c++ condition variable notification not working as expected
std::async returns a std::future holding result of function execution. In your case, it is a temporary object which is immideatly destroyed. The documentation for std::future says:
✔ the shared state was created by a call to std::async
✔ the shared state is not yet ready
✔ this was the last reference to the shared state
All of those are true, so destruction of that future will block until worker function will finish executing.
You can create detached thread to avoid this problem:
std::thread(worker_thread).detach();