Render string values as handlebars strings in resource_update

This adds two new secial handlebars strings:

* `CURRENT_TIME`
* `NULL`

farmbot_os/lib/farmbot_os/sys_calls/resource_update.ex

@@ -1,20 +1,26 @@
 defmodule FarmbotOS.SysCalls.ResourceUpdate do
+  require Logger
+
   alias FarmbotCore.{
     Asset,
     Asset.Private
   }
 
+  alias FarmbotOS.SysCalls.SendMessage
+
   @point_kinds ~w(Plant GenericPointer)
 
   def resource_update("Device", 0, params) do
     params
+    |> do_handlebars()
     |> Asset.update_device!()
     |> Private.mark_dirty!()
 
     :ok
   end
 
   def resource_update(kind, id, params) when kind in @point_kinds do
+    params = do_handlebars(params)
     point_resource_update(kind, id, params)
   end
 
@@ -36,4 +42,22 @@ defmodule FarmbotOS.SysCalls.ResourceUpdate do
       {:error, _changeset} -> {:error, "Failed to update #{type}.#{id}"}
     end
   end
+
+  @doc false
+  def do_handlebars(params) do
+    Map.new(params, fn
+      {key, value} when is_binary(value) ->
+        case SendMessage.render(value) do
+          {:ok, rendered} ->
+            {key, rendered}
+
+          _ ->
+            Logger.warn("failed to render #{key} => #{value} for resource_update")
+            {key, value}
+        end
+
+      {key, value} ->
+        {key, value}
+    end)
+  end
 end

farmbot_os/lib/farmbot_os/sys_calls/send_message.ex

@@ -23,7 +23,9 @@ defmodule FarmbotOS.SysCalls.SendMessage do
   def render(templ) do
     with {:ok, pos} <- pos(),
          {:ok, pins} <- pins(),
-         env <- Keyword.merge(pos, pins) do
+         {:ok, special} <- special(),
+         env <- Keyword.merge(pos, pins),
+         env <- Keyword.merge(env, special) do
       env = Map.new(env, fn {k, v} -> {to_string(k), to_string(v)} end)
 
       # Mini Mustache parser
@@ -62,4 +64,12 @@ defmodule FarmbotOS.SysCalls.SendMessage do
        {:"pin#{p}", FarmbotCeleryScript.FormatUtil.format_float(v)}
      end)}
   end
+
+  def special() do
+    {:ok,
+     [
+       {:NULL, nil},
+       {:CURRENT_TIME, DateTime.utc_now()}
+     ]}
+  end
 end