Make WordPress writable before, and read-only after, WordPress updates with this MU-Plugin

make-readonly.php

<?php
/**
 * Plugin Name: Make Read Only &amp; Clear Read Only WordPress
 * Plugin URI: https://www.saotn.org
 * Description: Makes all WordPress files read-only after updates, and writable just before updates. This should ease WordPress updates when it's read-only on the file system. Please donate <a rel="nofollow" target="_blank" href="https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&amp;hosted_button_id=J24FGGU96YSUY" title="donate to Sysadmins of the North in Dollars">$2.50 USD</a> (or <a rel="nofollow" target="_blank" href="https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&amp;hosted_button_id=QLX4RPPL3L8LY" title="donate to Sysadmins of the North in Euros">&euro; 2,50 Euro</a>) through PayPal to support me in my research time and hosting costs.
 * Network: True
 * Version: 1.0
 * Author: Jan Reilink
 * Author URI: https://www.saotn.org
 * License: GPLv2
 */

// change your path here:
$path = "d:/www/example.com/www";

function make_readonly($obj) {
	$chunks = explode('/', $obj);
	chmod($obj, '0600');
}
function make_writable($obj) {
	$chunks = explode('/', $obj);
	chmod($obj, '0755');
}
function pre_install_writeable($dir) {
	if($objs = glob($dir."/*")) {
		foreach($objs as $obj) {
			make_writable($obj);
			if(is_dir($obj)) pre_install_writeable($obj);
		}
	}
	error_log($dir ." made writable");
	return make_writable($dir);
}
function post_install_readonly($dir) {
	if($objs = glob($dir."/*")) {      
		foreach($objs as $obj) {
			make_readonly($obj);
			if(is_dir($obj)) post_install_readonly($obj);
		}
	}
	error_log($dir ." made readonly");
	return make_readonly($dir);
}

function _prefix_post_install_readonly() {
	global $path;
	post_install_readonly($path);
}

function _prefix_pre_install_writeable() {
	global $path;
	pre_install_writeable($path);
}

add_filter('upgrader_pre_install', '_prefix_pre_install_writeable', 10, 2);
add_filter('upgrader_post_install', '_prefix_post_install_readonly', 10, 2);
?>

DEMO code to make an already read-only WordPress site temporarily writable for WordPress Core-, Theme- and Plugin updates. Afterwards the WordPress site is made read-only again. Fun upgrader_pre_install() and upgrader_post_install() usage.

This code is tested only once to prove a point for myself and coworkers. Use this at your own risk.