Don't warn on module redefinition (#52)

lib/live_book/runtime/erl_dist/manager.ex

@@ -93,11 +93,22 @@ defmodule LiveBook.Runtime.ErlDist.Manager do
 
     Process.send_after(self(), :check_owner, @await_owner_timeout)
 
-    {:ok, %{owner: nil, evaluators: %{}}}
+    # Set `ignore_module_conflict` only for the Manager lifetime.
+    initial_ignore_module_conflict = Code.compiler_options()[:ignore_module_conflict]
+    Code.compiler_options(ignore_module_conflict: true)
+
+    {:ok,
+     %{
+       owner: nil,
+       evaluators: %{},
+       initial_ignore_module_conflict: initial_ignore_module_conflict
+     }}
   end
 
   @impl true
-  def terminate(_reason, _state) do
+  def terminate(_reason, state) do
+    Code.compiler_options(ignore_module_conflict: state.initial_ignore_module_conflict)
+
     ErlDist.unload_required_modules()
 
     :ok

test/live_book/runtime/erl_dist/manager_test.exs

@@ -38,5 +38,23 @@ defmodule LiveBook.Runtime.ErlDist.ManagerTest do
 
       Manager.stop(node())
     end
+
+    test "prevents from module redefinition warning being printed to standard error" do
+      Manager.start()
+      Manager.set_owner(node(), self())
+
+      stderr =
+        ExUnit.CaptureIO.capture_io(:stderr, fn ->
+          Manager.evaluate_code(node(), "defmodule Foo do end", :container1, :evaluation1)
+          Manager.evaluate_code(node(), "defmodule Foo do end", :container1, :evaluation2)
+
+          assert_receive {:evaluation_response, :evaluation1, _}
+          assert_receive {:evaluation_response, :evaluation2, _}
+        end)
+
+      assert stderr == ""
+
+      Manager.stop(node())
+    end
   end
 end