NixOS installation on multi-boot system with GRUB (from Arch installation) using an embedded configuration file

A while ago, I started playing around with Arch Linux on a spare computer. After following the (excellent) Beginners' guide I ended up with a rather nice LVM on LUKS setup. This spare computer has one physical (SSD) disk (/dev/sda), which is divided in /dev/sda1 (which is /boot, a bootable GRUB2 partition used and setup with Arch) and /dev/sda2 which is my cryptroot that, when unlocked, is a single large lvmpool with the full size of the partition /dev/sda2. I have installed Arch on 3 lvs (arch-root for the root filesystem, arch-home for the home folders and arch-swap for the swap partition), as seen in the image below:

Since I read lots of great things about NixOS, I am trying to install NixOS next to my current Arch install, so I created 3 more lvs similar to the arch-{root,home,swap} lvs above, and I followed the NixOS installation guide. I want to re-use my existing GRUB2 from Arch which resides on the (bootable) GRUB2 partition /dev/sda1. Of course, I am not the first one trying to achieve something like this or similar to this. However, the GRUB2 documentation on specifying devices is rather limited and cumbersome, especially in the more complex cases such as on my LVM on LUKS setup. With the help of the Unix & Linux StackExchange and some very useful information from Reddit user cookie_enthusiast on the /r/linuxquestions page on Reddit, I managed to get this working. To further spread and share this knowledge, as well as for my own future reference, I will write down my steps taken here, which are largely based on the aforementioned sources.

I installed NixOS by following the NixOS installation guide and modified the /etc/nixos/configuration.nix file to look like this:

# Use the GRUB 2 boot loader.
boot.loader.grub.enable = true;
boot.loader.grub.version = 2;
# Define on which hard drive you want to install Grub.
boot.loader.grub.device = "nodev";
boot.initrd.luks.devices = [ { name = "cryptroot"; device = "/dev/sda2"; preLVM = true; } ];

I also added nixpkgs.config.allowUnfree = true; to allow unfree packages which might be needed for your NixOS system configuration.

It turns out that GRUB2 works well with UUIDs, and not so well with device names. With this knowledge in mind, we need the following 4 UUIDs at hand, before we can (manually) create an extra GRUB menuentry to our NixOS grub.cfg configfile:

1. The UUID of the LUKS device.
2. The UUID of the LVM Volume Group.
3. The UUID of the LVM Logical Volume.
4. The UUID of the filesystem containing the grub.cfg file we want to load via the configfile directive in GRUB2.

I will list here how to obtain these four UUIDs:

1. Execute cryptsetup luksUUID /dev/sda2 and remove all the dashes (-) from the UUID, a0cb535a-8468-485f-a220-a5f49e85c9f4 would become a0cb535a8468485fa220a5f49e85c9f4 in my case.
2. Execute vgdisplay and look for the VG UUID, lvmpool with UUID 5atKN9-PQBi-T9wb-Iyz8-qP4y-HN2E-c5uLOT in my case.
3. Execute lvdisplay and look for the LV UUID of the LV Name or LV Path that contains your grub.cfg file, nix-root or /dev/lvmpool/nix-root with UUID C9zkjF-IHu0-qQkP-KgLf-8rAy-TVPu-HQ7gtj in my case.
4. Execute lsblk -p -o +UUID and look for the UUID of the Device Path that contains your grub.cfg file, /dev/mapper/lvmpool-nix--root with UUID cc6a06bb-336f-4e9f-a5f0-fdd43e7f548f in my case.

This will allow you to create the following extra GRUB menuentry for referencing our NixOS grub.cfg configfile, which is on my nix-root lv and because of the boot.loader.grub.device = "nodev"; in my /etc/nixos/configuration.nix there is no GRUB installed for my NixOS installation (in Arch, this would go into /etc/grub.d/40_custom):

menuentry 'NixOS' {
    insmod crypto
    insmod cryptodisk
    insmod luks
    insmod lvm
    cryptomount -u a0cb535a8468485fa220a5f49e85c9f4
    set root='lvmid/5atKN9-PQBi-T9wb-Iyz8-qP4y-HN2E-c5uLOT/C9zkjF-IHu0-qQkP-KgLf-8rAy-TVPu-HQ7gtj'
    search --fs-uuid --set=root cc6a06bb-336f-4e9f-a5f0-fdd43e7f548f
    configfile '/boot/grub/grub.cfg'
}

To clarify this even more, this contains some literal values such as lvmid which is not supposed to be replaced with the name or ID of your LVM. This is not properly documented anywhere, it seems. The same issue applies for when your put the UUID of your LUKS device in the cryptomount -u line with dashes in it, GRUB will just tell you Press any key to continue, which is (obviously) not very helpful.

The bare template for a manual GRUB menuentry to boot from crypt -> LVM -> root with an LVM on LUKS setup would thus be:

menuentry 'NixOS' {
    insmod crypto
    insmod cryptodisk
    insmod luks
    insmod lvm
    cryptomount -u <LUKS UUID without dashes>
    set root='lvmid/<LVM Volume Group UUID>/<LVM Logical Volume UUID>'
    search --fs-uuid --set=root <Filesystem UUID>
    configfile '/boot/grub/grub.cfg'
}

Again, I would like to owe a big thank you to Reddit user cookie_enthusiast and Alexander Batischev for helping me out here, as well as all other sources referenced earlier.