浏览器取消请求时出现 ASP.NET Web API OperationCanceledException

Question

当用户加载页面时，它会发出一个或多个 ajax 请求，这些请求会命中 ASP.NET Web API 2 Controller 。如果用户导航到另一个页面，在这些 ajax 请求完成之前，浏览器会取消这些请求。然后，我们的 ELMAH HttpModule 会为每个取消的请求记录两个错误:

错误1:

System.Threading.Tasks.TaskCanceledException: A task was canceled.
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.Filters.AuthorizationFilterAttribute.<ExecuteAuthorizationFilterAsyncCore>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at System.Web.Http.Controllers.ExceptionFilterResult.<ExecuteAsync>d__0.MoveNext()

错误2:

System.OperationCanceledException: The operation was canceled.
   at System.Threading.CancellationToken.ThrowIfCancellationRequested()
   at System.Web.Http.WebHost.HttpControllerHandler.<WriteBufferedResponseContentAsync>d__1b.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.WebHost.HttpControllerHandler.<CopyResponseAsync>d__7.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.Http.WebHost.HttpControllerHandler.<ProcessRequestAsyncCore>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Web.TaskAsyncHelper.EndTask(IAsyncResult ar)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

查看堆栈跟踪，我发现异常是从这里抛出的:https://github.com/ASP-NET-MVC/aspnetwebstack/blob/master/src/System.Web.Http.WebHost/HttpControllerHandler.cs#L413

我的问题是:如何处理和忽略这些异常？

它似乎超出了用户代码...

注释:

• 我正在使用 ASP.NET Web API 2
• Web API 端点是异步和非异步方法的混合体。
• 无论我在哪里添加错误日志记录，我都无法捕获用户代码中的异常
  • Global.asax Applicaiton_Error
  • TaskScheduler.UnobservedTaskException
  • ELMAH 错误过滤void ErrorLog_Filtering ( https://code.google.com/p/elmah/wiki/ErrorFiltering )

Answer 1

这是 ASP.NET Web API 2 中的一个错误，不幸的是，我认为没有一种解决方法总是能成功。我们提交了bug在我们这边修复它。

最终，问题在于我们在这种情况下将取消的任务返回给 ASP.NET，而 ASP.NET 将取消的任务视为未处理的异常(它将问题记录在应用程序事件日志中)。

同时，您可以尝试类似下面的代码。它添加了一个顶级消息处理程序，该处理程序在取消 token 触发时删除内容。如果响应没有内容，则不应触发该错误。发生这种情况的可能性仍然很小，因为客户端可能会在消息处理程序检查取消 token 之后但在更高级别的 Web API 代码执行相同检查之前立即断开连接。但我认为这在大多数情况下都会有帮助。

大卫

config.MessageHandlers.Add(new CancelledTaskBugWorkaroundMessageHandler());

class CancelledTaskBugWorkaroundMessageHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        HttpResponseMessage response = await base.SendAsync(request, cancellationToken);

        // Try to suppress response content when the cancellation token has fired; ASP.NET will log to the Application event log if there's content in this case.
        if (cancellationToken.IsCancellationRequested)
        {
            return new HttpResponseMessage(HttpStatusCode.InternalServerError);
        }

        return response;
    }
}