Bootstrap/Load: Change shutdown handler naming to final fatal error handler and allow disabling the handler entirely via a constant.

The `WP_Shutdown_Handler` name plus related function names were premature when originally committed, as there can be multiple shutdown handlers in PHP, and WordPress makes use of that feature. This changeset modifies the name to a more appropriate `WP_Fatal_Error_Handler`, and related to that changes the following names:

* The drop-in to override the handler is now called `fatal-error-handler.php`.
* The internal function `wp_register_premature_shutdown_handler` is now called `wp_register_fatal_error_handler()`.

In addition to these naming changes, a new constant `WP_DISABLE_FATAL_ERROR_HANDLER` is introduced that can be set in `wp-config.php` to entirely disable the fatal error handler. That constant's value is and should be accessed indirectly via a new `wp_is_fatal_error_handler_enabled()` function and is filterable via a new `wp_fatal_error_handler_enabled` hook. Note that disabling the fatal error handler will skip the new functionality entirely, including the potentially used `fatal-error-handler.php` drop-in.

The new set of constant, filter and function provide for an easier-to-use mechanism to disable the fatal error handler altogether, rather than requiring developers to implement a drop-in for purely that purpose.

Props afragen, flixos90, joyously, knutsp, markjaquith, ocean90, schlessera, spacedmonkey.
Fixes #46047. See #44458.


git-svn-id: https://develop.svn.wordpress.org/trunk@44674 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/error-protection.php

@@ -148,19 +148,48 @@ function wp_should_handle_error( $error ) {
 }
 
 /**
- * Registers the WordPress premature shutdown handler.
+ * Registers the shutdown handler for fatal errors.
+ *
+ * The handler will only be registered if {@see wp_is_fatal_error_handler_enabled()} returns true.
  *
  * @since 5.1.0
  */
-function wp_register_premature_shutdown_handler() {
+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 . '/shutdown-handler.php' ) ) {
-		$handler = include WP_CONTENT_DIR . '/shutdown-handler.php';
+	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_Shutdown_Handler();
+		$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 5.1.0
+ *
+ * @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 5.1.0
+	 *
+	 * @param bool $enabled True if the fatal error handler is enabled, false otherwise.
+	 */
+	return apply_filters( 'wp_fatal_error_handler_enabled', $enabled );
+}