Parse nixos extlinux boot and boot a vm with vftool

run_vftool.rb

require 'optparse'
require 'tempfile'
require 'json'

def parse_extlinux(file)
  globals = {}
  labels = {}

  target = globals

  file.lines.each do |line|
    line = line.strip
    next if line.empty?
    next if line =~ /^#/

    key, value = line.split(' ', 2)
    if key == 'LABEL'
      target = labels[value] = {}
    else
      target[key] = value
    end
  end

  [globals, labels]
end

def default_config(globals, labels)
  default = globals.fetch('DEFAULT') { labels.each_key.first }
  labels[default].merge(globals)
end

options = {}

OptionParser.new do |opts|
  opts.on('-b', '--boot BOOT', 'Boot disk image (fat32)')
  opts.on('-r', '--root ROOT', 'Root disk image')
  # vftool arguments
  opts.on('-c', '--cdrom path', 'cdrom image path')
  opts.on('-p', '--processors count', 'number of processors')
  opts.on('-m', '--memory size', 'memory size in mb')
  opts.on('-t', '--tty type', 'tty type')
end.permute!(into: options)

config_file = `mcopy -i "#{options[:boot]}" "::/extlinux/extlinux.conf" -`
raise 'no config' unless $?.success?

config = default_config(*parse_extlinux(config_file))

kernel = Tempfile.new
system('mcopy', '-n', '-i', options[:boot], "::/extlinux/#{config.fetch('LINUX')}", kernel.path) or raise('no kernel')

initrd = Tempfile.new
system('mcopy', '-n', '-i', options[:boot], "::/extlinux/#{config.fetch('INITRD')}", initrd.path) or raise('no initrd')

vftool_args = [
  '-k', kernel.path,
  '-i', initrd.path,
  '-a', config.fetch('APPEND'),
  '-d', options[:root],
  '-d', options[:boot]
]

opt_mapping = { cdrom: '-c', processors: '-p', memory: '-m', tty: '-t' }
opt_mapping.each do |option, arg|
  if (opt = options[option])
    vftool_args <<  arg << opt
  end
end

STDERR.puts "vftool #{vftool_args.join(' ')}"
exec('vftool', *vftool_args)