HI,

I would like to know what the best way to validate runtime configuration before the supervisor starts their children. Essentially I need to check that if one environment variable is set, another is also set, if not the application should exit with 1:

defmodule MyApp.MyAppSupervisor do
  # Automatically defines child_spec/1
  use Supervisor
  alias Vapor.Provider.{File, Env}

  def start_link(init_arg) do
    Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
  end

  @impl true
  def init(_init_arg) do
    providers = [
      %Env{bindings: [
              {:proxy_enable, "PROXY_ENABLE", default: false, map: fn s -> String.upcase(s) == "TRUE" end},
              {:proxy_upstream, "PROXY_UPSTREAM", default: ""},
              {:proxy_environment, "PROXY_ENVIRONMENT", default: 0, map: &String.to_integer/1},
              {:proxy_timeout, "PROXY_TIMEOUT", default: 5000, map: &String.to_integer/1},
              {:proxy_max_connections, "PROXY_MAX_CONNECTIONS", default: 100, map: &String.to_integer/1},
          ]},
      # %File{path: "config.yaml", bindings: [kafka_brokers: "kafka.brokers"]},
    ]

    # If values could not be found we raise an exception and halt the boot
    # process
    config = Vapor.load!(providers)


    Application.put_env(:my_app, MyApp.PlugProxy, [
      upstream: config[:proxy_upstream],
      environment: config[:proxy_environment],
      timeout: config[:proxy_timeout]
    ])

    children = [
    ]

    children =
      if config.proxy_enable do

        if config.proxy_upstream == "" do
          IO.puts "You must configure Proxy Upstream"
          exit {:shutdown, 1}
        end


        child_specs =
          [
            Cables.child_spec(:http_proxy_pool, config.proxy_upstream, :http, max_connections: config.proxy_max_connections),
            MyApp.ProxyEndpoint
          ]
        children ++ child_specs
      else
        children
      end

    Supervisor.init(children, strategy: :one_for_one)
  end
end

However, I get a rather ugly output and also a crash_dump.

>>> mix release
...
>>>  _build/dev/rel/myapp/bin/myapp start
You must configure Proxy Upstream
[info] Application myapp exited: MyApp.Application.start(:normal, []) returned an error: shutdown: failed to start child: MyApp.MyAppSupervisor
    ** (EXIT) shutdown
[os_mon] memory supervisor port (memsup): Erlang has closed
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed
{"Kernel pid terminated",application_controller,"{application_start_failure,myapp,{{shutdown,{failed_to_start_child,'Elixir.MyApp.MyAppSupervisor',shutdown}},{'Elixir.MyApp.Application',start,[normal,[]]}}}"}
Kernel pid terminated (application_controller) ({application_start_failure,myapp,{{shutdown,{failed_to_start_child,'Elixir.MyApp.MyAppSupervisor',shutdown}},{'Elixir.MyApp

Crash dump is being written to: erl_crash.dump...done

What is the best way to not show the error message and prevent it from writing a crash dump?

I would like the output of my application to be:

>>>  _build/dev/rel/myapp/bin/myapp start
You must configure Proxy Upstream
>>> ehco $?
1