Mount LUKS Btrfs for arch-chroot

LUKS and BTRFS make for an interesting combination if you ever need to perform some work on another drive. This is a step-by-step procedure I wrote for myself and thought it would be useful to share with others.

The ArchWiki already covers how to do this if you combine a few separate articles, but I wanted a unified document.

Mounting

Identify the drive to mount.

 $ sudo lsblk
 NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
 ...output trimmed...
 nvme1n1     259:0    0 931.5G  0 disk 
 ├─nvme1n1p1 259:1    0     1G  0 part 
 └─nvme1n1p2 259:2    0 930.5G  0 part

 $ export DRIVE="nvme1n1"

I have trimmed the output to show only the drive applicable here. I am also exporting it as a variable to use later.

Decrpyt the drive, if encrypted, otherwise skip this step.
```
 $ sudo --preserve-env=DRIVE cryptsetup luksOpen /dev/"${DRIVE}p2" luksvol
 Enter passphrase for /dev/nvme1n1p2: 
```
Note: If the prompt appears to freeze, be patient. It takes a few seconds to decrypt.
Mount the @ subvolume.
```
 $ sudo mount --options subvol=@ /dev/mapper/luksvol /mnt
```
The @ subvolume is the / directory. It needs to be mounted first.

Mount the @home subvolume.

 $ sudo mount --options subvol=@home /dev/mapper/luksvol /mnt/home

Mount the @pkg subvolume.

 $ sudo mount --options subvol=@pkg /dev/mapper/luksvol /mnt/var/cache/pacman/pkg

Mount the @log subvolume.

 $ sudo mount --options subvol=@log /dev/mapper/luksvol /mnt/var/log

Mount the @snapshots subvolume.

 $ sudo mount --options subvol=@snapshots /dev/mapper/luksvol /mnt/.snapshots

The following message may appear if this subvolume doesn’t exist. You can safely proceed.

 mount: /mnt/.snapshots: fsconfig() failed: No such file or directory.

Mount the boot partition. This is critical if you want to update the kernel or bootloader.
```
 $ sudo --preserve-env=DRIVE mount /dev/"${DRIVE}p1" /mnt/boot
```
Finally you can arch-chroot into the mounted system.
```
 $ sudo arch-chroot /mnt
```

Cleanup

After exiting the arch-chroot, unmount everything.

$ sudo umount --recursive /mnt

If the drive was encrypted, reencrypt.

$ sudo cryptsetup close luksvol

References:

https://wiki.archlinux.org/title/Dm-crypt
https://wiki.archlinux.org/title/Chroot#Running_on_Btrfs