From 0392742ceb52f5b8eefdd8138387b2c8f2e9fd48 Mon Sep 17 00:00:00 2001
From: Dmitri Vereshchagin <dmitri.vereshchagin@gmail.com>
Date: Wed, 28 Aug 2019 07:40:56 +0300
Subject: Disable the default logger handler in shell if required

To avoid a conflict the Kernel's default handler have to be disabled
before the new default handler is added

    {kernel,
     [{logger,
       [{handler, default, undefined}
       ]}
     ]},
    {my_app,
     [{logger,
       [{handler, default, my_handler, #{}}
       ]}
     ]}

To support this behavior in shell it's necessary to process the
handler-default-undefined tuple.

It's not clear what to do with the simple handler (logger_simple_h)
however.  It's added at the start and removed if a default handler was
added.  Should it be added in reread_logger_config/1 if there's no
default handler?
---
 src/rebar_utils.erl | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/src/rebar_utils.erl b/src/rebar_utils.erl
index 5c7da92..7b267e3 100644
--- a/src/rebar_utils.erl
+++ b/src/rebar_utils.erl
@@ -502,15 +502,26 @@ reread_logger_config() ->
             Primary = #{level => LogLvlPrimary,
                         filter_default => FilterDefault,
                         filters => Filters},
-            %% Load the correct handlers based on their individual config.
-            [case Id of
-                 default -> logger:update_handler_config(Id, Cfg);
-                 _ -> logger:add_handler(Id, Mod, Cfg)
-             end || {handler, Id, Mod, Cfg} <- LogCfg],
+            lists:foreach(fun maybe_reset_logger_handler/1, LogCfg),
             logger:set_primary_config(Primary),
             ok
     end.
 
+%% @private add or update handlers based on their individual config,
+%% also remove default handler if needed.
+maybe_reset_logger_handler({handler, Id, Mod, Cfg}) ->
+    case logger:add_handler(Id, Mod, Cfg) of
+        {error, {already_exist, Id}} ->
+            logger:update_handler_config(Id, Cfg);
+        _ ->
+            ok
+    end;
+maybe_reset_logger_handler({handler, default, undefined}) ->
+    _ = logger:remove_handler(default),
+    ok;
+maybe_reset_logger_handler(_) ->
+    ok.
+
 
 %% @doc Given env. variable `FOO' we want to expand all references to
 %% it in `InStr'. References can have two forms: `$FOO' and `${FOO}'
-- 
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