User:Halscode/Atomic Alpine

Warning: This article is an early draft, it may be incomplete, and it may still miss some things. You should refer instead to Immutable root with atomic upgrades.
Specifically, this is a draft I (Halscode) am working on as I build my own system based on the linked guide. See Talk:Immutable root with atomic upgrades to see the changes I made to get the guide to work.

Todo: Add the/an everywhere, use Template:Cat and Template:Cmd instead of pre tags where appropriate

What?

This article provides a basic guide to setting up a read-only-root-based Alpine Linux system with several boot environments and atomic upgrades using a modern bootloader and btrfs.

Why?

Read-only root and atomic upgrades with the ability to easily rollback or boot previous configurations is a concept that has been gaining popularity recently. Distributions providing and promoting such features, for example, are Fedora Silverblue, Opensuse MicroOS, NixOS and GNU Guix.

While Alpine Linux has its killer features, it lacks the ones mentioned above on default setup. This is a proof of concept that it's possible to implement them in a minimal way on a minimal system.

Note: Alpine Linux can also boot from RAM in diskless mode (see Installation) which supports preserving changes between reboots using lbu.

Preparation

You should have bootable Alpine media. The process to obtain it is described on the installation page.

If you cannot connect to the Internet via Ethernet, you will need to install and set up wpa_supplicant to connect to the Internet. Follow the linked instructions until the header "Automatic Configuration on System Boot"; since the installer system is not permanent, that step is not necessary. The standard image does not include iwd, so you must use wpa_supplicant for the setup process.

The installer will need several packages that are not available on the base images. Set up your network and repository:

# setup-interfaces  # if you've already set up wifi, you can skip this
# setup-apkrepos

Partitioning disks

In this guide, it's assumed that you have a fresh UEFI system without an OS and have just booted into a live Alpine system using a USB flash drive or CD.

Note: If you want to reuse a partition for data, it must have a btrfs filesystem using subvolumes on it already (since it will store /var and /home), or you must already have separate partitions for /var and /home.

If you aren't planning to re-use an existing partition, create a new, clean partition table on your target storage device (we'll use /dev/sda):

# apk add gptfdisk
# gdisk /dev/sda
> o ↵  # creates a new GPT partition table
> y ↵  # confirm
> w ↵  # commits this new table to disk
> y ↵  # confirm

Now we can define the partitions:

# cgdisk /dev/sda

Partition creation process consists of several steps:

1. Start sector - you can safely use default value by pressing ↵
2. Size
3. Type (as hex code) - EFI is ef00, Linux filesystem is 8300, Swap is 8200.

Result table:

Part.     #     Size        Partition Type            Partition Name
----------------------------------------------------------------
1               200.0 MiB   EFI System                EFI
2               200.0 GiB   Linux filesystem          ROOT
3               32.0 GiB    Linux swap                SWAP

ROOT partition name will later be used in rEFInd configuration to identify boot volume.

Note: We recommend giving your root partition at least 20 GiB of space.

Tip: If you want to put the swap partition at the end, set its start sector to, for example, -16G. This gives it 16 GiB of space to work with, so the size should remain default (which will be 16GiB). The swap size should be between as much RAM as you have and double that.
If you don't use swap, you may encounter performance issues and you will not be able to suspend to disk.

You will likely need to manually re-read the partition table to get the block devices needed for the next step. To do this:

# partprobe /dev/sda
# mdev -s

Next step is creating filesystems:

# apk add btrfs-progs  # includes mkfs.btrfs
# mkfs.vfat -F32 /dev/sda1  # EFI
# mkfs.btrfs /dev/sda2  # ROOT
# mkswap /dev/sda3  # SWAP

Now we can mount our root volume:

# mount -t btrfs /dev/sda2 /mnt

File system structure

Now we should create the file structure that would provide reliable atomic system upgrades.
Start with the following directories:

# mkdir /mnt/next

Stores next current link, is necessary due to how busybox mv does atomic link replacement.

# mkdir /mnt/commons

Stores common non-snapshotting subvolumes.
We may populate it right away:

# btrfs subvolume create /mnt/commons/@var
# btrfs subvolume create /mnt/commons/@home

Let's go ahead and add a couple needed folders for /var before they become an issue:

# mkdir /mnt/commons/@var/empty
# mkdir /mnt/commons/@var/tmp  # this will be mounted over

Next, the most important directories:

# mkdir /mnt/snapshots

Stores directories containing snapshots belonging to one generation.

