C++ semaphores cause a deadlock if a GC happens while they're locked

[davidgiven]

...or at least, I think.

I was using a semaphore to wait for a thread pool to empty, and would find that my program would get some work done and then deadlock. Looking at the stack traces, I could see my master thread waiting on the semaphore, my worker threads waiting inside hx::ExitGCFreeZone, and the garbage collector waiting for all the threads to halt.

Finding the mutex and semaphore implementations, I found this in hxMutex:

        void Acquire()
        {
                hx::EnterGCFreeZone();
                mMutex.Lock();
                hx::ExitGCFreeZone();
        }

...and this in hxSemaphore:

  void Acquire() {
#if HX_WINDOWS
        WaitForSingleObject(sem, INFINITE);
#elif defined(POSIX_SEMAPHORE)
    sem_wait(&sem);
#elif defined(APPLE_SEMAPHORE)
    dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
#endif
  }

That is, no calls to EnterGCFreeZone() and ExitGCFreeZone(). I tried adding them without really knowing what I was doing and now my program runs fine.

[Simn]

Try with git hxcpp, this was somewhat recently fixed in 51f1a6c