Only lookup `config.log_level` for stdlib `::Logger`. Closes #11665.

This prevents Rails from assigning meaningless log levels to third
party loggers like log4r. If `Rails.logger` is not `kind_of?(::Logger)`
we simply assign the `config.log_level` as is.

This bug was introduced by #11665.

3 files changed, 26 insertions, 2 deletions

diff --git a/railties/CHANGELOG.md b/railties/CHANGELOG.md
index de3c1074ef..83747470bf 100644
--- a/railties/CHANGELOG.md
+++ b/railties/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Only lookup `config.log_level` for stdlib `::Logger` instances.
+    Assign it as is for third party loggers like `Log4r::Logger`.
+
+    Fixes #13421.
+
+    *Yves Senn*
+
 *   The `Gemfile` of new applications depends on SDoc ~> 0.4.0.
 
     *Xavier Noria*
diff --git a/railties/lib/rails/application/bootstrap.rb b/railties/lib/rails/application/bootstrap.rb
index a26d41c0cf..33bcab1e57 100644
--- a/railties/lib/rails/application/bootstrap.rb
+++ b/railties/lib/rails/application/bootstrap.rb
@@ -53,7 +53,11 @@ INFO
           logger
         end
 
-        Rails.logger.level = ActiveSupport::Logger.const_get(config.log_level.to_s.upcase)
+        if ::Logger === Rails.logger
+          Rails.logger.level = ActiveSupport::Logger.const_get(config.log_level.to_s.upcase)
+        else
+          Rails.logger.level = config.log_level
+        end
       end
 
       # Initialize cache early in the stack so railties can make use of it.
diff --git a/railties/test/application/configuration_test.rb b/railties/test/application/configuration_test.rb
index 6158c416d7..02d8b2c91d 100644
--- a/railties/test/application/configuration_test.rb
+++ b/railties/test/application/configuration_test.rb
@@ -754,7 +754,7 @@ module ApplicationTests
       end
     end
 
-    test "config.log_level with custom logger" do
+    test "lookup config.log_level with custom logger (stdlib Logger)" do
       make_basic_app do |app|
         app.config.logger = Logger.new(STDOUT)
         app.config.log_level = :info
@@ -762,6 +762,19 @@ module ApplicationTests
       assert_equal Logger::INFO, Rails.logger.level
     end
 
+    test "assign log_level as is with custom logger (third party logger)" do
+      logger_class = Class.new do
+        attr_accessor :level
+      end
+      logger_instance = logger_class.new
+      make_basic_app do |app|
+        app.config.logger = logger_instance
+        app.config.log_level = :info
+      end
+      assert_equal logger_instance, Rails.logger
+      assert_equal :info, Rails.logger.level
+    end
+
     test "respond_to? accepts include_private" do
       make_basic_app