Rails 5.2: ActionController::Live does not stream responses, instead returns entire response at once

[anaulin]

Steps to reproduce

In a clean Rails app (generated with rails new), with Rails 5.2.4.1 or Rails 5.4.2.2, add the following controller:

# in file app/controllers/streaming_controllers.rb

class StreamingController < ApplicationController
  include ActionController::Live

  def index
    response.headers['Content-Type'] = 'text/event-stream'
    5.times {
      response.stream.write "hello world\n"
      sleep 1
    }
  ensure
    response.stream.close
  end
end

Routes:

# in file config/routes.rb
Rails.application.routes.draw do
  get 'streaming' => 'streaming#index'
end

Run the server locally, then go to http://localhost:3000/streaming

Observed behavior: The server waits for about 5 seconds, then spits out 5 lines of hello world.
Expected behavior: The server streams one line of hello world every second.

On Rails 5.0.7.2, the behavior of this controller is as expected -- streams a line every second.

---

Detailed Gemfile.lock contents for clean Rails app that exhibits this issue:

GEM
  remote: https://rubygems.org/
  specs:
    actioncable (5.2.4.2)
      actionpack (= 5.2.4.2)
      nio4r (~> 2.0)
      websocket-driver (>= 0.6.1)
    actionmailer (5.2.4.2)
      actionpack (= 5.2.4.2)
      actionview (= 5.2.4.2)
      activejob (= 5.2.4.2)
      mail (~> 2.5, >= 2.5.4)
      rails-dom-testing (~> 2.0)
    actionpack (5.2.4.2)
      actionview (= 5.2.4.2)
      activesupport (= 5.2.4.2)
      rack (~> 2.0, >= 2.0.8)
      rack-test (>= 0.6.3)
      rails-dom-testing (~> 2.0)
      rails-html-sanitizer (~> 1.0, >= 1.0.2)
    actionview (5.2.4.2)
      activesupport (= 5.2.4.2)
      builder (~> 3.1)
      erubi (~> 1.4)
      rails-dom-testing (~> 2.0)
      rails-html-sanitizer (~> 1.0, >= 1.0.3)
    activejob (5.2.4.2)
      activesupport (= 5.2.4.2)
      globalid (>= 0.3.6)
    activemodel (5.2.4.2)
      activesupport (= 5.2.4.2)
    activerecord (5.2.4.2)
      activemodel (= 5.2.4.2)
      activesupport (= 5.2.4.2)
      arel (>= 9.0)
    activestorage (5.2.4.2)
      actionpack (= 5.2.4.2)
      activerecord (= 5.2.4.2)
      marcel (~> 0.3.1)
    activesupport (5.2.4.2)
      concurrent-ruby (~> 1.0, >= 1.0.2)
      i18n (>= 0.7, < 2)
      minitest (~> 5.1)
      tzinfo (~> 1.1)
    addressable (2.7.0)
      public_suffix (>= 2.0.2, < 5.0)
    archive-zip (0.12.0)
      io-like (~> 0.3.0)
    arel (9.0.0)
    bindex (0.8.1)
    bootsnap (1.4.6)
      msgpack (~> 1.0)
    builder (3.2.4)
    byebug (11.1.1)
    capybara (3.31.0)
      addressable
      mini_mime (>= 0.1.3)
      nokogiri (~> 1.8)
      rack (>= 1.6.0)
      rack-test (>= 0.6.3)
      regexp_parser (~> 1.5)
      xpath (~> 3.2)
    childprocess (3.0.0)
    chromedriver-helper (2.1.1)
      archive-zip (~> 0.10)
      nokogiri (~> 1.8)
    coffee-rails (4.2.2)
      coffee-script (>= 2.2.0)
      railties (>= 4.0.0)
    coffee-script (2.4.1)
      coffee-script-source
      execjs
    coffee-script-source (1.12.2)
    concurrent-ruby (1.1.6)
    crass (1.0.6)
    erubi (1.9.0)
    execjs (2.7.0)
    ffi (1.12.2)
    globalid (0.4.2)
      activesupport (>= 4.2.0)
    i18n (1.8.2)
      concurrent-ruby (~> 1.0)
    io-like (0.3.1)
    jbuilder (2.10.0)
