diff -r 3d4e9c994f10 -r a86126ab1dd4 wp/wp-admin/update.php
--- a/wp/wp-admin/update.php	Tue Oct 22 16:11:46 2019 +0200
+++ b/wp/wp-admin/update.php	Tue Dec 15 13:49:49 2020 +0100
@@ -6,21 +6,25 @@
  * @subpackage Administration
  */
 
-if ( ! defined( 'IFRAME_REQUEST' ) && isset( $_GET['action'] ) && in_array( $_GET['action'], array( 'update-selected', 'activate-plugin', 'update-selected-themes' ) ) ) {
+if ( ! defined( 'IFRAME_REQUEST' )
+	&& isset( $_GET['action'] ) && in_array( $_GET['action'], array( 'update-selected', 'activate-plugin', 'update-selected-themes' ), true )
+) {
 	define( 'IFRAME_REQUEST', true );
 }
 
 /** WordPress Administration Bootstrap */
-require_once( dirname( __FILE__ ) . '/admin.php' );
+require_once __DIR__ . '/admin.php';
 
-include_once( ABSPATH . 'wp-admin/includes/class-wp-upgrader.php' );
+require_once ABSPATH . 'wp-admin/includes/class-wp-upgrader.php';
+
+wp_enqueue_script( 'wp-a11y' );
 
 if ( isset( $_GET['action'] ) ) {
 	$plugin = isset( $_REQUEST['plugin'] ) ? trim( $_REQUEST['plugin'] ) : '';
 	$theme  = isset( $_REQUEST['theme'] ) ? urldecode( $_REQUEST['theme'] ) : '';
 	$action = isset( $_REQUEST['action'] ) ? $_REQUEST['action'] : '';
 
-	if ( 'update-selected' == $action ) {
+	if ( 'update-selected' === $action ) {
 		if ( ! current_user_can( 'update_plugins' ) ) {
 			wp_die( __( 'Sorry, you are not allowed to update plugins for this site.' ) );
 		}
@@ -48,7 +52,7 @@
 
 		iframe_footer();
 
-	} elseif ( 'upgrade-plugin' == $action ) {
+	} elseif ( 'upgrade-plugin' === $action ) {
 		if ( ! current_user_can( 'update_plugins' ) ) {
 			wp_die( __( 'Sorry, you are not allowed to update plugins for this site.' ) );
 		}
@@ -60,7 +64,7 @@
 		$submenu_file = 'plugins.php';
 
 		wp_enqueue_script( 'updates' );
-		require_once( ABSPATH . 'wp-admin/admin-header.php' );
+		require_once ABSPATH . 'wp-admin/admin-header.php';
 
 		$nonce = 'upgrade-plugin_' . $plugin;
 		$url   = 'update.php?action=upgrade-plugin&plugin=' . urlencode( $plugin );
@@ -68,9 +72,9 @@
 		$upgrader = new Plugin_Upgrader( new Plugin_Upgrader_Skin( compact( 'title', 'nonce', 'url', 'plugin' ) ) );
 		$upgrader->upgrade( $plugin );
 
-		include( ABSPATH . 'wp-admin/admin-footer.php' );
+		require_once ABSPATH . 'wp-admin/admin-footer.php';
 
-	} elseif ( 'activate-plugin' == $action ) {
+	} elseif ( 'activate-plugin' === $action ) {
 		if ( ! current_user_can( 'update_plugins' ) ) {
 			wp_die( __( 'Sorry, you are not allowed to update plugins for this site.' ) );
 		}
@@ -91,18 +95,18 @@
 			echo '<p>' . __( 'Plugin failed to reactivate due to a fatal error.' ) . '</p>';
 
 			error_reporting( E_CORE_ERROR | E_CORE_WARNING | E_COMPILE_ERROR | E_ERROR | E_WARNING | E_PARSE | E_USER_ERROR | E_USER_WARNING | E_RECOVERABLE_ERROR );
-			@ini_set( 'display_errors', true ); //Ensure that Fatal errors are displayed.
+			ini_set( 'display_errors', true ); // Ensure that fatal errors are displayed.
 			wp_register_plugin_realpath( WP_PLUGIN_DIR . '/' . $plugin );
-			include( WP_PLUGIN_DIR . '/' . $plugin );
+			include WP_PLUGIN_DIR . '/' . $plugin;
 		}
 		iframe_footer();
-	} elseif ( 'install-plugin' == $action ) {
+	} elseif ( 'install-plugin' === $action ) {
 
 		if ( ! current_user_can( 'install_plugins' ) ) {
 			wp_die( __( 'Sorry, you are not allowed to install plugins on this site.' ) );
 		}
 
