Risk of a data race in mustache::set_loader() when called concurrently with mustache::load_text()

[jwolak]

The documentation (docs/getting_started/a_simple_webpage.md) shows that Mustache templates may be loaded from a route handler while the application is running:

int main()
{
    crow::SimpleApp app;

    CROW_ROUTE(app, "/")([](){
        auto page = crow::mustache::load_text("fancypage.html");
        return page;
    });

    app.port(18080).multithreaded().run();
}

However, the documentation does not describe set_loader() or specify whether it may be called while the server is handling requests.

        namespace detail
        {
            inline std::function<std::string(std::string)>& get_loader_ref()
            {
                static std::function<std::string(std::string)> loader = default_loader;
                return loader;
            }
        } // namespace detail

set_loader() assigns a process-wide std::function:

        inline void set_loader(std::function<std::string(std::string)> loader)
        {
            detail::get_loader_ref() = std::move(loader);
        }

At the same time, load() and load_text() invoke the same global function:

        inline template_t load(const std::string& filename)
        {
            std::string filename_sanitized(filename);
            utility::sanitize_filename(filename_sanitized);
            return compile(detail::get_loader_ref()(filename_sanitized));
        }

        inline std::string load_text(const std::string& filename)
        {
            std::string filename_sanitized(filename);
            utility::sanitize_filename(filename_sanitized);
            return detail::get_loader_ref()(filename_sanitized);
        }

If set_loader() is called from one thread while another thread calls load() or load_text(), ThreadSanitizer reports a data race on the global std::function.