Handling exceptions with thin, eventmachine, and em-http-request.

config.ru

require 'rubygems'
require 'eventmachine'
require 'fiber'

EM.error_handler { |e| p "EM.error_handler: #{ e.message }" }
run(lambda do |env|
  case env['PATH_INFO']

  ###
  # Hits EM.error_handler
  #
  #   $ bundle exec thin start
  #   >> Using rack adapter
  #   >> Thin web server (v1.2.11 codename Bat-Shit Crazy)
  #   >> Maximum connections set to 1024
  #   >> Listening on 0.0.0.0:3000, CTRL+C to stop
  #   "yielding"
  #   "throwing"
  #   "done sleeping"
  #   "EM.error_handler: die"
  when '/sleep'
    Fiber.new do
      fiber = Fiber.current
      EM::Timer.new(1) { p 'done sleeping'; fiber.resume }

      p 'yielding'
      Fiber.yield

      raise 'die'
    end.resume

    p 'throwing'
    throw :async

  ###
  # Crashes app server using eventmachine 0.12.10 and em-http-request 0.3.0.
  # EM.error_handler catches the exception using em-http-request master.
  #
  #   $ bundle exec thin start
  #   >> Using rack adapter
  #   >> Thin web server (v1.2.11 codename Bat-Shit Crazy)
  #   >> Maximum connections set to 1024
  #   >> Listening on 0.0.0.0:3000, CTRL+C to stop
  #   "yielding"
  #   "throwing"
  #   "resuming"
  #   /Users/Larry/Sites/pollex/config.ru:57:in `block (3 levels) in <main>': die (RuntimeError)
  when '/http'
    require 'em-http'
    Fiber.new do
      f = Fiber.current

      http = EM::HttpRequest.new('http://getcloudapp.com').get
      http.callback { p 'resuming'; f.resume(http) }

      p 'yielding'
      Fiber.yield

      raise 'die'
    end.resume

    p 'throwing'
    throw :async

  ###
  # Crashes app server using eventmachine 0.12.10 and em-http-request 0.3.0.
  # EM.error_handler catches the exception using em-http-request master.
  #
  #   $ bundle exec thin start
  #   >> Using rack adapter
  #   >> Thin web server (v1.2.11 codename Bat-Shit Crazy)
  #   >> Maximum connections set to 1024
  #   >> Listening on 0.0.0.0:3000, CTRL+C to stop
  #   "throwing"
  #   "http.callback"
  #   /Users/Larry/Sites/pollex/config.ru:79:in `block (3 levels) in <main>': die (RuntimeError)
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/eventmachine-0.12.10/lib/em/deferrable.rb:134:in `call'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/eventmachine-0.12.10/lib/em/deferrable.rb:134:in `set_deferred_status'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/eventmachine-0.12.10/lib/em/deferrable.rb:173:in `succeed'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/em-http-request-0.3.0/lib/em-http/client.rb:307:in `unbind'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/eventmachine-0.12.10/lib/eventmachine.rb:1417:in `event_callback'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run_machine'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/thin-1.2.11/lib/thin/backends/base.rb:61:in `start'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/thin-1.2.11/lib/thin/server.rb:159:in `start'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/thin-1.2.11/lib/thin/controllers/controller.rb:86:in `start'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/thin-1.2.11/lib/thin/runner.rb:185:in `run_command'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/thin-1.2.11/lib/thin/runner.rb:151:in `run!'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/gems/thin-1.2.11/bin/thin:6:in `<top (required)>'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/bin/thin:19:in `load'
  #           from /Users/Larry/.rvm/gems/ruby-1.9.2-p180@pollex/bin/thin:19:in `<main>'
  when '/http-no-fiber'
    require 'em-http'

    http = EM::HttpRequest.new('http://getcloudapp.com').get
    p "EM.reactor_thread?: #{ EM.reactor_thread? }"
    http.callback do
      p 'http.callback'
      p "EM.reactor_thread?: #{ EM.reactor_thread? }"

      raise 'die'
    end

    p 'throwing'
    throw :async

  when '/defer'
    operation = lambda { p 'operation' }
    callback  = lambda { |result| raise 'callback' }

    EM.defer operation, callback

    p 'throwing'
    throw :async

  ###
  # Caught by thin, maybe?
  #
  #   $ bundle exec thin start
  #   >> Using rack adapter
  #   >> Thin web server (v1.2.11 codename Bat-Shit Crazy)
  #   >> Maximum connections set to 1024
  #   >> Listening on 0.0.0.0:3000, CTRL+C to stop
  #   !! Unexpected error while processing request: die
  when '/die'
    raise 'die'

  ###
  # Crashes app server as expected.
  #
  #   $ bundle exec thin start
  #   >> Using rack adapter
  #   >> Thin web server (v1.2.11 codename Bat-Shit Crazy)
  #   >> Maximum connections set to 1024
  #   >> Listening on 0.0.0.0:3000, CTRL+C to stop
  #   "throwing"
  #   /Users/Larry/Sites/pollex/config.ru:94:in `block (3 levels) in <main>': thread (RuntimeError)
  when '/thread'
    Thread.new do
      sleep 1
      raise 'thread'
    end

    p 'throwing'
    throw :async
  end
end)

Gemfile

source :gemcutter

gem 'em-http-request', :git => 'https://github.com/igrigorik/em-http-request.git'
gem 'thin'

Gemfile.lock

GIT
  remote: https://github.com/igrigorik/em-http-request.git
  revision: ddfb713b29570d2f93713f619d94e5960ea8f627
  specs:
    em-http-request (1.0.0.beta.3)
      addressable (>= 2.2.3)
      em-socksify
      eventmachine (>= 1.0.0.beta.3)
      http_parser.rb (>= 0.5.1)

GEM
  remote: http://rubygems.org/
  specs:
    addressable (2.2.6)
    daemons (1.1.3)
    em-socksify (0.1.0)
      eventmachine
    eventmachine (1.0.0.beta.3)
    http_parser.rb (0.5.1)
    rack (1.3.0)
    thin (1.2.11)
      daemons (>= 1.0.9)
      eventmachine (>= 0.12.6)
      rack (>= 1.0.0)

PLATFORMS
  ruby

DEPENDENCIES
  em-http-request!
  thin

My guess is that it is because the exception is being raised in a thread. What happens if you put this line at the top of file?

Thread.abort_on_exception = true

Thanks for the suggestion! No dice, though. I'm looking through em-http-request and eventmachine to see if they're doing anything with threads.

Yeah, I realize now it was a bad guess on my part. Threads should be involved here. I'm stumped. Ping Ilya Grigorik on Twitter (igrigorik). He will know.

I updated the gist to add a test case for raising an exception in a thread with Thread.abort_on_exception = true. The server dies in the same way as /http.

Looks like it has nothing to do with fibers. I added /http-no-fiber that raises an exception in HttpClient#callback and it also takes down thin.

Larry, can you try building eventmachine from master? When I run your example, I get:

"throwing"
"http.callback"
"Thread.abort_on_exception: true"
"EM.error_handler: die"

As in, it doesn't blow up.

Fantastic! Thin crashed out after only upgrading to eventmachine master but using it along with em-http-request master works like a champ.

Awesome. :)