#!/bin/bash

# copy resumably with dd via network

script_name=$(basename "$0")

usage() {
  >&2 printf 'usage:\n'
  >&2 printf '  %s in-host in-dev out-dev log-file\n' "${script_name}"
  >&2 printf '  %s -s in-dev start-position remote-port\n' "${script_name}"
  exit 1
}

dd_options='bs=1M'

for needed in dd nc ss screen pgrep kill ssh; do
  if ! which "${needed}" >/dev/null; then
    >&2 printf 'please install required program %s\n' "${needed}"
    exit 1
  fi
done

if [ "x$1" = 'x-s' ]; then
  shift
  if [ $# -ne 3 ]; then
    >&2 printf 'expected 3 arguments, got %s:\n' "$#"
    >&2 printf '"%s" ' "$@"
    >&2 printf '\n'
    usage
  fi
  dev="$1"
  start="$2"
  remote_ip="${SSH_CLIENT%% *}"
  if [ -z "${remote_ip}" ]; then
    >&2 printf 'cannot obtain remote ip\n'
    exit 1
  fi
  remote_port="$3"
  if [ ! -r "${dev}" ]; then
    >&2 printf 'cannot read from %s\n\n' "${dev}"
    usage
  fi
  set +o pipefail
  dd if="${dev}" ${dd_options} skip="${start}" 2>/dev/null | \
    nc -cn "${remote_ip}" "${remote_port}"
  exit $?
fi

if [ $# -ne 4 ]; then
  usage
fi
remote_host="$1"
remote_dev="$2"
local_dev="$3"
log_file="$4"

if [ ! -w "${log_file%/*}" ] && [ ! -w "${log_file}" ]; then
  >&2 printf 'cannot write/create log-file %s\n\n' "${log_file}"
  usage
fi

if [ ! -w "${local_dev}" ]; then
  >&2 printf 'cannot write to out-dev %s\n\n' "${local_dev}"
  usage
fi

start=0
if [ -f "${log_file}" ]; then
  start=$(
    sed -n '
      1 {
        s/^start: //
        p
        q
      }
    ' "${log_file}"
  )
  copied=$(
    sed '
      s/^\([0-9]\+\)+0 records out$/\1/
      t
      d
    ' "${log_file}" | \
    sort -n | \
    tail -n1
  )
  if [ -n "${copied}" ]; then
    start=$((start+copied))
  fi
fi

if pgrep -ax dd || pgrep -ax nc; then
  >&2 printf 'another dd or nc is already running - this won'"'"'t work\n'
  exit 1
fi

printf 'start: %s\n' "${start}" > \
  "${log_file}"

screen -d -m bash -c '
  set +o pipefail
  nc -l -c | \
    dd of="'"${local_dev}"'" '"${dd_options}"' seek='"${start}"' 2>>"'"${log_file}"'" >/dev/null
  printf '"'"'\nwrite-pipe-exit: %s\n'"'"' $? >>"'"${log_file}"'"
'

for i in {1..20}; do
  if [ -n "${port}" ]; then
    break
  elif [ ${i} -gt 1 ]; then
    sleep 0.1
  fi
  nc_pid=$(
    pgrep -x nc
  )
  dd_pid=$(
    pgrep -x dd
  )
  port=$(
    ss -nlp | \
      grep -F ",pid=${nc_pid}," | \
      awk '{print $5}' | \
      sed 's/^.*://'
  )
done
if [ -z "${port}" ]; then
  >&2 printf 'could not determine listening port\n'
  exit 1
fi
screen -d -m bash -c '
  while kill -SIGUSR1 '"${dd_pid}"'; do
    sleep 1
  done
'

ssh "${remote_host}" "${script_name} -s ${remote_dev} ${start} ${port}"
printf '\nssh-exit: %s\n' $? >>"${log_file}"