Recovering from FreeBSD bootloader crash

A couple of days ago, after switching on my laptop, the boot process stopped prematurely still within the FreeBSD boot loader, showing me this obscure message ZFS: i/o error - all block copies unavailable:

No further interaction was possible so it basically "paniced" within the boot loader. And of course I did panic too, at least a bit, as the last full backup was before I left home roughly two month ago. The night before the crash I was doing a forceful package update using pkg upgrade -f mainly because the update to FreeBSD 13.2-RELEASE did not went too well thanks to my own stupidity of updating the packages before doing the second stage of freebsd-update install. Most likely, during pkg upgrade -f, the ZFS filesystem got exhausted and for some unknown reason destroyed the boot loader. Or some other obscure corruption happend.

How to fix the corrupt boot loader

The article Recover a FreeBSD system using a liveUSB helped a lot.

First step: I got a FreeBSD live USB image (FreeBSD 13.1, as 13.2 did not boot properly) and booted into it.

Then: Before trying to fix anything, I first checked if I can still mount my ZFS root filesystem and data partitions:

# The root ZFS file system is on my NVME SSD (nvd0)
# located in the 4th GPT partition (nvd0p4).
# It is `geli` encrypted, so we first need to attach it.
geli attach /dev/nvd0p4

# Import the zroot pool
zpool import -fR /mnt zroot

# Mount the ROOT filesystem
mount -t zfs zroot/ROOT/default /mnt

Luckily this did not show any corruption. So I continued with re-initializing the boot loader as can be seen on the second half of the following screenshot:

Or in text form:

# Show where freebsd-boot resides
gpart show nvd0

# Re-initialize it (partition 2)
gpart bootcode -b /boot/zfsboot -i 2 nvd0

After executing these commands I was able to boot normally into FreeBSD.