Handle general form posts

amp.php

@@ -106,7 +106,6 @@ function amp_after_setup_theme() {
  * @global string $pagenow
  */
 function amp_init() {
-	global $pagenow;
 
 	/**
 	 * Triggers on init when AMP plugin is active.
@@ -128,9 +127,7 @@ function amp_init() {
 	if ( class_exists( 'Jetpack' ) && ! ( defined( 'IS_WPCOM' ) && IS_WPCOM ) ) {
 		require_once AMP__DIR__ . '/jetpack-helper.php';
 	}
-	if ( isset( $pagenow ) && 'wp-comments-post.php' === $pagenow ) {
-		amp_prepare_comment_post();
-	}
+	amp_handle_xhr_request();
 }
 
 // Make sure the `amp` query var has an explicit value.

includes/amp-helper-functions.php

@@ -400,23 +400,36 @@ function amp_print_schemaorg_metadata() {
 }
 
 /**
- * Hook into a comment submission of an AMP XHR post request.
- *
- * This only runs on wp-comments-post.php.
+ * Hook into a form submissions, such as comment the form or some other .
  *
  * @since 0.7.0
+ * @global string $pagenow
  */
-function amp_prepare_comment_post() {
+function amp_handle_xhr_request() {
+	global $pagenow;
 	if ( ! isset( $_GET['__amp_source_origin'] ) ) { // WPCS: CSRF ok. Beware of AMP_Theme_Support::purge_amp_query_vars().
 		return;
 	}
 
-	// Add amp comment hooks.
-	add_filter( 'comment_post_redirect', function() {
+	if ( isset( $pagenow ) && 'wp-comments-post.php' === $pagenow ) {
 		// We don't need any data, so just send a success.
-		wp_send_json_success();
-	}, PHP_INT_MAX, 2 );
+		add_filter( 'comment_post_redirect', function() {
+			// We don't need any data, so just send a success.
+			wp_send_json_success();
+		}, PHP_INT_MAX, 2 );
+		amp_handle_xhr_headers_output();
+	} elseif ( isset( $_GET['_wp_amp_action_xhr_converted'] ) ) { // WPCS: CSRF ok.
+		add_filter( 'wp_redirect', 'amp_intercept_post_request_redirect', PHP_INT_MAX, 2 );
+		amp_handle_xhr_headers_output();
+	}
+}
 
+/**
+ * Handle the AMP XHR headers and output errors.
+ *
+ * @since 0.7.0
+ */
+function amp_handle_xhr_headers_output() {
 	// Add die handler for AMP error display.
 	add_filter( 'wp_die_handler', function() {
 		/**
@@ -429,12 +442,50 @@ function amp_prepare_comment_post() {
 			if ( is_wp_error( $error ) ) {
 				$error = $error->get_error_message();
 			}
-			$error = strip_tags( $error, 'strong' );
-			wp_send_json( compact( 'error' ) );
+			$amp_mustache_allowed_html_tags = array( 'strong', 'b', 'em', 'i', 'u', 's', 'small', 'mark', 'del', 'ins', 'sup', 'sub' );
+			wp_send_json( array(
+				'error' => wp_kses( $error, array_fill_keys( $amp_mustache_allowed_html_tags, array() ) ),
+			) );
 		};
 	} );
 
 	// Send AMP header.
 	$origin = esc_url_raw( wp_unslash( $_GET['__amp_source_origin'] ) ); // WPCS: CSRF ok.
 	header( 'AMP-Access-Control-Allow-Source-Origin: ' . $origin, true );
 }
+
+/**
+ * Intercept the response to a non-comment POST request.
+ *
+ * @since 0.7.0
+ * @param string $location The location to redirect to.
+ */
+function amp_intercept_post_request_redirect( $location ) {
+
+	// Make sure relative redirects get made absolute.
+	$parsed_location = array_merge(
+		array(
+			'scheme' => 'https',
+			'host'   => wp_parse_url( home_url(), PHP_URL_HOST ),
+			'path'   => strtok( wp_unslash( $_SERVER['REQUEST_URI'] ), '?' ),
+		),
+		wp_parse_url( $location )
+	);
+
+	$absolute_location = $parsed_location['scheme'] . '://' . $parsed_location['host'];
+	if ( isset( $parsed_location['port'] ) ) {
+		$absolute_location .= ':' . $parsed_location['port'];
+	}
+	$absolute_location .= $parsed_location['path'];
+	if ( isset( $parsed_location['query'] ) ) {
+		$absolute_location .= '?' . $parsed_location['query'];
+	}
+	if ( isset( $parsed_location['fragment'] ) ) {
+		$absolute_location .= '#' . $parsed_location['fragment'];
+	}
+
+	header( 'AMP-Redirect-To: ' . $absolute_location );
+	header( 'Access-Control-Expose-Headers: AMP-Redirect-To' );
+	// Send json success as no data is required.
+	wp_send_json_success();
+}

includes/class-amp-theme-support.php

@@ -102,7 +102,7 @@ public static function init() {
 			self::register_paired_hooks();
 		}
 