# mkdir /mnt/links

Stores generations of directories containing links to snapshot generations.

Let's create the first generation and populate it with one OS root snapshot @:

# NEWSNAPSHOTS="$(date -u +"%Y%m%d%H%M%S")$(cat /dev/urandom | tr -dc 'a-zA-Z' | fold -w 8 | head -n 1)"
# mkdir "/mnt/snapshots/$NEWSNAPSHOTS"
# btrfs subvolume create /mnt/snapshots/$NEWSNAPSHOTS/@
# btrfs subvolume create /mnt/snapshots/$NEWSNAPSHOTS/@etc

Populate links:

# NEWLINKS="$(date -u +"%Y%m%d%H%M%S")$(cat /dev/urandom | tr -dc 'a-zA-Z' | fold -w 8 | head -n 1)"
# mkdir "/mnt/links/$NEWLINKS"
# ln -s "../../snapshots/$NEWSNAPSHOTS" "/mnt/links/$NEWLINKS/0"
# ln -s "../../snapshots/$NEWSNAPSHOTS" "/mnt/links/$NEWLINKS/1"
# ln -s "../../snapshots/$NEWSNAPSHOTS" "/mnt/links/$NEWLINKS/2"
# ln -s "../../snapshots/$NEWSNAPSHOTS" "/mnt/links/$NEWLINKS/3"

You can have as many links as you like, just apply changes to rEFInd config and upgrade scripts described below accordingly.

Link that will point to latest links generation:

# ln -s "./links/$NEWLINKS" /mnt/current

This setup allows us to just have static rEFInd config that points to to /current/0/@, /current/1/@, etc. while the actual underlying boot environment will change with each upgrade.

But how will fs mounting services know which snapshot generation is currently loaded?
The answer is common fstab in the btrfs root.

Get UUIDs of the partitions first (this puts them in the file for easy access):

# blkid > /mnt/fstab

Tip: You may want to install vim or nano here to make editing files easier.

Now edit fstab accordingly:

# vi /mnt/fstab

Replace the UUIDs below, but it should look like:

UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 / btrfs subvol=CURRENT_SNAPSHOTS_PATH/@,ro,noatime 0 0
UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 /etc btrfs subvol=CURRENT_SNAPSHOTS_PATH/@etc,rw,noatime 0 0
UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 /var btrfs subvol=/commons/@var,rw,noatime 0 0
UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 /home btrfs subvol=/commons/@home,rw,noatime 0 0
# UUID=2FE6-837A /boot/efi vfat rw,noatime,discard 0 2
tmpfs /tmp tmpfs mode=1777,noatime,nosuid,nodev,size=2G 0 0
tmpfs /var/tmp tmpfs mode=1777,noatime,nosuid,nodev,size=2G 0 0
UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 swap swap rw,noatime,discard 0 0

CURRENT_SNAPSHOTS_PATH will be replaced by sysmut with, for example, /snapshots/20210411212549sdBXyLxg, and the result will be piped into /etc/fstab of a created snapshot while applying it.

Tip: If you want to persistently edit fstab later, remount the full root partition (i.e. mount -t btrfs /dev/sda2 /mnt) and edit /mnt/fstab. You do not have to do this within sysmut, but it is recommended to do it that way, since your changes will then be applied for the next boot when you exit.

Here's what the root btrfs volume structure mounted on /mnt should look like now:

|--mnt
| |--commons
| | |--@var
| | |--@home
| |--current
| |--fstab
| |--links
| | |--20210411213742qwrXAJBz
| | | |--0
| | | |--1
| | | |--2
| | | |--3
| |--next
| |--snapshots
| | |--20210411212549sdBXyLxg
| | | |--@
| | | |--@etc

Base system install

With the directory structure prepared, we can begin installing a basic Alpine Linux system.
Considering that installation is done from Alpine system, we only need following parts of the process:

# apk -X https://dl-cdn.alpinelinux.org/alpine/latest-stable/main -U --allow-untrusted -p /mnt/snapshots/20210411212549sdBXyLxg/@ --initdb add alpine-base

Copy the contents of /etc to the etc subvolume:

