puma on random port

app.rb

require 'rack'
require 'puma'

def my_app
  return ->(env) {
    [200, {"content-type"=>"text/plain"},
     ["hello world"]]
  }
end

_,_,_,my_address = UDPSocket.new.tap {|u| u.connect("8.8.8.8", 53) }.addr

puma = Rack::Handler.get("puma")
puma.run(my_app, {Host: "0.0.0.0", Port: 0}) do |s|
  # oh Demeter ...
  realport = s.binder.ios.first.local_address.ip_port
  FileUtils.mkdir_p("run")
  File.open("run/local_address", "w") do |f|
    local_endpoint="#{my_address}:#{realport}"
    f.write("#{local_endpoint}\n")
    $stderr.write("Wrote #{local_endpoint.inspect} to run/local_address\n")
  end
end