implement automatic receiving of tun messages from elixir code

lib/p2p_chat/transport/gen_tun.ex

@@ -0,0 +1,52 @@
+defmodule P2pChat.Transport.GenTun do
+  require Logger
+  use Rustler, otp_app: :p2p_chat, crate: "p2pchat_transport_gen_tun"
+
+  @behaviour GenServer
+
+  defstruct [:tun_handle]
+
+  #
+  # Server API
+  # 
+  @impl true
+  def init(args) do
+    {:ok, tun_handle} = make_tun_device()
+    state = %__MODULE__{tun_handle: tun_handle}
+    { :ok, state, { :continue, :recv } }
+  end
+
+  @impl true
+  def handle_continue(:recv, state) do
+    case tun_recv(state.tun_handle) do
+       {:ok, buf} ->
+         Logger.debug("Received #{byte_size(buf)} bytes")
+       {:error, :would_block} -> {}
+       {:error, e} ->
+         Logger.error("Error during receive: #{e}")
+    end
+    
+    {:noreply, state, { :continue, :recv }}
+  end
+
+  @impl true
+  def handle_call({:recv}, _from, state) do
+    data = tun_recv(state.tun_handle)
+    { :reply, data, state }
+  end
+
+  #
+  # Client API
+  # 
+  def open() do
+    GenServer.start_link(__MODULE__, nil)
+  end  
+
+  #
+  # NIFs
+  # 
+
+  defp make_tun_device(), do: :erlang.nif_error(:nif_not_loaded)
+  defp tun_recv(_handle, _bufsize \\ 2*16), do: :erlang.nif_error(:nif_not_loaded)
+end
+