Example showing a Sinatra RuntimeError upon shutdown

config.ru

require 'sinatra/base'

class Application < Sinatra::Base
  get '/' do
    'Hello World!'
  end
end

run Application.run!

# A fix is to instead say
#
#   run Application
#
# But then some of Sinatra's settings, such as the port, are not respected...

output_with_puma.txt

$ rackup test.ru 
== Sinatra (v2.0.0) has taken the stage on 4567 for development with backup from Puma
Puma starting in single mode...
* Version 3.9.1 (ruby 2.2.6-p396), codename: Private Caller
* Min threads: 0, max threads: 16
* Environment: development
* Listening on tcp://localhost:4567
Use Ctrl-C to stop
^C== Sinatra has ended his set (crowd applauds)
- Gracefully stopping, waiting for requests to finish
=== puma shutdown: 2017-08-01 15:02:08 +0200 ===
- Goodbye!
/usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/builder.rb:146:in `to_app': missing run or map statement (RuntimeError)
	from /home/jrs/Projects/wordcabin/test.ru:11:in `<main>'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/builder.rb:49:in `eval'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/builder.rb:49:in `new_from_string'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/builder.rb:40:in `parse_file'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/server.rb:319:in `build_app_and_options_from_config'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/server.rb:219:in `app'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/server.rb:354:in `wrapped_app'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/server.rb:283:in `start'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/server.rb:148:in `start'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/bin/rackup:4:in `<top (required)>'
	from /usr/bin/rackup:23:in `load'
	from /usr/bin/rackup:23:in `<main>'

output_with_webrick.txt

$ rackup test.ru 
[2017-08-01 15:10:33] INFO  WEBrick 1.3.1
[2017-08-01 15:10:33] INFO  ruby 2.2.6 (2016-11-15) [x86_64-linux]
== Sinatra (v2.0.0) has taken the stage on 4567 for development with backup from WEBrick
[2017-08-01 15:10:33] INFO  WEBrick::HTTPServer#start: pid=12522 port=4567
^C== Sinatra has ended his set (crowd applauds)
[2017-08-01 15:10:37] INFO  going to shutdown ...
[2017-08-01 15:10:37] INFO  WEBrick::HTTPServer#start done.
/usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/builder.rb:146:in `to_app': missing run or map statement (RuntimeError)
	from /home/jrs/Projects/wordcabin/test.ru:11:in `<main>'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/builder.rb:49:in `eval'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/builder.rb:49:in `new_from_string'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/builder.rb:40:in `parse_file'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/server.rb:319:in `build_app_and_options_from_config'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/server.rb:219:in `app'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/server.rb:354:in `wrapped_app'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/server.rb:283:in `start'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/lib/rack/server.rb:148:in `start'
	from /usr/lib64/ruby/gems/2.2.0/gems/rack-2.0.3/bin/rackup:4:in `<top (required)>'
	from /usr/bin/rackup:23:in `load'
	from /usr/bin/rackup:23:in `<main>'