#!/bin/bash
#
# This script is the simplified and portable version of arch-chroot
# (https://wiki.archlinux.org/index.php/Change_root#Using_arch-chroot)
#

set -e

JUNEST_BASE="$(readlink -f $(dirname $(readlink -f "$0"))/..)"

source "${JUNEST_BASE}/lib/utils.sh"

################################ MAIN FUNCTIONS ###########################

chroot_add_mount() {
  mount "$@" && CHROOT_ACTIVE_MOUNTS=("$2" "${CHROOT_ACTIVE_MOUNTS[@]}")
}

chroot_maybe_add_mount() {
  local cond=$1; shift
  if eval "$cond"; then
    chroot_add_mount "$@"
  fi
}

chroot_setup() {
  CHROOT_ACTIVE_MOUNTS=()
  [[ $(trap -p EXIT) ]] && die '(BUG): attempting to overwrite existing EXIT trap'
  trap 'chroot_teardown' EXIT

  chroot_maybe_add_mount "! mountpoint -q '$1'" "$1" "$1" --bind &&
  chroot_add_mount proc "$1/proc" -t proc -o nosuid,noexec,nodev &&
  chroot_add_mount sys "$1/sys" -t sysfs -o nosuid,noexec,nodev,ro &&
  chroot_add_mount udev "$1/dev" -t devtmpfs -o mode=0755,nosuid &&
  chroot_add_mount devpts "$1/dev/pts" -t devpts -o mode=0620,gid=5,nosuid,noexec &&
  chroot_add_mount shm "$1/dev/shm" -t tmpfs -o mode=1777,nosuid,nodev &&
  chroot_add_mount run "$1/run" -t tmpfs -o nosuid,nodev,mode=0755 &&
  chroot_add_mount tmp "$1/tmp" -t tmpfs -o mode=1777,atime,nodev,nosuid &&
  mkdir -p "$1/$HOME" &&
  chroot_add_mount $HOME "$1/$HOME" --bind

  mkdir -p "$1/run/lock"
}

chroot_teardown() {
  umount "${CHROOT_ACTIVE_MOUNTS[@]}"
  unset CHROOT_ACTIVE_MOUNTS
}

usage() {
  cat <<EOF
usage: ${0##*/} chroot-dir [command]

    -h             Print this help message

If 'command' is unspecified, ${0##*/} will launch /bin/sh.

EOF
}

chroot_add_resolv_conf() {
  local chrootdir=$1 resolv_conf=$1/etc/resolv.conf

  # Handle resolv.conf as a symlink to somewhere else.
  if [[ -L $chrootdir/etc/resolv.conf ]]; then
    # readlink(1) should always give us *something* since we know at this point
    # it's a symlink. For simplicity, ignore the case of nested symlinks.
    resolv_conf=$(readlink "$chrootdir/etc/resolv.conf")
    if [[ $resolv_conf = /* ]]; then
      resolv_conf=$chrootdir$resolv_conf
    else
      resolv_conf=$chrootdir/etc/$resolv_conf
    fi

    # ensure file exists to bind mount over
    if [[ ! -f $resolv_conf ]]; then
      install -Dm644 /dev/null "$resolv_conf" || return 1
    fi
  fi

  chroot_add_mount /etc/resolv.conf "$resolv_conf" --bind
}

if [[ -z $1 || $1 == -h || $1 == --help ]]; then
  usage
  exit $(( $# ? 0 : 1 ))
fi

(( EUID == 0 )) || die 'This script must be run with root privileges'
chrootdir=$1
shift

[[ -d $chrootdir ]] || die "Can't create chroot on non-directory $chrootdir"

chroot_setup "$chrootdir" || die "failed to setup chroot $chrootdir"
chroot_add_resolv_conf "$chrootdir" || die "failed to setup resolv.conf"

SHELL="/bin/sh" chroot "$chrootdir" "$@"