Fix the LoggerSilence to work as described:

- Following the Rails guide which state that a logger needs to include
  the `ActiveSupport::LoggerSilence` as well as
  `ActiveSupport::LoggerThreadSafe` modules isn't enough and won't
  work.

  Here is a test cases with 3 tests that all fails
  https://gist.github.com/Edouard-chin/4a72930c2b1eafbbd72a80c66f102010

  The problems are the following:

  1) The logger needs to call `after_initialize` in order to setup
  some instance variables.
  2) The silence doesn't actually work because the bare ruby Logger
  `add` method checks for the instance variable `@logger`. We need to
  override the `add` (like we used to in the ActiveSupport::Logger
  class).
  3) Calling `debug?` `info?` etc... doesn't work as the bare ruby
  methods will check for the instance variable. Again we need to
  override this methods (like we used to in the ActiveSupport::Logger
  class)

  The LoggerSilence won't work without LoggerThreadSafe, but the later
  is not public API, the user shouldn't have to include it so I
  modified to include it automatically.
  Same for the `after_initialize` method. I find unuintitive to have
  to call it directly. I modified to instance the variables when the
  module get included.

6 files changed, 65 insertions, 22 deletions

diff --git a/activesupport/lib/active_support/logger.rb b/activesupport/lib/active_support/logger.rb
index 8152a182b4..b8555c887b 100644
--- a/activesupport/lib/active_support/logger.rb
+++ b/activesupport/lib/active_support/logger.rb
@@ -6,7 +6,6 @@ require "logger"
 
 module ActiveSupport
   class Logger < ::Logger
-    include ActiveSupport::LoggerThreadSafeLevel
     include LoggerSilence
 
     # Returns true if the logger destination matches one of the sources
@@ -81,20 +80,6 @@ module ActiveSupport
     def initialize(*args)
       super
       @formatter = SimpleFormatter.new
-      after_initialize if respond_to? :after_initialize
-    end
-
-    def add(severity, message = nil, progname = nil, &block)
-      return true if @logdev.nil? || (severity || UNKNOWN) < level
-      super
-    end
-
-    Logger::Severity.constants.each do |severity|
-      class_eval(<<-EOT, __FILE__, __LINE__ + 1)
-        def #{severity.downcase}?                # def debug?
-          Logger::#{severity} >= level           #   DEBUG >= level
-        end                                      # end
-      EOT
     end
 
     # Simple formatter which only displays the message.
diff --git a/activesupport/lib/active_support/logger_silence.rb b/activesupport/lib/active_support/logger_silence.rb
index 2f62cc13b9..b2444c1e34 100644
--- a/activesupport/lib/active_support/logger_silence.rb
+++ b/activesupport/lib/active_support/logger_silence.rb
@@ -2,6 +2,7 @@
 
 require "active_support/concern"
 require "active_support/core_ext/module/attribute_accessors"
+require "active_support/logger_thread_safe_level"
 
 module LoggerSilence
   extend ActiveSupport::Concern
@@ -22,6 +23,7 @@ module ActiveSupport
 
     included do
       cattr_accessor :silencer, default: true
+      include ActiveSupport::LoggerThreadSafeLevel
     end
 
     # Silences the logger for the duration of the block.
diff --git a/activesupport/lib/active_support/logger_thread_safe_level.rb b/activesupport/lib/active_support/logger_thread_safe_level.rb
index 22ab4cc28c..f16c90cfc6 100644
--- a/activesupport/lib/active_support/logger_thread_safe_level.rb
+++ b/activesupport/lib/active_support/logger_thread_safe_level.rb
@@ -1,14 +1,30 @@
 # frozen_string_literal: true
 
 require "active_support/concern"
+require "active_support/core_ext/module/attribute_accessors"
 require "concurrent"
 
 module ActiveSupport
   module LoggerThreadSafeLevel # :nodoc:
     extend ActiveSupport::Concern
 
