std::condition_variable and std::condition_variable_any

[johnsheu]

wait_for() and wait_until() for std::condition_variable and std::condition_variable_any take their wait duration or timeout parameters by const-reference. This can lead to unexpected behavior if the referent changes while the wait_for() or wait_until() blocks.

For example:

  template <class _Clock, class _Duration>
  _LIBCPP_HIDE_FROM_ABI cv_status
  wait_until(unique_lock<mutex>& __lk, const chrono::time_point<_Clock, _Duration>& __t) {
    using namespace chrono;
    using __clock_tp_ns = time_point<_Clock, nanoseconds>;

    typename _Clock::time_point __now = _Clock::now();
    if (__t <= __now)
      return cv_status::timeout;

    __clock_tp_ns __t_ns = __clock_tp_ns(std::__safe_nanosecond_cast(__t.time_since_epoch()));

    __do_timed_wait(__lk, __t_ns);
    return _Clock::now() < __t ? cv_status::no_timeout : cv_status::timeout;
  }

The duration parameter __t is accessed by-reference, and so its value may be different before and after the wait.

For std::condition_variable_any, this is an actual data race if the timeout parameter is protected by the lock, as the implementation accesses the timeout outside of the lock.

[johnsheu]

Pull request #148330