Bootstrap/Load: Introduce fatal error recovery mechanism allowing users to still log in to their admin dashboard.

This changeset introduces a `WP_Shutdown_Handler` class that detects fatal errors and which extension (plugin or theme) causes them. Such an error is then recorded, and an error message is displayed. Subsequently, in certain protected areas, for example the admin, the broken extension will be paused, ensuring that the website is still usable in the respective area. The major benefit is that this mechanism allows site owners to still log in to their website, to fix the problem by either disabling the extension or solving the bug and then resuming the extension.

Extensions are only paused in certain designated areas. The frontend for example stays unaffected, as it is impossible to know what pausing the extension would cause to be missing, so it might be preferrable to clearly see that the website is temporarily not accessible instead.

The fatal error recovery is especially important in scope of encouraging the switch to a maintained PHP version, as not necessarily every WordPress extension is compatible with all PHP versions. If problems occur now, non-technical site owners that do not have immediate access to the codebase are not locked out of their site and can at least temporarily solve the problem quickly.

Websites that have custom requirements in that regard can implement their own shutdown handler by adding a `shutdown-handler.php` drop-in that returns the handler instance to use, which must be based on a class that inherits `WP_Shutdown_Handler`. That handler will then be used in place of the default one.

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.

Props afragen, bradleyt, flixos90, ocean90, schlessera, SergeyBiryukov, spacedmonkey.
Fixes #44458.


git-svn-id: https://develop.svn.wordpress.org/trunk@44524 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/load.php

@@ -696,9 +696,117 @@ function wp_get_active_and_valid_plugins() {
 			$plugins[] = WP_PLUGIN_DIR . '/' . $plugin;
 		}
 	}
+
+	/*
+	 * Remove plugins from the list of active plugins when we're on an endpoint
+	 * that should be protected against WSODs and the plugin is paused.
+	 */
+	if ( is_protected_endpoint() ) {
+		$plugins = wp_skip_paused_plugins( $plugins );
+	}
+
 	return $plugins;
 }
 
+/**
+ * Filters a given list of plugins, removing any paused plugins from it.
+ *
+ * @since 5.1.0
+ *
+ * @param array $plugins List of absolute plugin main file paths.
+ * @return array Filtered value of $plugins, without any paused plugins.
+ */
+function wp_skip_paused_plugins( array $plugins ) {
+	$paused_plugins = wp_paused_plugins()->get_all();
+
+	if ( empty( $paused_plugins ) ) {
+		return $plugins;
+	}
+
+	foreach ( $plugins as $index => $plugin ) {
+		list( $plugin ) = explode( '/', plugin_basename( $plugin ) );
+
+		if ( array_key_exists( $plugin, $paused_plugins ) ) {
+			unset( $plugins[ $index ] );
+
+			// Store list of paused plugins for displaying an admin notice.
+			$GLOBALS['_paused_plugins'][ $plugin ] = $paused_plugins[ $plugin ];
+		}
+	}
+
+	return $plugins;
+}
+
+/**
+ * Retrieves an array of active and valid themes.
+ *
+ * While upgrading or installing WordPress, no themes are returned.
+ *
+ * @since 5.1.0
+ * @access private
+ *
+ * @return array Array of paths to theme directories.
+ */
+function wp_get_active_and_valid_themes() {
+	global $pagenow;
+
+	$themes = array();
+
+	if ( wp_installing() && 'wp-activate.php' !== $pagenow ) {
+		return $themes;
+	}
+
+	if ( TEMPLATEPATH !== STYLESHEETPATH ) {
+		$themes[] = STYLESHEETPATH;
+	}
+
+	$themes[] = TEMPLATEPATH;
+
+	/*
+	 * Remove themes from the list of active themes when we're on an endpoint
+	 * that should be protected against WSODs and the theme is paused.
+	 */
+	if ( is_protected_endpoint() ) {
+		$themes = wp_skip_paused_themes( $themes );
+
+		// If no active and valid themes exist, skip loading themes.
+		if ( empty( $themes ) ) {
+			add_filter( 'wp_using_themes', '__return_false' );
+		}
+	}
+
+	return $themes;
+}
+
+/**
+ * Filters a given list of themes, removing any paused themes from it.
+ *
+ * @since 5.1.0
+ *
+ * @param array $themes List of absolute theme directory paths.
+ * @return array Filtered value of $themes, without any paused themes.
+ */
+function wp_skip_paused_themes( array $themes ) {
+	$paused_themes = wp_paused_themes()->get_all();
+
+	if ( empty( $paused_themes ) ) {
+		return $themes;
+	}
+
+	foreach ( $themes as $index => $theme ) {
+		$theme = basename( $theme );
+
+		if ( array_key_exists( $theme, $paused_themes ) ) {
+			unset( $themes[ $index ] );
+
+			// Store list of paused themes for displaying an admin notice.
+			$GLOBALS['_paused_themes'][ $theme ] = $paused_themes[ $theme ];
+		}
+	}
+
+	return $themes;
+}
+
 /**
  * Set internal encoding.
  *
@@ -1163,6 +1271,106 @@ function wp_doing_ajax() {
 	return apply_filters( 'wp_doing_ajax', defined( 'DOING_AJAX' ) && DOING_AJAX );
 }
 
