Can't configure ssl to load custom session cache module

Questions / Help
,
1

I’m attempting to use a custom session cache module for the ssl application. I’ve got the following in config/config.exs:

config :ssl,
  session_cb: TlsServer.SessionCache

The module itself looks like this:

defmodule TlsServer.SessionCache do
  @behaviour :ssl_session_cache_api

  defdelegate init(options), to: :ssl_session_cache
  defdelegate terminate(ref), to: :ssl_session_cache
  defdelegate lookup(ref, key), to: :ssl_session_cache
  defdelegate update(ref, key, session), to: :ssl_session_cache
  defdelegate delete(ref, key), to: :ssl_session_cache
  defdelegate foldl(fun, acc, ref), to: :ssl_session_cache
  defdelegate select_session(ref, port), to: :ssl_session_cache
  defdelegate size(ref), to: :ssl_session_cache
end

…yes, it just delegates everything at the moment.

But when I run it with PORT=15350 iex -S mix run --no-halt, I get the following error:

17:19:01.821 [info]  Application ssl exited: :ssl_app.start(:normal, []) returned an error: shutdown: failed to start child: :ssl_admin_sup
    ** (EXIT) shutdown: failed to start child: :ssl_manager
        ** (EXIT) an exception was raised:
            ** (UndefinedFunctionError) function TlsServer.SessionCache.init/1 is undefined (module TlsServer.SessionCache is not available)
                TlsServer.SessionCache.init([role: :client])
                (ssl 9.5.3) ssl_manager.erl:237: :ssl_manager.init/1
                (stdlib 3.11.2) gen_server.erl:374: :gen_server.init_it/2
                (stdlib 3.11.2) gen_server.erl:342: :gen_server.init_it/6
                (stdlib 3.11.2) proc_lib.erl:249: :proc_lib.init_p_do_apply/3

Is there something I need to do differently to configure an Elixir module as a callback to an Erlang module? Or is there some problem with my module not being available at the correct time, somehow?

Do I have a problem that my application depends on ranch, which depends on ssl, which depends on a module from my application? That wouldn’t matter in Erlang, but does Elixir lazily-load my module or something?

2

If I run my application with the following:

PORT=15350 iex -pa _build/dev/lib/tls_server/ebin/ \
    -pa _build/dev/lib/ranch/ebin/ \
    --erl "-ssl session_cb 'Elixir.TlsServer.SessionCache'"

…and then use Application.ensure_all_started(:tls_server), then it all works.

But that’s awkward.

Update: It looks like it’s the mix part that’s breaking. I can’t even run mix release with that configuration item present. Which implies that mix is doing something lazy w.r.t. updating the code path, and that it’s also starting ssl (too early).

3

If I move the offending configuration to config/releases.exs and then use mix release, etc., then it works correctly.

This is slower than just using iex -S mix run. Can I get it to load extra config, but only immediately before starting the application?

4

Yes, your module works just fine as long as it is in the load path when :ssl starts. But outside of a release (including at compile time and release build time) it may be a catch 22.

If you don’t mind a hack, you can add…

  Application.put_env(:ssl, :session_cb, TlsServer.SessionCache)
  :ssl.stop
  :ssl.start

…to the top of your Application.start/2

5

I got it to work in two different ways:

  1. Using mix release and putting the setting in config/releases.ex, where it’s only used by the release.
  2. Not using mix run to run the app. Instead, I used iex --app thusly:
ERL_LIBS=_build/dev/lib PORT=15350 iex --erl-config sys.config --app tls_server

…where sys.config looks like this:

[{ssl,
    [{session_cb, 'Elixir.TlsServer.SessionCache'}]
}].

Note that this doesn’t load config/config.exs, so you’ll need to merge that into sys.config. I did it with a custom compiler.

6

The root of the issue is that, in any project that has any Hex dependencies, Mix starts Hex. This happens even when passing --no-compile --no-deps-check. Hex requires :ssl, which therefore gets started before the load paths have been set up for the application.

1 Like
7

Thanks for the clarification. I suspected it was something like that.