Configuration not working with Elixir 1.9 releases

[José Francisco Calvo]

I'm using Elixir 1.9 and configuring the release using the recommended config/releases.exs file with the following line:

config :cors_plug, origin: [System.fetch_env!("CORS_ORIGIN")]

Looks like the previous line is not working as expected and the built release is not allowing requests from the origin set on the CORS_ORIGIN env var.

I'm doing something wrong? The other configs are working fine so I think this is an specific problem with cors_plug.

If I define the following line on prod.exs the requests works as expected (but of course I lose the possibility of configure the origin executing the release):

config :cors_plug, origin: ["http://192.168.1.128:8080"]

[Thiago Santos]

@jfcalvo As far as I know that happens because the place where the config is fetched, the init/1 callback of the plug runs at compile-time. So when the release starts and update the config, the previous value is already inlined by the compiler.

What I usually do is to pass a function that returns the allowed origins, as described in the docs. And in there, I make sure to fetch the config at runtime.

# endpoint.ex
plug CORSPlug, origin: &MyCors.my_fun/0

# my_cors.ex
defmodule MyCors do
  def my_fun do
    Application.fetch_env!(:my_app, :my_allowed_origins)
  end
end

# config/releases.exs
config :my_app, my_allowed_origins: System.fetch_env!("MY_CORS_ORIGIN")

[Mika Cohen]

with elixir 1.9 releases you put the dynamic stuff in config/releases.exs like this:

import Config
config :cors_plug, origin: [System.fetch_env!("CORS_ORIGIN")]

https://hexdocs.pm/mix/Mix.Tasks.Release.html#module-runtime-configuration

if you imported Mix.Config instead of Config it would reproduce the behavior you're seeing, I'm pretty sure.

[Michael Schaefermeyer]

I'm considering this closed for now, please let me know if there is anything I can do on my end and I'll reopen!