#!/bin/sh
# Mount what a brintOS boot keeps in memory: /run and /var/volatile on every
# boot, as /etc/fstab declares them, and a tmpfs over each login's home and
# over /var/spool and /var/cache when that directory is read-only, seeded from
# the image so what it ships stays visible. A snapshot launch on brintos.io
# boots with a read-only root; a writable launch keeps its directories.
# Run once per boot, before anything writes to these directories.

mounted() {
	awk -v m="$1" '$2 == m { f = 1 } END { exit !f }' /proc/mounts
}

# A write probe, not test -w: on a snapshot the guest's superblock stays
# read-write and only the backend refuses, with EROFS on the site and EIO under
# Node. A mkdir that reports failure but leaves the directory (linux#73) means
# the directory is writable.
writable() {
	_p=$1/.brintos-volatile.$$
	mkdir "$_p" 2>/dev/null || [ -d "$_p" ] || return 1
	rmdir "$_p" 2>/dev/null
	return 0
}

# Give $2 the mode and owner of $1.
same_attr() {
	chown -h "$(stat -c %u:%g "$1")" "$2" &&
		{ [ -L "$2" ] || chmod "$(stat -c %a "$1")" "$2"; }
}

# Copy the tree under $1 into $2 with GNU tar, which reads with read(2).
# busybox tar, cp and cat copy through sendfile(2), which on a file not yet
# read this boot never returns or leaks a restart sentinel, so without GNU tar
# the copy goes entry by entry, the file data through dd.
# BRINTOS-WORKAROUND: glibc#14 remove -- when sendfile(2) no longer leaks the restart sentinel, any tar will do.
seed() {
	if tar --version 2>/dev/null | grep -q 'GNU tar'; then
		tar -C "$1" -cf - . | tar -C "$2" -xpf -
	else
		(cd "$1" && find . ! -name .) | while IFS= read -r _e; do
			if [ -L "$1/$_e" ]; then
				ln -s "$(readlink "$1/$_e")" "$2/$_e"
			elif [ -d "$1/$_e" ]; then
				mkdir "$2/$_e"
			elif [ -f "$1/$_e" ]; then
				dd if="$1/$_e" of="$2/$_e" bs=64k 2>/dev/null
			else
				continue
			fi && same_attr "$1/$_e" "$2/$_e"
		done
	fi
	[ "$(cd "$1" && find . | sort)" = "$(cd "$2" && find . | sort)" ] ||
		echo "brintos-volatile: $1: not every entry was copied" >&2
}

# The directory's mode and owner, as tmpfs mount options.
dir_opts() {
	set -- $(stat -c '%a %u %g' "$1" 2>/dev/null)
	[ $# = 3 ] && echo "mode=$1,uid=$2,gid=$3"
}

# A tmpfs of its own over a home: mounted at a staging point, seeded, then moved.
cover_home() {
	[ -d "$1" ] && ! mounted "$1" && ! writable "$1" || return 0
	_n=$((_n + 1))
	_s=/run/brintos-volatile/home$_n
	mkdir "$_s" &&
		mount -t tmpfs -o "$(dir_opts "$1")" tmpfs "$_s" &&
		seed "$1" "$_s" &&
		mount --move "$_s" "$1" &&
		rmdir "$_s"
}

# /var/<dir> from a copy in /var/volatile, bound over it.
cover_var() {
	[ -d "/var/$1" ] && ! mounted "/var/$1" && ! writable "/var/$1" || return 0
	mkdir "/var/volatile/$1" &&
		same_attr "/var/$1" "/var/volatile/$1" &&
		seed "/var/$1" "/var/volatile/$1" &&
		mount --bind "/var/volatile/$1" "/var/$1"
}

mounted /run || mount -t tmpfs -o mode=0755,nodev,nosuid,strictatime tmpfs /run || exit 1
[ -d /run/lock ] || mkdir -m 1777 /run/lock
if mounted /var/volatile || mount -t tmpfs -o mode=0755 tmpfs /var/volatile; then
	[ -d /var/volatile/log ] || mkdir /var/volatile/log
	[ -d /var/volatile/tmp ] || mkdir -m 1777 /var/volatile/tmp
fi

[ -d /run/brintos-volatile ] || mkdir -m 0700 /run/brintos-volatile
_n=0
for h in $(awk -F: '($3 == 0 || ($3 >= 1000 && $3 < 65534)) && $6 != "/" { print $6 }' /etc/passwd); do
	cover_home "$h"
done
cover_var spool
cover_var cache
exit 0