Persist calls to add_backend and remove_backend

Author: José Valim

lib/logger/lib/logger.ex

@@ -123,11 +123,7 @@ defmodule Logger do
   is explored with detail below.
 
   The initial backends are loaded via the `:backends` configuration,
-  which must be set before the logger application is started. However,
-  backends can be added or removed dynamically via the `add_backend/2`,
-  `remove_backend/1` and `configure_backend/2` functions, although such
-  backends are not persisted (i.e. if the Logger supervision tree crashes,
-  they are not re-added).
+  which must be set before the logger application is started.
 
   ### Console backend
 
@@ -369,13 +365,27 @@ 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), backend)
+    case Logger.Watcher.watch(Logger, translate_backend(backend), backend) do
+      {:ok, _} = ok ->
+        Logger.Config.add_backend(backend)
+        ok
+      {:error, _} = error ->
+        error
+    end
   end
 
   @doc """
   Removes a backend.
+
+  ## Options
+
+    * `:flush` - when true, guarantees all messages currently sent
+      to both Logger and Erlang's `error_logger` are processed before
+      the backend is removed
   """
-  def remove_backend(backend) do
+  def remove_backend(backend, opts \\ []) do
+    _ = if opts[:flush], do: GenEvent.which_handlers(:error_logger)
+    Logger.Config.remove_backend(backend)
     Logger.Watcher.unwatch(Logger, translate_backend(backend))
   end
 

lib/logger/lib/logger/config.ex

@@ -22,6 +22,14 @@ defmodule Logger.Config do
     GenEvent.call(Logger, @name, {:remove_translator, translator})
   end
 
+  def add_backend(backend) do
+    GenEvent.call(Logger, @name, {:add_backend, backend})
+  end
+
+  def remove_backend(backend) do
+    GenEvent.call(Logger, @name, {:remove_backend, backend})
+  end
+
   def __data__() do
     Application.get_env(:logger, @data)
   end
@@ -88,8 +96,23 @@ defmodule Logger.Config do
     {:ok, :ok, state}
   end
 
+  def handle_call({:add_backend, backend}, state) do
+    update_backends(&[backend|List.delete(&1, backend)])
+    {:ok, :ok, state}
+  end
+
+  def handle_call({:remove_backend, backend}, state) do
+    update_backends(&List.delete(&1, backend))
+    {:ok, :ok, state}
+  end
+
   ## Helpers
 
+  defp update_backends(fun) do
+    backends = fun.(Application.get_env(:logger, :backends, []))
+    Application.put_env(:logger, :backends, backends)
+  end
+
   defp update_translators(%{translators: translators} = state, fun) do
     translators = fun.(translators)
     Application.put_env(:logger, :translators, translators)

lib/logger/test/logger_test.exs

@@ -32,6 +32,7 @@ defmodule LoggerTest do
 
   test "add_backend/1 and remove_backend/1" do
     assert :ok = Logger.remove_backend(:console)
+    assert Application.get_env(:logger, :backends) == []
     assert Logger.remove_backend(:console) ==
            {:error, :module_not_found}
 
@@ -40,8 +41,10 @@ defmodule LoggerTest do
     end) == ""
 
     assert {:ok, pid} = Logger.add_backend(:console)
+    assert Application.get_env(:logger, :backends) == [:console]
     assert Logger.add_backend(:console) ==
            {:error, {:already_started, pid}}
+    assert Application.get_env(:logger, :backends) == [:console]
   end
 
   test "add_backend/1 with {module, id}" do