In my config/runtime.exs I have the following:

timezone = System.get_env("TZ") || "Etc/UTC"

unless Timex.is_valid_timezone?(timezone) do
  raise """
  The timezone specified in the TZ environment variable is invalid.
  """
end

config :my_app, MyApp, timezone: timezone

My intention is that a valid Timezone will work as expected while an invalid one (such as Europe/Börlin) crashes the application early.In the rest of the App, I can then assume that the user-supplied timezone is always valid.

This works as expected in dev, on prod however, this results in a crash at runtime:

ERROR! Config provider Config.Reader failed with:
** (ArgumentError) errors were found at the given arguments:
  * 1st argument: the table identifier does not refer to an existing ETS table
    (stdlib 6.2) :ets.lookup(:tzdata_current_release, :release_version)
    (tzdata 1.1.3) lib/tzdata/release_reader.ex:74: Tzdata.ReleaseReader.current_release
    (tzdata 1.1.3) lib/tzdata/release_reader.ex:17: Tzdata.ReleaseReader.simple_lookup/1
    (tzdata 1.1.3) lib/tzdata/release_reader.ex:9: Tzdata.ReleaseReader.zone_and_link_li
    (tzdata 1.1.3) lib/tzdata.ex:61: Tzdata.zone_exists?/1
    (timex 3.7.13) lib/timezone/timezone.ex:230: Timex.Timezone.name_of/1
    (timex 3.7.13) lib/timex.ex:857: Timex.is_valid_timezone?/1
    /app/releases/0.1.0/runtime.exs:13: (file)

I’m assuming this happens because the Timex application isn’t started yet when the runtime configuration is evaluated.

So instead, I now have this workaround that achieves the same thing:

defmodule MyApp.Application do
  use Application

  @impl true
  def start(_type, _args) do
    with :ok <- validate_timezone_config() do
      # shortened
      Supervisor.start_link(children, opts)
    end
  end

  defp validate_timezone_config do
    timezone = Briefly.user_timezone()

    case Timex.is_valid_timezone?(timezone) do
      true -> :ok
      false -> {:error, "Configured timezone '#{timezone}' is invalid"}
    end
  end
end

If an invalid timezone is supplied, the app crashes early:

11:39:02.487 [notice] Application briefly exited: Briefly.Application.start(:normal, []) returned an error: "Configured timezone 'Europe/Börlin' is invalid"
** (exit) :terminating
    (kernel 10.2.1) application_controller.erl:511: :application_controller.call/2
    (kernel 10.2.1) application.erl:367: :application."-ensure_all_started/3-lc$^0/1-0-"/1
    (kernel 10.2.1) application.erl:367: :application.ensure_all_started/3
    (mix 1.18.2) lib/mix/tasks/app.start.ex:72: Mix.Tasks.App.Start.start/3
    (mix 1.18.2) lib/mix/task.ex:495: anonymous fn/3 in Mix.Task.run_task/5
    (mix 1.18.2) lib/mix/tasks/run.ex:129: Mix.Tasks.Run.run/5
    (mix 1.18.2) lib/mix/tasks/run.ex:85: Mix.Tasks.Run.run/1
    (mix 1.18.2) lib/mix/task.ex:495: anonymous fn/3 in Mix.Task.run_task/5

Is this considered an anti-pattern? Is there a more appropriate/easier way to validate runtime configuration during startup?