WEBrick TimeoutHandler race condition

webrick_race.rb

require 'webrick/utils'

# The initialize below is augmented to stall the @timeout_info iteration.
# The fix is uncommenting TimeoutMutex.synchronize{} around the @timeout_info iteration.
class WEBrick::Utils::TimeoutHandler
  def initialize
    @timeout_info = Hash.new
    Thread.start{
      while true
        now = Time.now
        # uncomment the block below to prevent a race
        # TimeoutMutex.synchronize{
          @timeout_info.each{|thread, ary|
            ary.dup.each{|info|
              puts "stalling timeout_info iteration"
              sleep 0.1
              time, exception = *info
              interrupt(thread, info.object_id, exception) if time < now
              puts "finished timeout_info iteration"
            }
          }
        # }
        sleep 0.5
      end
    }
  end
end

while true do
  loader_thread = Thread.new do
    print '.'
    WEBrick::Utils::TimeoutHandler.register(1, Timeout::Error)
  end
  loader_thread.join
  sleep 0.1
end

# Expect to see something like:
# ➥ ruby webrick_race.rb
# .....stalling timeout_info iteration
# .finished timeout_info iteration/Users/adacosta/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/webrick/utils.rb:197:in `register': can't add a new key into hash during iteration (RuntimeError)
# from /Users/adacosta/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/webrick/utils.rb:148:in `block in register'
# from /Users/adacosta/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/webrick/utils.rb:147:in `synchronize'
# from /Users/adacosta/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/webrick/utils.rb:147:in `register'
# from webrick_race.rb:32:in `block in <main>'