class Rack::Logger

def call(env)

crashing your web server.
Also, it might close your log file after the first request, potentially
which is forbidden in the SPEC and made it impossible to use Rack::Lint.
In Rack 1.2 and earlier, Rack::Logger called env['rack.errors'].close, 
def call(env)
  logger = ::Logger.new(env['rack.errors'])
  logger.level = @level
  env['rack.logger'] = logger
  @app.call(env)
end