contrib/dracut: 90: mount essential datasets under root

This partly mirrors what the i-t script does (though that mounts all
children, recursively) ‒ /etc, /usr, /lib*, and /bin are all essential,
if present, to successfully invoke the real init, which will then mount
everything else it might need in the right order

The following extreme-case set-up boots w/o issues now:
  /               zoot            zfs  rw,relatime,xattr,noacl
  ├─/etc          zoot/etc        zfs  rw,relatime,xattr,noacl
  ├─/usr          zoot/usr        zfs  rw,relatime,xattr,noacl
  │ └─/usr/local  zoot/usr/local  zfs  rw,relatime,xattr,noacl
  ├─/var          zoot/var        zfs  rw,relatime,xattr,noacl
  │ ├─/var/lib    zoot/var/lib    zfs  rw,relatime,xattr,noacl
  │ ├─/var/log    zoot/var/log    zfs  rw,relatime,xattr,posixacl
  │ ├─/var/cache  zoot/var/cache  zfs  rw,relatime,xattr,noacl
  │ └─/var/tmp    zoot/var/tmp    zfs  rw,relatime,xattr,noacl
  ├─/home         zoot/home       zfs  rw,relatime,xattr,noacl
  │ └─/home/nab   zoot/home/nab   zfs  rw,relatime,xattr,noacl
  ├─/boot         zoot/boot       zfs  rw,relatime,xattr,noacl
  ├─/root         zoot/home/root  zfs  rw,relatime,xattr,noacl
  ├─/opt          zoot/opt        zfs  rw,relatime,xattr,noacl
  └─/srv          zoot/srv        zfs  rw,relatime,xattr,noacl

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11898

contrib/dracut/90zfs/zfs-lib.sh.in

@@ -23,6 +23,7 @@ command -v getargbool >/dev/null || {
 OLDIFS="${IFS}"
 NEWLINE="
 "
+TAB="	"
 
 ZPOOL_IMPORT_OPTS=""
 if getargbool 0 zfs_force -y zfs.force -y zfsforce ; then
@@ -58,7 +59,7 @@ find_bootfs() {
 # import_pool POOL
 #   imports the given zfs pool if it isn't imported already.
 import_pool() {
-        pool="${1}"
+    pool="${1}"
 
     if ! zpool list -H "${pool}" > /dev/null 2>&1; then
         info "ZFS: Importing pool ${pool}..."
@@ -71,26 +72,62 @@ import_pool() {
     return 0
 }
 
+_mount_dataset_cb() {
+    mount -o zfsutil -t zfs "${1}" "${NEWROOT}${2}"
+}
+
 # mount_dataset DATASET
 #   mounts the given zfs dataset.
 mount_dataset() {
-        dataset="${1}"
+    dataset="${1}"
     mountpoint="$(zfs get -H -o value mountpoint "${dataset}")"
+    ret=0
 
     # We need zfsutil for non-legacy mounts and not for legacy mounts.
     if [ "${mountpoint}" = "legacy" ] ; then
-        mount -t zfs "${dataset}" "${NEWROOT}"
+        mount -t zfs "${dataset}" "${NEWROOT}" || ret=$?
     else
-        mount -o zfsutil -t zfs "${dataset}" "${NEWROOT}"
+        mount -o zfsutil -t zfs "${dataset}" "${NEWROOT}" || ret=$?
+
+        if [ "$ret" = "0" ]; then
+            for_relevant_root_children "${dataset}" _mount_dataset_cb || ret=$?
+        fi
     fi
 
-    return $?
+    return ${ret}
+}
+
+# for_relevant_root_children DATASET EXEC
+#   Runs "EXEC dataset mountpoint" for all children of DATASET that are needed for system bringup
+#   Used by zfs-generator.sh and friends, too!
+for_relevant_root_children() {
+    dataset="${1}"
+    exec="${2}"
+
+    zfs list -t filesystem -Ho name,mountpoint,canmount -r "${dataset}" |
+        (
+            _ret=0
+            while IFS="${TAB}" read -r dataset mountpoint canmount; do
+                [ "$canmount" != "on" ] && continue
+
+                case "$mountpoint" in
+                    /etc|/bin|/lib|/lib??|/libx32|/usr)
+                        # If these aren't mounted we may not be able to get to the real init at all, or pollute the dataset holding the rootfs
+                        "${exec}" "${dataset}" "${mountpoint}" || _ret=$?
+                        ;;
+                    *)
+                        # Up to the real init to remount everything else it might need
+                        ;;
+                esac
+            done
+            exit ${_ret}
+        )
 }
 
 # export_all OPTS
 #   exports all imported zfs pools.
 export_all() {
-        opts="${@}"
+    opts="${@}"
     ret=0
 
     IFS="${NEWLINE}"