Improve post processor response cache

includes/class-amp-theme-support.php

@@ -26,6 +26,34 @@ class AMP_Theme_Support {
 	 */
 	const RESPONSE_CACHE_GROUP = 'amp-response';
 
+	/**
+	 * Post-processor cache effectiveness group name.
+	 *
+	 * @var string
+	 */
+	const POST_PROCESSOR_CACHE_EFFECTIVENESS_GROUP = 'post_processor_cache_effectiveness_group';
+
+	/**
+	 * Post-processor cache effectiveness key name.
+	 *
+	 * @var string
+	 */
+	const POST_PROCESSOR_CACHE_EFFECTIVENESS_KEY = 'post_processor_cache_effectiveness';
+
+	/**
+	 * Cache miss threshold for determining when to disable post-processor cache.
+	 *
+	 * @var int
+	 */
+	const CACHE_MISS_THRESHOLD = 20;
+
+	/**
+	 * Cache miss URL option name.
+	 *
+	 * @var string
+	 */
+	const CACHE_MISS_URL_OPTION = 'amp_cache_miss_url';
+
 	/**
 	 * Sanitizer classes.
 	 *
@@ -1760,6 +1788,13 @@ public static function prepare_response( $response, $args = array() ) {
 		$current_url = amp_get_current_url();
 		$ampless_url = amp_remove_endpoint( $current_url );
 
+		// When response caching is enabled, determine if it should be turned off for cache misses.
+		$caches_for_url = null;
+		if ( true === $args['enable_response_caching'] ) {
+			list( $disable_response_caching, $caches_for_url ) = self::check_for_cache_misses();
+			$args['enable_response_caching']                   = ! $disable_response_caching;
+		}
+
 		// Return cache if enabled and found.
 		$cache_response = null;
 		if ( true === $args['enable_response_caching'] ) {
@@ -1802,7 +1837,15 @@ public static function prepare_response( $response, $args = array() ) {
 				return $response_cache['body'];
 			}
 
-			$cache_response = function( $body, $validation_results ) use ( $response_cache_key ) {
+			$cache_response = function( $body, $validation_results ) use ( $response_cache_key, $caches_for_url ) {
+				$caches_for_url[] = $response_cache_key;
+				wp_cache_set(
+					AMP_Theme_Support::POST_PROCESSOR_CACHE_EFFECTIVENESS_KEY,
+					$caches_for_url,
+					AMP_Theme_Support::POST_PROCESSOR_CACHE_EFFECTIVENESS_GROUP,
+					600 // 10 minute cache.
+				);
+
 				return wp_cache_set(
 					$response_cache_key,
 					compact( 'body', 'validation_results' ),
@@ -1956,6 +1999,67 @@ public static function prepare_response( $response, $args = array() ) {
 		return $response;
 	}
 
+	/**
+	 * Check for cache misses. When found, store in an option to retain the URL.
+	 *
+	 * @since 1.0
+	 *
+	 * @return array {
+	 *     State.
+	 *
+	 *     @type bool       Flag indicating if the threshold has been exceeded.
+	 *     @type string[]   Collection of URLs.
+	 * }
+	 */
+	private static function check_for_cache_misses() {
+		// If the cache miss threshold is exceeded, return true.
+		if ( self::exceeded_cache_miss_threshold() ) {
+			return array( true, null );
+		}
+
+		// Get the cache miss URLs.
+		$cache_miss_urls = wp_cache_get( self::POST_PROCESSOR_CACHE_EFFECTIVENESS_KEY, self::POST_PROCESSOR_CACHE_EFFECTIVENESS_GROUP );
+		$cache_miss_urls = is_array( $cache_miss_urls ) ? $cache_miss_urls : array();
+
+		$exceeded_threshold = (
+			! empty( $cache_miss_urls )
+			&&
+			count( $cache_miss_urls ) >= self::CACHE_MISS_THRESHOLD
+		);
+
+		if ( ! $exceeded_threshold ) {
+			return array( $exceeded_threshold, $cache_miss_urls );
+		}
+
+		// When the threshold is exceeded, store the URL for cache miss and turn off response caching.
+		update_option( self::CACHE_MISS_URL_OPTION, amp_get_current_url() );
+		AMP_Options_Manager::update_option( 'enable_response_caching', false );
+		return array( true, null );
+	}
+
+	/**
+	 * Reset the cache miss URL option.
+	 *
+	 * @since 1.0
+	 */
+	public static function reset_cache_miss_url_option() {
+		if ( get_option( self::CACHE_MISS_URL_OPTION ) ) {
+			delete_option( self::CACHE_MISS_URL_OPTION );
+		}
+	}
+
+	/**
+	 * Checks if cache miss threshold has been exceeded.
+	 *
+	 * @since 1.0
+	 *
+	 * @return bool
+	 */
+	public static function exceeded_cache_miss_threshold() {
+		$url = get_option( self::CACHE_MISS_URL_OPTION, false );
+		return ! empty( $url );
+	}
+
 	/**
 	 * Adds 'data-amp-layout' to the allowed <img> attributes for wp_kses().
 	 *

includes/options/class-amp-options-manager.php

@@ -31,6 +31,7 @@ class AMP_Options_Manager {
 		'disable_admin_bar'       => false,
 		'all_templates_supported' => true,
 		'supported_templates'     => array( 'is_singular' ),
+		'enable_response_caching' => true,
 	);
 
