Ensure backends receive the id when added, closes #2635

Author: José Valim

lib/logger/lib/logger.ex

@@ -237,7 +237,7 @@ defmodule Logger do
 
     handlers =
       for backend <- Application.get_env(:logger, :backends) do
-        {Logger, translate_backend(backend), []}
+        {Logger, translate_backend(backend), backend}
       end
 
     options  = [strategy: :rest_for_one, name: Logger.Supervisor]
@@ -355,7 +355,7 @@ defmodule Logger do
   """
   def add_backend(backend, opts \\ []) do
     _ = if opts[:flush], do: GenEvent.which_handlers(:error_logger)
-    Logger.Watcher.watch(Logger, translate_backend(backend), [])
+    Logger.Watcher.watch(Logger, translate_backend(backend), backend)
   end
 
   @doc """

lib/logger/test/logger_test.exs

@@ -44,6 +44,20 @@ defmodule LoggerTest do
            {:error, {:already_started, pid}}
   end
 
+  test "add_backend/1 with {module, id}" do
+    defmodule MyBackend do
+      use GenEvent
+
+      def init({MyBackend, :hello}) do
+        {:ok, :hello}
+      end
+    end
+
+    assert {:ok, _} = Logger.add_backend({MyBackend, :hello})
+    assert {:error, {:already_started, _}} = Logger.add_backend({MyBackend, :hello})
+    assert :ok = Logger.remove_backend({MyBackend, :hello})
+  end
+
   test "level/0" do
     assert Logger.level == :debug
   end