#!/bin/sh
#
# Copyright (C) 2014-2015  Codethink Limited
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; version 2 of the License.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License along
# with this program.  If not, see <http://www.gnu.org/licenses/>.
#
# =*= License: GPL-2 =*=


# Receive a data stream describing a sparse file, and reproduce it,
# either to a named file or stdout.
#
# The data stream is simple: it's a sequence of DATA or HOLE records:
#
#    DATA
#    123
#    <123 bytes of binary data, NOT including newline at the end>
#
#    HOLE
#    123
#
# This shell script can be executed over ssh (given to ssh as an arguemnt,
# with suitable escaping) on a different computer. This allows a large
# sparse file (e.g., disk image) be transferred quickly.
#
# This script should be called in one of the following ways:
#
#    recv-hole file FILENAME
#    recv-hole vbox FILENAME DISKSIZE
#
# In both cases, FILENAME is the pathname of the disk image on the
# receiving end. DISKSIZE is the size of the disk image in bytes. The
# first form is used when transferring a disk image to become an
# identical file on the receiving end.
#
# The second form is used when the disk image should be converted for
# use by VirtualBox. In this case, we want to avoid writing a
# temporary file on disk, and then calling the VirtualBox VBoxManage
# tool to do the conversion, since that would involve large amounts of
# unnecessary I/O and disk usage. Instead we pipe the file directly to
# VBoxManage, avoiding those issues. The piping is done here in this
# script, instead of in the caller, to make it easier to run things
# over ssh.
#
# However, since it's not possible seek in a Unix pipe, we have to
# explicitly write the zeroes into the pipe. This is not
# super-efficient, but the way to avoid that would be to avoid sending
# a sparse file, and do the conversion to a VDI on the sending end.
# That is out of scope for xfer-hole and recv-hole.


set -eu


die()
{
    echo "$@" 1>&2
    exit 1
}


recv_hole_to_file()
{
    local n

    read n
    truncate --size "+$n" "$1"
}


recv_data_to_file()
{
    local n
    read n

    local blocksize=1048576
    local blocks=$(($n / $blocksize))
    local extra=$(($n % $blocksize))

    xfer_data_to_stdout "$blocksize" "$blocks" >> "$1"
    xfer_data_to_stdout 1 "$extra" >> "$1"
}


recv_hole_to_stdout()
{
    local n
    read n
    (echo "$n"; cat /dev/zero) | recv_data_to_stdout
}


recv_data_to_stdout()
{
    local n
    read n

    local blocksize=1048576
    local blocks=$(($n / $blocksize))
    local extra=$(($n % $blocksize))

    xfer_data_to_stdout "$blocksize" "$blocks"
    xfer_data_to_stdout 1 "$extra"
}


xfer_data_to_stdout()
{
    local log="$(mktemp)"
    if ! dd "bs=$1" count="$2" iflag=fullblock status=noxfer 2> "$log"
    then
        cat "$log" 1>&2
        rm -f "$log"
        exit 1
    else
        rm -f "$log"
    fi
}


type="$1"
case "$type" in
    file)
        output="$2"
        truncate --size=0 "$output"
        while read what
        do
            case "$what" in
                DATA) recv_data_to_file "$output" ;;
                HOLE) recv_hole_to_file "$output" ;;
                *) die "Unknown instruction: $what" ;;
            esac
        done
        ;;
    vbox)
        output="$2"
        disk_size="$3"
        while read what
        do
            case "$what" in
                DATA) recv_data_to_stdout ;;
                HOLE) recv_hole_to_stdout ;;
                *) die "Unknown instruction: $what" ;;
            esac
        done |
        VBoxManage convertfromraw stdin "$output" "$disk_size"
        ;;
esac