How to Avoid Intermittent File Access Errors with Asynchronous Locking in C#?

Solving Intermittent File Access Errors with Asynchronous Locking in C#

Asynchronous locking, particularly when using a hashed URL and a class like AsyncDuplicateLock, can sometimes lead to intermittent file access errors. This often stems from improper semaphore handling within a concurrent dictionary. An initial flawed approach might look like this:

<code class="language-csharp">SemaphoreSlim locker;
if (SemaphoreSlims.TryRemove(s, out locker))
{
    locker.Release();
    locker.Dispose();
}</code>

The problem here is removing the semaphore before releasing it. This creates excessive semaphore churn, leading to ongoing semaphore usage even after its removal from the dictionary.

A robust solution employs reference counting to manage semaphore lifetimes:

<code class="language-csharp">public sealed class AsyncDuplicateLock
{
    private sealed class RefCounted<T>
    {
        public RefCounted(T value)
        {
            RefCount = 1;
            Value = value;
        }

        public int RefCount { get; set; }
        public T Value { get; private set; }
    }

    private static readonly Dictionary<object, RefCounted<SemaphoreSlim>> SemaphoreSlims
                        = new Dictionary<object, RefCounted<SemaphoreSlim>>();

    private SemaphoreSlim GetOrCreate(object key)
    {
        RefCounted<SemaphoreSlim> item;
        lock (SemaphoreSlims)
        {
            if (SemaphoreSlims.TryGetValue(key, out item))
            {
                ++item.RefCount;
            }
            else
            {
                item = new RefCounted<SemaphoreSlim>(new SemaphoreSlim(1, 1));
                SemaphoreSlims[key] = item;
            }
        }
        return item.Value;
    }

    public IDisposable Lock(object key)
    {
        GetOrCreate(key).Wait();
        return new Releaser { Key = key };
    }

    public async Task<IDisposable> LockAsync(object key)
    {
        await GetOrCreate(key).WaitAsync().ConfigureAwait(false);
        return new Releaser { Key = key };
    }

    private sealed class Releaser : IDisposable
    {
        public object Key { get; set; }

        public void Dispose()
        {
            RefCounted<SemaphoreSlim> item;
            lock (SemaphoreSlims)
            {
                item = SemaphoreSlims[Key];
                --item.RefCount;
                if (item.RefCount == 0)
                    SemaphoreSlims.Remove(Key);
            }
            item.Value.Release();
        }
    }
}</code>

This revised code uses a RefCounted<T> wrapper to track semaphore references. A semaphore is only removed from the dictionary when its reference count reaches zero, ensuring proper release and preventing premature disposal, thereby eliminating the intermittent file access errors.

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