render stream: true always use default locale in layout

[mfo]

Steps to reproduce

I have a waiting PR on this issue, but It's stalled. So I'm here to check if it's something you plan to support.

Here is some background :

1. render stream:true is implemented via ActionView::StreamingTemplateRenderer. The layouts is rendered within a Fiber:

fiber = Fiber.new do
  if layout
    layout.render(view, locals, output, &yielder)
  else
    # If you don't have a layout, just render the thing
    # and concatenate the final result. This is the same
    # as a layout with just <%= yield %>
    output.safe_concat view._layout_for
  end
end

But Fibers can't access Thread.current[]. ex:

irb
> Thread.current[:foo]= :bar
> Fiber.new { puts Thread.current[:foo] }.resume # puts nil...

2. Some parts of the stack rely on Thread.current[]... ex: the I18n which stores the current I18n config in Thread.current[:i18n_config].
3. the result is I18n(.locale, I18n.t, t(), ...) does not works within the layout (and all the call which rely on a Thread.current[]).

Expected behavior

You tell me.

A few ideas:

1. Instead of using a classic Fiber, use a custom one that copy the caller's I18n_config in its scope:

class FiberWithI18n < Fiber
      def self.new_with(caller_i18n_config = I18n.config, &block)
        new do
          I18n.config = caller_i18n_config
          block.call
        end
      end
    end

2. Instead of using a classic Fiber, use a custom one that copy the caller's Thread.current[] in its scope:

class FiberWithLocalsFromCallerThread < Fiber
  def self.new_with(caller_thread = Thread.current, &block)
    caller_thread_locals = extract_thread_locals(caller_thread)
    new do
      reapply_locals(Thread.current, caller_thread_locals)
      block.call
    end
  end

  def self.extract_thread_locals(thread)
    thread.keys.inject({}) do |memo, thread_local_varname|
      memo[thread_local_varname] = thread[thread_local_varname]
      memo
    end
  end

  def self.reapply_locals(thread, locals)
    locals.each_pair do|key, value|
      thread[key] = value
    end
  end
end

3. Drop the Fiber (which is the root cause of this bug) and use a threaded model.

Actual behavior

Within the layout we loose all track of Thread.current[]

System configuration

Rails version: I tested with 5.1.3
Ruby version: I tested with 2.4.1p111
But I'm not sure it's relevant, Fiber does not support access to Thread.current, so same behaviour since the introduction of the Fiber.

[mfo]

Ok maybe I'm simply wrong about the expected feature of the render stream :true. But I want to flush my response as earlier as possible (via a Transfer-Encoding: chunked).

I tried to reproduce this ?bug? again... with a brand new rails app. Its fails : https://github.com/mfo/rails_30361/tree/master/app.

So I should be wrong, anyone to give an hint ? What am I missing (I'll glady hear your worse feedback).