Update-grub fails to populate menu with existing ZFS root installation

[Chris Box]

I had an existing ZFS system running 18.04 which I upgraded to 20.04. It uses an ext4 /boot, but / is ZFS. So I am expecting /boot/grub/grub.cfg to contain this:

        set root='hd0,gpt2'
        linux /vmlinuz-5.4.0-31-generic root=ZFS=ssd/root ro boot=zfs rpool=ssd bootfs=ssd/root
        initrd /initrd.img-5.4.0-31-generic

However after do-release-upgrade, the above was absent and after reboot all I had was an option to go into UEFI bios settings, plus my 40_custom entries.

The cause of this is that both these sections were empty:

### BEGIN /etc/grub.d/10_linux ###
### END /etc/grub.d/10_linux ###

### BEGIN /etc/grub.d/10_linux_zfs ###
### END /etc/grub.d/10_linux_zfs ###

Looking at those scripts, 10_linux outputs nothing because it aborts early, due to the presence of 10_linux_zfs installed by focal's grub-common 2.04-1ubuntu26:

    xzfs)
        # We have a more specialized ZFS handler, with multiple system in 10_linux_zfs.
        if [ -e "`dirname $(readlink -f $0)`/10_linux_zfs" ]; then
          exit 0
        fi

However 10_linux_zfs doesn't output anything either, although it is hard to read so I'm not sure why. Is 10_linux_zfs supposed to cover the cases where /boot is on ext4?

To recover my system, I renamed 10_linux_zfs and re-ran update-grub. This then generated the correct entries under ### BEGIN /etc/grub.d/10_linux ###.

As I say, I'd first like to understand which script has the responsibility of finding bootable kernels for this kind of layout, then we can work from there.

Thanks,
Chris