class ActionDispatch::ContentSecurityPolicy::Middleware

def call(env)
  request = ActionDispatch::Request.new env
  _, headers, _ = response = @app.call(env)
  return response unless html_response?(headers)
  return response if policy_present?(headers)
  if policy = request.content_security_policy
    nonce = request.content_security_policy_nonce
    context = request.controller_instance || request
    headers[header_name(request)] = policy.build(context, nonce)
  end
  response
end