#!/bin/sh
# mrgrctrnl: configurable ssh tunneler
# Author: Case Duckworth <acdw@acdw.net>
# Version: 0.3.1
# License: MIT

PRGN="${0##*/}"

# entry point
main() {
	# flags
	_RUN=true
	_LOG=true
	_USE_CONFIG=true
	_RESTART=false
	# options
	__CONFIG="${XDG_CONFIG_HOME:-$HOME/.config}/$PRGN/config"
	__PIDF="/tmp/$PRGN.pid"
	__CMDF="/tmp/$PRGN.cmd"
	__SSH="$(command -v autossh || command -v ssh)"
	__SSH_ARGS="-nT -N \"\$user@\$machine\" -L \"\$local:\$remote\""

	# parse options
	while getopts hkrnqSAc:s: OPT; do
		case "$OPT" in
		h)
			usage
			exit 0
			;;
		k)
			untunnel
			pkill -x "$PRGN"
			exit "$?"
			;;
		r) _RESTART=true ;;
		n) _RUN=false ;;
		q) _LOG=false ;;
		S) __SSH='ssh' ;;
		A) __SSH='autossh' ;;
		c) __CONFIG="$OPTARG" ;;
		s)
			__SSH_ARGS="$OPTARG"
			_USE_CONFIG=false
			;;
		*) exit 4 ;;
		esac
	done
	shift $((OPTIND - 1))

	# save command invocation (for -r)
	if $_RESTART; then
		untunnel
		exec "$PRGN" $(cat "$__CMDF")
	else
		printf '%s\n' "$*" >"$__CMDF"
	fi

	# make sure ssh is installed
	if ! command -v "$__SSH" >/dev/null; then
		log "Not installed: $__SSH"
		exit 2
	fi

	# make sure the config is readable
	if $_USE_CONFIG && [ ! -r "$__CONFIG" ]; then
		log "Cannot find config file $__CONFIG; aborting."
		exit 3
	fi

	# load the config
	if $_USE_CONFIG; then
		awk '{sub(/#.*$/,"");print}' "$__CONFIG" |
			while read -r machine user local remote key rest; do
				if [ -z "$machine" ] ||
					[ -z "$user" ] ||
					[ -z "$local" ] ||
					[ -z "$remote" ]; then
					continue
				fi

				case "$local" in
				:*) local="localhost$local" ;;
				esac
				case "$remote" in
				:*) remote="localhost$remote" ;;
				esac

				# shellcheck disable=2030
				if [ -n "$key" ]; then
					__SSH_ARGS="$__SSH_ARGS -i \"$key\""
				fi
				if [ -n "$rest" ]; then
					__SSH_ARGS="$__SSH_ARGS $rest"
				fi

				eval "tunnel_${__SSH##*/} $__SSH_ARGS" &
			done
	else
		#shellcheck disable=2031
		eval "tunnel_${__SSH##*/} $__SSH_ARGS" &
	fi

	wait
}

usage() {
	cat <<END
$PRGN: make magic ssh tunnels
usage:	$PRGN [-h] [-k] [-r]
	$PRGN [-n] [-q] [-S|-A] [-c FILE | -s ARGS]

options:
	-h	show this help
	-k	demolish all tunnels and exit
	-r	demolish all tunnels and restart
	-n	dry run: just print what would happen
	-q	quiet: don't print anything

	-S	force use of 'ssh'
	-A	force use of 'autossh'
		default: use 'autossh' if installed, otherwise 'ssh'

	-c FILE	load FILE as config file
		default:	\$XDG_CONFIG_HOME/$PRGN/config

	-s ARGS	run CMD with ARGS, do not read config file
		default:	'-N USER@MACHINE -L LOCAL:REMOTE'
				(populated from config file)

config example:
	# a hash mark comments to the end of the line
	# if either 'local' or 'remote' begin with a colon,
	# they'll be prefixed with 'localhost'
	# machine	user	local	remote	key	other
	example.com	bill	:1299	:6667	# key and other are optional
END
}

tunnel_ssh() {
	set -- ssh "$@"
	log "$@"
	$_RUN || return

	touch "$__PIDF"
	while [ -e "$__PIDF" ]; do # quit if the pid file is removed
		# make sure we haven't tunneled this connection already
		if grep -q -e "$*" "$__PIDF"; then
			sleep 3
			continue
		fi

		# try to start ssh tunnel
		if "$@" & then
			printf '%s\t%s\n' "$!" "$*" >>"$__PIDF"
		else
			badpid="$!"
			kill "$badpid"
			sed "/^$badpid/d" "$__PIDF" >"$__PIDF~" &&
				mv "$__PIDF~" "$__PIDF"
			continue
		fi
		sleep 3
	done
}

tunnel_autossh() {
	set -- autossh -f "$@"
	log "$@"
	$_RUN || return

	"$@" && printf '%s\t%s\n' "$!" "$*" >"$__PIDF"
}

untunnel() {
	log "Killing tunnels"
	if [ -e "$__PIDF" ]; then
		while read -r pid _; do
			kill "$pid" 2>/dev/null
		done <"$__PIDF"
		rm "$__PIDF"
	else
		pkill -x ssh
		pkill -x autossh
	fi
	log "Done"
}

log() {
	$_LOG && printf '%s: %s\n' "$PRGN" "$*" >&2
}

main "$@"