#!/bin/bash -e

bootstrap() {
	[[ $(id -u) -eq 0 ]] || exec sudo env -uUNSHARE HOME="$HOME" "$0" "$@"

	if [[ -z ${UNSHARE} ]] ; then
		mount_args=
		if type -P unshare >&/dev/null ; then
			test_arg() { unshare "$@" -- true >&/dev/null && uargs+=( "$@" ) || :; }
			uargs=( -m )
			test_arg -u
			test_arg -i
			test_arg -p -f --mount-proc
			test_arg --propagation=private
			UNSHARE=true exec unshare "${uargs[@]}" -- "$0" "$@"
		fi
	else
		mount_args='-n'
	fi
	unset UNSHARE
}

is_mounted() {
	local dst=$1
	grep -sq "${dst}" /proc/mounts
}

maybe_mount() {
	local src=/$1 dst=${chroot}/${2:-$1}
	[[ -d ${src} ]] || return 0
	if ! mkdir -p "${dst}" ; then
		[[ -w ${chroot} ]] && exit 1 || return 0
	fi
	is_mounted "${dst}" || mount ${mount_args} --bind "${src}" "${dst}"
}

get_type() {
	case $(file "$1") in
	*x86-64*)             echo x86_64;;
	*"Intel 80386"*)      echo i386;;
	*32-bit*PowerPC*MSB*) echo ppc;;
	*64-bit*PowerPC*MSB*) echo ppc64;;
	*32-bit*PowerPC*LSB*) echo ppcle;;
	*64-bit*PowerPC*LSB*) echo ppc64le;;
	*32-bit*S/390*)       echo s390;;
	*64-bit*S/390*)       echo s390x;;
	*64-bit*MIPS*)        echo mips64;;
	*32-bit*MIPS*N32*)    echo mips64;;
	*32-bit*MIPS*)        echo mips;;
	*32-bit*PA-RISC*)     echo parisc;;
	*64-bit*PA-RISC*)     echo parisc64;;
	*32-bit*SPARC*)       echo sparc;;
	*64-bit*SPARC*)       echo sparc64;;
	esac
}

init_chroot() {
	[[ -w . ]] || return 0

	if [[ ! -L etc/mtab ]] ; then
		ln -sfT /proc/mounts etc/mtab
	fi
	local f dst
	local etc=(
		hosts
		locale.gen
		localtime
		resolv.conf
	)
	local home=(
		.inputrc
		.gdbinit
		.gitconfig
		.nanorc
	)
	for f in \
		$(printf 'etc/%s ' "${etc[@]}") \
	; do
		if [[ -e /${f} ]] ; then
			cp "/${f}" "${f}"
		fi
	done
	for f in "${home[@]}" ; do
		df="root/${f}"
		f="${HOME}/${f}"
		if [[ -e ${f} ]] ; then
			cp "${f}" "${df}"
		fi
	done

	if [[ ! -d root/.git ]] ; then
		f="${HOME}/.profile.d/aliases.sh"
		if [[ -e ${f} ]] ; then
			cat "${f}" > root/.bash_profile
		fi
	fi
}

usage() {
	cat <<-EOF
	Usage: ${0##*/} [options] [program to run]

	Sets up common mount points and then chroots in and runs a program.
	If no program is specified, then launch a login shell.

	Options:
	  -u          Unmount all paths in the chroot
	  -m <path>   Add path to mount list
	  -d <dir>    Use <dir> as chroot (defaults to ${0%/*})
	  -h          This help screen
	EOF
	exit
}

main() {
	bootstrap "$@"

	local mounts=(
		proc sys tmp dev dev/shm run
		usr/portage usr/portage/distfiles
		var/db/repos/gentoo var/cache/distfiles
		usr/local/src
	)

	local chroot=${0%/*}
	case ${chroot} in
	.) chroot=${PWD} ;;
	./*) chroot=${PWD}/${chroot#./} ;;
	esac

	local cmd
	while [[ -n $1 ]] ; do
		case $1 in
		-u) cmd='umount' ;;
		-m) mounts+=( "$2" ); shift ;;
		-d) chroot=$(realpath "$2"); shift ;;
		--help|-h) usage ;;
		-*) echo "${0##*/}: unknown option $1"; exit 1 ;;
		*)  break ;;
		esac
		shift
	done
	cd "${chroot}"

	case ${cmd} in
	umount) exec "${0%/*}/umount-tree" -y "${chroot}" ;;
	esac

	local m
	for m in "${mounts[@]}" ; do
		maybe_mount "${m}"
	done
	# Handle special mounts that we don't want to just bind mount.
	if ! is_mounted "${chroot}/dev/pts" ; then
		# Option order matters: on older kernels that don't recognize newinstance,
		# parsing stops as soon as it hits that.
		mount -t devpts devpts "${chroot}/dev/pts" \
			-o nosuid,noexec,mode=0620,gid=5,ptmxmode=0666,newinstance
	fi

	init_chroot

	local setarch
	if type -P setarch &>/dev/null ; then
		local bin_dst=$(get_type bin/bash)
		if [[ -n ${bin_dst} ]] ; then
			setarch="setarch ${bin_dst}"
		fi
	fi

	# Doubtful these settings we want to leak into the chroot.
	unset ROOT PORTAGE_CONFIGROOT LD_LIBRARY_PATH
	unset LS_COLORS # format changes over time
	[[ $# -eq 0 ]] && set -- env HOME=/root /bin/bash -l
	exec \
		${setarch} \
		chroot "${chroot}" \
		"$@"
}

main "$@"