Core_Upgrader::upgrade

	public function upgrade( $current, $args = array() ) {
		global $wp_filesystem;

		require ABSPATH . WPINC . '/version.php'; // $wp_version;

		$start_time = time();

		$defaults    = array(
			'pre_check_md5'                => true,
			'attempt_rollback'             => false,
			'do_rollback'                  => false,
			'allow_relaxed_file_ownership' => false,
		);
		$parsed_args = wp_parse_args( $args, $defaults );

		$this->init();
		$this->upgrade_strings();

		// Is an update available?
		if ( ! isset( $current->response ) || 'latest' === $current->response ) {
			return new WP_Error( 'up_to_date', $this->strings['up_to_date'] );
		}

		$res = $this->fs_connect( array( ABSPATH, WP_CONTENT_DIR ), $parsed_args['allow_relaxed_file_ownership'] );
		if ( ! $res || is_wp_error( $res ) ) {
			return $res;
		}

		$wp_dir = trailingslashit( $wp_filesystem->abspath() );

		$partial = true;
		if ( $parsed_args['do_rollback'] ) {
			$partial = false;
		} elseif ( $parsed_args['pre_check_md5'] && ! $this->check_files() ) {
			$partial = false;
		}

		/*
		 * If partial update is returned from the API, use that, unless we're doing
		 * a reinstallation. If we cross the new_bundled version number, then use
		 * the new_bundled zip. Don't though if the constant is set to skip bundled items.
		 * If the API returns a no_content zip, go with it. Finally, default to the full zip.
		 */
		if ( $parsed_args['do_rollback'] && $current->packages->rollback ) {
			$to_download = 'rollback';
		} elseif ( $current->packages->partial && 'reinstall' !== $current->response && $wp_version == $current->partial_version && $partial ) {
			$to_download = 'partial';
		} elseif ( $current->packages->new_bundled && version_compare( $wp_version, $current->new_bundled, '<' )
			&& ( ! defined( 'CORE_UPGRADE_SKIP_NEW_BUNDLED' ) || ! CORE_UPGRADE_SKIP_NEW_BUNDLED ) ) {
			$to_download = 'new_bundled';
		} elseif ( $current->packages->no_content ) {
			$to_download = 'no_content';
		} else {
			$to_download = 'full';
		}

		// Lock to prevent multiple Core Updates occurring.
		$lock = WP_Upgrader::create_lock( 'core_updater', 15 * MINUTE_IN_SECONDS );
		if ( ! $lock ) {
			return new WP_Error( 'locked', $this->strings['locked'] );
		}

		$download = $this->download_package( $current->packages->$to_download, true );

		// Allow for signature soft-fail.
		// WARNING: This may be removed in the future.
		if ( is_wp_error( $download ) && $download->get_error_data( 'softfail-filename' ) ) {
			// Outout the failure error as a normal feedback, and not as an error:
			/** This filter is documented in wp-admin/includes/update-core.php */
			apply_filters( 'update_feedback', $download->get_error_message() );

			// Report this failure back to WordPress.org for debugging purposes.
			wp_version_check(
				array(
					'signature_failure_code' => $download->get_error_code(),
					'signature_failure_data' => $download->get_error_data(),
				)
			);

			// Pretend this error didn't happen.
			$download = $download->get_error_data( 'softfail-filename' );
		}

		if ( is_wp_error( $download ) ) {
			WP_Upgrader::release_lock( 'core_updater' );
			return $download;
		}

		$working_dir = $this->unpack_package( $download );
		if ( is_wp_error( $working_dir ) ) {
			WP_Upgrader::release_lock( 'core_updater' );
			return $working_dir;
		}

