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.

Answer 3

Variation of @usr's answer that solved my slightly more general problem (after quite some time going down the rathole of trying to marry AvailableWaitHandle with Task...)

    class SemaphoreSlimExtensions
    
        public static Task AwaitButReleaseAsync(this SemaphoreSlim s) => 
            s.WaitAsync().ContinueWith(_t -> s.Release(), ct,
                TaskContinuationOptions.ExecuteSynchronously | TaskContinuationOptions.OnlyOnRanToCompletion,
                TaskScheduler.Default);
        public static bool TryTake(this SemaphoreSlim s) => 
            s.Wait(0);

In my use case, the await is just a trigger for synchronous logic that then walks the full set - the TryTake helper is in my case a natural way to handle the conditional acquisition of the semaphore and the processing that's contingent on that.

var sems = new[] { new SemaphoreSlim(1, 1), new SemaphoreSlim(1, 1) };

await Task.WhenAny(from s in sems select s.AwaitButReleaseAsync());

Putting it here as I believe it to be clean, clear and relatively efficient but would be happy to see improvements on it