#!/bin/sh
# vim: set ts=4:
#---help---
# Usage: alpine-chroot-install [options]
#
# This script installs Alpine Linux into a chroot and optionally sets up
# qemu-user and binfmt to emulate different architecture (e.g. armhf).
#
# If qemu-user and binfmt is needed, the script checks if both are available.
# If not, it tries to install them using apt-get. Beside this the script should
# work on any Linux system.
#
# It also creates script "enter-chroot" inside the chroot directory, that may
# be used to enter the chroot environment. That script do the following:
#
#   1. saves environment variables specified by $CHROOT_KEEP_VARS and PWD,
#   2. chroots into $CHROOT_DIR,
#   3. starts clean environment using "env -i",
#   4. switches user and simulates full login using "su -l",
#   5. loads saved environment variables and changes directory to saved PWD,
#   6. executes specified command or "sh" if not provided.
#
# Example:
#   sudo alpine-chroot-install -d /alpine -p build-base -p cmake
#	/alpine/enter-chroot -u $USER ./build
#
# Options and environment variables:
#   -a ARCH                CPU architecture for the chroot. If not set, then it's
#                          the same as the host's architecture. If it's different
#                          from the host's architecture, then it will be emulated
#                          using qemu-user. Options: x86_64, x86, aarch64, armhf,
#                          armv7, ppc64le, riscv64, s390x.
#
#   -b ALPINE_BRANCH       Alpine branch to install (default is latest-stable).
#
#   -d CHROOT_DIR          Absolute path to the directory where Alpine chroot
#                          should be installed (default is /alpine).
#
#   -i BIND_DIR            Absolute path to the directory on the host system that
#                          should be mounted on the same path inside the chroot
#                          (default is PWD, if it's under /home, or none).
#
#   -k CHROOT_KEEP_VARS... Names of the environment variables to pass from the
#                          host environment into chroot by the enter-chroot
#                          script. Name may be an extended regular expression.
#                          Default: ARCH CI QEMU_EMULATOR TRAVIS_.*.
#
#   -m ALPINE_MIRROR...    URI of the Aports mirror to fetch packages from
#                          (default is http://dl-cdn.alpinelinux.org/alpine).
#
#   -p ALPINE_PACKAGES...  Alpine packages to install into the chroot (default is
#                          build-base ca-certificates ssl_client).
#
#   -r EXTRA_REPOS...      Alpine repositories to be added to
#                          /etc/apk/repositories (main and community from
#                          $ALPINE_MIRROR are always added).
#
#   -t TEMP_DIR            Absolute path to the directory where to store temporary
#                          files (defaults to `mktemp -d`).
#
#   -h                     Show this help message and exit.
#
#   -v                     Print version and exit.
#
#   APK_TOOLS_URI          URL of apk.static to download. Default is x86_64
#                          binary from
#                          https://gitlab.alpinelinux.org/alpine/apk-tools/-/packages.
#
#   APK_TOOLS_SHA256       SHA-256 checksum of $APK_TOOLS_URI.
#
# Each option can be also provided by environment variable. If both option and
# variable is specified and the option accepts only one argument, then the
# option takes precedence.
#
# https://github.com/alpinelinux/alpine-chroot-install
#---help---
set -eu

#=======================  C o n s t a n t s  =======================#

: ${APK_TOOLS_URI:="https://gitlab.alpinelinux.org/api/v4/projects/5/packages/generic/v2.12.10/x86_64/apk.static"}
: ${APK_TOOLS_SHA256:="d7506bb11327b337960910daffed75aa289d8bb350feab624c52965be82ceae8"}

# Alpine APK keys for packages verification.
ALPINE_KEYS='
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'

# Version of alpine-chroot-install script.
VERSION='0.14.0'


#=======================  F u n c t i o n s  =======================#

die() {
	printf '\033[1;31mERROR:\033[0m %s\n' "$@" >&2  # bold red
	exit 1
}

einfo() {
	printf '\n\033[1;36m> %s\033[0m\n' "$@" >&2  # bold cyan
}

ewarn() {
	printf '\033[1;33m> %s\033[0m\n' "$@" >&2  # bold yellow
}

# Writes Alpine APK keys embedded in this script into directory $1.
dump_alpine_keys() {
	local dest_dir="$1"
	local content id line

	mkdir -p "$dest_dir"
	for line in $ALPINE_KEYS; do
		id=${line%%:*}
		content=${line#*:}

		printf -- "-----BEGIN PUBLIC KEY-----\n$content\n-----END PUBLIC KEY-----\n" \
			> "$dest_dir/alpine-devel@lists.alpinelinux.org-$id.rsa.pub"
	done
}

normalize_arch() {
	case "$1" in
		x86 | i[3456]86) echo 'i386';;
		armhf | armv[4-9]) echo 'arm';;
		*) echo "$1";;
	esac
}

fetch_url() {
	local url="$1"

	echo "fetch $url" >&2
	if command -v curl >/dev/null; then
		curl --remote-name --connect-timeout 10 -fsSL "$url"
	elif command -v wget >/dev/null; then
		wget -T 10 --no-verbose "$url"
	else
		die 'Cannot download a file: neither curl nor wget is available!'
	fi
}