		// Copy update-core.php from the new version into place.
		if ( ! $wp_filesystem->copy( $working_dir . '/wordpress/wp-admin/includes/update-core.php', $wp_dir . 'wp-admin/includes/update-core.php', true ) ) {
			$wp_filesystem->delete( $working_dir, true );
			WP_Upgrader::release_lock( 'core_updater' );
			return new WP_Error( 'copy_failed_for_update_core_file', __( 'The update cannot be installed because we will be unable to copy some files. This is usually due to inconsistent file permissions.' ), 'wp-admin/includes/update-core.php' );
		}
		$wp_filesystem->chmod( $wp_dir . 'wp-admin/includes/update-core.php', FS_CHMOD_FILE );

		wp_opcache_invalidate( ABSPATH . 'wp-admin/includes/update-core.php' );
		require_once ABSPATH . 'wp-admin/includes/update-core.php';

		if ( ! function_exists( 'update_core' ) ) {
			WP_Upgrader::release_lock( 'core_updater' );
			return new WP_Error( 'copy_failed_space', $this->strings['copy_failed_space'] );
		}

		$result = update_core( $working_dir, $wp_dir );

		// In the event of an issue, we may be able to roll back.
		if ( $parsed_args['attempt_rollback'] && $current->packages->rollback && ! $parsed_args['do_rollback'] ) {
			$try_rollback = false;
			if ( is_wp_error( $result ) ) {
				$error_code = $result->get_error_code();
				/*
				 * Not all errors are equal. These codes are critical: copy_failed__copy_dir,
				 * mkdir_failed__copy_dir, copy_failed__copy_dir_retry, and disk_full.
				 * do_rollback allows for update_core() to trigger a rollback if needed.
				 */
				if ( false !== strpos( $error_code, 'do_rollback' ) ) {
					$try_rollback = true;
				} elseif ( false !== strpos( $error_code, '__copy_dir' ) ) {
					$try_rollback = true;
				} elseif ( 'disk_full' === $error_code ) {
					$try_rollback = true;
				}
			}

			if ( $try_rollback ) {
				/** This filter is documented in wp-admin/includes/update-core.php */
				apply_filters( 'update_feedback', $result );

				/** This filter is documented in wp-admin/includes/update-core.php */
				apply_filters( 'update_feedback', $this->strings['start_rollback'] );

				$rollback_result = $this->upgrade( $current, array_merge( $parsed_args, array( 'do_rollback' => true ) ) );

				$original_result = $result;
				$result          = new WP_Error(
					'rollback_was_required',
					$this->strings['rollback_was_required'],
					(object) array(
						'update'   => $original_result,
						'rollback' => $rollback_result,
					)
				);
			}
		}

		/** This action is documented in wp-admin/includes/class-wp-upgrader.php */
		do_action(
			'upgrader_process_complete',
			$this,
			array(
				'action' => 'update',
				'type'   => 'core',
			)
		);

		// Clear the current updates.
		delete_site_transient( 'update_core' );

		if ( ! $parsed_args['do_rollback'] ) {
			$stats = array(
				'update_type'      => $current->response,
				'success'          => true,
				'fs_method'        => $wp_filesystem->method,
				'fs_method_forced' => defined( 'FS_METHOD' ) || has_filter( 'filesystem_method' ),
				'fs_method_direct' => ! empty( $GLOBALS['_wp_filesystem_direct_method'] ) ? $GLOBALS['_wp_filesystem_direct_method'] : '',
				'time_taken'       => time() - $start_time,
				'reported'         => $wp_version,
				'attempted'        => $current->version,
			);

			if ( is_wp_error( $result ) ) {
				$stats['success'] = false;
				// Did a rollback occur?
				if ( ! empty( $try_rollback ) ) {
					$stats['error_code'] = $original_result->get_error_code();
					$stats['error_data'] = $original_result->get_error_data();
					// Was the rollback successful? If not, collect its error too.
					$stats['rollback'] = ! is_wp_error( $rollback_result );
					if ( is_wp_error( $rollback_result ) ) {
						$stats['rollback_code'] = $rollback_result->get_error_code();
						$stats['rollback_data'] = $rollback_result->get_error_data();
					}
				} else {
					$stats['error_code'] = $result->get_error_code();
					$stats['error_data'] = $result->get_error_data();
				}
			}

			wp_version_check( $stats );
		}

		WP_Upgrader::release_lock( 'core_updater' );

		return $result;