For worker implementation of "run loop", I am looking to implement MessageQueue::waitForMessageWithTimeout(), where timeout would be computed from the the JS timer with nearest expiration. I am looking at using WTF::ThreadCondition::timedWait for implementation but it seems it is not implemented right... Before trying to send a patch, I'd like to ask here to validate the idea.
It is defined with 'secondsToWait': bool ThreadCondition::timedWait(Mutex& mutex, double secondsToWait) while a corresponding pthread function takes absolute time: int pthread_cond_timedwait(pthread_cond_t *, pthread_mutex_t *, const struct timespec *); The reason pthread function takes abs time is because pthread's function can return 'spuriously'. I think the reason is that most implementation lack the atomic "wake up and re-acquire the mutex" operation, so the conditions can change between waking up and re-acquiring the mutex - and the wait can return while in fact it should have not. Here is a piece of pthread doc ( https://computing.llnl.gov/tutorials/pthreads/man/pthread_cond_timedwait.txt ): When using condition variables there is always a Boolean predicate involving shared variables associated with each condition wait that is true if the thread should proceed. Spurious wakeups from the pthread_cond_timedwait() or pthread_cond_wait() functions may occur. Since the return from pthread_cond_timed- wait() or pthread_cond_wait() does not imply anything about the value of this predicate, the predicate should be re-evaluated upon such return. That means the waiting should always be done in a loop, checking if the thread can actually proceed and if not, call wait function again, like here: bool MessageQueue::waitForMessage() { MutexLocker lock(m_mutex); while (m_queue.isEmpty()) m_condition.wait(m_mutex); .. consume message ... } In case of timed wait it means the absolute time should be computed before such loop, and it explains why pthreads have timed wait function with absolute time as a parameter. That means WTF timedWait method should change prototype to take absolute time: bool ThreadCondition::timedWait(Mutex& mutex, const struct timespec *timeoutTime) Then it's possible to implement blocking MessageQueue::waitForMessageWithTimeout() to return exactly after specified timeout or with a message. Ok to change? Dmitry
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