Source

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

protected function after_core_update( $update_result ) {
		$wp_version = get_bloginfo( 'version' );

		$core_update = $update_result->item;
		$result      = $update_result->result;

		if ( ! is_wp_error( $result ) ) {
			$this->send_email( 'success', $core_update );
			return;
		}

		$error_code = $result->get_error_code();

		// Any of these WP_Error codes are critical failures, as in they occurred after we started to copy core files.
		// We should not try to perform a background update again until there is a successful one-click update performed by the user.
		$critical = false;
		if ( 'disk_full' === $error_code || false !== strpos( $error_code, '__copy_dir' ) ) {
			$critical = true;
		} elseif ( 'rollback_was_required' === $error_code && is_wp_error( $result->get_error_data()->rollback ) ) {
			// A rollback is only critical if it failed too.
			$critical        = true;
			$rollback_result = $result->get_error_data()->rollback;
		} elseif ( false !== strpos( $error_code, 'do_rollback' ) ) {
			$critical = true;
		}

		if ( $critical ) {
			$critical_data = array(
				'attempted'  => $core_update->current,
				'current'    => $wp_version,
				'error_code' => $error_code,
				'error_data' => $result->get_error_data(),
				'timestamp'  => time(),
				'critical'   => true,
			);
			if ( isset( $rollback_result ) ) {
				$critical_data['rollback_code'] = $rollback_result->get_error_code();
				$critical_data['rollback_data'] = $rollback_result->get_error_data();
			}
			update_site_option( 'auto_core_update_failed', $critical_data );
			$this->send_email( 'critical', $core_update, $result );
			return;
		}

		/*
		 * Any other WP_Error code (like download_failed or files_not_writable) occurs before
		 * we tried to copy over core files. Thus, the failures are early and graceful.
		 *
		 * We should avoid trying to perform a background update again for the same version.
		 * But we can try again if another version is released.
		 *
		 * For certain 'transient' failures, like download_failed, we should allow retries.
		 * In fact, let's schedule a special update for an hour from now. (It's possible
		 * the issue could actually be on WordPress.org's side.) If that one fails, then email.
		 */
		$send               = true;
		$transient_failures = array( 'incompatible_archive', 'download_failed', 'insane_distro', 'locked' );
		if ( in_array( $error_code, $transient_failures, true ) && ! get_site_option( 'auto_core_update_failed' ) ) {
			wp_schedule_single_event( time() + HOUR_IN_SECONDS, 'wp_maybe_auto_update' );
			$send = false;
		}

		$n = get_site_option( 'auto_core_update_notified' );
		// Don't notify if we've already notified the same email address of the same version of the same notification type.
		if ( $n && 'fail' === $n['type'] && get_site_option( 'admin_email' ) === $n['email'] && $n['version'] == $core_update->current ) {
			$send = false;
		}

		update_site_option(
			'auto_core_update_failed',
			array(
				'attempted'  => $core_update->current,
				'current'    => $wp_version,
				'error_code' => $error_code,
				'error_data' => $result->get_error_data(),
				'timestamp'  => time(),
				'retry'      => in_array( $error_code, $transient_failures, true ),
			)
		);

		if ( $send ) {
			$this->send_email( 'fail', $core_update, $result );
		}
	}