-		self::purge_amp_query_vars(); // Note that amp_prepare_comment_post() still looks at $_GET['__amp_source_origin'].
+		self::purge_amp_query_vars(); // Note that amp_prepare_xhr_post() still looks at $_GET['__amp_source_origin'].
 		self::register_hooks();
 		self::$embed_handlers    = self::register_content_embed_handlers();
 		self::$sanitizer_classes = amp_get_content_sanitizers();
@@ -214,6 +214,7 @@ public static function register_hooks() {
 	public static function purge_amp_query_vars() {
 		$query_vars = array(
 			'__amp_source_origin',
+			'_wp_amp_action_xhr_converted',
 			'amp_latest_update_time',
 		);
 

includes/sanitizers/class-amp-form-sanitizer.php

@@ -87,7 +87,11 @@ public function sanitize() {
 			} elseif ( 'post' === $method ) {
 				$node->removeAttribute( 'action' );
 				if ( ! $xhr_action ) {
+					// record that action was converted tp action-xhr.
+					$action_url = add_query_arg( '_wp_amp_action_xhr_converted', 1, $action_url );
 					$node->setAttribute( 'action-xhr', $action_url );
+					// Append error handler if not found.
+					$this->ensure_submit_error_element( $node );
 				} elseif ( 'http://' === substr( $xhr_action, 0, 7 ) ) {
 					$node->setAttribute( 'action-xhr', substr( $xhr_action, 5 ) );
 				}
@@ -108,4 +112,30 @@ public function sanitize() {
 			}
 		}
 	}
+
+	/**
+	 * Checks if the form has an error handler else create one if not.
+	 *
+	 * @link https://www.ampproject.org/docs/reference/components/amp-form#success/error-response-rendering
+	 * @since 0.7
+	 *
+	 * @param DOMElement $form The form node to check.
+	 */
+	public function ensure_submit_error_element( $form ) {
+		$templates = $form->getElementsByTagName( 'template' );
+		for ( $i = $templates->length - 1; $i >= 0; $i-- ) {
+			if ( $templates->item( $i )->parentNode->hasAttribute( 'submit-error' ) ) {
+				return; // Found error template, do nothing.
+			}
+		}
+
+		$div      = $this->dom->createElement( 'div' );
+		$template = $this->dom->createElement( 'template' );
+		$mustache = $this->dom->createTextNode( '{{{error}}}' );
+		$div->setAttribute( 'submit-error', '' );
+		$template->setAttribute( 'type', 'amp-mustache' );
+		$template->appendChild( $mustache );
+		$div->appendChild( $template );
+		$form->appendChild( $div );
+	}
 }

tests/test-amp-form-sanitizer.php

@@ -46,7 +46,7 @@ public function get_data() {
 			),
 			'form_with_post_method_http_action_and_no_target' => array(
 				'<form method="post" action="http://example.org/example-page/"></form>',
-				'<form method="post" action-xhr="//example.org/example-page/" target="_top"></form>',
+				'<form method="post" action-xhr="//example.org/example-page/?_wp_amp_action_xhr_converted=1" target="_top"><div submit-error=""><template type="amp-mustache">{{{error}}}</template></div></form>',
 			),
 			'form_with_post_method_http_action_and_blank_target' => array(
 				'<form method="post" action-xhr="http://example.org/example-page/" target="_blank"></form>',
@@ -58,7 +58,7 @@ public function get_data() {
 			),
 			'form_with_post_method_https_action_and_custom_target' => array(
 				'<form method="post" action="https://example.org/" target="some_other_target"></form>',
-				'<form method="post" target="_blank" action-xhr="https://example.org/"></form>',
+				'<form method="post" target="_blank" action-xhr="https://example.org/?_wp_amp_action_xhr_converted=1"><div submit-error=""><template type="amp-mustache">{{{error}}}</template></div></form>',
 			),
 		);
 	}