Add flush to Logger.add_backend/2

Author: José Valim

lib/elixir/test/elixir/gen_event_test.exs

@@ -98,8 +98,7 @@ defmodule GenEventTest do
     assert GenEvent.which_handlers(pid) == []
     assert GenEvent.stop(pid) == :ok
   after
-    :gen_event.which_handlers(:error_logger)
-    Logger.add_backend(:console)
+    Logger.add_backend(:console, flush: true)
   end
 
   test "sync stream/2" do

lib/elixir/test/elixir/task/supervisor_test.exs

@@ -10,10 +10,7 @@ defmodule Task.SupervisorTest do
 
   setup do
     Logger.remove_backend(:console)
-    on_exit fn ->
-      :gen_event.which_handlers(:error_logger)
-      Logger.add_backend(:console)
-    end
+    on_exit fn -> Logger.add_backend(:console, flush: true) end
     :ok
   end
 

lib/elixir/test/elixir/task_test.exs

@@ -5,10 +5,7 @@ defmodule TaskTest do
 
   setup do
     Logger.remove_backend(:console)
-    on_exit fn ->
-      :gen_event.which_handlers(:error_logger)
-      Logger.add_backend(:console)
-    end
+    on_exit fn -> Logger.add_backend(:console, flush: true) end
     :ok
   end
 

lib/logger/lib/logger.ex

@@ -338,8 +338,16 @@ defmodule Logger do
 
   @doc """
   Adds a new backend.
+
+  ## Options
+
+    * `:flush` - when true, guarantees all messages currently sent
+      to both Logger and Erlang's `error_logger` are processed before
+      the backend is added
+
   """
-  def add_backend(backend) do
+  def add_backend(backend, opts \\ []) do
+    _ = if opts[:flush], do: GenEvent.which_handlers(:error_logger)
     Logger.Watcher.watch(Logger, translate_backend(backend), [])
   end
 

lib/mix/test/mix/tasks/app.start_test.exs

@@ -28,8 +28,7 @@ defmodule Mix.Tasks.App.StartTest do
       on_exit fn ->
         :application.stop(app)
         :application.unload(app)
-        :gen_event.which_handlers(:error_logger)
-        Logger.add_backend(:console)
+        Logger.add_backend(:console, flush: true)
       end
     end