#!/bin/sh

# isopad [+] [-] isofile...
#
# The Linux IDE CD driver in 2.6 tries to read ahead, even past the end of the
# CD or DVD.  Even when the program issuing the original read request was only
# trying to read legitimate parts of the disc (albeit near the end).
# The result is spurious I/O errors and read failures.
# It does not seem that this driver bug is going to be fixed soon.
#
# This program is intended to facilitate a workaround.  It can pad (or unpad)
# a .iso file so that, when it is burned, the resulting disc will allow
# reads past the end of the content to succeed.
#
# "+" means pad the following .iso files.
# "-" means remove all padding.
# neither means test file and iso sizes.
#
# To see how much readahead is enabled on a drive: hdparm -a /dev/hdc
#
# Why do the padding in place, rather than on a copy of the file?
# .iso files are usually quite large so copying takes a lot of time and space.
#
# Copyright 2005 D. Hugh Redelmeier
# License: GPL
# Version: Sat Jun 18 02:31:48 EDT 2005

# stop at the least sign of trouble
set -u -e

# op is "", "-", or "+": operation to be performed
op=""

for fn
do
	case "$fn"
	in
	"-h"|"--help")
		echo "Usage: $0 [-|+|] isofile..."
		;;
	"+"|"-")
		op="$fn"
		;;
	*)
		isosize -x "$fn"
		isz=`isosize "$fn"`
		fsz=`stat --format='%s' "$fn"`

		# conventional block size for CDs
		bs=2048

		# my guess at a sufficient amount of padding (in blocks)
		pb=64

		if [ $fsz -lt $isz ]
		then
			echo "$fn is shorter ($fsz) than it should be ($isz)" >&2
			exit 3
		elif [ ` expr $fsz % $bs ` -ne 0 ]
		then
			echo "$fn file size ($fsz) is not a multiple of $bs" >&2
			exit 4
		elif [ ` expr $isz % $bs ` -ne 0 ]
		then
			echo "$fn isosize ($isz) is not a multiple of $bs" >&2
			exit 5
		else
			case "$op" in
			"")
				if [ $fsz -eq $isz ]
				then
					echo "$fn: isosize == file size == $fsz"
				else
					echo "$fn: isosize $isz; file size $fsz"
				fi
				;;
			"+")
				echo "$fn: padding with $pb blocks of $bs zero bytes"
				dd if=/dev/zero bs=$bs count=$pb >>"$fn"
				;;
			"-")
				if [ $fsz -eq $isz ]
				then
					echo "$fn: already $fsz bytes"
				else
					echo "$fn: truncating from $fsz to $isz bytes"
					dd if=/dev/null of="$fn" seek=$isz bs=1
				fi
				;;
			esac
		fi
		;;
	esac
done