Do you need clean up one-shot timers?

Raymond Chen

The CreateTimerQueueTimer function allows you to create one-shot timers by passing the WT_EXECUTEONLYONCE flag. The documentation says that you need to call the DeleteTimerQueueTimer function when you no longer need the timer.

Why do you need to clean up one-shot timers?

To answer this, I would like to introduce you to one of my favorite rhetorical questions when trying to puzzle out API design: “What would the world be like if this were true?”

Imagine what the world would be like if you didn’t need to clean up one-shot timers.

Well, for one thing, it means that the behavior of the function would be confusing. The caller of the the CreateTimerQueueTimer function would have to keep track of whether the timer was one-shot or not, to know whether or not the handle needed to be deleted.

But far, far worse is that if one-shot timers were self-deleting, it would be impossible to use them correctly.

Suppose you have an object that creates a one-shot timer, and you want to clean it up in your destructor if it hasn’t fired yet. If one-shot timers were self-deleting, then it would be impossible to write this object.

class Sample {
 HANDLE m_hTimer;
 Sample() : m_hTimer(NULL) { CreateTimerQueueTimer(&m_hTimer, …); }
 ~Sample() { … what to write here? … }
};

You might say, “Well, I’ll have my callback null out the m_hTimer variable. That way, the destructor will know that the timer has fired.”

Except that’s a race condition.

Sample::Callback(void *context)
{
  /// RACE WINDOW HERE
  ((Sample*)context)->m_hTimer = NULL;
  …
}

If the callback is pre-empted during the race window and the object is destructed, and one-shot timers were self-deleting, then the object would attempt to use an invalid handle.

This race window is uncloseable since the race happens even before you get a chance to execute a single line of code.

So be glad that you have to delete handles to one-shot timers.

Raymond Chen

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