Add plug to generate Matrix error

lib/matrix_server_web/plug/authenticate.ex

@@ -0,0 +1,36 @@
+defmodule MatrixServerWeb.Plug.Authenticate do
+  import MatrixServerWeb.Plug.Error
+  import Plug.Conn
+  import Phoenix.Controller, only: [json: 2]
+
+  alias MatrixServer.Account
+  alias Plug.Conn
+
+  def init(opts), do: opts
+
+  def call(%Conn{params: %{"access_token" => access_token}} = conn, _opts) do
+    authenticate(conn, access_token)
+  end
+
+  def call(%Conn{req_headers: headers} = conn, _opts) do
+    case List.keyfind(headers, "authorization", 0) do
+      {_, "Bearer " <> access_token} ->
+        authenticate(conn, access_token)
+
+      _ ->
+        put_error(conn, :missing_token)
+    end
+  end
+
+  defp authenticate(conn, access_token) do
+    case Account.get_by_access_token(access_token) do
+      %Account{devices: [device]} = account ->
+        conn
+        |> assign(:account, account)
+        |> assign(:device, device)
+
+      nil ->
+        put_error(conn, :unknown_token)
+    end
+  end
+end