 	/**
@@ -48,6 +49,7 @@ public static function register_settings() {
 
 		add_action( 'update_option_' . self::OPTION_NAME, array( __CLASS__, 'maybe_flush_rewrite_rules' ), 10, 2 );
 		add_action( 'admin_notices', array( __CLASS__, 'persistent_object_caching_notice' ) );
+		add_action( 'admin_notices', array( __CLASS__, 'render_cache_miss_notice' ) );
 	}
 
 	/**
@@ -78,6 +80,7 @@ public static function get_options() {
 		if ( empty( $options ) ) {
 			$options = array();
 		}
+		self::$defaults['enable_response_caching'] = wp_using_ext_object_cache();
 		return array_merge( self::$defaults, $options );
 	}
 
@@ -198,6 +201,16 @@ public static function validate_options( $new_options ) {
 		// Store the current version with the options so we know the format.
 		$options['version'] = AMP__VERSION;
 
+		// Handle the caching option.
+		$options['enable_response_caching'] = (
+			wp_using_ext_object_cache()
+			&&
+			! empty( $new_options['enable_response_caching'] )
+		);
+		if ( $options['enable_response_caching'] ) {
+			AMP_Theme_Support::reset_cache_miss_url_option();
+		}
+
 		return $options;
 	}
 
@@ -325,4 +338,43 @@ public static function persistent_object_caching_notice() {
 			);
 		}
 	}
+
+	/**
+	 * Render the cache miss admin notice.
+	 *
+	 * @return void
+	 */
+	public static function render_cache_miss_notice() {
+		if ( 'toplevel_page_' . self::OPTION_NAME !== get_current_screen()->id ) {
+			return;
+		}
+
+		if ( ! self::show_response_cache_disabled_notice() ) {
+			return;
+		}
+
+		printf(
+			'<div class="notice notice-warning is-dismissible"><p>%s <a href="%s">%s</a></p></div>',
+			esc_html__( "The AMP plugin's post-processor cache disabled due to the detection of highly-variable content.", 'amp' ),
+			esc_url( 'https://github.com/Automattic/amp-wp/wiki/Post-Processor-Cache' ),
+			esc_html__( 'More details', 'amp' )
+		);
+	}
+
+	/**
+	 * Show the response cache disabled notice.
+	 *
+	 * @since 1.0
+	 *
+	 * @return bool
+	 */
+	public static function show_response_cache_disabled_notice() {
+		return (
+			wp_using_ext_object_cache()
+			&&
+			! self::get_option( 'enable_response_caching' )
+			&&
+			AMP_Theme_Support::exceeded_cache_miss_threshold()
+		);
+	}
 }

includes/options/class-amp-options-menu.php

@@ -109,6 +109,19 @@ public function add_menu_items() {
 			)
 		);
 
+		if ( wp_using_ext_object_cache() ) {
+			add_settings_field(
+				'caching',
+				__( 'Caching', 'amp' ),
+				array( $this, 'render_caching' ),
+				AMP_Options_Manager::OPTION_NAME,
+				'general',
+				array(
+					'class' => 'amp-caching-field',
+				)
+			);
+		}
+
 		$submenus = array(
 			new AMP_Analytics_Options_Submenu( AMP_Options_Manager::OPTION_NAME ),
 		);
@@ -390,6 +403,34 @@ function updateFieldsetVisibility() {
 		<?php
 	}
 
+	/**
+	 * Render the caching settings section.
+	 *
+	 * @since 1.0
+	 *
+	 * @todo Change the messaging and description to be user-friendly and helpful.
+	 */
+	public function render_caching() {
+		?>
+		<fieldset>
+			<?php if ( AMP_Options_Manager::show_response_cache_disabled_notice() ) : ?>
+				<div class="notice notice-info notice-alt inline">
+					<p><?php esc_html_e( 'The post-processor cache was disabled due to detecting randomly generated content found on', 'amp' ); ?> <a href="<?php echo esc_url( get_option( AMP_Theme_Support::CACHE_MISS_URL_OPTION, '' ) ); ?>"><?php esc_html_e( 'on this web page.', 'amp' ); ?></a></p>
+					<p><?php esc_html_e( 'Randomly generated content was detected on this web page.  To avoid filling up the cache with unusable content, the AMP plugin\'s post-processor cache was automatically disabled.', 'amp' ); ?>
+						<a href="<?php echo esc_url( 'https://github.com/Automattic/amp-wp/wiki/Post-Processor-Cache' ); ?>"><?php esc_html_e( 'Read more', 'amp' ); ?></a>.</p>
+				</div>
+			<?php endif; ?>
+			<p>
+				<label for="enable_response_caching">
+					<input id="enable_response_caching" type="checkbox" name="<?php echo esc_attr( AMP_Options_Manager::OPTION_NAME . '[enable_response_caching]' ); ?>" <?php checked( AMP_Options_Manager::get_option( 'enable_response_caching' ) ); ?>>
+					<?php esc_html_e( 'Enable post-processor caching.', 'amp' ); ?>
+				</label>
+			</p>
+			<p class="description"><?php esc_html_e( 'This will enable post-processor caching to speed up processing an AMP response after WordPress renders a template.', 'amp' ); ?></p>
+		</fieldset>
+		<?php
+	}
+
 	/**
 	 * List template conditional options.
 	 *