Run a NervesHub script on device (#207)

* Run a NervesHub script on device

Evaluate a script locally on device from NervesHub instead of relying on
the console channel. Bumps the api version so NervesHub is aware this is
possible on the connecting device.

* Run script capture in a separate process

In case of bad code we won't crash the socket process or potentially
leave the socket process with a bad group leader

lib/nerves_hub_link/configurator.ex

@@ -3,7 +3,7 @@ defmodule NervesHubLink.Configurator do
   alias __MODULE__.Config
   require Logger
 
-  @device_api_version "2.0.0"
+  @device_api_version "2.1.0"
   @console_version "2.0.0"
 
   defmodule Config do

lib/nerves_hub_link/script.ex

@@ -0,0 +1,46 @@
+defmodule NervesHubLink.Script do
+  use GenServer
+
+  @doc """
+  Run a script from NervesHub and capture its output
+  """
+  def capture(text, ref) do
+    _ = GenServer.start_link(__MODULE__, {self(), text, ref})
+
+    :ok
+  end
+
+  @impl GenServer
+  def init({pid, text, ref}) do
+    state = %{pid: pid, text: text, ref: ref}
+    {:ok, state, {:continue, :capture}}
+  end
+
+  @impl GenServer
+  def handle_continue(:capture, state) do
+    # Inspired from ExUnit.CaptureIO
+    # https://github.com/elixir-lang/elixir/blob/main/lib/ex_unit/lib/ex_unit/capture_io.ex
+    {:ok, string_io} = StringIO.open("")
+
+    Process.group_leader(self(), string_io)
+
+    {result, output} =
+      try do
+        Code.eval_string(state.text)
+      catch
+        kind, reason ->
+          {:ok, {_input, output}} = StringIO.close(string_io)
+          result = Exception.format_banner(kind, reason, __STACKTRACE__)
+          output = output <> "\n" <> result
+          {nil, output}
+      else
+        {result, _binding} ->
+          {:ok, {_input, output}} = StringIO.close(string_io)
+          {result, output}
+      end
+
+    send(state.pid, {"scripts/run", state.ref, output, result})
+
+    {:stop, :normal, state}
+  end
+end

lib/nerves_hub_link/socket.ex

@@ -9,6 +9,7 @@ defmodule NervesHubLink.Socket do
   alias NervesHubLink.Client
   alias NervesHubLink.Configurator
   alias NervesHubLink.Configurator.SharedSecret
+  alias NervesHubLink.Script
   alias NervesHubLink.UpdateManager
   alias NervesHubLink.UploadFile
 
@@ -306,6 +307,12 @@ defmodule NervesHubLink.Socket do
     {:ok, socket}
   end
 
+  def handle_message(@device_topic, "scripts/run", params, socket) do
+    # See related handle_info for pushing back the script result
+    :ok = Script.capture(params["text"], params["ref"])
+    {:ok, socket}
+  end
+
   def handle_message(@device_topic, "archive", params, socket) do
     {:ok, info} = NervesHubLink.Message.ArchiveInfo.parse(params)
     _ = ArchiveManager.apply_archive(info, socket.assigns.config.archive_public_keys)
@@ -410,6 +417,17 @@ defmodule NervesHubLink.Socket do
     end
   end
 
+  def handle_info({"scripts/run", ref, output, return}, socket) do
+    _ =
+      push(socket, @device_topic, "scripts/run", %{
+        ref: ref,
+        output: output,
+        return: inspect(return, pretty: true)
+      })
+
+    {:noreply, socket}
+  end
+
   def handle_info({:tty_data, data}, socket) do
     _ = push(socket, @console_topic, "up", %{data: data})
     {:noreply, set_iex_timer(socket)}