download_file() (
	local url="$1"
	local sha256="$2"
	local dest="${3:-.}"

	mkdir -p "$dest" \
		&& cd "$dest" \
		&& rm -f "${url##*/}" \
		&& fetch_url "$url" \
		&& echo "$sha256  ${url##*/}" | sha256sum -c
)

usage() {
	sed -En '/^#---help---/,/^#---help---/p' "$0" | sed -E 's/^# ?//; 1d;$d;'
}

gen_chroot_script() {
	cat <<-EOF
		#!/bin/sh
		set -e

		ENV_FILTER_REGEX='($(echo "$CHROOT_KEEP_VARS" | tr -s ' ' '|'))'
	EOF
	if [ -n "$QEMU_EMULATOR" ]; then
		printf 'export QEMU_EMULATOR="%s"' "$QEMU_EMULATOR"
	fi
	cat <<-'EOF'

		user='root'
		if [ $# -ge 2 ] && [ "$1" = '-u' ]; then
		    user="$2"; shift 2
		fi
		oldpwd="$(pwd)"
		[ "$(id -u)" -eq 0 ] || _sudo='sudo'

		tmpfile="$(mktemp)"
		chmod 644 "$tmpfile"
		export | sed -En "s/^([^=]+ ${ENV_FILTER_REGEX}=)('.*'|\".*\")$/\1\3/p" > "$tmpfile" || true

		cd "$(dirname "$0")"
		$_sudo mv "$tmpfile" env.sh
		$_sudo chroot . /usr/bin/env -i su -l "$user" \
		    sh -c ". /etc/profile; . /env.sh; cd '$oldpwd' 2>/dev/null; \"\$@\"" \
		    -- "${@:-sh}"
	EOF
	# NOTE: ash does not load login profile when run with QEMU user-mode
	# emulation (I have no clue why), that's why /etc/profile is sourced here.
}

gen_destroy_script() {
	cat <<-'EOF'
		#!/bin/sh
		set -e

		remove=no
		case "$1" in
			-r | --remove) remove=yes;;
			'') ;;
			*) echo "Usage: $0 [-r | --remove]"; exit 1;;
		esac

		SCRIPT_DIR=$(cd "$(dirname "$0")" && pwd)
		[ "$(id -u)" -eq 0 ] || _sudo='sudo'

		# Unmounts all filesystem under the specified directory tree.
		cat /proc/mounts | cut -d' ' -f2 | grep "^$SCRIPT_DIR." | sort -r | while read path; do
			echo "Unmounting $path" >&2
			$_sudo umount -fn "$path" || exit 1
		done

		if [ "$remove" = yes ]; then
			rm_opts=''
			# Just to be extra careful...
			rm --help 2>&1 | grep -Fq 'one-file-system' && rm_opts='--one-file-system'

			echo "Removing $SCRIPT_DIR" >&2
			$_sudo rm -Rf $rm_opts "$SCRIPT_DIR"
		else
			echo "If you want to remove $SCRIPT_DIR directory, run: $0 --remove" >&2
		fi
	EOF
}

#------------------------- Debian/Ubuntu ---------------------------#

APT_UPDATED=no

apt_install() {
	if [ "$APT_UPDATED" != yes ]; then
		apt-get update
		APT_UPDATED=yes
	fi
	# -o=Dpkg::Use-Pty=0 - get rid of annoying "Reading database ..." messages.
	apt-get install -y -o=Dpkg::Use-Pty=0 --no-install-recommends "$@"
}

# Installs and enables binfmt-support on Debian/Ubuntu host.
install_binfmt_support() {
	apt_install binfmt-support \
		|| die 'Failed to install binfmt-support using apt-get!'

	update-binfmts --enable \
		|| die 'Failed to enable binfmt!'
}

# Installs QEMU user mode emulation binaries on Debian/Ubuntu host.
install_qemu_user() {
	apt_install qemu-user-static \
		|| die 'Failed to install qemu-user-static using apt-get!'
}


#============================  M a i n  ============================#

while getopts 'a:b:d:i:k:m:p:r:t:hv' OPTION; do
	case "$OPTION" in
		a) ARCH="$OPTARG";;
		b) ALPINE_BRANCH="$OPTARG";;
		d) CHROOT_DIR="$OPTARG";;
		i) BIND_DIR="$OPTARG";;
		k) CHROOT_KEEP_VARS="${CHROOT_KEEP_VARS:-} $OPTARG";;
		m) ALPINE_MIRROR="$OPTARG";;
		p) ALPINE_PACKAGES="${ALPINE_PACKAGES:-} $OPTARG";;
		r) EXTRA_REPOS="${EXTRA_REPOS:-} $OPTARG";;
		t) TEMP_DIR="$OPTARG";;
		h) usage; exit 0;;
		v) echo "alpine-chroot-install $VERSION"; exit 0;;
	esac
done

