gitlab-org--gitlab-foss/lib/constraints/user_url_constrainer.rb

class UserUrlConstrainer
  def matches?(request)
    full_path = request.params[:username]

    return false unless NamespacePathValidator.valid_path?(full_path)

    User.find_by_full_path(full_path, follow_redirects: request.get?).present?
  end
end