基于密钥的异步锁定 [英] Asynchronous locking based on a key
问题描述
我正在尝试找出ImageProcessor库在此处提出的问题将项目添加到缓存时出现间歇性的文件访问错误.
I'm attempting to figure out an issue that has been raised with my ImageProcessor library here where I am getting intermittent file access errors when adding items to the cache.
System.IO.IOException:该进程无法访问文件'D:\ home \ site \ wwwroot \ app_data \ cache \ 0 \ 6 \ 5 \ f \ 2 \ 7 \ 065f27fc2c8e843443d210a1e84d1ea28bbab6c4.webp'通过另一个过程.
System.IO.IOException: The process cannot access the file 'D:\home\site\wwwroot\app_data\cache\0\6\5\f\2\7\065f27fc2c8e843443d210a1e84d1ea28bbab6c4.webp' because it is being used by another process.
我写了一个类，旨在根据散列url生成的密钥执行异步锁定，但是看来我在实现中错过了一些东西.
I wrote a class designed to perform an asynchronous lock based upon a key generated by a hashed url but it seems I have missed something in the implementation.
我的锁定课程
public sealed class AsyncDuplicateLock
{
/// <summary>
/// The collection of semaphore slims.
/// </summary>
private static readonly ConcurrentDictionary<object, SemaphoreSlim> SemaphoreSlims
= new ConcurrentDictionary<object, SemaphoreSlim>();
/// <summary>
/// Locks against the given key.
/// </summary>
/// <param name="key">
/// The key that identifies the current object.
/// </param>
/// <returns>
/// The disposable <see cref="Task"/>.
/// </returns>
public IDisposable Lock(object key)
{
DisposableScope releaser = new DisposableScope(
key,
s =>
{
SemaphoreSlim locker;
if (SemaphoreSlims.TryRemove(s, out locker))
{
locker.Release();
locker.Dispose();
}
});
SemaphoreSlim semaphore = SemaphoreSlims.GetOrAdd(key, new SemaphoreSlim(1, 1));
semaphore.Wait();
return releaser;
}
/// <summary>
/// Asynchronously locks against the given key.
/// </summary>
/// <param name="key">
/// The key that identifies the current object.
/// </param>
/// <returns>
/// The disposable <see cref="Task"/>.
/// </returns>
public Task<IDisposable> LockAsync(object key)
{
DisposableScope releaser = new DisposableScope(
key,
s =>
{
SemaphoreSlim locker;
if (SemaphoreSlims.TryRemove(s, out locker))
{
locker.Release();
locker.Dispose();
}
});
Task<IDisposable> releaserTask = Task.FromResult(releaser as IDisposable);
SemaphoreSlim semaphore = SemaphoreSlims.GetOrAdd(key, new SemaphoreSlim(1, 1));
Task waitTask = semaphore.WaitAsync();
return waitTask.IsCompleted
? releaserTask
: waitTask.ContinueWith(
(_, r) => (IDisposable)r,
releaser,
CancellationToken.None,
TaskContinuationOptions.ExecuteSynchronously,
TaskScheduler.Default);
}
/// <summary>
/// The disposable scope.
/// </summary>
private sealed class DisposableScope : IDisposable
{
/// <summary>
/// The key
/// </summary>
private readonly object key;
/// <summary>
/// The close scope action.
/// </summary>
private readonly Action<object> closeScopeAction;
/// <summary>
/// Initializes a new instance of the <see cref="DisposableScope"/> class.
/// </summary>
/// <param name="key">
/// The key.
/// </param>
/// <param name="closeScopeAction">
/// The close scope action.
/// </param>
public DisposableScope(object key, Action<object> closeScopeAction)
{
this.key = key;
this.closeScopeAction = closeScopeAction;
}
/// <summary>
/// Disposes the scope.
/// </summary>
public void Dispose()
{
this.closeScopeAction(this.key);
}
}
}
用法-在HttpModule中
private readonly AsyncDuplicateLock locker = new AsyncDuplicateLock();
using (await this.locker.LockAsync(cachedPath))
{
// Process and save a cached image.
}
有人可以发现我出了错吗?我担心我误解了一些基本知识.
Can anyone spot where I have gone wrong? I'm worried that I am misunderstanding something fundamental.
该库的完整资源存储在Github上此处
The full source for the library is stored on Github here
推荐答案
如指出的其他答复者所示，原始代码在释放信号量之前从ConcurrentDictionary中删除SemaphoreSlim.因此，您的信号量不断增加-当它们仍可以使用时(它们尚未被获取，但已经从字典中检索出来)，它们已从字典中删除.
As the other answerer noted, the original code is removing the SemaphoreSlim from the ConcurrentDictionary before it releases the semaphore. So, you've got too much semaphore churn going on - they're being removed from the dictionary when they could still be in use (not acquired, but already retrieved from the dictionary).
这种映射锁"的问题在于，很难知道何时不再需要该信号量.一种选择是根本不丢弃信号灯.那是简单的解决方案，但在您的情况下可能不可接受.另一个选择-如果信号量实际上与对象实例相关，而不与值(如字符串)相关，则可以使用星历表将其附加；但是，我相信在您的情况下此选项也不可接受.
The problem with this kind of "mapping lock" is that it's difficult to know when the semaphore is no longer necessary. One option is to never dispose the semaphores at all; that's the easy solution, but may not be acceptable in your scenario. Another option - if the semaphores are actually related to object instances and not values (like strings) - is to attach them using ephemerons; however, I believe this option would also not be acceptable in your scenario.
因此，我们很难做到这一点. :)
So, we do it the hard way. :)
有几种可行的方法.我认为从引用计数的角度(对字典中的每个信号量进行引用计数)来看待它是有意义的.另外，我们希望使减数计数和删除操作成为原子操作，所以我只使用一个lock(使并发字典变得多余):
There are a few different approaches that would work. I think it makes sense to approach it from a reference-counting perspective (reference-counting each semaphore in the dictionary). Also, we want to make the decrement-count-and-remove operation atomic, so I just use a single lock (making the concurrent dictionary superfluous):
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();
}
}
}
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