: ${ALPINE_BRANCH:="latest-stable"}
: ${ALPINE_MIRROR:="http://dl-cdn.alpinelinux.org/alpine"}
: ${ALPINE_PACKAGES:="build-base ca-certificates ssl_client"}
: ${ARCH:=}
: ${BIND_DIR:=}
: ${CHROOT_DIR:="/alpine"}
: ${CHROOT_KEEP_VARS:="ARCH CI QEMU_EMULATOR TRAVIS_.*"}
: ${EXTRA_REPOS:=}
: ${TEMP_DIR:=$(mktemp -d || echo /tmp/alpine)}

# Note: Binding $PWD into chroot as default was a bad idea. It's convenient
# on Travis, but dangerous in general. However, all existing .travis.yml relies
# on this behaviour, so we can't (shouldn't) remove it completely.
[ "$BIND_DIR" ] || case "$(pwd)" in
	/home/*) BIND_DIR="$(pwd)";;
esac


if [ "$(id -u)" -ne 0 ]; then
	die 'This script must be run as root!'
fi

mkdir -p "$CHROOT_DIR"
cd "$CHROOT_DIR"


# Install QEMU user mode emulation if needed (works only on Debian and derivates)

QEMU_EMULATOR=''
if [ -n "$ARCH" ] && [ $(normalize_arch $ARCH) != $(normalize_arch $(uname -m)) ]; then
	qemu_arch="$(normalize_arch $ARCH)"
	QEMU_EMULATOR="/usr/bin/qemu-$qemu_arch-static"

	if [ ! -x "$QEMU_EMULATOR" ]; then
		einfo 'Installing qemu-user-static on host system...'
		install_qemu_user
	fi

	if [ ! -e /proc/sys/fs/binfmt_misc/qemu-$qemu_arch ]; then
		einfo 'Installing and enabling binfmt-support on host system...'
		install_binfmt_support
	fi

	mkdir -p usr/bin
	cp -v "$QEMU_EMULATOR" usr/bin/
fi


einfo 'Downloading static apk-tools'

download_file "$APK_TOOLS_URI" "$APK_TOOLS_SHA256" "$TEMP_DIR"
APK="$TEMP_DIR/apk.static"
chmod +x "$APK"


einfo "Installing Alpine Linux $ALPINE_BRANCH ($ARCH) into chroot"

mkdir -p "$CHROOT_DIR"/etc/apk
cd "$CHROOT_DIR"

printf '%s\n' \
	"$ALPINE_MIRROR/$ALPINE_BRANCH/main" \
	"$ALPINE_MIRROR/$ALPINE_BRANCH/community" \
	$EXTRA_REPOS \
	> etc/apk/repositories

dump_alpine_keys etc/apk/keys/

cp /etc/resolv.conf etc/resolv.conf

"$APK" add \
	--root . --update-cache --initdb --no-progress \
	${ARCH:+--arch $ARCH} \
	alpine-baselayout apk-tools busybox busybox-suid musl-utils

if "$APK" info --root . --no-progress --quiet alpine-release >/dev/null; then
	"$APK" add --root . --no-progress alpine-release
else
	# In Alpine <3.17, this package contains /etc/os-release,
	# /etc/alpine-release and /etc/issue, but we don't wanna install all
	# its dependencies (e.g. openrc).
	"$APK" fetch --root . --no-progress --stdout alpine-base \
		| tar -xz etc
fi

gen_chroot_script > enter-chroot
gen_destroy_script > destroy
chmod +x enter-chroot destroy


einfo 'Binding filesystems into chroot'

mount -v -t proc none proc
mount -v --rbind /sys sys
mount --make-rprivate sys
mount -v --rbind /dev dev
mount --make-rprivate dev

# Some systems (Ubuntu?) symlinks /dev/shm to /run/shm.
if [ -L /dev/shm ] && [ -d /run/shm ]; then
	mkdir -p run/shm
	mount -v --bind /run/shm run/shm
	mount --make-private run/shm
fi

if [ -d "$BIND_DIR" ]; then
	mkdir -p "${CHROOT_DIR}${BIND_DIR}"
	mount -v --bind "$BIND_DIR" "${CHROOT_DIR}${BIND_DIR}"
	mount --make-private "${CHROOT_DIR}${BIND_DIR}"
fi


einfo 'Setting up Alpine'

./enter-chroot <<-EOF
	set -e
	apk update
	apk add $ALPINE_PACKAGES

	if [ -d /etc/sudoers.d ] && [ ! -e /etc/sudoers.d/wheel ]; then
		echo '%wheel ALL=(ALL) NOPASSWD: ALL' > /etc/sudoers.d/wheel
	fi

	if [ -n "${SUDO_USER:-}" ]; then
		adduser -u "${SUDO_UID:-1000}" -G users -s /bin/sh -D "${SUDO_USER:-}" || true
	fi
EOF

cat >&2 <<-EOF
	---
	Alpine installation is complete
	Run $CHROOT_DIR/enter-chroot [-u <user>] [command] to enter the chroot
	and $CHROOT_DIR/destroy [--remove] to destroy it.
EOF