-		include_once( ABSPATH . 'wp-admin/includes/plugin-install.php' ); //for plugins_api..
+		include_once ABSPATH . 'wp-admin/includes/plugin-install.php'; // For plugins_api().
 
 		check_admin_referer( 'install-plugin_' . $plugin );
 		$api = plugins_api(
@@ -122,8 +126,9 @@
 		$title        = __( 'Plugin Installation' );
 		$parent_file  = 'plugins.php';
 		$submenu_file = 'plugin-install.php';
-		require_once( ABSPATH . 'wp-admin/admin-header.php' );
+		require_once ABSPATH . 'wp-admin/admin-header.php';
 
+		/* translators: %s: Plugin name and version. */
 		$title = sprintf( __( 'Installing Plugin: %s' ), $api->name . ' ' . $api->version );
 		$nonce = 'install-plugin_' . $plugin;
 		$url   = 'update.php?action=install-plugin&plugin=' . urlencode( $plugin );
@@ -131,14 +136,14 @@
 			$url .= '&from=' . urlencode( stripslashes( $_GET['from'] ) );
 		}
 
-		$type = 'web'; //Install plugin type, From Web or an Upload.
+		$type = 'web'; // Install plugin type, From Web or an Upload.
 
 		$upgrader = new Plugin_Upgrader( new Plugin_Installer_Skin( compact( 'title', 'url', 'nonce', 'plugin', 'api' ) ) );
 		$upgrader->install( $api->download_link );
 
-		include( ABSPATH . 'wp-admin/admin-footer.php' );
+		require_once ABSPATH . 'wp-admin/admin-footer.php';
 
-	} elseif ( 'upload-plugin' == $action ) {
+	} elseif ( 'upload-plugin' === $action ) {
 
 		if ( ! current_user_can( 'upload_plugins' ) ) {
 			wp_die( __( 'Sorry, you are not allowed to install plugins on this site.' ) );
@@ -151,23 +156,47 @@
 		$title        = __( 'Upload Plugin' );
 		$parent_file  = 'plugins.php';
 		$submenu_file = 'plugin-install.php';
-		require_once( ABSPATH . 'wp-admin/admin-header.php' );
+		require_once ABSPATH . 'wp-admin/admin-header.php';
 
-		$title = sprintf( __( 'Installing Plugin from uploaded file: %s' ), esc_html( basename( $file_upload->filename ) ) );
+		/* translators: %s: File name. */
+		$title = sprintf( __( 'Installing plugin from uploaded file: %s' ), esc_html( basename( $file_upload->filename ) ) );
 		$nonce = 'plugin-upload';
 		$url   = add_query_arg( array( 'package' => $file_upload->id ), 'update.php?action=upload-plugin' );
-		$type  = 'upload'; //Install plugin type, From Web or an Upload.
+		$type  = 'upload'; // Install plugin type, From Web or an Upload.
 
-		$upgrader = new Plugin_Upgrader( new Plugin_Installer_Skin( compact( 'type', 'title', 'nonce', 'url' ) ) );
-		$result   = $upgrader->install( $file_upload->package );
+		$overwrite = isset( $_GET['overwrite'] ) ? sanitize_text_field( $_GET['overwrite'] ) : '';
+		$overwrite = in_array( $overwrite, array( 'update-plugin', 'downgrade-plugin' ), true ) ? $overwrite : '';
+
+		$upgrader = new Plugin_Upgrader( new Plugin_Installer_Skin( compact( 'type', 'title', 'nonce', 'url', 'overwrite' ) ) );
+		$result   = $upgrader->install( $file_upload->package, array( 'overwrite_package' => $overwrite ) );
 
