bug_repro.rb

# current HEAD
require 'sinatra/base'

class MyApp < Sinatra::Base
  get '/' do
    'test'
  end
end

app = Sinatra.new(MyApp) do
end

thread = Thread.new(app) do |a|
  a.run!
end

while !app.running?
  # block until the app spins up...
end


# This never gets here.
puts "The Sinatra app has booted up finally!"
thread.join
# wait for ctrl+c for exit.

explanation_and_fix.txt

For the fix: http://github.com/dbalatero/sinatra/commit/7d492b2311502ebe398a1fa860c13920640cf29b

The reason that the relevant line in that commit had to move is because Sinatra 
never makes it past the  handler.run() Proc, as calling handler.run() will join 
on the Thin/Mongrel/etc server until someone sends a Ctrl + C to stop it.

By setting running to true inside the Proc, we are guaranteed that to happen before 
blocking happens on the web server.