rotating_file_sink_mt doesn't work in fork mode

[maordadush]

I set up spdlog in the parent before fork()

auto file_sink = std::make_shared<spdlog::sinks::rotating_file_sink_mt>(file_logger_path, 10  * 1024 * 1024, 0);
file_sink->set_pattern("[%m/%d/%Y %H:%M:%S.%e] [%^%l%$] %v");
auto file_logger = std::make_shared<spdlog::logger>("file_logger", file_sink);

Log rotation doesn't work if I run my program with a non-root user, but with a root user, it works.
if I disable the fork mode and run my program with a non-root user the log rotation works

[tt4g]

Do not set up spdlog in the parent process before fork().
After fork(), resources created with C++ std::thread and std::mutex will not work properly.

See:

• https://stackoverflow.com/questions/39890363/what-happens-when-a-thread-forks
• https://stackoverflow.com/questions/6078712/is-it-safe-to-fork-from-within-a-thread/6079669#6079669

[tt4g]

See also: #2874 (comment)

[maordadush]

Thanks!