feat(server): add client connection handler

lib/excache/server/connection.ex

@@ -0,0 +1,59 @@
+defmodule Excache.Server.Connection do
+  @moduledoc false
+
+  use GenServer, restart: :temporary
+
+  alias Excache.RESP.{Parser, Encoder}
+  alias Excache.Command
+  alias Excache.Command.Transaction
+
+  def start_link(socket), do: GenServer.start_link(__MODULE__, socket)
+
+  @impl true
+  def init(socket) do
+    base = %{socket: socket, buffer: "", subscriptions: MapSet.new()}
+    {:ok, Map.merge(base, Transaction.initial_state())}
+  end
+
+  @impl true
+  def handle_info(:socket_ready, state) do
+    :inet.setopts(state.socket, active: :once)
+    {:noreply, state}
+  end
+
+  def handle_info({:tcp, socket, data}, state) do
+    {output, state} = consume(state.buffer <> data, %{state | buffer: ""})
+
+    unless output == [] do
+      :gen_tcp.send(socket, output)
+    end
+
+    :inet.setopts(socket, active: :once)
+    {:noreply, state}
+  end
+
+  def handle_info({:tcp_closed, _socket}, state), do: {:stop, :normal, state}
+  def handle_info({:tcp_error, _socket, _reason}, state), do: {:stop, :normal, state}
+
+  def handle_info({:pubsub, channel, payload}, state) do
+    message = Encoder.encode({:array, [{:bulk, "message"}, {:bulk, channel}, {:bulk, payload}]})
+    :gen_tcp.send(state.socket, message)
+    {:noreply, state}
+  end
+
+  defp consume(buffer, state) do
+    case Parser.parse_command(buffer) do
+      :incomplete ->
+        {[], %{state | buffer: buffer}}
+
+      {:error, reason} ->
+        {[Encoder.encode({:error, "ERR Protocol error: #{reason}"})], %{state | buffer: ""}}
+
+      {:ok, args, rest} ->
+        {replies, state} = Command.execute(args, state)
+        encoded = Enum.map(replies, &Encoder.encode/1)
+        {more, state} = consume(rest, state)
+        {[encoded | more], state}
+    end
+  end
+end