HOWTO Freeze Linux

home directory is on a btrfs partition (/dev/sda6 in this example)
/mnt is available on boot for temporaryly mounting a fs
read-only version of home is a read-only snapshot named home-ro in btrfs root
the name home-rw is not used on the btrfs fs
wipe is installed in /usr/local/bin
systemd is used and wipe.service is installed in etc/systemd/system
run systemd enable wipe.service to enable wiping
if anything fails on boot, home is mounted from fstab (99% of cases; except when exit code is 4)
user login is delayed until wiping is done

#!/bin/bash
if mount /dev/sda6 /mnt; then
	btrfs subvolume delete /mnt/home-rw
	if btrfs subvolume snapshot /mnt/home-ro /mnt/home-rw; then
		if umount /home; then
			if mount -o subvol=home-rw /dev/sda6 /home; then
				ret=0
			else
				ret=4
			fi
		else
			ret=3
		fi
	else
		ret=2
	fi
else
	ret=1
fi
umount /mnt
if [ $? -ne 0 ]; then
	ret=1"$ret"
fi
 
exit $ret

wipe.service

[Unit]
Description=Wipe home directory modifications
Requires=local-fs.target
After=local-fs.target
Before=systemd-user-sessions.service
 
[Install]
WantedBy=systemd-user-sessions.service
 
[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/local/bin/wipe