app.rb

require 'eventmachine'

def check_checksum
  # some checksum processing here... let's say it fails
  raise Exception.new("invalid checksum: 28fad31")
end 
      
def notify(ev)
  if ev == :download_complete then
    check_checksum
  end 
end 

def exception_handler
  yield
rescue Exception => e
  puts "handling exception #{e.message} by logging it"
  # send an email etc
end

def start_deployment
  exception_handler do
    # wait for some special events (that prevent EventMachine from returning)
    EventMachine.add_timer(10) do
      puts "slow wait ending"
    end

    # start "fake" download
    EventMachine.add_timer(1) do
      # "fake download" complete
      notify(:download_complete)
    end
  end
end

app_spec.rb

require './app'
require 'eventmachine'

describe 'the app' do
  it 'should raise exception' do
    stub!(:exception_handler) do
      # 1. check that the exception message matches "invalid checksum"
      # how?
  
      # 2. call EM.stop
      EM.stop
    end

    EM.run do
      start_deployment
    end
  end
end