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/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 );
+}