      activesupport (>= 5.0.0)
    listen (3.1.5)
      rb-fsevent (~> 0.9, >= 0.9.4)
      rb-inotify (~> 0.9, >= 0.9.7)
      ruby_dep (~> 1.2)
    loofah (2.4.0)
      crass (~> 1.0.2)
      nokogiri (>= 1.5.9)
    mail (2.7.1)
      mini_mime (>= 0.1.1)
    marcel (0.3.3)
      mimemagic (~> 0.3.2)
    method_source (1.0.0)
    mimemagic (0.3.4)
    mini_mime (1.0.2)
    mini_portile2 (2.4.0)
    minitest (5.14.0)
    msgpack (1.3.3)
    nio4r (2.5.2)
    nokogiri (1.10.9)
      mini_portile2 (~> 2.4.0)
    public_suffix (4.0.3)
    puma (3.12.4)
    rack (2.2.2)
    rack-test (1.1.0)
      rack (>= 1.0, < 3)
    rails (5.2.4.2)
      actioncable (= 5.2.4.2)
      actionmailer (= 5.2.4.2)
      actionpack (= 5.2.4.2)
      actionview (= 5.2.4.2)
      activejob (= 5.2.4.2)
      activemodel (= 5.2.4.2)
      activerecord (= 5.2.4.2)
      activestorage (= 5.2.4.2)
      activesupport (= 5.2.4.2)
      bundler (>= 1.3.0)
      railties (= 5.2.4.2)
      sprockets-rails (>= 2.0.0)
    rails-dom-testing (2.0.3)
      activesupport (>= 4.2.0)
      nokogiri (>= 1.6)
    rails-html-sanitizer (1.3.0)
      loofah (~> 2.3)
    railties (5.2.4.2)
      actionpack (= 5.2.4.2)
      activesupport (= 5.2.4.2)
      method_source
      rake (>= 0.8.7)
      thor (>= 0.19.0, < 2.0)
    rake (13.0.1)
    rb-fsevent (0.10.3)
    rb-inotify (0.10.1)
      ffi (~> 1.0)
    regexp_parser (1.7.0)
    ruby_dep (1.5.0)
    rubyzip (2.3.0)
    sass (3.7.4)
      sass-listen (~> 4.0.0)
    sass-listen (4.0.0)
      rb-fsevent (~> 0.9, >= 0.9.4)
      rb-inotify (~> 0.9, >= 0.9.7)
    sass-rails (5.1.0)
      railties (>= 5.2.0)
      sass (~> 3.1)
      sprockets (>= 2.8, < 4.0)
      sprockets-rails (>= 2.0, < 4.0)
      tilt (>= 1.1, < 3)
    selenium-webdriver (3.142.7)
      childprocess (>= 0.5, < 4.0)
      rubyzip (>= 1.2.2)
    spring (2.1.0)
    spring-watcher-listen (2.0.1)
      listen (>= 2.7, < 4.0)
      spring (>= 1.2, < 3.0)
    sprockets (3.7.2)
      concurrent-ruby (~> 1.0)
      rack (> 1, < 3)
    sprockets-rails (3.2.1)
      actionpack (>= 4.0)
      activesupport (>= 4.0)
      sprockets (>= 3.0.0)
    sqlite3 (1.4.2)
    thor (1.0.1)
    thread_safe (0.3.6)
    tilt (2.0.10)
    turbolinks (5.2.1)
      turbolinks-source (~> 5.2)
    turbolinks-source (5.2.0)
    tzinfo (1.2.6)
      thread_safe (~> 0.1)
    uglifier (4.2.0)
      execjs (>= 0.3.0, < 3)
    web-console (3.7.0)
      actionview (>= 5.0)
      activemodel (>= 5.0)
      bindex (>= 0.4.0)
      railties (>= 5.0)
    websocket-driver (0.7.1)
      websocket-extensions (>= 0.1.0)
    websocket-extensions (0.1.4)
    xpath (3.2.0)
      nokogiri (~> 1.8)

PLATFORMS
  ruby

DEPENDENCIES
  bootsnap (>= 1.1.0)
  byebug
  capybara (>= 2.15)
  chromedriver-helper
  coffee-rails (~> 4.2)
  jbuilder (~> 2.5)
  listen (>= 3.0.5, < 3.2)
  puma (~> 3.11)
  rails (~> 5.2.4, >= 5.2.4.1)
  sass-rails (~> 5.0)
  selenium-webdriver
  spring
  spring-watcher-listen (~> 2.0.0)
  sqlite3
  turbolinks (~> 5)
  tzinfo-data
  uglifier (>= 1.3.0)
  web-console (>= 3.3.0)

RUBY VERSION
   ruby 2.6.4p104

BUNDLED WITH
   2.1.4

[anaulin]

This appears to be due to rack/rack#1619 , and is not a Rails issue, marking this as closed.

This change in my Gemfile fixed the issue for me:

gem 'rack', '<2.2'

[omerbn]

Thanks !