Add device features support

Start of device features (previously "extensions") which allow for
specialized features on device to report data and interactions
safely outside the update mechanism.

lib/nerves_hub_web/channels/device_channel.ex

@@ -104,6 +104,11 @@ defmodule NervesHubWeb.DeviceChannel do
       schedule_health_check()
     end
 
+    # device.allow_features?
+    if true do
+      push(socket, "features:get", %{})
+    end
+
     {:noreply, socket}
   end
 

lib/nerves_hub_web/channels/device_socket.ex

@@ -11,6 +11,7 @@ defmodule NervesHubWeb.DeviceSocket do
 
   channel("console", NervesHubWeb.ConsoleChannel)
   channel("device", NervesHubWeb.DeviceChannel)
+  channel("features", NervesHubWeb.FeaturesChannel)
 
   # Default 90 seconds max age for the signature
   @default_max_hmac_age 90

lib/nerves_hub_web/channels/features_channel.ex

@@ -0,0 +1,36 @@
+defmodule NervesHubWeb.FeaturesChannel do
+  use Phoenix.Channel
+
+  alias Phoenix.Socket.Broadcast
+
+  @impl Phoenix.Channel
+  def join("features", payload, socket) do
+    enable_list =
+      for {feature, ver} <- payload, into: %{} do
+        {feature, allowed?(feature, ver)}
+      end
+
+    topic = "device:#{socket.assigns.device.id}:features"
+    Phoenix.PubSub.subscribe(NervesHub.PubSub, topic)
+
+    {:ok, enable_list, socket}
+  end
+
+  defp allowed?(feature, ver) do
+    # TODO: Some conditions for allow/disallow feature
+    true
+  end
+
+  @impl Phoenix.Channel
+  def handle_in(event, payload, socket) do
+    dbg({event, payload})
+    {:noreply, socket}
+  end
+
+  @impl Phoenix.Channel
+  # TODO: Get specific on messages passed to device
+  def handle_info(%Broadcast{event: event, payload: payload}, socket) do
+    push(socket, event, payload)
+    {:noreply, socket}
+  end
+end