 		if ( $result || is_wp_error( $result ) ) {
 			$file_upload->cleanup();
 		}
 
-		include( ABSPATH . 'wp-admin/admin-footer.php' );
+		require_once ABSPATH . 'wp-admin/admin-footer.php';
+
+	} elseif ( 'upload-plugin-cancel-overwrite' === $action ) {
+		if ( ! current_user_can( 'upload_plugins' ) ) {
+			wp_die( __( 'Sorry, you are not allowed to install plugins on this site.' ) );
+		}
+
+		check_admin_referer( 'plugin-upload-cancel-overwrite' );
 
-	} elseif ( 'upgrade-theme' == $action ) {
+		// Make sure the attachment still exists, or File_Upload_Upgrader will call wp_die()
+		// that shows a generic "Please select a file" error.
+		if ( ! empty( $_GET['package'] ) ) {
+			$attachment_id = (int) $_GET['package'];
+
+			if ( get_post( $attachment_id ) ) {
+				$file_upload = new File_Upload_Upgrader( 'pluginzip', 'package' );
+				$file_upload->cleanup();
+			}
+		}
+
+		wp_redirect( self_admin_url( 'plugin-install.php' ) );
+		exit;
+	} elseif ( 'upgrade-theme' === $action ) {
 
 		if ( ! current_user_can( 'update_themes' ) ) {
 			wp_die( __( 'Sorry, you are not allowed to update themes for this site.' ) );
@@ -180,7 +209,7 @@
 		$title        = __( 'Update Theme' );
 		$parent_file  = 'themes.php';
 		$submenu_file = 'themes.php';
-		require_once( ABSPATH . 'wp-admin/admin-header.php' );
+		require_once ABSPATH . 'wp-admin/admin-header.php';
 
 		$nonce = 'upgrade-theme_' . $theme;
 		$url   = 'update.php?action=upgrade-theme&theme=' . urlencode( $theme );
@@ -188,8 +217,8 @@
 		$upgrader = new Theme_Upgrader( new Theme_Upgrader_Skin( compact( 'title', 'nonce', 'url', 'theme' ) ) );
 		$upgrader->upgrade( $theme );
 
-		include( ABSPATH . 'wp-admin/admin-footer.php' );
-	} elseif ( 'update-selected-themes' == $action ) {
+		require_once ABSPATH . 'wp-admin/admin-footer.php';
+	} elseif ( 'update-selected-themes' === $action ) {
 		if ( ! current_user_can( 'update_themes' ) ) {
 			wp_die( __( 'Sorry, you are not allowed to update themes for this site.' ) );
 		}
@@ -216,13 +245,13 @@
 		$upgrader->bulk_upgrade( $themes );
 
 		iframe_footer();
-	} elseif ( 'install-theme' == $action ) {
+	} elseif ( 'install-theme' === $action ) {
 
 		if ( ! current_user_can( 'install_themes' ) ) {
 			wp_die( __( 'Sorry, you are not allowed to install themes on this site.' ) );
 		}
 
-		include_once( ABSPATH . 'wp-admin/includes/class-wp-upgrader.php' ); //for themes_api..
+		include_once ABSPATH . 'wp-admin/includes/class-wp-upgrader.php'; // For themes_api().
 
 		check_admin_referer( 'install-theme_' . $theme );
 		$api = themes_api(
@@ -234,7 +263,7 @@
 					'tags'     => false,
 				),
 			)
-		); //Save on a bit of bandwidth.
+		); // Save on a bit of bandwidth.
 
 		if ( is_wp_error( $api ) ) {
 			wp_die( $api );
@@ -243,19 +272,20 @@
 		$title        = __( 'Install Themes' );
 		$parent_file  = 'themes.php';
 		$submenu_file = 'themes.php';
-		require_once( ABSPATH . 'wp-admin/admin-header.php' );
+		require_once ABSPATH . 'wp-admin/admin-header.php';
 
+		/* translators: %s: Theme name and version. */
 		$title = sprintf( __( 'Installing Theme: %s' ), $api->name . ' ' . $api->version );
 		$nonce = 'install-theme_' . $theme;
 		$url   = 'update.php?action=install-theme&theme=' . urlencode( $theme );
-		$type  = 'web'; //Install theme type, From Web or an Upload.
+		$type  = 'web'; // Install theme type, From Web or an Upload.
 
