Fix race when two process load the NIF at the same time

Interestingly, the unit test was able to segfault before this fix.

Author: fhunleth

lib/i2c/i2c_nif.ex

@@ -1,15 +1,19 @@
 defmodule Circuits.I2C.Nif do
   @moduledoc false
 
-  defp load_nif() do
+  defp load_nif_and_apply(fun, args) do
     nif_binary = Application.app_dir(:circuits_i2c, "priv/i2c_nif")
-    :erlang.load_nif(to_charlist(nif_binary), 0)
+
+    # 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
   end
 
   def open(device) do
-    with :ok <- load_nif() do
-      apply(__MODULE__, :open, [device])
-    end
+    load_nif_and_apply(:open, [device])
   end
 
   def read(_ref, _address, _count, _retries), do: :erlang.nif_error(:nif_not_loaded)
@@ -21,7 +25,6 @@ defmodule Circuits.I2C.Nif do
   def close(_ref), do: :erlang.nif_error(:nif_not_loaded)
 
   def info() do
-    :ok = load_nif()
-    apply(__MODULE__, :info, [])
+    load_nif_and_apply(:info, [])
   end
 end

test/circuits_i2c_test.exs

@@ -84,4 +84,20 @@ defmodule Circuits.I2CTest do
     assert info.backend == Circuits.I2C.I2CDev
     assert info.test?
   end
+
+  test "racing to load the NIF" do
+    # Make sure the NIF isn't loaded
+    _ = :code.delete(Circuits.I2C.Nif)
+    _ = :code.purge(Circuits.I2C.Nif)
+
+    # Try to hit the race by having 32 processes race to load the NIF
+    tasks =
+      for _ <- 0..31 do
+        Task.async(fn ->
+          _ = Circuits.I2C.info()
+        end)
+      end
+
+    Enum.each(tasks, &Task.await/1)
+  end
 end