How to Solve Intermittent File Access Errors in Asynchronous Locking?

Key-based asynchronous locking

This article discusses an issue encountered in the ImageProcessor library: an intermittent file access error during cache addition. The error message indicates that the file in question is being used by another process. To solve this problem, a key-based asynchronous locking mechanism is implemented. However, it was later discovered that this implementation had an oversight.

The problem

The original locking class AsyncDuplicateLock attempts to perform asynchronous locking by retrieving the semaphore from the concurrent dictionary using the key. However, it mistakenly removes the semaphore from the dictionary before releasing it. This means that the semaphore was deleted while still in use, leading to the observed file access error.

Solution

To solve this problem, the locking class was modified. Instead of using a concurrent dictionary-based lock-free approach, the updated version uses a more traditional approach that combines reference counting and a single lock. This ensures that semaphores are only released when they are no longer used.

The following is the modified lock class code:

<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 updated implementation maintains a reference count for each semaphore and ensures that the semaphore is only removed from the dictionary when no more active locks refer to it. This approach effectively resolves intermittent file access errors encountered in previous implementations.

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