Simpler implementation of ActionController::Live

application_controller.rb

class ApplicationController < ActionController::Base
  private

  def stream_body(async: false, **options, &block)
    self.status ||= 200

    self.headers["Cache-Control"] = "no-cache"
    self.headers.delete("Content-Length")

    stream_class = async ? AsyncStream : Stream

    self.response_body = stream_class.new(**options, &block)
  end

  class AsyncStream
    def initialize(queue: SizedQueue.new(10), **options, &block)
      @stream = Stream.new(**options, &block)
      @queue  = queue
    end

    def each
      Thread.new do
        Thread.current.abort_on_exception = true
        
        @stream.each { |chunk| @queue.push(chunk) }
        @queue.close
      rescue ClosedQueueError
        # download request terminated early, we let the thread finish
      end

      while (chunk = @queue.pop)
        yield chunk
      end
    end

    def close
      @queue.close
    end
  end

  class Stream
    def initialize(**, &block)
      @block = block
    end

    def each(&chunk)
      writer = Writer.new(&chunk)

      @block.call(writer)
    end

    class Writer
      def initialize(&chunk)
        @chunk = chunk
      end

      def write(data)
        @chunk.call(data)
        data.bytesize # makes it work with IO.copy_stream
      end
      alias << write
    end
  end
end

files_controller.rb

class FilesController < ApplicationController
  def download
    self.headers.merge!(
      "Content-Disposition" => ContentDisposition.attachment("file-streaming.txt"),
      "Content-Type"        => "text/plain",
      "Last-Modified"       => Time.now.httpdate,
    )

    stream_body(async: true) do |out|
      sleep 1 # mimic delay
      out << "foo"
      sleep 1 # mimic delay
      out << "bar"
      sleep 1 # mimic delay
      out << "baz"
    end
  end
end