Race condition between Cache Dispose(bool disposing) and CleanupLoop()

[AnyhowStep]

When running tests that use PersistentQueue, I sometimes see this,

Unhandled exception. System.ObjectDisposedException: The CancellationTokenSource has been disposed.
   at Persistent.Queue.Cache.Cache`2.CleanupLoop()
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_1(Object state)
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()
The active test run was aborted. Reason: Test host process crashed : Unhandled exception. System.ObjectDisposedException: The CancellationTokenSource has been disposed.
   at Persistent.Queue.Cache.Cache`2.CleanupLoop()
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_1(Object state)
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()

Or,

Unhandled exception. System.ObjectDisposedException: The CancellationTokenSource has been disposed.
   at System.Threading.CancellationTokenSource.get_Token()
   at Persistent.Queue.Cache.Cache`2.CleanupLoop()
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_1(Object state)
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()
The active test run was aborted. Reason: Test host process crashed : Unhandled exception. System.ObjectDisposedException: The CancellationTokenSource has been disposed.
   at System.Threading.CancellationTokenSource.get_Token()
   at Persistent.Queue.Cache.Cache`2.CleanupLoop()
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_1(Object state)
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()

The race condition is somewhere here,

PersistentQueue/PersistentQueue/Cache/Cache.cs

Lines 123 to 155 in 91015ed

	protected virtual void Dispose(bool disposing)
	{
	if (disposing)
	{
	_cts.Cancel();
	RemoveAll();
	// Wait until write lock becomes available.
	// If this is the case, The CleanupLoop must have finished.
	_lock.EnterWriteLock();
	_lock.ExitWriteLock();
	_lock.Dispose();
	_cts.Dispose();
	}
	}
	private async void CleanupLoop()
	{
	try
	{
	while (!_cts.IsCancellationRequested)
	{
	RemoveOldItems();
	await Task.Delay(_ttl / 2, _cts.Token);
	}
	}
	catch (OperationCanceledException)
	{
	// ignore
	}
	}

1. CleanupLoop() while-loop has not exited yet
2. _cts.Cancel();
3. _cts.Dispose();
4. !_cts.IsCancellationRequested <-- ObjectDisposedException

Or,

1. CleanupLoop() while-loop has not exited yet
2. _cts.Cancel();
3. _cts.Dispose();
4. _cts.Token <-- ObjectDisposedException

Just because _lock.EnterWriteLock(); succeeds, does not mean CleanupLoop won't reference _cts again soon after