Super-simple Rack streaming with Thin and other EventMachine-based servers

This is the absolute simplest way to turn any Rack app into a streaming- and deferrable-capable service using Thin. It handles the necessary async calls to make Thin start streaming, then delivers your response body on each next tick until sent. If you're sending something big, make sure it responds to each in chunks:

class FileStreamer
  def initialize(file)
    @file = file
  end

  def each
    while !@file.eof?
      yield @file.read(8192)
    end
  end
end

# then respond with a `FileStreamer`

def call(env)
  # ... do stuff ...

  [ 200, {}, FileStreamer.new(File.open('big-file.mpg')) ]
end

Only one thing to configure, the error handler if something explodes during the deferred callback (since you no longer have your Rack handlers at that point):

# for Rails:

config.middleware.insert_before(::Rack::Lock, ::Rack::EMStream) do |exception, environment|
  # do something when there's a deferred error
end

# for Rack::Builder and derivatives:

use Rack::EMStream do |exception, environment|
  # do something when there's a deferred error
end

This, of course, means that you need to push all of your in-app error handing to happen before your response object hits each, but that's the price you pay for super-simple deferred streaming.

I'm still pretty n00b to async stuff, so if you have suggestions, let me know!