UberQueue

UberQueue.cs

class UberQueue<T>
{
    private readonly SemaphoreSlim _semaphore;
    private readonly IAsyncQueue<T>[] _queues;

    public UberQueue(IAsyncQueue<T>[] queues)
    {
        _semaphore = new SemaphoreSlim(1, 1);
        _queues = queues;
    }

    private Task<T>[]? PendingTasks { get; set; }

    public async Task<T> DequeueAsync()
    {
        try
        {
            await _semaphore.WaitAsync();

            if (PendingTasks == null || PendingTasks.Length == 0)
            {
                PendingTasks = _queues.Select(q => q.DequeueAsync()).ToArray();
            }

            var completedTask = await Task.WhenAny(PendingTasks);

            PendingTasks = PendingTasks.Where(t => t != completedTask).ToArray();

            return await completedTask;
        }
        finally
        {
            _semaphore.Release();
        }
    }
}

interface IAsyncQueue<T>
{
    Task<T> DequeueAsync();
}