Fix Mix suite with async_run and await_run

Author: josevalim

lib/ex_unit/lib/ex_unit.ex

@@ -355,6 +355,31 @@ defmodule ExUnit do
     ExUnit.Runner.run(options, nil)
   end
 
+  @doc """
+  Starts tests asynchronously while test cases are still loading.
+
+  It returns a task that must be given to `await_run/0` when a result
+  is desired.
+  """
+  @doc since: "1.12.0"
+  @spec async_run() :: Task.t()
+  def async_run() do
+    Task.async(fn ->
+      options = persist_defaults(configuration())
+      ExUnit.Runner.run(options, nil)
+    end)
+  end
+
+  @doc """
+  Awaits for a test suite that has been started with `async_run/0`.
+  """
+  @doc since: "1.12.0"
+  @spec await_run(Task.t()) :: suite_result()
+  def await_run(task) do
+    ExUnit.Server.modules_loaded()
+    Task.await(task, :infinity)
+  end
+
   @doc """
   Sets a callback to be executed after the completion of a test suite.
 

lib/ex_unit/test/ex_unit_test.exs

@@ -27,7 +27,8 @@ defmodule ExUnitTest do
     ExUnit.Server.modules_loaded()
 
     assert capture_io(fn ->
-             assert ExUnit.run() == %{failures: 0, skipped: 0, total: 0, excluded: 0}
+             assert ExUnit.async_run() |> ExUnit.await_run() ==
+                      %{failures: 0, skipped: 0, total: 0, excluded: 0}
            end) =~ "\n0 failures\n"
   end
 

lib/mix/lib/mix/compilers/test.ex

@@ -34,16 +34,15 @@ defmodule Mix.Compilers.Test do
     if test_files == [] do
       :noop
     else
-      task = Task.async(ExUnit, :run, [])
+      task = ExUnit.async_run()
 
       try do
         case Kernel.ParallelCompiler.require(test_files, parallel_require_callbacks) do
           {:ok, _, _} -> :ok
           {:error, _, _} -> exit({:shutdown, 1})
         end
 
-        ExUnit.Server.modules_loaded()
-        %{failures: failures} = results = Task.await(task, :infinity)
+        %{failures: failures} = results = ExUnit.await_run(task)
 
         if failures == 0 do
           agent_write_manifest(stale_manifest_pid)