diff -r 490d5cc509ed -r cf61fcea0001 wp/wp-admin/includes/class-wp-ajax-upgrader-skin.php
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-admin/includes/class-wp-ajax-upgrader-skin.php	Mon Oct 14 17:39:30 2019 +0200
@@ -0,0 +1,126 @@
+<?php
+/**
+ * Upgrader API: WP_Ajax_Upgrader_Skin class
+ *
+ * @package WordPress
+ * @subpackage Upgrader
+ * @since 4.6.0
+ */
+
+/**
+ * Upgrader Skin for Ajax WordPress upgrades.
+ *
+ * This skin is designed to be used for Ajax updates.
+ *
+ * @since 4.6.0
+ *
+ * @see Automatic_Upgrader_Skin
+ */
+class WP_Ajax_Upgrader_Skin extends Automatic_Upgrader_Skin {
+
+	/**
+	 * Holds the WP_Error object.
+	 *
+	 * @since 4.6.0
+	 * @var null|WP_Error
+	 */
+	protected $errors = null;
+
+	/**
+	 * Constructor.
+	 *
+	 * @since 4.6.0
+	 *
+	 * @param array $args Options for the upgrader, see WP_Upgrader_Skin::__construct().
+	 */
+	public function __construct( $args = array() ) {
+		parent::__construct( $args );
+
+		$this->errors = new WP_Error();
+	}
+
+	/**
+	 * Retrieves the list of errors.
+	 *
+	 * @since 4.6.0
+	 *
+	 * @return WP_Error Errors during an upgrade.
+	 */
+	public function get_errors() {
+		return $this->errors;
+	}
+
+	/**
+	 * Retrieves a string for error messages.
+	 *
+	 * @since 4.6.0
+	 *
+	 * @return string Error messages during an upgrade.
+	 */
+	public function get_error_messages() {
+		$messages = array();
+
+		foreach ( $this->errors->get_error_codes() as $error_code ) {
+			if ( $this->errors->get_error_data( $error_code ) && is_string( $this->errors->get_error_data( $error_code ) ) ) {
+				$messages[] = $this->errors->get_error_message( $error_code ) . ' ' . esc_html( strip_tags( $this->errors->get_error_data( $error_code ) ) );
+			} else {
+				$messages[] = $this->errors->get_error_message( $error_code );
+			}
+		}
+
+		return implode( ', ', $messages );
+	}
+
+	/**
+	 * Stores a log entry for an error.
+	 *
+	 * @since 4.6.0
+	 *
+	 * @param string|WP_Error $errors Errors.
+	 */
+	public function error( $errors ) {
+		if ( is_string( $errors ) ) {
+			$string = $errors;
+			if ( ! empty( $this->upgrader->strings[ $string ] ) ) {
+				$string = $this->upgrader->strings[ $string ];
+			}
+
+			if ( false !== strpos( $string, '%' ) ) {
+				$args = func_get_args();
+				$args = array_splice( $args, 1 );
+				if ( ! empty( $args ) ) {
+					$string = vsprintf( $string, $args );
+				}
+			}
+
+			// Count existing errors to generate an unique error code.
+			$errors_count = count( $this->errors->get_error_codes() );
+			$this->errors->add( 'unknown_upgrade_error_' . $errors_count + 1 , $string );
+		} elseif ( is_wp_error( $errors ) ) {
+			foreach ( $errors->get_error_codes() as $error_code ) {
+				$this->errors->add( $error_code, $errors->get_error_message( $error_code ), $errors->get_error_data( $error_code ) );
+			}
+		}
+
+		$args = func_get_args();
+		call_user_func_array( array( $this, 'parent::error' ), $args );
+	}
+
+	/**
+	 * Stores a log entry.
+	 *
+	 * @since 4.6.0
+	 *
+	 * @param string|array|WP_Error $data Log entry data.
+	 */
+	public function feedback( $data ) {
+		if ( is_wp_error( $data ) ) {
+			foreach ( $data->get_error_codes() as $error_code ) {
+				$this->errors->add( $error_code, $data->get_error_message( $error_code ), $data->get_error_data( $error_code ) );
+			}
+		}
+
+		$args = func_get_args();
+		call_user_func_array( array( $this, 'parent::feedback' ), $args );
+	}
+}