apt upgrade failed

Main PC runs Ubuntu which is updated (apt update && apt upgrade) regularly.
The PC uses ZFS for everything, including boot.
Today’s upgrade failed, not enough space in the boot pool.
My bad really, several updates without running apt autoremove.

If you get stuck then use zsysctl to manually remove saved state, e.g.

# zfs list -r -t snapshot -o name,used,referenced,creation bpool/BOOT
# zsysctl state remove y3xem1 --system

(where y3xem1 is the state name, change this to the oldest state name returned by zfs list)