#!/bin/sh
# Copyright (C) 2023  Umorpha Systems
# SPDX-License-Identifier: AGPL-3.0-or-later

# This script is intended to work both in an initramfs and as a
# developer-friendly tool on a real OS.  Therefore, it has very few
# dependencies and must be tolerant of different implementations:
#
#  - sh     (bash or busybox)
#  - getopt (busybox or util-linux)
#  - mkdir  (busybox or coreutils)
#  - mount  (util-linux)

set -eu

usage() {
	echo "Usage: $0 OPTIONS"
	echo "Mount devises using the Umorpha overlay-partition scheme."
	echo
	echo "OPTIONS:"
	echo "  -h, --help                             Show this help text"
	echo "      --root SPEC             (required) The read-only root device ant its mount arguments"
	echo "      --overlay SPEC          (required) The writable overlay device ant its mount arguments"
	echo "      --boot SPEC             (optional) The device to mount at /boot"
	echo "      --basedir DIR           (required) Scratch directory to mount the root device and the overlay device"
	echo "      --final-mountpoint DIR  (optional) Where to mount the overlayfs (default: \${basedir}/mnt"
	echo
	echo "A SPEC is of the form"
	echo "    DEVICE[:[FSTYPE][:OPTS]]"
	echo "If FSTYPE value isn't present, or is empty, then \`auto\` is used."
}

main() {
	mode=run
	if O=$(getopt -n "${0##*/}" -l help,root:,overlay:,boot:,basedir:,final-mountpoint: -o h -- "$@"); then
		eval "set -- $O"
		while true; do
			case "$1" in
				--help|-h) usage; exit 0;;
				--root) root_all=$2; shift 2;;
				--overlay) overlay_all=$2; shift 2;;
				--boot) boot_all=$2; shift 2;;
				--basedir) basedir=$2; shift 2;;
				--final-mountpoint) union_mnt=$2; shift 2;;
				--) shift; break;;
			esac
		done
		if [[ "$#" -gt 0 ]]; then
			echo >&2 "${0##*/}: unexpected positional arguments: $*"
			mode=error
		fi
		if [[ -z "${root_all:-}" ]]; then
			echo >&2 "${0##*/}: missing required --root flag"
			mode=error
		fi
		if [[ -z "${overlay_all:-}" ]]; then
			echo >&2 "${0##*/}: missing required --overlay flag"
			mode=error
		fi
		if [[ -z "${basedir:-}" ]]; then
			echo >&2 "${0##*/}: missing required --basedir flag"
			mode=error
		fi
	else
		mode=error
	fi
	if [[ $mode == error ]]; then
		echo "Try '${0##*/} --help' for more information."
		exit 2
	fi

	IFS=: read -r root_dev root_typ root_opt < <(echo "$root_all")
	root_mnt=$basedir/root

	IFS=: read -r overlay_dev overlay_typ overlay_opt < <(echo "$overlay_all")
	overlay_mnt=$basedir/overlay

	union_dev=umorpha-rootfs
	union_typ=overlay
	# BusyBox ash supports weird Bash-isms like "<(process
	# substitution)" and the special "[[" syntax, but supporting
	# "+=" to append to a variable is too much bloat!??
	union_opt=''
	union_opt="${union_opt:+${union_opt},}lowerdir=${root_mnt}"
	union_opt="${union_opt:+${union_opt},}upperdir=${overlay_mnt}/upperdir"
	union_opt="${union_opt:+${union_opt},}workdir=${overlay_mnt}/workdir"
	# > Offline changes to the lower tree are only allowed if the
	# > "metadata only copy up", "inode index", "xino" and
	# > "redirect_dir" features have not been used.
	#
	# -- https://www.kernel.org/doc/html/v6.6/filesystems/overlayfs.html#changes-to-underlying-filesystems
	union_opt="${union_opt:+${union_opt},}metacopy=off"
	union_opt="${union_opt:+${union_opt},}index=off"
	union_opt="${union_opt:+${union_opt},}xino=off"
	union_opt="${union_opt:+${union_opt},}redirect_dir=off"
	union_mnt=${union_mnt:-${basedir:-}/mnt}

	if [[ -n "${boot_all:-}" ]]; then
		IFS=: read -r boot_dev boot_typ boot_opt < <(echo "$boot_all")
		boot_mnt=$root_mnt/boot
	fi

	set -x
	mount --mkdir -t "${root_typ:-auto}" -o "$root_opt" "$root_dev" "$root_mnt"
	mount --mkdir -t "${overlay_typ:-auto}" -o "$overlay_opt" "$overlay_dev" "$overlay_mnt"
	mkdir -p -- "$overlay_mnt/upperdir" "$overlay_mnt/lowerdir"
	mount --mkdir -t "${union_typ:-auto}" -o "$union_opt" "$union_dev" "$union_mnt"
	if [[ -n "${boot_all:-}" ]]; then
		mount --mkdir -t "${boot_typ:-auto}" -o "$boot_opt" "$boot_dev" "$boot_mnt"
	fi
}

main "$@"