Source

File: wp-admin/includes/class-wp-automatic-updater.php

public function update( $type, $item ) {
		$skin = new Automatic_Upgrader_Skin;

		switch ( $type ) {
			case 'core':
				// The Core upgrader doesn't use the Upgrader's skin during the actual main part of the upgrade, instead, firing a filter.
				add_filter( 'update_feedback', array( $skin, 'feedback' ) );
				$upgrader = new Core_Upgrader( $skin );
				$context  = ABSPATH;
				break;
			case 'plugin':
				$upgrader = new Plugin_Upgrader( $skin );
				$context  = WP_PLUGIN_DIR; // We don't support custom Plugin directories, or updates for WPMU_PLUGIN_DIR.
				break;
			case 'theme':
				$upgrader = new Theme_Upgrader( $skin );
				$context  = get_theme_root( $item->theme );
				break;
			case 'translation':
				$upgrader = new Language_Pack_Upgrader( $skin );
				$context  = WP_CONTENT_DIR; // WP_LANG_DIR;
				break;
		}

		// Determine whether we can and should perform this update.
		if ( ! $this->should_update( $type, $item, $context ) ) {
			return false;
		}

		/**
		 * Fires immediately prior to an auto-update.
		 *
		 * @since 4.4.0
		 *
		 * @param string $type    The type of update being checked: 'core', 'theme', 'plugin', or 'translation'.
		 * @param object $item    The update offer.
		 * @param string $context The filesystem context (a path) against which filesystem access and status
		 *                        should be checked.
		 */
		do_action( 'pre_auto_update', $type, $item, $context );

		$upgrader_item = $item;
		switch ( $type ) {
			case 'core':
				/* translators: %s: WordPress version. */
				$skin->feedback( __( 'Updating to WordPress %s' ), $item->version );
				/* translators: %s: WordPress version. */
				$item_name = sprintf( __( 'WordPress %s' ), $item->version );
				break;
			case 'theme':
				$upgrader_item = $item->theme;
				$theme         = wp_get_theme( $upgrader_item );
				$item_name     = $theme->Get( 'Name' );
				/* translators: %s: Theme name. */
				$skin->feedback( __( 'Updating theme: %s' ), $item_name );
				break;
			case 'plugin':
				$upgrader_item = $item->plugin;
				$plugin_data   = get_plugin_data( $context . '/' . $upgrader_item );
				$item_name     = $plugin_data['Name'];
				/* translators: %s: Plugin name. */
				$skin->feedback( __( 'Updating plugin: %s' ), $item_name );
				break;
			case 'translation':
				$language_item_name = $upgrader->get_name_for_update( $item );
				/* translators: %s: Project name (plugin, theme, or WordPress). */
				$item_name = sprintf( __( 'Translations for %s' ), $language_item_name );
				/* translators: 1: Project name (plugin, theme, or WordPress), 2: Language. */
				$skin->feedback( sprintf( __( 'Updating translations for %1$s (%2$s)…' ), $language_item_name, $item->language ) );
				break;
		}

		$allow_relaxed_file_ownership = false;
		if ( 'core' === $type && isset( $item->new_files ) && ! $item->new_files ) {
			$allow_relaxed_file_ownership = true;
		}

		// Boom, this site's about to get a whole new splash of paint!
		$upgrade_result = $upgrader->upgrade(
			$upgrader_item,
			array(
				'clear_update_cache'           => false,
				// Always use partial builds if possible for core updates.
				'pre_check_md5'                => false,
				// Only available for core updates.
				'attempt_rollback'             => true,
				// Allow relaxed file ownership in some scenarios.
				'allow_relaxed_file_ownership' => $allow_relaxed_file_ownership,
			)
		);

		// If the filesystem is unavailable, false is returned.
		if ( false === $upgrade_result ) {
			$upgrade_result = new WP_Error( 'fs_unavailable', __( 'Could not access filesystem.' ) );
		}

		if ( 'core' === $type ) {
			if ( is_wp_error( $upgrade_result )
				&& ( 'up_to_date' === $upgrade_result->get_error_code()
					|| 'locked' === $upgrade_result->get_error_code() )
			) {
				// These aren't actual errors, treat it as a skipped-update instead
				// to avoid triggering the post-core update failure routines.
				return false;
			}

			// Core doesn't output this, so let's append it so we don't get confused.
			if ( is_wp_error( $upgrade_result ) ) {
				$skin->error( __( 'Installation failed.' ), $upgrade_result );
			} else {
				$skin->feedback( __( 'WordPress updated successfully.' ) );
			}
		}

		$this->update_results[ $type ][] = (object) array(
			'item'     => $item,
			'result'   => $upgrade_result,
			'name'     => $item_name,
			'messages' => $skin->get_upgrade_messages(),
		);

		return $upgrade_result;
	}