[Feature] Full-Page Caching for 404s

src/alley/wp/alleyvate/features/class-full-page-cache-404.php

@@ -0,0 +1,243 @@
+<?php
+/**
+ * Class file for Full Page Cache for 404s
+ *
+ * (c) Alley <[email protected]>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ *
+ * @package wp-alleyvate
+ */
+
+namespace Alley\WP\Alleyvate\Features;
+
+use Alley\WP\Alleyvate\Feature;
+
+/**
+ * Full Page Cache for 404s.
+ */
+final class Full_Page_Cache_404 implements Feature {
+
+	/**
+	 * Cache group.
+	 *
+	 * @var string
+	 */
+	const CACHE_GROUP = 'alleyvate';
+
+	/**
+	 * Cache key.
+	 *
+	 * @var string
+	 */
+	const CACHE_KEY = 'alleyvate_404_cache';
+
+	/**
+	 * Cache key for stale cache.
+	 *
+	 * @var string
+	 */
+	const STALE_CACHE_KEY = 'alleyvate_404_cache_stale';
+
+	/**
+	 * Cache time.
+	 *
+	 * @var int
+	 */
+	const CACHE_TIME = HOUR_IN_SECONDS;
+
+	/**
+	 * Stale cache time.
+	 *
+	 * @var int
+	 */
+	const STALE_CACHE_TIME = DAY_IN_SECONDS;
+
+	/**
+	 * Guaranteed 404 URI.
+	 * Used for populating the cache.
+	 */
+	const GUARANTEED_404_URI = '/wp-alleyvate-this-is-a-404-page';
+
+	/**
+	 * Boot the feature.
+	 */
+	public function boot(): void {
+
+		// Return 404 page cache on template_redirect.
+		add_action( 'template_redirect', [ $this, 'action__template_redirect' ], 9999 );
+
+		// Add HTTP header for debugging.
+		add_action( 'send_headers', [ $this, 'action__send_headers' ] );
+
+		// Force the Guaranteed 404 page to be a 404, because this is the page we will cache.
+		add_action( 'pre_get_posts', [ $this, 'action__pre_get_posts' ] );
+
+		// For the Guaranteed 404 page, hook in on WP to start output buffering, to capture the HTML.
+		add_action( 'wp', [ $this, 'action__wp' ] );
+
+		// Replenish the cache every hour.
+		if ( ! wp_next_scheduled( 'alleyvate_404_cache' ) ) {
+			wp_schedule_event( time(), 'hourly', 'alleyvate_404_cache' );
+		}
+		// Callback for Cron Event.
+		add_action( 'alleyvate_404_cache', [ $this, 'trigger_404_page_cache' ] );
+		add_action( 'alleyvate_404_cache_single', [ $this, 'trigger_404_page_cache' ] );
+	}
+
+	/**
+	 * Get 404 Page Cache and return early if found.
+	 */
+	public function action__template_redirect() {
+
+		// Allow 404s for the Admin.
+		if ( is_admin() ) {
+			return;
+		}
+
+		// Don't cache if not a 404.
+		if ( ! is_404() ) {
+			return;
+		}
+
+		// Allow this URL through, as this request will populate the cache.
+		if ( isset( $_SERVER['REQUEST_URI'] ) && self::GUARANTEED_404_URI === $_SERVER['REQUEST_URI'] ) {
+			return;
+		}
+
+		$cache = self::get_cache();
+
+		if ( false === $cache ) {
+			$cache = self::get_stale_cache();
+		}
+		if ( $cache ) {
+			// Cached content is already escaped.
+			echo $cache; // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
+			exit;
+		} else {
+			// Schedule a single event to generate the cache immediately.
+			if ( ! wp_next_scheduled( 'alleyvate_404_cache_single' ) ) {
+				wp_schedule_single_event( time(), 'alleyvate_404_cache_single' );
+			}
+			// If no cache, return an empty string.
+			echo '';
+			exit;
+		}
+	}
+
+	/**
+	 * Send Headers.
+	 */
+	public function action__send_headers() {
+		if ( ! is_404() ) {
+			return;
+		}
+		if ( isset( $_SERVER['REQUEST_URI'] ) && self::GUARANTEED_404_URI === $_SERVER['REQUEST_URI'] ) {
+			return;
+		}
+		if ( self::get_cache() ) {
+			header( 'X-Alleyvate-404-Cache: HIT' );
+		} elseif ( self::get_stale_cache() ) {
+			header( 'X-Alleyvate-404-Cache: HIT (stale)' );
+		} else {
+			header( 'X-Alleyvate-404-Cache: MISS' );
+		}
+	}
+
+	/**
+	 * Ensure that the 404 page is always a 404.
+	 * We cache this page, so need to make sure it's always a 404.
+	 *
+	 * @param \WP_Query $query WP Query.
+	 */
+	public function action__pre_get_posts( $query ) {
+		if ( isset( $_SERVER['REQUEST_URI'] ) && self::GUARANTEED_404_URI === $_SERVER['REQUEST_URI'] ) {
+			global $wp_query;
+			$wp_query->set_404();
+		}
+	}
+
+
+	/**
+	 * Start output buffering, so we can cache the 404 page.
+	 */
+	public function action__wp() {
+		if ( isset( $_SERVER['REQUEST_URI'] ) && self::GUARANTEED_404_URI === $_SERVER['REQUEST_URI'] ) {
+			ob_start( [ self::class, 'finish_output_buffering' ] );
+		}
+	}
+
+	/**
+	 * Finish output buffering.
+	 *
+	 * @param string $buffer Buffer.
+	 */
+	public function finish_output_buffering( $buffer ) {
+		global $wp_query;
+		if ( ! $wp_query->is_404() ) {
+			return $buffer;
+		}
+		if ( ! $this->get_cache() ) {
+			self::set_cache( $buffer );
+		}
+		return $buffer;
+	}
+
+	/**
+	 * Get cache.
+	 *
+	 * @return mixed
+	 */
+	public function get_cache(): mixed {
+		return wp_cache_get( self::CACHE_KEY, self::CACHE_GROUP );
+	}
+
+	/**
+	 * Get stale cache.
+	 *
+	 * @return mixed
+	 */
+	public function get_stale_cache() {
+		return wp_cache_get( self::STALE_CACHE_KEY, self::CACHE_GROUP );
+	}
+
+	/**
+	 * Set cache.
+	 *
+	 * @param string $buffer The Output Buffer.
+	 *
+	 * @return void
+	 */
+	public function set_cache( string $buffer ): void {
+		wp_cache_set( self::CACHE_KEY, $buffer, self::CACHE_GROUP, self::CACHE_TIME ); // phpcs:ignore WordPressVIPMinimum.Performance.LowExpiryCacheTime.CacheTimeUndetermined
+		wp_cache_set( self::STALE_CACHE_KEY, $buffer, self::CACHE_GROUP, self::STALE_CACHE_TIME ); // phpcs:ignore WordPressVIPMinimum.Performance.LowExpiryCacheTime.CacheTimeUndetermined
+	}
+
+	/**
+	 * Delete cache.
+	 */
+	public function delete_cache() {
+		wp_cache_delete( self::CACHE_KEY, self::CACHE_GROUP );
+		wp_cache_delete( self::STALE_CACHE_KEY, self::CACHE_GROUP );
+	}
+
+	/**
+	 * Populate cache.
+	 */
+	public function populate_cache() {
+		ob_start( [ self::class, 'finish_output_buffering' ] );
+	}
+
+	/**
+	 * Spin up a request to the guaranteed 404 page to populate the cache.
+	 */
+	public function trigger_404_page_cache() {
+		$url = home_url( self::GUARANTEED_404_URI, 'https' );
+		// replace http with https to ensure the styles don't get blocked due to insecure content.
+		$url = str_replace( 'http://', 'https://', $url );
+
+		// This request will populate the cache using output buffering.
+		wpcom_vip_file_get_contents( $url );
+	}
+}

