feat: initial implementation

lib/init.ex

@@ -1,2 +1,19 @@
 defmodule Init do
+  @moduledoc """
+  Run a function synchronously as part of a `Supervisor`
+  """
+  use GenServer, restart: :transient
+
+  def start_link(arg, opts \\ []) do
+    GenServer.start_link(__MODULE__, mfa!(arg), opts)
+  end
+
+  @impl true
+  def init({mod, fun, args}) do
+    apply(mod, fun, args)
+    :ignore
+  end
+
+  defp mfa!({_m, _f, _a} = mfa), do: mfa
+  defp mfa!(fun) when is_function(fun, 0), do: {Kernel, :apply, [fun, []]}
 end

test/init_test.exs

@@ -0,0 +1,60 @@
+defmodule Echo do
+  def ping(pid, value, delay) do
+    Process.sleep(delay)
+    send(pid, value)
+  end
+end
+
+defmodule InitTest do
+  use ExUnit.Case, async: true
+
+  describe "when passed a module-function-arguments tuple" do
+    test "it runs to completion before other children of a Supervisor" do
+      children = [
+        Supervisor.child_spec({Init, {Echo, :ping, [self(), :foo, 20]}}, id: {Init, 1}),
+        Supervisor.child_spec({Init, {Echo, :ping, [self(), :bar, 10]}}, id: {Init, 2})
+      ]
+
+      start_supervised!(%{
+        id: Supervisor,
+        start: {Supervisor, :start_link, [children, [strategy: :one_for_one]]}
+      })
+
+      assert_receive :foo
+      assert_receive :bar
+    end
+  end
+
+  describe "when passed a 0-arity anonymous function" do
+    test "it runs to completion before other children of a Supervisor" do
+      self = self()
+
+      children = [
+        Supervisor.child_spec(
+          {Init,
+           fn ->
+             Process.sleep(20)
+             send(self, :foo)
+           end},
+          id: {Init, 1}
+        ),
+        Supervisor.child_spec(
+          {Init,
+           fn ->
+             Process.sleep(10)
+             send(self, :bar)
+           end},
+          id: {Init, 2}
+        )
+      ]
+
+      start_supervised!(%{
+        id: Supervisor,
+        start: {Supervisor, :start_link, [children, [strategy: :one_for_one]]}
+      })
+
+      assert_receive :foo
+      assert_receive :bar
+    end
+  end
+end