.envrc

# Load environment variables from `nix-shell` and export it out.
#
# Usage: use_nix [-s <nix-expression>] [-w <path>] [-w <path>] ...
#   -s nix-expression: The nix expression to use for building the shell environment.
#   -w path: watch a file for changes. It can be specified multiple times. The
#      shell specified with -s is automatically watched.
#
#   If no nix-expression were given with -s, it will attempt to find and load
#   the shell using the following files in order: shell.nix and default.nix.
#
# Example:
#   -  use_nix
#   -  use_nix -s shell.nix -w .nixpkgs-version.json
#
# The dependencies pulled by nix-shell are added to Nix's garbage collector
# roots, such that the environment remains persistent.
#
# Nix-shell is invoked only once per environment, and the output is cached for
# better performance. If any of the watched files change, then the environment
# is rebuilt.
#
# To remove old environments, and allow the GC to collect their dependencies:
# rm -f .direnv
#
use_nix() {
	if ! validate_version; then
		echo "This .envrc requires direnv version 2.18.2 or above."
		exit 1
	fi

	# define all local variables
	local shell
	local files_to_watch=()

	local opt OPTARG OPTIND # define vars used by getopts locally
	while getopts ":n:s:w:" opt; do
		case "${opt}" in
			s)
				shell="${OPTARG}"
				files_to_watch=("${files_to_watch[@]}" "${shell}")
				;;
			w)
				files_to_watch=("${files_to_watch[@]}" "${OPTARG}")
				;;
			:)
				fail "Invalid option: $OPTARG requires an argument"
				;;
			\?)
				fail "Invalid option: $OPTARG"
				;;
		esac
	done
	shift $((OPTIND -1))

	if [[ -z "${shell}" ]]; then
		if [[ -f shell.nix ]]; then
			shell=shell.nix
			files_to_watch=("${files_to_watch[@]}" shell.nix)
		elif [[ -f default.nix ]]; then
			shell=default.nix
			files_to_watch=("${files_to_watch[@]}" default.nix)
		else
			fail "ERR: no shell was given"
		fi
	fi

	local f
	for f in "${files_to_watch[@]}"; do
		if ! [[ -f "${f}" ]]; then
			fail "cannot watch file ${f} because it does not exist"
		fi
	done

	# compute the hash of all the files that makes up the development environment
	local env_hash="$(hash_contents "${files_to_watch[@]}")"

	# define the paths
	local dir="$(direnv_layout_dir)"
	local wd="${dir}/wd-${env_hash}"
	local drv="${wd}/env.drv"
	local dump="${wd}/dump.env"

	# Generate the environment if we do not have one generated already.
	if [[ ! -f "${drv}" ]]; then
		mkdir -p "${wd}"

		log_status "use nix: deriving new environment"
		IN_NIX_SHELL=1 nix-instantiate --add-root "${drv}" --indirect "${shell}" > /dev/null
		nix-store -r $(nix-store --query --references "${drv}") --add-root "${wd}/dep" --indirect > /dev/null
		if [[ "${?}" -ne 0 ]] || [[ ! -f "${drv}" ]]; then
			rm -rf "${wd}"
			fail "use nix: was not able to derive the new environment. Please run 'direnv reload' to try again."
		fi

		log_status "use nix: updating cache"
		nix-shell --pure "${drv}" --show-trace --run "$(join_args "$direnv" dump bash)" > "${dump}"
		if [[ "${?}" -ne 0 ]] || [[ ! -f "${dump}" ]] || ! grep -q IN_NIX_SHELL "${dump}"; then
			rm -rf "${wd}"
			fail "use nix: was not able to update the cache of the environment. Please run 'direnv reload' to try again."
		fi
	fi

	# evaluate the dump created by nix-shell earlier, but have to merge the PATH
	# with the current PATH
	# NOTE: we eval the dump here as opposed to direnv_load it because we don't
	# want to persist environment variables coming from the shell at the time of
	# the dump. See https://github.com/direnv/direnv/issues/405 for context.
	local path_backup="${PATH}"
	eval $(cat "${dump}")
	export PATH="${PATH}:${path_backup}"

	# cleanup the environment of variables that are not requried, or are causing issues.
	unset shellHook  # when shellHook is present, then any nix-shell'd script will execute it!

	# watch all the files we were asked to watch for the environment
	for f in "${files_to_watch[@]}"; do
		watch_file "${f}"
	done
}

fail() {
	log_error "${@}"
	exit 1
}

hash_contents() {
	if has md5sum; then
		cat "${@}" | md5sum | cut -c -32
	elif has md5; then
		cat "${@}" | md5 -q
	fi
}

hash_file() {
	if has md5sum; then
		md5sum "${@}" | cut -c -32
	elif has md5; then
		md5 -q "${@}"
	fi
}

validate_version() {
	local version="$("${direnv}" version)"
	local major="$(echo "${version}" | cut -d. -f1)"
	local minor="$(echo "${version}" | cut -d. -f2)"
	local patch="$(echo "${version}" | cut -d. -f3)"

	if [[ "${major}" -gt 2 ]]; then return 0; fi
	if [[ "${major}" -eq 2 ]] && [[ "${minor}" -gt 18 ]]; then return 0; fi
	if [[ "${major}" -eq 2 ]] && [[ "${minor}" -eq 18 ]] && [[ "${patch}" -ge 2 ]]; then return 0; fi
	return 1
}

use_nix -s shell.nix