Task.WhenAny and SemaphoreSlim class

Question

When using WaitHandle.WaitAny and Semaphore class like the following:

var s1 = new Semaphore(1, 1);
var s2 = new Semaphore(1, 1);

var handles = new [] { s1, s2 };

var index = WaitHandle.WaitAny(handles);

handles[index].Release();

It seems guaranteed that only one semaphore is acquired by WaitHandle.WaitAny.

Is it possible to obtain similar behavior for asynchronous (async/await) code?

Answer 1

I cannot think of a built-in solution. I'd do it like this:

var s1 = new SemaphoreSlim(1, 1);
var s2 = new SemaphoreSlim(1, 1);

var waits = new [] { s1.WaitAsync(), s2.WaitAsync() };

var firstWait = await Task.WhenAny(waits);

//The wait is still running - perform compensation.
if (firstWait == waits[0])
 waits[1].ContinueWith(_ => s2.Release());
if (firstWait == waits[1])
 waits[0].ContinueWith(_ => s1.Release());

This acquires both semaphores but it immediately releases the one that came second. This should be equivalent. I cannot think of a negative consequence of acquiring a semaphore needlessly (except performance of course).

Answer 2

Here is a generalized implementation of a WaitAnyAsync method, that acquires asynchronously any of the supplied semaphores:

/// <summary>
/// Asynchronously waits to enter any of the semaphores in the specified array.
/// </summary>
public static async Task<SemaphoreSlim> WaitAnyAsync(SemaphoreSlim[] semaphores,
    CancellationToken cancellationToken = default)
{
    // Fast path
    cancellationToken.ThrowIfCancellationRequested();
    var acquired = semaphores.FirstOrDefault(x => x.Wait(0));
    if (acquired != null) return acquired;

    // Slow path
    using var cts = CancellationTokenSource.CreateLinkedTokenSource(
        cancellationToken);
    Task<SemaphoreSlim>[] acquireTasks = semaphores
        .Select(async s => { await s.WaitAsync(cts.Token); return s; })
        .ToArray();

    Task<SemaphoreSlim> acquiredTask = await Task.WhenAny(acquireTasks);

    cts.Cancel(); // Cancel all other tasks

    var releaseOtherTasks = acquireTasks
        .Where(task => task != acquiredTask)
        .Select(async task => (await task).Release());

    try { await Task.WhenAll(releaseOtherTasks); }
    catch (OperationCanceledException) { } // Ignore
    catch
    {
        // Consider any other error (possibly SemaphoreFullException or
        // ObjectDisposedException) as a failure, and propagate the exception.
        try { (await acquiredTask).Release(); } catch { }
        throw;
    }

    try { return await acquiredTask; }
    catch (OperationCanceledException)
    {
        // Propagate an exception holding the correct CancellationToken
        cancellationToken.ThrowIfCancellationRequested();
        throw; // Should never happen
    }
}

This method becomes increasingly inefficient as the contention gets higher and higher, so I wouldn't recommend using it in hot paths.