src/alley/wp/alleyvate/load.php

@@ -35,6 +35,7 @@ function load(): void {
 		'disallow_file_edit'            => new Features\Disallow_File_Edit(),
 		'redirect_guess_shortcircuit'   => new Features\Redirect_Guess_Shortcircuit(),
 		'user_enumeration_restrictions' => new Features\User_Enumeration_Restrictions(),
+		'full_page_cache_404'           => new Features\Full_Page_Cache_404(),
 	];
 
 	foreach ( $features as $handle => $feature ) {

tests/alley/wp/alleyvate/features/test-full-page-cache-404.php

@@ -0,0 +1,96 @@
+<?php
+/**
+ * Class file for Test_Disable_Comments
+ *
+ * (c) Alley <[email protected]>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ *
+ * @package wp-alleyvate
+ */
+
+namespace Alley\WP\Alleyvate\Features;
+
+use Alley\WP\Alleyvate\Feature;
+use Mantle\Testkit\Test_Case;
+
+/**
+ * Tests for fully disabling comment functionality.
+ */
+final class Test_Full_Page_Cache_404 extends Test_Case {
+	use \Mantle\Testing\Concerns\Admin_Screen;
+
+	/**
+	 * Feature instance.
+	 *
+	 * @var Feature
+	 */
+	private Feature $feature;
+
+	/**
+	 * Set up.
+	 */
+	protected function setUp(): void {
+		parent::setUp();
+		$this->feature = new Full_Page_Cache_404();
+	}
+
+	/**
+	 * Test full page cache 404.
+	 */
+	public function test_full_page_cache_404_returns_cache() {
+		$this->feature->boot();
+		$response = $this->get( '/this-is-a-404-page' );
+
+		// Expect empty string if cache isn't set.
+		$response->assertNoContent( 404 );
+
+		// Expect cron job to be scheduled.
+		$this->assertTrue( wp_next_scheduled( 'alleyvate_404_cache_single' ) > 0 );
+
+		$this->set_404_cache();
+
+		// Expect the cache to be returned.
+		$response = $this->get( '/this-is-a-404-page' );
+		$response->assertSee( $this->get_404_html() );
+		$response->assertStatus( 404 );
+	}
+
+	/**
+	 * Test that a post request returns the correct content.
+	 */
+	public function test_full_page_cache_not_returned_for_non_404() {
+		$this->feature->boot();
+		$post_id  = self::factory()->post->create( [ 'post_title' => 'Hello World' ] );
+		$response = $this->get( get_the_permalink( $post_id ) );
+		$response->assertHeaderMissing( 'X-Alleyvate-404-Cache' );
+		$response->assertSee( 'Hello World' );
+	}
+
+	/**
+	 * Set the cache.
+	 */
+	private function set_404_cache() {
+		$html = $this->get_404_html();
+		$this->feature->set_cache( $html );
+	}
+
+	/**
+	 * Get the 404 HTML.
+	 *
+	 * @return string
+	 */
+	private function get_404_html() {
+		return '<html><head></head><body><h1>404 Not Found</h1><p>The requested URL was not found on this server.</p></body></html>';
+	}
+
+	/**
+	 * Tear down.
+	 */
+	public function tearDown(): void {
+		$this->feature->delete_cache();
+		parent::tearDown();
+	}
+
+}