In honor of National Puzzle Day, I leave you today with a puzzle based on an actual customer problem.

Part One: The customer explains the problem.

We have this DLL, and during its startup, it creates a thread with the following thread procedure:

DWORD CALLBACK ThreadFunction(void *)
{
  HANDLE HandleArray[2];
  HandleArray[0] = SetUpStuff();
  if (HandleArray[0]) {
    HandleArray[1] = ShutdownEvent;
    while (WaitForMultipleObjects(2, HandleArray,
                             FALSE, INFINITE) == WAIT_OBJECT_0) {
      ProcessStuff();
    }
    CleanUpStuff(HandleArray[0]);
  }
  SetEvent(ThreadCompleteEvent);
  FreeLibraryAndExitThread(ThisLibrary, 0);
}

During process shutdown, the following function is called as part of DLL_PROCESS_DETACH handling:

void StopWorkerThread()
{
  // tell the thread to stop
  SetEvent(ShutdownEvent);

  // wait for it to stop
  WaitForSingleObject(ThreadCompleteEvent, INFINITE);
  // Clean up
  CloseHandle(ShutdownEvent);
  ShutdownEvent = NULL;
  CloseHandle(ThreadCompleteEvent);
  ThreadCompleteEvent = NULL;
}

The above function is hanging at the call to WaitForSingleObject. If we break in, we see that the thread that is supposed to be running the ThreadFunction is gone. I verified that the thread was successfully created, but by the time we get around to waiting for it, it’s already gone.

I checked, and nobody sets the ThreadCompleteEvent except the StopWorkerThread function. I stepped through SetUpStuff, and it succeeded. However, a breakpoint on CleanUpStuff was never hit. No exceptions were thrown either.

I am completely stumped as to how this thread disappeared.

You already know enough to explain how the thread disappeared.

Part Two: After providing your explanation, the customer came up with this solution.

Thank you for your explanation. We’ve made the following changes to fix the problem. Again, thank you for your help.

DWORD CALLBACK ThreadFunction(void *)
{
  HANDLE HandleArray[2];
  HandleArray[0] = SetUpStuff();
  if (HandleArray[0]) {
    HandleArray[1] = ShutdownEvent;
    while (WaitForMultipleObjects(2, HandleArray,
                             FALSE, INFINITE) == WAIT_OBJECT_0) {
      ProcessStuff();
    }
    CleanUpStuff(HandleArray[0]);
  }
  // SetEvent(ThreadCompleteEvent);
  FreeLibraryAndExitThread(ThisLibrary, 0);
}

void StopWorkerThread()
{
  // tell the thread to stop
  SetEvent(ShutdownEvent);
  // wait for the thread
  WaitForSingleObject(ThreadHandle, INFINITE);
  // Clean up
  CloseHandle(ShutdownEvent);
  ShutdownEvent = NULL;
}

Criticize this proposed solution.

Part Three: Even though the proposed solution is flawed, explain why it doesn’t cause a problem in practice. (I.e., explain why the customer is always lucky.)