Fix TaggedLogging functionality when broadcasting:

- This PR fixes two issues with the Tagged Logging feature in
  conjunction with broadcasting logs.

  For the sake of clarity I'll define the "main logger" and
  the "broadcasted logger" in this snippet:

  ```ruby
    main_logger = ActiveSupport::TaggedLogging.new(ActiveSupport::Logger.new(io))
    broadcaster_logger = ActiveSupport::TaggedLogging.new(ActiveSupport::Logger.new(io))

    main_logger.extend(Activesupport::Logger.broadcast(broadcasted_logger))
  ```

  1) The first issue that this PR fixes, is that the tags on the "main logger"
     don't propagate to the "broadcasted logger" when you pass in a block.

     ```ruby
     main_logger.tagged("FOO") { |logger| logger.info("Hello") }

     # Outputs:
     # [Foo] Hello <- By the main logger
     # Hello       <- By the broadcasted logger
     ```

     A fix was made in 70af536
     but that only works for the non block version

  2) It's quite common for the "broadcasted logger" to have a diffent
     log formatter that the "main logger". In example you'd want to
     output JSON logs in one and raw text in the other.

     That wasn't possible before. All loggers had to have the same
     instance of the formatter. The formatter was set on all loggers
     thanks to [this](https://github.com/rails/rails/blob/3fc9d12875dd434e454f74472da02014f6fa5d72/activesupport/lib/active_support/logger.rb#L45-L48) and it's [associated test](https://github.com/rails/rails/blob/3fc9d12875dd434e454f74472da02014f6fa5d72/activesupport/test/broadcast_logger_test.rb#L58-L64)
     This requirement was needed to make the Tagged Logging feature
     work; the tags being set in a thread variable whose name
     uses the `object_id` https://github.com/rails/rails/blob/3fc9d12875dd434e454f74472da02014f6fa5d72/activesupport/lib/active_support/tagged_logging.rb#L59
     (different formatter instance -> different object_id -> different variables)

     In this PR, I have removed the code that sets the same formatter
     instance on all logger. The "broadcaster logger" just need to
     have the `current_tags` point to the `current_tags` of the
     "main logger", I'm doing that by redefing the `current_tags`
     method each time the "main logger" uses a different formatter.

     The advantages by doing so is that custom made formatter
     can now call this `current_tags` method, which will return
     all the tags and process them the way they want.

     ```ruby
       class JSONLogFormatter
         def call(_, _, _, msg)
	   tags = current_tags # Can now retrieve the tags

	   { message: msg, tags: tags }.to_json
	 end
       end

       broadcasted_logger = Logger.new(io)
       broadcaster_logger.formatter = JSONLogFormatter.new
       main_logger = Logger.new(io)
       main_logger.extend(ActiveSupport::Logger.broadcast(broadcasted_logger))
     ```

     The behavior remains the same as before if a logger uses the
     Rails vanilla formatter or the Tagged Logging formatter.

activesupport/lib/active_support/logger.rb

@@ -22,6 +22,10 @@ def self.logger_outputs_to?(logger, *sources)
     # Broadcasts logs to multiple loggers.
     def self.broadcast(logger) # :nodoc:
       Module.new do
+        define_singleton_method(:extended) do |base|
+          base.public_send(:broadcast_to, logger) if base.respond_to?(:broadcast_to)
+        end
+
         define_method(:add) do |*args, &block|
           logger.add(*args, &block)
           super(*args, &block)
@@ -42,11 +46,6 @@ def self.broadcast(logger) # :nodoc:
           super(name)
         end
 
-        define_method(:formatter=) do |formatter|
-          logger.formatter = formatter
-          super(formatter)
-        end
-
         define_method(:level=) do |level|
           logger.level = level
           super(level)

activesupport/lib/active_support/tagged_logging.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "active_support/core_ext/module/delegation"
+require "active_support/core_ext/module/redefine_method"
 require "active_support/core_ext/object/blank"
 require "logger"
 require "active_support/logger"
@@ -94,6 +95,20 @@ def self.new(logger)
 
     delegate :push_tags, :pop_tags, :clear_tags!, to: :formatter
 
+    def broadcast_to(other_logger) # :nodoc:
+      define_singleton_method(:formatter=) do |formatter|
+        other_logger.formatter ||= formatter
+
+        other_logger.formatter.singleton_class.redefine_method(:current_tags) do
+          formatter.current_tags
+        end
+
+        super(formatter)
+      end
+
+      self.formatter = self.formatter.clone
+    end
+
     def tagged(*tags)
       if block_given?
         formatter.tagged(*tags) { yield self }

activesupport/test/broadcast_logger_test.rb

@@ -55,14 +55,6 @@ class BroadcastLoggerTest < TestCase
       assert_equal ::Logger::FATAL, log2.progname
     end
 
