One option is a condvar, or CONDition VARiable. Condvars are a nifty way to block a thread, pending a change in some Boolean condition. One difficulty is that condvars are associated exclusively with mutexes, but in this example, we don't mind all that much.
The way a condvar works is that, after taking a lock on a mutex, you pass the MutexGuard into Condvar::wait, which blocks the thread. Other threads may go through this same process, blocking on the same condition. Some other thread will take the same exclusive lock and eventually call either notify_one or notify_all on the condvar. The first wakes up a single thread, the second wakes up all threads. Condvars are subject to spurious wakeup, meaning the thread may leave its block without a notification being sent to it. For this reason condvars check their conditions in a loop. But, once the condvar wakes, you are guaranteed to hold the mutex, which prevents deadlocks on spurious wakeup.
Let's adapt...