Rewrite start_supervised already started into duplicate_child_name

Closes #9089.

Author: José Valim

lib/ex_unit/lib/ex_unit/callbacks.ex

@@ -344,7 +344,15 @@ defmodule ExUnit.Callbacks do
           raise ArgumentError, "start_supervised/2 can only be invoked from the test process"
       end
 
-    Supervisor.start_child(sup, Supervisor.child_spec(child_spec_or_module, opts))
+    child_spec = Supervisor.child_spec(child_spec_or_module, opts)
+
+    case Supervisor.start_child(sup, child_spec) do
+      {:error, {:already_started, _pid}} ->
+        {:error, {:duplicate_child_name, child_spec.id}}
+
+      other ->
+        other
+    end
   end
 
   @doc """
@@ -367,9 +375,13 @@ defmodule ExUnit.Callbacks do
     end
   end
 
-  defp start_supervised_error({{:EXIT, reason}, _info}), do: Exception.format_exit(reason)
-  defp start_supervised_error({reason, _info}), do: Exception.format_exit(reason)
-  defp start_supervised_error(reason), do: Exception.format_exit(reason)
+  defp start_supervised_error({{:EXIT, reason}, info}) when is_tuple(info),
+    do: Exception.format_exit(reason)
+
+  defp start_supervised_error({reason, info}) when is_tuple(info),
+    do: Exception.format_exit(reason)
+
+  defp start_supervised_error(reason), do: Exception.format_exit({:start_spec, reason})
 
   @doc """
   Stops a child process started via `start_supervised/2`.

lib/ex_unit/test/ex_unit/supervised_test.exs

@@ -73,8 +73,19 @@ defmodule ExUnit.SupervisedTest do
 
   test "starts a supervised process with ID checks" do
     {:ok, pid} = start_supervised({MyAgent, 0})
-    assert {:error, {:already_started, ^pid}} = start_supervised({MyAgent, 0})
+
+    assert {:error, {:duplicate_child_name, ExUnit.SupervisedTest.MyAgent}} =
+             start_supervised({MyAgent, 0})
+
     assert {:error, {{:already_started, ^pid}, _}} = start_supervised({MyAgent, 0}, id: :another)
+
+    assert_raise RuntimeError, ~r"Reason: bad child specification", fn ->
+      start_supervised!(%{id: 1, start: :oops})
+    end
+
+    assert_raise RuntimeError, ~r"Reason: already started", fn ->
+      start_supervised!({MyAgent, 0}, id: :another)
+    end
   end
 
   test "stops a supervised process" do