Conditional WebSocket connection - is there a way to transform my websocket module to a valid Plug handler?

Questions / Help
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1

Hi!
I am trying to create a proxy server with elixir, plug and cowboy to filter out websocket messages from a particular application.

However, the source application relies on the Upgrade header on the / route to upgrade the connection to a websocket one and returns the index.html file if the header is not present. I have not been able to upgrade the connection conditionally using Plug and Cowboy. I tested by doing the following:

defmodule MyPlug do
  import Plug.Conn

  def init(options) do
    options
  end

  def call(conn, opts) do
    IO.puts("Proxying '#{conn.request_path}'")

    case get_req_header(conn, "upgrade") do
      ["websocket"] ->
        Plug.forward(conn, conn.path_info, WebSocketPlug, opts)

      _http ->
        conn
        |> send_resp(200, "an html file")
    end
  end
end

And for the Websocket plug which is actually not one since it is a Cowboy handler:

defmodule WebSocketPlug do
  @behaviour :cowboy_websocket

  def init(request, _state) do
      {:cowboy_websocket, request, %{}}
    end
  end

  def websocket_handle({:text, _json}, state) do
    {:reply, {:text, "hey!"}, state}
  end

  def websocket_info(info, state) do
    {:reply, {:text, info}, state}
  end

  def websocket_init(state) do
    {:ok, state}
  end
end

Is there a way to transform my websocket module to a valid Plug handler ?

Thanks for your help!