Bootstrap/Load: Introduce fatal error handler.

This changeset introduces a `WP_Fatal_Error_Handler` class that detects fatal errors and displays a more user-friendly message about the site experiencing technical difficulties.

Websites that have custom requirements in that regard can implement their own fatal error handler by adding a `fatal-error-handler.php` drop-in that returns the handler instance to use, which must be based on a class that inherits `WP_Fatal_Error_Handler`. That handler will then be used in place of the default one. Alternatively, the fatal error handler feature can be completely disable through a constant `WP_DISABLE_FATAL_ERROR_HANDLER`.

Websites that would like to modify specifically the error template displayed in the frontend can add a `php-error.php` drop-in that works similarly to the existing `db-error.php` drop-in. For more granular customization, the fatal error handler also includes new filters `wp_should_handle_php_error`, `wp_php_error_message` and `wp_php_error_args`.

Props afragen, bradleyt, flixos90, ocean90, schlessera, SergeyBiryukov, spacedmonkey, timothyblynjacobs.
See #46130, #44458.


git-svn-id: https://develop.svn.wordpress.org/trunk@44962 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-fatal-error-handler.php

@@ -0,0 +1,185 @@
+<?php
+/**
+ * Error Protection API: WP_Fatal_Error_Handler class
+ *
+ * @package WordPress
+ * @since   5.2.0
+ */
+
+/**
+ * Core class used as the default shutdown handler for fatal errors.
+ *
+ * A drop-in 'fatal-error-handler.php' can be used to override the instance of this class and use a custom
+ * implementation for the fatal error handler that WordPress registers. The custom class should extend this class and
+ * can override its methods individually as necessary. The file must return the instance of the class that should be
+ * registered.
+ *
+ * @since 5.2.0
+ */
+class WP_Fatal_Error_Handler {
+
+	/**
+	 * Runs the shutdown handler.
+	 *
+	 * This method is registered via `register_shutdown_function()`.
+	 *
+	 * @since 5.2.0
+	 */
+	public function handle() {
+		// Bail if WordPress executed successfully.
+		if ( defined( 'WP_EXECUTION_SUCCEEDED' ) && WP_EXECUTION_SUCCEEDED ) {
+			return;
+		}
+
+		try {
+			// Bail if no error found.
+			$error = $this->detect_error();
+			if ( ! $error ) {
+				return;
+			}
+
+			// Display the PHP error template.
+			$this->display_error_template();
+		} catch ( Exception $e ) {
+			// Catch exceptions and remain silent.
+		}
+	}
+
+	/**
+	 * Detects the error causing the crash if it should be handled.
+	 *
+	 * @since 5.2.0
+	 *
+	 * @return array|null Error that was triggered, or null if no error received or if the error should not be handled.
+	 */
+	protected function detect_error() {
+		$error = error_get_last();
+
+		// No error, just skip the error handling code.
+		if ( null === $error ) {
+			return null;
+		}
+
+		// Bail if this error should not be handled.
+		if ( ! $this->should_handle_error( $error ) ) {
+			return null;
+		}
+
+		return $error;
+	}
+
+	/**
+	 * Determines whether we are dealing with an error that WordPress should handle
+	 * in order to protect the admin backend against WSODs.
+	 *
+	 * @since 5.2.0
+	 *
+	 * @param array $error Error information retrieved from error_get_last().
+	 * @return bool Whether WordPress should handle this error.
+	 */
+	protected function should_handle_error( $error ) {
+		$error_types_to_handle = array(
+			E_ERROR,
+			E_PARSE,
+			E_USER_ERROR,
+			E_COMPILE_ERROR,
+			E_RECOVERABLE_ERROR,
+		);
+
+		if ( isset( $error['type'] ) && in_array( $error['type'], $error_types_to_handle, true ) ) {
+			return true;
+		}
+
+		/**
+		 * Filters whether a given thrown error should be handled by the fatal error handler.
+		 *
+		 * This filter is only fired if the error is not already configured to be handled by WordPress core. As such,
+		 * it exclusively allows adding further rules for which errors should be handled, but not removing existing
+		 * ones.
+		 *
+		 * @since 5.2.0
+		 *
+		 * @param bool  $should_handle_error Whether the error should be handled by the fatal error handler.
+		 * @param array $error               Error information retrieved from error_get_last().
+		 */
+		return (bool) apply_filters( 'wp_should_handle_php_error', false, $error );
+	}
+
+	/**
+	 * Displays the PHP error template and sends the HTTP status code, typically 500.
+	 *
+	 * A drop-in 'php-error.php' can be used as a custom template. This drop-in should control the HTTP status code and
+	 * print the HTML markup indicating that a PHP error occurred. Note that this drop-in may potentially be executed
+	 * very early in the WordPress bootstrap process, so any core functions used that are not part of
+	 * `wp-includes/load.php` should be checked for before being called.
+	 *
+	 * If no such drop-in is available, this will call {@see WP_Fatal_Error_Handler::display_default_error_template()}.
+	 *
+	 * @since 5.2.0
+	 */
+	protected function display_error_template() {
+		if ( defined( 'WP_CONTENT_DIR' ) ) {
+			// Load custom PHP error template, if present.
+			$php_error_pluggable = WP_CONTENT_DIR . '/php-error.php';
+			if ( is_readable( $php_error_pluggable ) ) {
+				require_once $php_error_pluggable;
+
+				return;
+			}
+		}
+
+		// Otherwise, display the default error template.
+		$this->display_default_error_template();
+	}
+
+	/**
+	 * Displays the default PHP error template.
+	 *
+	 * This method is called conditionally if no 'php-error.php' drop-in is available.
+	 *
+	 * It calls {@see wp_die()} with a message indicating that the site is experiencing technical difficulties and a
+	 * login link to the admin backend. The {@see 'wp_php_error_message'} and {@see 'wp_php_error_args'} filters can
+	 * be used to modify these parameters.
+	 *
+	 * @since 5.2.0
+	 */
+	protected function display_default_error_template() {
+		if ( ! function_exists( '__' ) ) {
+			wp_load_translations_early();
+		}
+
+		if ( ! function_exists( 'wp_die' ) ) {
+			require_once ABSPATH . WPINC . '/functions.php';
+		}
+
+		$message = __( 'The site is experiencing technical difficulties.' );
+
+		$args = array(
+			'response' => 500,
+			'exit'     => false,
+		);
+
+		/**
+		 * Filters the message that the default PHP error template displays.
+		 *
+		 * @since 5.2.0
+		 *
+		 * @param string $message HTML error message to display.
+		 */
+		$message = apply_filters( 'wp_php_error_message', $message );
+
+		/**
+		 * Filters the arguments passed to {@see wp_die()} for the default PHP error template.
+		 *
+		 * @since 5.2.0
+		 *
+		 * @param array $args Associative array of arguments passed to `wp_die()`. By default these contain a
+		 *                    'response' key, and optionally 'link_url' and 'link_text' keys.
+		 */
+		$args = apply_filters( 'wp_php_error_args', $args );
+
+		$error = new WP_Error( 'internal_server_error', $message );
+
+		wp_die( $error, '', $args );
+	}
+}

