Remove deferred loading of NIF

It was possible to crash the BEAM by loading a completely trimmed down
shared library with multiple processes. This converts the NIF loader
back to the traditional `@on_load` way that's serialized by the code
loader. The benefits of delaying the load time are less now that the
shared library has been tested quite a bit.

lib/gpio/gpio_nif.ex

@@ -5,26 +5,15 @@
 defmodule Circuits.GPIO.Nif do
   @moduledoc false
 
-  defp load_nif_and_apply(fun, args) do
-    nif_binary = Application.app_dir(:circuits_gpio, "priv/gpio_nif")
+  @on_load {:load_nif, 0}
+  @compile {:autoload, false}
 
-    # Optimistically load the NIF. Handle the possible race.
-    case :erlang.load_nif(to_charlist(nif_binary), 0) do
-      :ok -> apply(__MODULE__, fun, args)
-      {:error, {:reload, _}} -> apply(__MODULE__, fun, args)
-      error -> error
-    end
+  def load_nif() do
+    :erlang.load_nif(:code.priv_dir(:circuits_gpio) ++ ~c"/gpio_nif", 0)
   end
 
-  def open(gpio_spec, resolved_gpio_spec, direction, initial_value, pull_mode) do
-    load_nif_and_apply(:open, [
-      gpio_spec,
-      resolved_gpio_spec,
-      direction,
-      initial_value,
-      pull_mode
-    ])
-  end
+  def open(_gpio_spec, _resolved_gpio_spec, _direction, _initial_value, _pull_mode),
+    do: :erlang.nif_error(:nif_not_loaded)
 
   def close(_gpio), do: :erlang.nif_error(:nif_not_loaded)
   def read(_gpio), do: :erlang.nif_error(:nif_not_loaded)
@@ -36,16 +25,7 @@ defmodule Circuits.GPIO.Nif do
   def set_direction(_gpio, _direction), do: :erlang.nif_error(:nif_not_loaded)
   def set_pull_mode(_gpio, _pull_mode), do: :erlang.nif_error(:nif_not_loaded)
   def info(_gpio), do: :erlang.nif_error(:nif_not_loaded)
-
-  def status(resolved_gpio_spec) do
-    load_nif_and_apply(:status, [resolved_gpio_spec])
-  end
-
-  def backend_info() do
-    load_nif_and_apply(:backend_info, [])
-  end
-
-  def enumerate() do
-    load_nif_and_apply(:enumerate, [])
-  end
+  def status(_resolved_gpio_spec), do: :erlang.nif_error(:nif_not_loaded)
+  def backend_info(), do: :erlang.nif_error(:nif_not_loaded)
+  def enumerate(), do: :erlang.nif_error(:nif_not_loaded)
 end

test/circuits_gpio_test.exs

@@ -482,21 +482,4 @@ defmodule Circuits.GPIO2Test do
     assert GPIO.write_one({@gpiochip, 1}, 1) == :ok
     assert GPIO.read_one({@gpiochip, 0}) == 1
   end
-
-  test "racing to load the NIF" do
-    # Make sure the NIF isn't loaded
-    assert true == :code.delete(Circuits.GPIO.Nif)
-    assert false == :code.purge(Circuits.GPIO.Nif)
-
-    # Try to hit the race by having 32 processes race to load the NIF
-    tasks =
-      for index <- 0..31 do
-        Task.async(fn ->
-          {:ok, gpio} = GPIO.open({@gpiochip, index}, :input)
-          GPIO.close(gpio)
-        end)
-      end
-
-    Enum.each(tasks, &Task.await/1)
-  end
 end