-    test "#formatter= assigns to all the loggers" do
-      assert_nil logger.formatter
-      logger.formatter = ::Logger::FATAL
-
-      assert_equal ::Logger::FATAL, log1.formatter
-      assert_equal ::Logger::FATAL, log2.formatter
-    end
-
     test "#local_level= assigns the local_level to all loggers" do
       assert_equal ::Logger::DEBUG, logger.local_level
       logger.local_level = ::Logger::FATAL

activesupport/test/tagged_logging_test.rb

@@ -226,4 +226,62 @@ class TaggedLoggingWithoutBlockTest < ActiveSupport::TestCase
     assert_equal "[OMG] Broadcasting...\n", @output.string
     assert_equal "[OMG] Broadcasting...\n", broadcast_output.string
   end
+
+  test "keeps broadcasting functionality when passed a block" do
+    broadcast_output = StringIO.new
+    broadcast_logger = ActiveSupport::TaggedLogging.new(Logger.new(broadcast_output))
+    @logger.extend(ActiveSupport::Logger.broadcast(broadcast_logger))
+
+    @logger.tagged("OMG") { |logger| logger.info "Broadcasting..." }
+
+    assert_equal "[OMG] Broadcasting...\n", @output.string
+    assert_equal "[OMG] Broadcasting...\n", broadcast_output.string
+  end
+
+  test "broadcasting when passing a block works and  keeps formatter untouched" do
+    broadcast_output = StringIO.new
+    broadcast_logger = ActiveSupport::TaggedLogging.new(Logger.new(broadcast_output))
+    my_formatter = Class.new do
+      def call(_, _, _, msg)
+        ActiveSupport::JSON.encode(message: msg, tags: current_tags)
+      end
+    end
+    broadcast_logger.formatter = my_formatter.new
+
+    @logger.extend(ActiveSupport::Logger.broadcast(broadcast_logger))
+    @logger.tagged("OMG") { |logger| logger.info "Broadcasting..." }
+
+    assert_equal "[OMG] Broadcasting...\n", @output.string
+    assert_equal "{\"message\":\"Broadcasting...\",\"tags\":[\"OMG\"]}", broadcast_output.string
+  end
+
+  test "broadcasting without passing a block works and keeps formatter untouched" do
+    broadcast_output = StringIO.new
+    broadcast_logger = ActiveSupport::TaggedLogging.new(Logger.new(broadcast_output))
+    my_formatter = Class.new do
+      def call(_, _, _, msg)
+        ActiveSupport::JSON.encode(message: msg, tags: current_tags)
+      end
+    end
+    broadcast_logger.formatter = my_formatter.new
+
+    @logger.extend(ActiveSupport::Logger.broadcast(broadcast_logger))
+    tagger_logger1 = @logger.tagged("OMG")
+    tagger_logger2 = tagger_logger1.tagged("FOO")
+    tagger_logger2.info("Broadcasting...")
+
+    assert_equal "[OMG] [FOO] Broadcasting...\n", @output.string
+    assert_equal "{\"message\":\"Broadcasting...\",\"tags\":[\"OMG\",\"FOO\"]}", broadcast_output.string
+  end
+
+  test "broadcasting on a non tagged logger" do
+    broadcast_output = StringIO.new
+    broadcast_logger = ActiveSupport::Logger.new(broadcast_output)
+
+    @logger.extend(ActiveSupport::Logger.broadcast(broadcast_logger))
+    @logger.tagged("OMG") { |logger| logger.info "Broadcasting..." }
+
+    assert_equal "[OMG] Broadcasting...\n", @output.string
+    assert_equal "Broadcasting...\n", broadcast_output.string
+  end
 end