+/**
+ * Determines whether the current request should use themes.
+ *
+ * @since 5.1.0
+ *
+ * @return bool True if themes should be used, false otherwise.
+ */
+function wp_using_themes() {
+	/**
+	 * Filters whether the current request should use themes.
+	 *
+	 * @since 5.1.0
+	 *
+	 * @param bool $wp_using_themes Whether the current request should use themes.
+	 */
+	return apply_filters( 'wp_using_themes', defined( 'WP_USE_THEMES' ) && WP_USE_THEMES );
+}
+
+/**
+ * Determines whether we are currently on an endpoint that should be protected against WSODs.
+ *
+ * @since 5.1.0
+ *
+ * @return bool True if the current endpoint should be protected.
+ */
+function is_protected_endpoint() {
+	// Protect login pages.
+	if ( isset( $GLOBALS['pagenow'] ) && 'wp-login.php' === $GLOBALS['pagenow'] ) {
+		return true;
+	}
+
+	// Protect the admin backend.
+	if ( is_admin() && ! wp_doing_ajax() ) {
+		return true;
+	}
+
+	// Protect AJAX actions that could help resolve a fatal error should be available.
+	if ( is_protected_ajax_action() ) {
+		return true;
+	}
+
+	/**
+	 * Filters whether the current request is against a protected endpoint.
+	 *
+	 * This filter is only fired when an endpoint is requested which is not already protected by
+	 * WordPress core. As such, it exclusively allows providing further protected endpoints in
+	 * addition to the admin backend, login pages and protected AJAX actions.
+	 *
+	 * @since 5.1.0
+	 *
+	 * @param bool $is_protected_endpoint Whether the currently requested endpoint is protected. Default false.
+	 */
+	return (bool) apply_filters( 'is_protected_endpoint', false );
+}
+
+/**
+ * Determines whether we are currently handling an AJAX action that should be protected against WSODs.
+ *
+ * @since 5.1.0
+ *
+ * @return bool True if the current AJAX action should be protected.
+ */
+function is_protected_ajax_action() {
+	if ( ! wp_doing_ajax() ) {
+		return false;
+	}
+
+	if ( ! isset( $_REQUEST['action'] ) ) {
+		return false;
+	}
+
+	$actions_to_protect = array(
+		'edit-theme-plugin-file', // Saving changes in the core code editor.
+		'heartbeat',              // Keep the heart beating.
+		'install-plugin',         // Installing a new plugin.
+		'install-theme',          // Installing a new theme.
+		'search-plugins',         // Searching in the list of plugins.
+		'search-install-plugins', // Searching for a plugin in the plugin install screen.
+		'update-plugin',          // Update an existing plugin.
+		'update-theme',           // Update an existing theme.
+	);
+
+	/**
+	 * Filters the array of protected AJAX actions.
+	 *
+	 * This filter is only fired when doing AJAX and the AJAX request has an 'action' property.
+	 *
+	 * @since 5.1.0
+	 *
+	 * @param array $actions_to_protect Array of strings with AJAX actions to protect.
+	 */
+	$actions_to_protect = (array) apply_filters( 'wp_protected_ajax_actions', $actions_to_protect );
+
+	if ( ! in_array( $_REQUEST['action'], $actions_to_protect, true ) ) {
+		return false;
+	}
+
+	return true;
+}
+
 /**
  * Determines whether the current request is a WordPress cron request.
  *