异步/等待和TaskCompletionSource导致堆栈跟踪增长异常

Question

异步/等待和TaskCompletionSource导致堆栈跟踪增长异常

c#.netasynchronousasync-awaittask-parallel-library

9

以下是C#代码示例：

class Program
{
    static readonly List<TaskCompletionSource<bool>> buffer = 
                new List<TaskCompletionSource<bool>>();
    static Timer timer;

    public static void Main()
    {
        var outstanding = Enumerable.Range(1, 10)
            .Select(Enqueue)
            .ToArray();

        timer = new Timer(x => Flush(), null, 
                         TimeSpan.FromSeconds(1),
                         TimeSpan.FromMilliseconds(-1));
        try
        {
            Task.WaitAll(outstanding);
        }
        catch {}

        Console.ReadKey();
    }

    static Task Enqueue(int i)
    {
        var task = new TaskCompletionSource<bool>();
        buffer.Add(task);
        return task.Task;
    }

    static void Flush()
    {
        try
        {
            throw new ArgumentException("test");
        }
        catch (Exception e)
        {
            foreach (var each in buffer)
            {
                var lenBefore = e.StackTrace.Length;
                each.TrySetException(e);
                var lenAfter = e.StackTrace.Length;
                Console.WriteLine($"Before - After: {lenBefore} - {lenAfter}");
                Console.WriteLine(e.StackTrace);

            }
        }
    }
}

生成：

Before - After: 149 - 149
Before - After: 149 - 149
Before - After: 149 - 149
Before - After: 149 - 149
Before - After: 149 - 149
Before - After: 149 - 149
Before - After: 149 - 149
Before - After: 149 - 149
Before - After: 149 - 149
Before - After: 149 - 149

但是当我将Enqueue方法改为异步时：

static async Task Enqueue(int i)
{
    var task = new TaskCompletionSource<bool>();
    buffer.Add(task);
    return await task.Task;
}

结果如下：

Before - After: 149 - 643
Before - After: 643 - 1137
Before - After: 1137 - 1631
Before - After: 1631 - 2125
Before - After: 2125 - 2619
Before - After: 2619 - 3113
Before - After: 3113 - 3607
Before - After: 3607 - 4101
Before - After: 4101 - 4595
Before - After: 4595 - 5089

看起来堆栈跟踪会为每个缓冲项目递归增加。对于第一个项目，异常堆栈跟踪将是：

   at Program.Flush() in C:\src\Program.cs:line 41
   --- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotificati...
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Program.<Enqueue>d__3.MoveNext() in C:\src\Program.cs:line 34

第二项的格式如下，以此类推：

   at Program.Flush() in C:\src\Program.cs:line 41
   --- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotificati...
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Program.<Enqueue>d__3.MoveNext() in C:\src\Program.cs:line 34
   --- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotificati...
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Program.<Enqueue>d__3.MoveNext() in C:\src\Program.cs:line 34

这里发生了什么，如何修复？

- Yevhen Bobrov

更新：最终我创建了一个名为BatchException的特殊异常，并将原始异常作为内部异常传递，正如@Dmitry所建议的那样。在我看来，这是解决这个问题最简洁和正确的方法。直到更好的时候... - Yevhen Bobrov

2个回答

1

问题在于您每个任务都重复使用相同的异常，因此它会将所有堆栈附加在一起，假设这是它们按顺序进行的序列。

如果您改为为每个任务创建新的异常

var ex = new Exception(e.Message, e);
each.TrySetException(ex);

然后你会得到

Before - After: 87 - 341
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Program.<Enqueue>d__3.MoveNext() in C:\src\Program.cs:line 34
Before - After: 87 - 341
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Program.<Enqueue>d__3.MoveNext() in C:\src\Program.cs:line 34
Before - After: 87 - 341
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Program.<Enqueue>d__3.MoveNext() in C:\src\Program.cs:line 34
Before - After: 87 - 341

如果您也在使用Ben.Demystifier库

var ex = new Exception(e.Message, e);
each.TrySetException(ex);
var lenAfter = ex.Demystify().StackTrace.Length;

然后，这将进一步降低:

Before - After: 87 - 105
   at async Task Program.Enqueue(int i) in C:\src\Program.cs:line 34
Before - After: 92 - 105
   at async Task Program.Enqueue(int i) in C:\src\Program.cs:line 34
Before - After: 92 - 105
   at async Task Program.Enqueue(int i) in C:\src\Program.cs:line 34
Before - After: 92 - 105

- Ben Adams

谢谢，本！Demystifier 真是太棒了！)) - Yevhen Bobrov

我明白这个解决方案，但它并不能完全解决问题。想象一下，如果那个异常来自第三方代码（比如Azure SDK），我就不能为每个任务创建新的实例。最终，我只能创建特殊的BatchItemException，但这仍然是一个有损的解决方案。 - Yevhen Bobrov

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原文链接

- Dmytro Vakulenko · Accepted Answer

简短回答： await 尝试解包结果，而没有使用 await 的方法则不尝试访问任务结果。

更长的回答：

调用栈的重复部分如下所示：

TaskAwaiter 的 ValidateEnd 方法被内联，HandleNonSuccessAndDebuggerNotification 调用 ThrowForNonSuccess，这似乎也被内联了，并且由于单个异常用于设置 10 个 TaskCompletionSource 的结果，因此异常堆栈增长的原因可以在此处看到。

简单的解决方案是在每次 TrySetException 调用中使用 new Exception("Some descriptive message", originalException)。