// Mutex protecting queued_data only. Note that if you want to hold both this
// and <mutex> below, you will need to take <mutex> before this one.
mutable pthread_mutex_t queued_data_mutex;
// Mutex protecting queued_data only. Note that if you want to hold both this
// and <mutex> below, you will need to take <mutex> before this one.
mutable pthread_mutex_t queued_data_mutex;