# cp -r /mnt/snapshots/20210411212549sdBXyLxg/@/etc/* /mnt/snapshots/20210411212549sdBXyLxg/@etc

Now we can setup basic chroot to complete the installation process:

# export SNP="/mnt/snapshots/20210411212549sdBXyLxg/@"
#     # tip: use tab to fill in the correct snapshot; this one is just an example

# mount -o bind /dev $SNP/dev
# mount -t proc none $SNP/proc
# mount -t sysfs sys $SNP/sys

# sed "s#CURRENT_SNAPSHOTS_PATH#/snapshots/20210411212549sdBXyLxg#g" /mnt/fstab > "$SNP/etc/fstab"

# cp -L /etc/resolv.conf "$SNP/etc/"  # so DNS still works
# chroot "$SNP" /bin/sh

# mount -a

As soon as you're in chroot, define repositories:

# echo "https://dl-cdn.alpinelinux.org/alpine/latest-stable/main" >> /etc/apk/repositories

This example shows only main, but you should also add testing and community if you need any packages in those. You can do this by repeating that command, but with testing or community instead of main accordingly.

Now it's time for the firmware, kernel, and btrfs packages:

# setup-apkrepos
# apk add -U linux-firmware linux-lts btrfs-progs

Tip: You may want to change linux-firmware to a custom set of firmware packages suitable for your system, for example, linux-firmware-amd linux-firmware-amd-ucode linux-firmware-amdgpu linux-firmware-ath10k linux-firmware-qca for a typical AMD laptop.

It's also important to add the btrfs feature to mkinitfs.conf and run mkinitfs manually:

# vi /etc/mkinitfs/mkinitfs.conf
# mkinitfs "$(ls /lib/modules)"

These steps prepare the kernel and generate the initramfs, which will be used later to boot from our first snapshot.

Now, you should add the sysmut and syscln, scripts, which you'll use to upgrade the system, add new software if you need, and clean up old snapshots.

Warning: If you do not add sysmut, you'll have to do its steps manually in the future to update your system. Please add it, or find or create a suitable alternative that is tailored to this configuration.

This would be a good time to:

• install and set up doas
• run setup-desktop
• install NetworkManager if you installed GNOME with setup-desktop (and if you need WiFi, networkmanager-wifi as well)
• Make sure you install execline so you can run sysmut and syscln!
• install wpa_supplicant or iwd to use WiFi
• install alternate shells, such as zsh, bash, and/or fish (and chsh them as you wish)

Warning: In case your PC only has wireless connection, make sure you install wireless software, like iwd or wpa_supplicant, so you will not end up severed from network on your first boot. The network connection from the installer does not carry over onto the installed system! So if you need WiFi and you don't do this, you may not be able to get it back!

Configure the system

Start with setting a password for the root:

# passwd root

Don't forget to add essential services to their respective runlevels:

rc-update add devfs sysinit
rc-update add dmesg sysinit
rc-update add mdev sysinit
rc-update add hwdrivers sysinit

rc-update add hwclock boot
rc-update add modules boot
rc-update add sysctl boot
rc-update add hostname boot
rc-update add bootmisc boot
rc-update add syslog boot

rc-update add mount-ro shutdown
rc-update add killprocs shutdown
rc-update add savecache shutdown

With the snapshot prepared and configured, we can chroot out of it and unmount everything:

# umount -a
# exit

Todo: There's still something weird here with the mounts. Running umount -a may make the next commands fail. Maybe we need to be more specific about what should be umounted (/etc, /home, /var, /dev, /proc, /sys?)

Finish editing the snapshot by setting the ro flag and unmounting the root volume:

# btrfs property set -ts "/mnt/snapshots/20210411212549sdBXyLxg/@" ro true
# umount /mnt

Bootloader installation

Mount the EFI partition:

# mount -t vfat /dev/sda1 /mnt
# mkdir /mnt/EFI

Bootloader configuration

There are 2 options as examples of the bootloader installation: rEFInd and GRUB. Sometimes one of them will refuse to work on a system for no particular reason, in this case try the other one.

rEFInd

Check the latest version number of the refind package:

# echo "https://dl-cdn.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories
# apk update
# apk info refind

Manually download the latest version (replace 0.13.2-r3 in the example below) of the refind package:

# wget https://dl-cdn.alpinelinux.org/alpine/edge/testing/x86_64/refind-0.13.2-r3.apk

Unpack the prepared rEFInd archive and copy relevant files to /mnt/EFI/

# tar -xzf refind-0.13.2-r3.apk
# cp -r usr/share/refind /mnt/EFI/
# cd /mnt/EFI/refind

Rename config file and edit it:

# mv refind.conf-sample refind.conf
# vi refind.conf

And append following to the end of the file, remember to replace example UUIDs with your own for root (btrfs partition) and resume (swap partition). Keep in mind that if you named the btrfs volume other than ROOT during "Partitioning disks" stage, you have to change the volume field below accordingly.

