#!/bin/sh

PreservedFileForConfigsDir="/usr/syno/synoinstall/space-preserve-for-configs"
PreservedFilePrefix="temp-config"

PreservedFileNum=100
PreservedDirSizeInKB=0 # there are no valid estimation of dir size on btrfs root
PreservedFileSizeInKB=100
PreservedFileTotalSizeInKB=$(( PreservedFileNum*PreservedFileSizeInKB + PreservedDirSizeInKB ))
MarginSizeInKB=$(( 100*1024 )) # 100 MB


Help() {
cat << EOF
usage: $0 options rootMnt
      --ensure   ensure the preserved space on rootMnt.
                 should not use this option in junior mode.
      --cleanup  cleanup the preserved space on rootMnt.
      --help     print this.
EOF

	return 0
}


ErrorLog() {
	/bin/logger -p "error" -s -t "$0" "$@"
}


GetAvailSizeInKB() {
	local rootMnt=$1

	local avail_block_num
	if ! avail_block_num=$(/bin/stat -f "$rootMnt" -c "%a"); then
		ErrorLog "Failed to get available block num on $rootMnt"
		return 1
	fi

	local block_size
	if ! block_size=$(/bin/stat -f "$rootMnt" -c "%S"); then
		ErrorLog "Failed to get block size on $rootMnt"
		return 1
	fi

	local avail_size
	avail_size=$(( avail_block_num*block_size/1024 )) # from B to KB
	/bin/echo "$avail_size"
}


GetSizeInKB() {
	local path="$1"

	local size=0
	if [ -e "$path" ]; then
		local stat_result
		if ! stat_result=$(/bin/find "$path" -print0 | /bin/xargs -0 -I{} /bin/stat -c %s {}); then
			ErrorLog "Failed to find all files and calculate size"
			return 1
		fi

		# NOTE: "/bin/awk" is not in junior mode
		if ! size=$(/bin/echo "$stat_result" | /bin/awk '{sum += $1} END {print sum}'); then
			ErrorLog "Failed to sum up size"
			return 1
		fi
	fi

	/bin/echo "$((size / 1024))"
}


RemoveFiles() {
	if ! /bin/rm -rf "$1"; then
		ErrorLog "Failed to remove files"
		return 1
	fi
}


CheckExistBackingFiles() {
	local path="$1"; shift

	if [ ! -d "$path" ]; then
		ErrorLog "dir is not exist [$path]"
		return 1
	fi

	local size=0
	if ! size=$(GetSizeInKB "$path"); then
		ErrorLog "Failed to get size [$path]"
		return 1
	fi
	[ "$PreservedFileTotalSizeInKB" -le "$size" ]
}


EnsureBackingFiles() {
	local rootMnt="$1"
	local path="$rootMnt$PreservedFileForConfigsDir"

	if [ ! -x /bin/stat ] || [ ! -x /bin/awk ]; then
		ErrorLog "There is no executable /bin/stat or /bin/awk. This option (\"--ensure\") should not be used in junior mode."
		return 1
	fi

	if CheckExistBackingFiles "$path"; then
		return 0
	fi

	RemoveFiles "$path"

	local avail_size=0
	if ! avail_size=$(GetAvailSizeInKB "$rootMnt"); then
		ErrorLog "Failed to get available size"
		return 1
	fi

	local guaranteed_size=$(( PreservedFileTotalSizeInKB + MarginSizeInKB ))
	if [ "$avail_size" -lt "$guaranteed_size" ]; then
		ErrorLog "No enough space left on \"$rootMnt\" [$avail_size, $guaranteed_size]"
		return 1
	fi

	if ! /bin/mkdir -p "$path"; then
		ErrorLog "Failed to create dir [$path]"
		return 1
	fi

	for i in $(/bin/seq 1 "$PreservedFileNum"); do
		local file="$path/$PreservedFilePrefix-$i"
		/bin/dd if=/dev/zero of="$file" bs="1K" count="$PreservedFileSizeInKB" status=none
	done

	if ! CheckExistBackingFiles "$path"; then
		ErrorLog "Failed to dd files for preserved space [$path]"
		return 1
	fi

	return 0
}


CleanBackingFiles() {
	local rootMnt="$1"; shift
	if ! RemoveFiles "$rootMnt$PreservedFileForConfigsDir"; then
		ErrorLog "Failed to clean backing files"
		return 1
	fi
}


main() {
	local action="$1"; shift
	local rootMnt="${1:-/}"; shift

	case "$action" in
		# NOTE: should not use --ensure in junior mode
		--ensure)
			if ! EnsureBackingFiles "$rootMnt"; then
				ErrorLog "Failed to ensure preserved space"
				CleanBackingFiles "$rootMnt"
				return 1
			fi
			;;
		--cleanup)
			if ! CleanBackingFiles "$rootMnt"; then
				ErrorLog "Failed to cleanup preserved space"
				return 1
			fi
			;;
		--help)
			Help
			;;
		*)
			ErrorLog "Invalid action; should be [--ensure|--cleanup|--help]"
			return 1
	esac

	return 0
}


main "$@"