OTP 21 support

To support OTP 21 we manually start error_logger if it is not present
(and lager is configured to install an error_logger handler) and we add
error_logger as a logger handler.

Longer term we should switch to installing our own logger handler, but
that is a larger task as we'd have to redo all the event parsing, if
that is even possible.

rebar.config

@@ -21,7 +21,7 @@
 
 {erl_opts, [
     {lager_extra_sinks, ['__lager_test_sink']},
-    {platform_define, "(19|20)", test_statem},
+    {platform_define, "(19|20|21)", test_statem},
     {platform_define, "(18)", 'FUNCTION_NAME', unavailable},
     {platform_define, "(18)", 'FUNCTION_ARITY', 0},
     debug_info,

src/lager_app.erl

@@ -158,24 +158,30 @@ start_error_logger_handler(true, HWM, WhiteList) ->
                         throw({error, bad_config})
                 end,
 
+    case whereis(error_logger) of
+        undefined ->
+            %% On OTP 21 and above, error_logger is deprecated in favor of 'logger'
+            %% As a band-aid, boot up error_logger anyway and install it as a logger handler
+            %% we can't use error_logger:add_report_handler because we want supervision of the handler
+            %% so we have to manually add the logger handler
+            %%
+            %% Longer term we should be installing a logger handler instead, but this will bridge the gap
+            %% for now.
+            error_logger:start(),
+            _ = logger:add_handler(error_logger,error_logger,#{level=>info,filter_default=>log});
+        _ ->
+            ok
+    end,
 
-    _ = case supervisor:start_child(lager_handler_watcher_sup, [error_logger, error_logger_lager_h, [HWM, GlStrategy]]) of
+    %% capture which handlers we removed from error_logger so we can restore them when lager stops
+    OldHandlers = case supervisor:start_child(lager_handler_watcher_sup, [error_logger, error_logger_lager_h, [HWM, GlStrategy]]) of
         {ok, _} ->
             [begin error_logger:delete_report_handler(X), X end ||
                 X <- gen_event:which_handlers(error_logger) -- [error_logger_lager_h | WhiteList]];
         {error, _} ->
             []
     end,
-
-    Handlers = case application:get_env(lager, handlers) of
-        undefined ->
-            [{lager_console_backend, [{level, info}]},
-             {lager_file_backend, [{file, "log/error.log"},   {level, error}, {size, 10485760}, {date, "$D0"}, {count, 5}]},
-             {lager_file_backend, [{file, "log/console.log"}, {level, info}, {size, 10485760}, {date, "$D0"}, {count, 5}]}];
-        {ok, Val} ->
-            Val
-    end,
-    Handlers.
+    OldHandlers.
 
 configure_sink(Sink, SinkDef) ->
     lager_config:new_sink(Sink),