# ONLY include the below line on x86_64 systems: this works around a bug in rEFInd
scan_driver_dirs EFI/refind/drivers_x86_64

menuentry "Alpine Linux" {
    icon /EFI/refind/icons/os_linux.png
    volume "ROOT"
    loader /current/0/@/boot/vmlinuz-lts
    initrd /current/0/@/boot/initramfs-lts
    options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/0/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"

    submenuentry "Boot fallback 1" {
        loader /current/1/@/boot/vmlinuz-lts
        initrd /current/1/@/boot/initramfs-lts
        options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/1/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"
    }

    submenuentry "Boot fallback 2" {
        loader /current/2/@/boot/vmlinuz-lts
        initrd /current/2/@/boot/initramfs-lts
        options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/2/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"
    }

    submenuentry "Boot fallback 3" {
        loader /current/3/@/boot/vmlinuz-lts
        initrd /current/3/@/boot/initramfs-lts
        options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/3/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"
    }
}

Note: "ROOT" is the PARTLABEL of the btrfs partition. You may also use PARTUUID instead. To get both blkid from blkid package can be used. blkid included in busybox does not provide this information.
You cannot use labels in options. You'll need to replace it with the partition's UUID.

All you have to do now is reboot, remove your media, and you should be done:

# umount /mnt
# reboot

Using the overlaytmpfs option

Note: This section is optional.

You can use overlayfs with tmpfs built into Alpine's init script to allow changes in the rootfs which will be automatically reverted upon reboot. To make use of this, add overlaytmpfs to the kernel boot options in refind.conf, e.g.:

...
    initrd /current/0/@/boot/initramfs-lts
    options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/0/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 overlaytmpfs quiet splash"

    submenuentry "Boot fallback 1" {
...

You may want to do this to each submenuentry as well, or instead create separate submenuentries for each snapshot. So instead, the file might look like:

# ONLY include the below line on x86_64 systems: this works around a bug in rEFInd
scan_driver_dirs EFI/refind/drivers_x86_64

menuentry "Alpine Linux" {
    icon /EFI/refind/icons/os_linux.png
    volume "ROOT"
    loader /current/0/@/boot/vmlinuz-lts
    initrd /current/0/@/boot/initramfs-lts
    options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/0/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"

    submenuentry "Boot fallback 1" {
        loader /current/1/@/boot/vmlinuz-lts
        initrd /current/1/@/boot/initramfs-lts
        options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/1/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"
    }

    submenuentry "Boot fallback 2" {
        loader /current/2/@/boot/vmlinuz-lts
        initrd /current/2/@/boot/initramfs-lts
        options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/2/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"
    }

    submenuentry "Boot fallback 3" {
        loader /current/3/@/boot/vmlinuz-lts
        initrd /current/3/@/boot/initramfs-lts
        options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/3/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"
    }

    submenuentry "Boot current with overlaytmpfs" {
        loader /current/0/@/boot/vmlinuz-lts
        initrd /current/0/@/boot/initramfs-lts
        options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/0/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 overlaytmpfs quiet splash"
    }

    submenuentry "Boot fallback 1 with overlaytmpfs" {
        loader /current/1/@/boot/vmlinuz-lts
        initrd /current/1/@/boot/initramfs-lts
        options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/1/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 overlaytmpfs quiet splash"
    }

    submenuentry "Boot fallback 2 with overlaytmpfs" {
        loader /current/2/@/boot/vmlinuz-lts
        initrd /current/2/@/boot/initramfs-lts
        options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/2/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 overlaytmpfs quiet splash"
    }

    submenuentry "Boot fallback 3 with overlaytmpfs" {
        loader /current/3/@/boot/vmlinuz-lts
        initrd /current/3/@/boot/initramfs-lts
        options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/3/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 overlaytmpfs quiet splash"
    }
}

GRUB

# apk add grub-efi

GRUB requires two configuration files this time as we will use grub-mkstandalone. The first configuration file is internal and should only point to the second file, where we store the menu:

# cd /tmp
# vi grub_internal.cfg

Set the contents to the following, but make sure to replace 2FE6-837A with your own EFI partition UUID:

insmod part_gpt
insmod fat
search --set efi --fs-uuid 2FE6-837A
configfile (${efi})/EFI/grub/grub.cfg

The second config file is the main config where we describe the entire boot menu.