 		$upgrader = new Theme_Upgrader( new Theme_Installer_Skin( compact( 'title', 'url', 'nonce', 'plugin', 'api' ) ) );
 		$upgrader->install( $api->download_link );
 
-		include( ABSPATH . 'wp-admin/admin-footer.php' );
+		require_once ABSPATH . 'wp-admin/admin-footer.php';
 
-	} elseif ( 'upload-theme' == $action ) {
+	} elseif ( 'upload-theme' === $action ) {
 
 		if ( ! current_user_can( 'upload_themes' ) ) {
 			wp_die( __( 'Sorry, you are not allowed to install themes on this site.' ) );
@@ -269,22 +299,46 @@
 		$parent_file  = 'themes.php';
 		$submenu_file = 'theme-install.php';
 
-		require_once( ABSPATH . 'wp-admin/admin-header.php' );
+		require_once ABSPATH . 'wp-admin/admin-header.php';
 
-		$title = sprintf( __( 'Installing Theme from uploaded file: %s' ), esc_html( basename( $file_upload->filename ) ) );
+		/* translators: %s: File name. */
+		$title = sprintf( __( 'Installing theme from uploaded file: %s' ), esc_html( basename( $file_upload->filename ) ) );
 		$nonce = 'theme-upload';
 		$url   = add_query_arg( array( 'package' => $file_upload->id ), 'update.php?action=upload-theme' );
-		$type  = 'upload'; //Install plugin type, From Web or an Upload.
+		$type  = 'upload'; // Install theme type, From Web or an Upload.
 
-		$upgrader = new Theme_Upgrader( new Theme_Installer_Skin( compact( 'type', 'title', 'nonce', 'url' ) ) );
-		$result   = $upgrader->install( $file_upload->package );
+		$overwrite = isset( $_GET['overwrite'] ) ? sanitize_text_field( $_GET['overwrite'] ) : '';
+		$overwrite = in_array( $overwrite, array( 'update-theme', 'downgrade-theme' ), true ) ? $overwrite : '';
+
+		$upgrader = new Theme_Upgrader( new Theme_Installer_Skin( compact( 'type', 'title', 'nonce', 'url', 'overwrite' ) ) );
+		$result   = $upgrader->install( $file_upload->package, array( 'overwrite_package' => $overwrite ) );
 
 		if ( $result || is_wp_error( $result ) ) {
 			$file_upload->cleanup();
 		}
 
-		include( ABSPATH . 'wp-admin/admin-footer.php' );
+		require_once ABSPATH . 'wp-admin/admin-footer.php';
+
+	} elseif ( 'upload-theme-cancel-overwrite' === $action ) {
+		if ( ! current_user_can( 'upload_themes' ) ) {
+			wp_die( __( 'Sorry, you are not allowed to install themes on this site.' ) );
+		}
+
+		check_admin_referer( 'theme-upload-cancel-overwrite' );
 
+		// Make sure the attachment still exists, or File_Upload_Upgrader will call wp_die()
+		// that shows a generic "Please select a file" error.
+		if ( ! empty( $_GET['package'] ) ) {
+			$attachment_id = (int) $_GET['package'];
+
+			if ( get_post( $attachment_id ) ) {
+				$file_upload = new File_Upload_Upgrader( 'themezip', 'package' );
+				$file_upload->cleanup();
+			}
+		}
+
+		wp_redirect( self_admin_url( 'theme-install.php' ) );
+		exit;
 	} else {
 		/**
 		 * Fires when a custom plugin or theme update request is received.
@@ -295,6 +349,6 @@
 		 *
 		 * @since 2.8.0
 		 */
-		do_action( "update-custom_{$action}" );
+		do_action( "update-custom_{$action}" ); // phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores
 	}
 }