Comment 2 for bug 1945874

Sure. This is a mode I've been using lately where I'm using legacy
mountpoints on datasets out of fstab. I suspect this would do the same
thing with a traditional inherited mount hierarchy.

# cat /etc/fstab
tank/zroot / zfs defaults 0 0
tank/home /home zfs defaults 0 0
tank/usr/src /usr/src zfs defaults 0 0
tank/var/mail /var/mail zfs defaults 0 0
tank/home/mason /home/mason zfs defaults 0 0
tank/var/log /var/log zfs defaults 0 0
tank/tmp /tmp zfs defaults 0 0
/dev/md0 /boot ext4 defaults 0 1
/dev/sda2 /boot/efi0 vfat defaults 0 1
/dev/sdb2 /boot/efi1 vfat defaults 0 1
/dev/mapper/swap none swap sw 0 0

# update-grub
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
+ set -e
+ prefix=/usr
+ datarootdir=/usr/share
+ ubuntu_recovery=1
+ quiet_boot=1
+ quick_boot=1
+ gfxpayload_dynamic=1
+ vt_handoff=1
+ . /usr/share/grub/grub-mkconfig_lib
+ prefix=/usr
+ exec_prefix=/usr
+ datarootdir=/usr/share
+ datadir=/usr/share
+ bindir=/usr/bin
+ sbindir=/usr/sbin
+ [ x/usr/share/grub = x ]
+ test x = x
+ grub_probe=/usr/sbin/grub-probe
+ test x = x
+ grub_file=/usr/bin/grub-file
+ test x = x
+ grub_mkrelpath=/usr/bin/grub-mkrelpath
+ which gettext
+ :
+ grub_tab=
+ export TEXTDOMAIN=grub
+ export TEXTDOMAINDIR=/usr/share/locale
+ set -u
+ which zfs
+
+ imported_pools=
+ mktemp -d /tmp/zfsmnt.XXXXXX
+ MNTDIR=/tmp/zfsmnt.TssEDF
+ mktemp -d /tmp/zfstmp.XXXXXX
+ ZFSTMP=/tmp/zfstmp.v6r2Ln
+ uname -m
+ machine=x86_64
+ GENKERNEL_ARCH=x86_64
+ RC=0
+ trap on_exit EXIT INT QUIT ABRT PIPE TERM
+ GRUB_LINUX_ZFS_TEST=
+ import_pools
+ zpool list
+ awk {if (NR>1) print $1}
+ local initial_pools=tank
+ local all_pools=
+ local imported_pools=
+ local err=
+ set +e
+ zpool import -f -a -o cachefile=none -o readonly=on -N
+ err=no pools available to import
+ [ 0 -ne 0 ]
+ set -e
+ zpool list
+ awk {if (NR>1) print $1}
+ all_pools=tank
+ echo tank
+ grep -wq tank
+ continue
+ echo
+ imported_pools=
+ bootlist /tmp/zfsmnt.TssEDF
+ local mntdir=/tmp/zfsmnt.TssEDF
+ local boot_list=
+ get_root_datasets
+ zpool list
+ awk {if (NR>1) print $1}
+ local pools=tank
+ zpool get -H altroot tank
+ awk {print $3}
+ local rel_pool_root=-
+ [ - = - ]
+ rel_pool_root=/
+ zfs list -H -o name,canmount,mountpoint -t filesystem
+ awk {print $1}
+ grep -E ^tank(\s|/[[:print:]]*\s)(on|noauto)\s/$
+ echo
+ boot_list=
+ generate_grub_menu_metadata
+ local bootlist=
+ get_machines_sorted
+ local bootlist=
+ echo
+ + awk {print $3}
sort -u
+ local machineids=
+ + sort -nr
awk {print $2}
+ menu_metadata=
+ generate_grub_menu
+ local menu_metadata=
+ local last_section=
+ local main_dataset_name=
+ local main_dataset=
+ local have_zsys=
+ [ -z ]
+ return
+ grub_menu=
+ [ -n ]
+ on_exit
+ mountpoint -q /tmp/zfsmnt.TssEDF
+ true
+ rmdir /tmp/zfsmnt.TssEDF
+ rm -rf /tmp/zfstmp.v6r2Ln
+ exit 0
done

To reiterate, though, 10_linux does the right thing if it's allowed to
probe for entries. Maybe a good solution would be to let it, and save a
few lines and some complexity in 10_linux_zfs.