Commit ef69971 introduced caching of the Sinatra request object. Unfortunately this has the side effect of breaking routing when you have a setup consisting of sinatra middleware -> rack::builder -> sinatra application. For example, assume the following two classes:

class MyMiddleware < Sinatra::Base
end

class MyApp < Sinatra::Base
  get "/foo" do
    "hello"
  end
end

If we have the following config.ru then a request to /test/foo works as expected:

builder = Rack::Builder.new do
  map "/test" do
    run MyApp
  end
end
run builder

However, if we introduce the middleware, then a request to /test/foo generates a 404:

builder = Rack::Builder.new do
  use MyMiddleware
  map "/test" do
    run MyApp
  end
end
run builder

This is because MyMiddleware creates a cached copy of the Sinatra request in env['sinatra.request'] with the PATH_INFO set to /test/foo. Rack builder modifies the PATH_INFO and SCRIPT_NAME before MyApp runs, but MyApp uses the cached information from env['sinatra.request'].