+    included do
+      cattr_accessor :local_levels, default: Concurrent::Map.new(initial_capacity: 2), instance_accessor: false
+    end
+
+    Logger::Severity.constants.each do |severity|
+      class_eval(<<-EOT, __FILE__, __LINE__ + 1)
+        def #{severity.downcase}?                # def debug?
+          Logger::#{severity} >= level           #   DEBUG >= level
+        end                                      # end
+      EOT
+    end
+
     def after_initialize
-      @local_levels = Concurrent::Map.new(initial_capacity: 2)
+      ActiveSupport::Deprecation.warn(
+        "Logger don't need to call #after_initialize directly anymore. It will be deprecated without replacement in " \
+        "Rails 6.1."
+      )
     end
 
     def local_log_id
@@ -16,19 +32,24 @@ module ActiveSupport
     end
 
     def local_level
-      @local_levels[local_log_id]
+      self.class.local_levels[local_log_id]
     end
 
     def local_level=(level)
       if level
-        @local_levels[local_log_id] = level
+        self.class.local_levels[local_log_id] = level
       else
-        @local_levels.delete(local_log_id)
+        self.class.local_levels.delete(local_log_id)
       end
     end
 
     def level
       local_level || super
     end
+
+    def add(severity, message = nil, progname = nil, &block) # :nodoc:
+      return true if @logdev.nil? || (severity || UNKNOWN) < level
+      super
+    end
   end
 end
diff --git a/activesupport/test/broadcast_logger_test.rb b/activesupport/test/broadcast_logger_test.rb
index ea2e388b8f..02bc317c66 100644
--- a/activesupport/test/broadcast_logger_test.rb
+++ b/activesupport/test/broadcast_logger_test.rb
@@ -123,6 +123,8 @@ module ActiveSupport
     end
 
     class CustomLogger
+      include ActiveSupport::LoggerSilence
+
       attr_reader :adds, :closed, :chevrons
       attr_accessor :level, :progname, :formatter, :local_level
 
@@ -174,7 +176,6 @@ module ActiveSupport
     end
 
     class FakeLogger < CustomLogger
-      include ActiveSupport::LoggerSilence
     end
   end
 end
diff --git a/activesupport/test/silence_logger_test.rb b/activesupport/test/silence_logger_test.rb
new file mode 100644
index 0000000000..fc5c5c1161
--- /dev/null
+++ b/activesupport/test/silence_logger_test.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+require "active_support/logger_silence"
+require "logger"
+
+class LoggerSilenceTest < ActiveSupport::TestCase
+  class MyLogger < ::Logger
+    include LoggerSilence
+  end
+
+  setup do
+    @io = StringIO.new
+    @logger = MyLogger.new(@io)
+  end
+
+  test "#silence silences the log" do
+    @logger.silence(Logger::ERROR) do
+      @logger.info("Foo")
+    end
+    @io.rewind
+
+    assert_empty @io.read
+  end
+
+  test "#debug? is true when setting the temporary level to Logger::DEBUG" do
+    @logger.level = Logger::INFO
+
+    @logger.silence(Logger::DEBUG) do
+      assert_predicate @logger, :debug?
+    end
+
+    assert_predicate @logger, :info?
+  end
+end
diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index d23af2ca57..812565b207 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -119,11 +119,10 @@ defaults to `:debug` for all environments. The available log levels are: `:debug
 * `config.logger` is the logger that will be used for `Rails.logger` and any related Rails logging such as `ActiveRecord::Base.logger`. It defaults to an instance of `ActiveSupport::TaggedLogging` that wraps an instance of `ActiveSupport::Logger` which outputs a log to the `log/` directory. You can supply a custom logger, to get full compatibility you must follow these guidelines:
   * To support a formatter, you must manually assign a formatter from the `config.log_formatter` value to the logger.
   * To support tagged logs, the log instance must be wrapped with `ActiveSupport::TaggedLogging`.
-  * To support silencing, the logger must include `ActiveSupport::LoggerSilence` and `ActiveSupport::LoggerThreadSafeLevel` modules. The `ActiveSupport::Logger` class already includes these modules.
+  * To support silencing, the logger must include `ActiveSupport::LoggerSilence`  module. The `ActiveSupport::Logger` class already includes these modules.
 
     ```ruby
     class MyLogger < ::Logger
-      include ActiveSupport::LoggerThreadSafeLevel
       include ActiveSupport::LoggerSilence
     end