src/wp-includes/error-protection.php

@@ -0,0 +1,54 @@
+<?php
+/**
+ * Error Protection API: Functions
+ *
+ * @package WordPress
+ * @since   5.2.0
+ */
+
+/**
+ * Registers the shutdown handler for fatal errors.
+ *
+ * The handler will only be registered if {@see wp_is_fatal_error_handler_enabled()} returns true.
+ *
+ * @since 5.2.0
+ */
+function wp_register_fatal_error_handler() {
+	if ( ! wp_is_fatal_error_handler_enabled() ) {
+		return;
+	}
+
+	$handler = null;
+	if ( defined( 'WP_CONTENT_DIR' ) && is_readable( WP_CONTENT_DIR . '/fatal-error-handler.php' ) ) {
+		$handler = include WP_CONTENT_DIR . '/fatal-error-handler.php';
+	}
+
+	if ( ! is_object( $handler ) || ! is_callable( array( $handler, 'handle' ) ) ) {
+		$handler = new WP_Fatal_Error_Handler();
+	}
+
+	register_shutdown_function( array( $handler, 'handle' ) );
+}
+
+/**
+ * Checks whether the fatal error handler is enabled.
+ *
+ * A constant `WP_DISABLE_FATAL_ERROR_HANDLER` can be set in `wp-config.php` to disable it, or alternatively the
+ * {@see 'wp_fatal_error_handler_enabled'} filter can be used to modify the return value.
+ *
+ * @since 5.2.0
+ *
+ * @return bool True if the fatal error handler is enabled, false otherwise.
+ */
+function wp_is_fatal_error_handler_enabled() {
+	$enabled = ! defined( 'WP_DISABLE_FATAL_ERROR_HANDLER' ) || ! WP_DISABLE_FATAL_ERROR_HANDLER;
+
+	/**
+	 * Filters whether the fatal error handler is enabled.
+	 *
+	 * @since 5.2.0
+	 *
+	 * @param bool $enabled True if the fatal error handler is enabled, false otherwise.
+	 */
+	return apply_filters( 'wp_fatal_error_handler_enabled', $enabled );
+}

src/wp-settings.php

@@ -17,9 +17,14 @@ define( 'WPINC', 'wp-includes' );
 
 // Include files required for initialization.
 require( ABSPATH . WPINC . '/load.php' );
+require( ABSPATH . WPINC . '/class-wp-fatal-error-handler.php' );
+require( ABSPATH . WPINC . '/error-protection.php' );
 require( ABSPATH . WPINC . '/default-constants.php' );
 require_once( ABSPATH . WPINC . '/plugin.php' );
 
+// Make sure we register the shutdown handler for fatal errors as soon as possible.
+wp_register_fatal_error_handler();
+
 /*
  * These can't be directly globalized in version.php. When updating,
  * we're including version.php from another installation and don't want
@@ -528,3 +533,12 @@ if ( is_multisite() ) {
  * @since 3.0.0
  */
 do_action( 'wp_loaded' );
+
+/*
+ * Store the fact that we could successfully execute the entire WordPress
+ * lifecycle. This is used to skip the premature shutdown handler, as it cannot
+ * be unregistered.
+ */
+if ( ! defined( 'WP_EXECUTION_SUCCEEDED' ) ) {
+	define( 'WP_EXECUTION_SUCCEEDED', true );
+}