# vi grub.cfg

Set to contain, but replace UUIDs:

set timeout=3
menuentry "Alpine Linux Current" {
	search --set root --fs-uuid b9ff5e7b-e128-4e64-861a-2fdd794a9828
	linux /current/0/@/boot/vmlinuz-edge root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/0/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash
	initrd /current/0/@/boot/initramfs-edge
}
menuentry "Alpine Linux Snapshot 1" {
	search --set root --fs-uuid b9ff5e7b-e128-4e64-861a-2fdd794a9828
	linux /current/1/@/boot/vmlinuz-edge root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/1/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash
	initrd /current/1/@/boot/initramfs-edge
}
menuentry "Alpine Linux Snapshot 2" {
	search --set root --fs-uuid b9ff5e7b-e128-4e64-861a-2fdd794a9828
	linux /current/2/@/boot/vmlinuz-edge root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/2/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash
	initrd /current/2/@/boot/initramfs-edge
}
menuentry "Alpine Linux Snapshot 3" {
	search --set root --fs-uuid b9ff5e7b-e128-4e64-861a-2fdd794a9828
	linux /current/3/@/boot/vmlinuz-edge root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/3/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash
	initrd /current/3/@/boot/initramfs-edge
}

Generate the grubx64.efi binary:

# grub-mkstandalone -O x86_64-efi -o grubx64.efi "boot/grub/grub.cfg=/tmp/grub_internal.cfg"
# mkdir /mnt/EFI/grub
# mv grubx64.efi /mnt/EFI/grub/
# mv grub.cfg /mnt/EFI/grub/

Adding EFI boot entry

To add the chosen bootloader to UEFI, efibootmgr is a suitable tool. The following example is for rEFInd, but could be easily adjusted for GRUB:

# apk add efibootmgr
# efibootmgr --create --disk /dev/sda --part 1 --loader /EFI/refind/refind_x64.efi --label "rEFInd" --verbose

/dev/sda is our disk device and 1 is the number of the FAT32 partition containing the bootloader data.

Updating or altering the system

Warning: Without the following step or an alternative you will have no easy way to mutate the installed system.

Warning: These examples are implemented using execline and require the execline package in the system.

Note: These could surely be implemented in POSIX shell, however, execline provides a number of runtime advantages and the resulting script is much more readable.

# touch /usr/sbin/sysmut
# chmod +x /usr/sbin/sysmut
# vi /usr/sbin/sysmut

Example script to mutate the system:

