I cannot get a stream to stay open (which is necessary for Server-Sent Events to work) with Sinatra running on Puma. I don't really have any idea why, but the connections close right away.

require 'sinatra'

set :server, :puma
set :connections, []

get '/stream', provides: 'text/event-stream' do
  stream :keep_open do |out|
    settings.connections << out
    out.callback do
      puts 'stream closed'
      settings.connections.delete(out)
    end
  end
end

Then I run curl http://localhost:4567/stream and the output is

$ ruby example.rb 
Puma 2.13.4 starting...
* Min threads: 0, max threads: 16
* Environment: development
* Listening on tcp://localhost:4567
== Sinatra (v1.4.6) has taken the stage on 4567 for development with backup from Puma
stream closed
127.0.0.1 - - [14/Sep/2015 09:45:33] "GET /stream HTTP/1.1" 200 - 0.0047

If I have the server as :thin, it works fine. I don't want to use Thin though, for other reasons. And if I understand puma/puma#3 correctly, Puma can do this kind of streaming just fine.

This could be related to the problems on #962, but I'm using MRI (tested on 2.1.5p273) so it might be unrelated to any problems JRuby has.