Suggested fix: 10_linux_zfs will not detect kernels if /boot/grub/grubenv exists on the root dataset

[Sam Van den Eynde]

Since a few weeks update-grub was no longer detecting my root-on-zfs install. This system does not follow the bpool/rpool logic as it got installed a long time ago.

I was able to pinpoint the issue to this part of the 10_linux_zfs script:

    if [ -n "$(ls ${candidate_path} 2>/dev/null)" ]; then
        echo "${candidate_path}"
        return
    fi

This code seems to identify candidate locations for /boot directories, and expects them to be empty. ZFS does not require /boot to be empty to mount the target boot dataset on it (overlay=on), but I assume this way some other candidate paths can easily be skipped quickly.

So I think I basically ran into this issue: https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1881442.

When grubenv gets created on the zfs root dataset during a failed boot sequence, 10_linux_zfs will skip it from then on, leaving grub.cfg without valid boot entries.

I was able to reproduce the issue by creating and removing /boot/grub on my root dataset, and fix it by allowing the 10_linux_zfs script to continue if only grub exists for candidate_path:

    if [ -n "$(ls ${candidate_path} 2>/dev/null)" ] && [ "$(ls ${candidate_path} 2>/dev/null)" != "grub" ]; then
        echo "${candidate_path}"
        return
    fi

Even when the grub-initrd-fallback.service bug gets addressed, I would allow for /boot/grub to exist on the root dataset. If not, that bug becomes quite critical so it seems.