#!/bin/execlineb -W
unshare --mount
importas -D 0 source 1
define mnt /media/root
if { mkdir -p ${mnt} }
if { mount -t btrfs -o rw,noatime UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 ${mnt} }
foreground {
	backtick -E dt {
		date -u +%Y%m%d%H%M%S
	}
	backtick -E rnd {
		pipeline { cat /dev/urandom }
		pipeline { tr -dc a-zA-Z }
		pipeline { fold -w 8 }
		head -n 1
	}
	define newsnap ${dt}${rnd}
	if { mkdir -p ${mnt}/snapshots/${newsnap} }
	if { btrfs subvolume snapshot ${mnt}/current/${source}/@ ${mnt}/snapshots/${newsnap}/@ }
	if { btrfs subvolume snapshot ${mnt}/current/${source}/@etc ${mnt}/snapshots/${newsnap}/@etc }
	if {
		redirfd -w 1 ${mnt}/snapshots/${newsnap}/@etc/fstab
			sed s#CURRENT_SNAPSHOTS_PATH#/snapshots/${newsnap}#g ${mnt}/fstab
	}
	if { mount -t proc none ${mnt}/snapshots/${newsnap}/@/proc }
	if { mount -t sysfs sys ${mnt}/snapshots/${newsnap}/@/sys }
	if { mount -o bind,ro /dev ${mnt}/snapshots/${newsnap}/@/dev }
	foreground {
		foreground { mount -o bind,ro /etc/resolv.conf ${mnt}/snapshots/${newsnap}/@etc/resolv.conf }
		foreground {
			chroot ${mnt}/snapshots/${newsnap}/@
			foreground { mount -a }
            foreground { echo "=== Entering sysmut (make your changes now)" }
			foreground { sh }
			importas apply ?
			foreground { umount -a }
			exit ${apply}
		}
		importas apply ?
		foreground { redirfd -w 2 /dev/null umount ${mnt}/snapshots/${newsnap}/@etc/resolv.conf }
		ifelse { exit ${apply} } {
			if { btrfs property set -ts ${mnt}/snapshots/${newsnap}/@ ro true }
			define newlink ${dt}${rnd}
			if { mkdir -p ${mnt}/links/${newlink} }
			if { ln -s ../../snapshots/${newsnap} ${mnt}/links/${newlink}/0 }
			if { cp -P ${mnt}/current/0 ${mnt}/links/${newlink}/1 }
			if { cp -P ${mnt}/current/1 ${mnt}/links/${newlink}/2 }
			if { cp -P ${mnt}/current/2 ${mnt}/links/${newlink}/3 }
			if { mkdir -p ${mnt}/next }
			if { ln -sfn ./links/${newlink} ${mnt}/next/current }
			if { mv ${mnt}/next/current ${mnt}/ }
			echo "=== Changes applied"
		}
        # Remove failed snapshot
        btrfs subvolume delete ${mnt}/snapshots/${newsnap}/@
        btrfs subvolume delete ${mnt}/snapshots/${newsnap}/@etc
        rm -r ${mnt}/snapshots/${newsnap}
		echo "=== Changes discarded"
	}
	foreground { redirfd -w 2 /dev/null umount ${mnt}/snapshots/${newsnap}/@/proc }
	foreground { redirfd -w 2 /dev/null umount ${mnt}/snapshots/${newsnap}/@/sys }
	redirfd -w 2 /dev/null umount ${mnt}/snapshots/${newsnap}/@/dev
}
umount ${mnt}

It will get you into the root shell chrooted into the new snapshot, where you can apply any change you like. The origin of the new snapshot is defined by the first and only argument, in form of number. If no argument provided the 0 (current latest) is taken as origin.
If chroot shell exits with an error, there will be no switch to the new snapshots. This means you can manually discard changes while in the chroot by:

# exit 1

Deleting unused snapshots

Unused snapshots can be garbage-collected by:

# touch /usr/sbin/syscln
# chmod +x /usr/sbin/syscln
# vi /usr/sbin/syscln

#!/bin/execlineb -W
unshare --mount
define mnt /media/root
if { mkdir -p ${mnt} }
if { mount -t btrfs -o rw,noatime,compress=zstd:3 UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 ${mnt} }
foreground {
	foreground {
		pipeline {
			foreground {
				pipeline {
					find -H ${mnt}/snapshots/ -maxdepth 1 -mindepth 1 -print0
				}
				xargs -0 -r realpath
			}
			pipeline {
				find -H ${mnt}/current/ -maxdepth 1 -mindepth 1 -print0
			}
			xargs -0 -r realpath
		}
		pipeline { tr \\n \\0 }
		pipeline { sort -z }
		pipeline { uniq -u -z }
		pipeline { xargs -0 -r -n 1 -I [] find -H [] -maxdepth 1 -mindepth 1 -print0 }
		xargs -0 -r btrfs subvolume delete
	}
	foreground { find -H ${mnt}/snapshots/ -maxdepth 1 -mindepth 1 -empty -type d -delete }
	foreground {
		pipeline {
			foreground {
				pipeline {
					find -H ${mnt}/links/ -maxdepth 1 -mindepth 1 -print0
				}
				xargs -0 -r realpath
			}
			realpath ${mnt}/current
		}
		pipeline { tr \\n \\0 }
		pipeline { sort -z }
		pipeline { uniq -u -z }
		pipeline { xargs -0 -r -n 1 -I [] find -H [] -maxdepth 1 -mindepth 1 -print0 }
		xargs -0 -r -n 1 unlink
	}
	find -H ${mnt}/links/ -maxdepth 1 -mindepth 1 -empty -type d -delete
}
umount ${mnt}