcdn.me',
+			'https://my-cdn.cdnservice.com/',
+			'http://cdn.example.com',
+			'/some/path/with/no/domain',
+			'test/tests',
+			'8901.wicked-fast-cdn.com/path/to/my/files',
+			'https://another.cdn.com/with/a/path',
+		],
+		'expected-html' => <<<HTML
+<link rel='dns-prefetch' href='//s.w.org' />
+<link href='//123456.rocketcdn.me' rel='preconnect' />
+<link href='https://my-cdn.cdnservice.com' rel='preconnect' />
+<link href='http://cdn.example.com' rel='preconnect' />
+<link href='//8901.wicked-fast-cdn.com' rel='preconnect' />
+<link href='https://another.cdn.com' rel='preconnect' />
+HTML
+	],
+];

tests/Integration/inc/Engine/CDN/Subscriber/TestCase.php

@@ -2,7 +2,7 @@
 
 namespace WP_Rocket\Tests\Integration\inc\Engine\CDN\Subscriber;
 
-use WPMedia\PHPUnit\Integration\TestCase as BaseTestCase;
+use WP_Rocket\Tests\Integration\TestCase as BaseTestCase;
 
 abstract class TestCase extends BaseTestCase {
 	protected $cnames;

tests/Integration/inc/Engine/CDN/Subscriber/addPreconnectCdn.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace WP_Rocket\Tests\Integration\inc\Engine\CDN\Subscriber;
+
+/**
+ * @covers \WP_Rocket\Engine\CDN\Subscriber::add_preconnect_cdn
+ * @uses   \WP_Rocket\Engine\CDN\CDN::get_cdn_urls
+ * @group  CDN
+ */
+class Test_addPreconnectCdn extends TestCase {
+
+	public function setUp() {
+		$this->unregisterAllCallbacksExcept( 'wp_resource_hints', 'add_preconnect_cdn', 10 );
+
+		parent::setUp();
+	}
+
+	public function tearDown() {
+		$this->restoreWpFilter( 'wp_resource_hints' );
+
+		parent::tearDown();
+	}
+
+	/**
+	 * @dataProvider providerTestData
+	 */
+	public function testShouldAddPreconnectCdn($cnames, $expected) {
+		$this->cnames = $cnames;
+
+		add_filter( 'pre_get_rocket_option_cdn', [ $this, 'return_true'] );
+		add_filter( 'rocket_cdn_cnames', [ $this, 'setCnames' ] );
+
+		ob_start();
+		wp_resource_hints();
+
+		$this->assertSame(
+			$this->format_the_html($expected),
+			$this->format_the_html(ob_get_clean())
+		);
+	}
+
+	public function providerTestData() {
+		return $this->getTestData( __DIR__, 'add-preconnect-cdn' );
+	}
+}