I wrote a GenServer that wraps a gen_tcp socket and does some long running tasks involving reading and writing on the socket.

defmodule MyClient do
  use GenServer

  @impl true
  def init(_) do
    {:ok, nil, {:continue, :connect}}
  end

  @impl true
  def handle_continue(:connect, _) do
    {:ok, socket} = :gen_tcp.connect(@ip, @port, [
      mode: :binary,  # <-- NOTICE THIS CONFIG
      packet: 0,
      keepalive: true,
      active: false,
      reuseaddr: true,
      send_timeout: 5000,
      send_timeout_close: true
    ])
    send(self(), :work)
    {:noreply, socket}
  end

  @impl true
  def handle_info(:work, socket) do
    {:ok, data} = :gen_tcp.recv(socket, 4)  # <-- The problematic line
    ...
    {:noreply, socket}
  end
end

When I set the mode to :list, it works fine.

When I set the mode to :binary, the line :gen_tcp.recv always returns {:error, :closed}. I wonder why. Did I messed up the configuration?