iis回收后实体框架异步方法突然抛出异常

Question

从昨天早上开始，在我们的夜间站点在 03:00 UTC 回收之后，实体框架在不同类型的异步方法调用（CountAsync、ToArrayAsync 等）上抛出 NullReferenceException。

没有发生任何代码或配置更改（根据 Git 日志），服务器本身也没有运行任何自动更新。

现在，我不知道是什么导致了这种行为，有什么想法吗？

尝试了以下

• 重新部署现有代码
• 重启
• Windows 更新
• 新网站

在我们将生产部署到的测试服务器上工作（服务器是 Windows Server 2012 R2，全部托管在 AWS 上）。

切换到非异步方法可以解决问题，但目前并非无处不在。

通知控制器的堆栈跟踪，但作业控制器上的行为相同等等。

  at System.Data.Entity.Core.Objects.Internal.BufferedDataReader.ReadAsync(CancellationToken cancellationToken)
   at System.Data.Entity.Core.Common.Internal.Materialization.Shaper`1.<StoreReadAsync>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at System.Data.Entity.Utilities.TaskExtensions.CultureAwaiter`1.GetResult()
   at System.Data.Entity.Core.Common.Internal.Materialization.Shaper`1.SimpleEnumerator.<MoveNextAsync>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at System.Data.Entity.Utilities.TaskExtensions.CultureAwaiter`1.GetResult()
   at System.Data.Entity.Internal.LazyAsyncEnumerator`1.<FirstMoveNextAsync>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at System.Data.Entity.Utilities.TaskExtensions.CultureAwaiter`1.GetResult()
   at System.Data.Entity.Infrastructure.IDbAsyncEnumerableExtensions.<SingleAsync>d__2d`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Web.Notifications.AlertService.UnmappedCheck`4.<Check>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Web.Notifications.AlertService.<Check>d__12.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at Web.ApiControllers.NotificationsController.<Count>d__14.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at System.Threading.Tasks.TaskHelpersExtensions.<CastToObject>d__3`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Web.Http.Controllers.ExceptionFilterResult.<ExecuteAsync>d__0.MoveNext()

Answer 1

经过大量测试和搜索，我们发现罪魁祸首是SetExecutionStrategy 与 webapi 和对数据库的异步调用结合使用。

一段时间以来，我们在某些情况下使用这种方法来制定策略：

https://msdn.microsoft.com/en-us/data/dn307226.aspx#transactions_workarounds

public class MyConfiguration : DbConfiguration 
{ 
    public MyConfiguration() 
    { 
        this.SetExecutionStrategy("System.Data.SqlClient", () => SuspendExecutionStrategy 
          ? (IDbExecutionStrategy)new DefaultExecutionStrategy() 
          : new SqlAzureExecutionStrategy()); 
    } 
  
    public static bool SuspendExecutionStrategy 
    { 
        get 
        { 
            return (bool?)CallContext.LogicalGetData("SuspendExecutionStrategy") ?? false; 
        } 
        set 
        { 
            CallContext.LogicalSetData("SuspendExecutionStrategy", value); 
        } 
    } 
} 

为什么这会导致错误，直到最近正常回收后，我仍然没有答案。