Add a contrib script to snapshot the BE prior to boot

contrib/snapshot-teardown.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+
+## A simple teardown hook to take a clean snapshot of the boot environment
+## prior to launching the new kernel. This requires that your pool be read-write
+## and will always export it.
+##
+## To use, put this script somewhere, make sure it is executable, and add the
+## path to the `zfsbootmenu_teardown` space-separated list with, e.g.,
+##
+##     zfsbootmenu_teardown+=" <path to script> "
+##
+## in a dracut.conf(5) file inside the directory specified for the option
+## `Global.DracutConfDir` in the ZFSBootMenu `config.yaml`.
+
+# exit early if we're missing our env var
+[ -n "${ZBM_SELECTED_BE}" ] || exit
+
+for src in /lib/zfsbootmenu-core.sh /lib/kmsg-log-lib.sh ; do
+  # shellcheck disable=SC1090
+  source "${src}" || exit
+done
+
+zpool="${ZBM_SELECTED_BE%%/*}"
+
+if ! zpool list -H -o name "${zpool}" >/dev/null 2>&1 ; then
+  read_write=true import_pool "${zpool}"
+else
+  set_rw_pool "${zpool}"
+fi
+
+printf -v snapshot "${ZBM_SELECTED_BE}@preboot-%(%Y-%m-%d-%H%M%S)T"
+zfs snapshot "${snapshot}" 
+export_pool "${zpool}"