shell.sh

#!/usr/bin/env sh

# Assumption #1: non-GNU, non-BSD linux distros are out-of-scope
# Otherwise we at least need to get rid of 'local' keyword
# Assumption #2: coreutils are present in $PATH
# Assumption #3: there is internet access (assumption to be removed)

# '-o pipefail' is not POSIX compliant, will fail in some sh's
set -eu

realpath () {
  case "$(uname)" in
    Linux)
      readlink -f -- "$1"
    ;;
    Darwin)
      greadlink -f -- "$1"
    ;;
  esac
}

# Find current script location
{
  __FILE__=$(dirname "$(realpath $0)")
} || {
  echo "Unable to determine current script location.."
  echo "Will assume '.'"
  __FILE__="."
}

fail() {
  echo "$0:" "$@" >&2
  exit 1
}

# Nicked from https://releases.nixos.org/nix/nix-2.5.1/install
require_util() {
  command -v "$1" > /dev/null 2>&1 ||
    fail "you do not have '$1' installed, which I need to $2"
}

readonly USER_HOME="${HOME:-"HOME variable has not been set"}"
readonly CACHE_ROOT="${__FILE__}/../.cache"
readonly NIX_USER_CHROOT_VERSION="1.2.2"
readonly NIX_USER_CHROOT_DIR="${CACHE_ROOT}/nix-user-chroot"
readonly NIX_USER_CHROOT_BIN="${NIX_USER_CHROOT_DIR}/nix-user-chroot"
readonly NIX_STORE="${CACHE_ROOT}/nix-store"
readonly NIX_VERSION="2.5.1"
readonly NIX_INSTALL_SCRIPT="${CACHE_ROOT}/nix-${NIX_VERSION}-install.sh"
readonly NIX_EXTRA_CONF_PATH="${__FILE__}/nix.conf"
readonly NIX_DIRENV_CONF_PATH="${CACHE_ROOT}/direnv.toml"

IS_NIX_INSTALLED=false

# Check if running on NixOS
if [ -e "/etc/os-release" ]; then
  case "$(cat /etc/os-release)" in 
    *NixOS*)
      readonly IS_NIXOS=true;
    ;;
    *)
      readonly IS_NIXOS=false;
    ;;
  esac
else
  readonly IS_NIXOS=false;
fi

preflightCheck() {
  require_util curl "download dependencies"
  require_util tar "decompress nix installation package"
  case "$(uname)" in
    Darwin)
      require_util nix "make the magic happen. You are running on OSX, please make sure Nix is installed by running the following: sh <(curl -L https://nixos.org/nix/install)."
    ;;
    Linux)
      require_util unshare "verify kernel support for user namespaces"
    ;;
    *)
      echo "Platform not yet supported"
    ;;
  esac
}

setup_nix_user_chroot() {
  # Verify that user namespaces for unprivileged users are enabled
  local readonly are_user_ns_enabled="$(unshare\
   --user --pid echo -n YES \
  )"
  if [ "$are_user_ns_enabled" != "YES" ]; then
    # TODO: Implement fallback to proot
    fail "Current kernel settings do not allow\
     running user namespaces for unprivileged users."
  fi

  local readonly arch=$(uname -m)
  case "$arch" in
    "aarch64")
    local readonly download_url="https://github.com/nix-community\
/nix-user-chroot/releases/download/${NIX_USER_CHROOT_VERSION}\
/nix-user-chroot-bin-${NIX_USER_CHROOT_VERSION}-aarch64-unknown-linux-musl"
    ;;
    "x86_64")
    local readonly download_url="https://github.com/nix-community\
/nix-user-chroot/releases/download/${NIX_USER_CHROOT_VERSION}\
/nix-user-chroot-bin-${NIX_USER_CHROOT_VERSION}-x86_64-unknown-linux-musl"
    ;;
    *)
    fail "Arch '${arch}' is not supported at the moment."
    ;;
  esac

  mkdir -p "${NIX_USER_CHROOT_DIR}" || {
    fail "Unable to create '${NIX_USER_CHROOT_DIR}'"
  }
  curl -L -o "${NIX_USER_CHROOT_BIN}" "${download_url}" || {
    fail "Unable to download nix-user-chroot (url: ${download_url})"
  }
  chmod +x "${NIX_USER_CHROOT_BIN}"
}

setup_nix() {
  mkdir -p "${NIX_STORE}" || {
    fail "Unable to create '${NIX_STORE}'"
  }
  $NIX_USER_CHROOT_BIN "${NIX_STORE}" sh -c\
   "curl -L https://releases.nixos.org/nix/nix-${NIX_VERSION}/install > ${NIX_INSTALL_SCRIPT} && sh ${NIX_INSTALL_SCRIPT}\
    --no-daemon\
    --no-modify-profile\
    --nix-extra-conf-file ${NIX_EXTRA_CONF_PATH}"
}

ensure_direnv_is_configured() {
  local readonly project_root=$(dirname $(realpath ${__FILE__}/../docs))
  mkdir -p "${CACHE_ROOT}"
  echo "[whitelist]" > ${NIX_DIRENV_CONF_PATH}
  echo "prefix = [ \"${project_root}\" ]" >> ${NIX_DIRENV_CONF_PATH}
}

ensure_nix_is_present() {
  if $IS_NIXOS; then
    # On nixos, nix is installed by default
    return
  fi

  if command -v "nix" >/dev/null 2>&1; then
    IS_NIX_INSTALLED=true;
  else
    IS_NIX_INSTALLED=false;
  fi
  # We need to distinguish between single-user and multi-user installs.
  # This is difficult because there's no official way to do this.
  # Details: https://github.com/lilyball/nix-env.fish/blob/00c6cc762427efe08ac0bd0d1b1d12048d3ca727/conf.d/nix-env.fish

  # stat is not portable. Splitting the output of ls -nd is reliable on most platforms.
  if $IS_NIX_INSTALLED; then
    local readonly nix_store_owner=$(ls -nd /nix/store | cut -d' ' -f3)
    if [ "${nix_store_owner}" -eq 0 ]; then
      local readonly is_nix_multiuser_install=true;
    else
      local readonly is_nix_multiuser_install=false;
    fi
  else
    local readonly is_nix_multiuser_install=false;
  fi

  # Global, single-user installation
  if $IS_NIX_INSTALLED && ! $is_nix_multiuser_install; then
    return
  fi

  if $IS_NIX_INSTALLED && $is_nix_multiuser_install; then
    # TODO: Find a solution
    fail "Daemon-based nix installation is not supported"
  fi

  if [ -d "${NIX_STORE}" ]; then
    # Nix has already been set up
    echo "Detected previous setup in ${CACHE_ROOT}. Will attempt to use it."
    return
  fi

  # No nix installed or nix is multi-user installation
  setup_nix_user_chroot
  setup_nix
}

preflightCheck
ensure_nix_is_present
ensure_direnv_is_configured

if $IS_NIXOS || $IS_NIX_INSTALLED; then
  NIX_USER_CONF_FILES=${NIX_EXTRA_CONF_PATH}\
   nix-shell --pure "${__FILE__}/shell.nix"
else
  # Explicitly source nix profile in bash invocation
  # In future: remove dependency on user HOME
  $NIX_USER_CHROOT_BIN "${NIX_STORE}" bash -c\
    ". ${HOME}/.nix-profile/etc/profile.d/nix.sh\
      && NIX_USER_CONF_FILES=${NIX_EXTRA_CONF_PATH}\
      nix-shell --pure '${__FILE__}/shell.nix'"
fi