Question

在以下代码中，Task.WhenAll行上发生死锁：

[Fact]
public async Task GetLdapEntries_ReturnsLdapEntries()
{
    var ldapEntries = _fixture.CreateMany<LdapEntryDto>(2).ToList();
    var creationTasks = new List<Task>();
    foreach (var led in ldapEntries)
    {
        var task = _attributesServiceClient.CreateLdapEntry(led);
        creationTasks.Add(task);
    }
    await Task.WhenAll(creationTasks); // <- deadlock here

    var result = await _ldapAccess.GetLdapEntries();

    result.Should().BeEquivalentTo(ldapEntries);
}

public async Task<LdapEntryDto> CreateLdapEntry(LdapEntryDto ldapEntryDto)
{
    using (var creationResponse = await _httpClient.PostAsJsonAsync<LdapEntryDto>("", ldapEntryDto))
    {
        if (creationResponse.StatusCode == HttpStatusCode.Created)
        {
            return await creationResponse.Content.ReadAsAsync<LdapEntryDto>();
        }

        return null;
    }
}

xUnit测试试图通过调用本身await来自Web服务的响应的异步方法来异步创建测试数据。 _httpClient是通过HttpClient在内存TestServer中创建的真实TestServer.CreateClient()。

在using line方法中的CreateLdapEntry上设置断点时，将命中两次。永远不会命中状态代码检查的断点。在断开Task.WhenAll()并检查creationTasks时，两个任务都处于状态WaitingForActivation：

creationTasks
Count = 2
    [0]: Id = 32, Status = WaitingForActivation, Method = "{null}", Result = "{Not yet computed}"
    [1]: Id = 33, Status = WaitingForActivation, Method = "{null}", Result = "{Not yet computed}"

当不使用Task.WhenAll()而是分别等待每个任务时，不会发生死锁：

foreach (var led in ldapEntries)
{
    await _attributesServiceClient.CreateLdapEntry(led);
}

我知道有人问过similar question，但是那里的代码示例使用的是.Result，而不是await Task.WhenAll()。

我想了解为什么在使用Task.WhenAll()时会发生死锁。

编辑：添加了锁定线程的调用堆栈

Not Flagged     3992    11  Worker Thread   Worker Thread   Microsoft.AspNetCore.Routing.dll!Microsoft.AspNetCore.Builder.RouterMiddleware.Invoke
                        [Managed to Native Transition]
                        Microsoft.AspNetCore.Routing.dll!Microsoft.AspNetCore.Builder.RouterMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext httpContext)
                        ShibbolethAttributes.Service.dll!RoleManager.Service.Middleware.ApiKeyHandlerMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 38
                        Microsoft.AspNetCore.Diagnostics.dll!Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context)
                        System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncMethodBuilderCore.Startd__7>(ref Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.d__7 stateMachine)
                        Microsoft.AspNetCore.Diagnostics.dll!Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context)
                        Microsoft.AspNetCore.Hosting.dll!Microsoft.AspNetCore.Hosting.Internal.HostingApplication.ProcessRequestAsync(Microsoft.AspNetCore.Hosting.Internal.HostingApplication.Context context)
                        Microsoft.AspNetCore.TestHost.dll!Microsoft.AspNetCore.TestHost.TestServer.ApplicationWrapper.ProcessRequestAsync(Microsoft.AspNetCore.Hosting.Internal.HostingApplication.Context context)
                        Microsoft.AspNetCore.TestHost.dll!Microsoft.AspNetCore.TestHost.HttpContextBuilder.SendAsync.AnonymousMethod__0()
                        System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncMethodBuilderCore.Startc__DisplayClass10_0.b__0>d stateMachine)
                        Microsoft.AspNetCore.TestHost.dll!Microsoft.AspNetCore.TestHost.HttpContextBuilder.SendAsync.AnonymousMethod__0()
                        System.Private.CoreLib.dll!System.Threading.Tasks.Task.InnerInvoke()
                        System.Private.CoreLib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state)
                        System.Private.CoreLib.dll!System.Threading.Tasks.Task.ExecuteWithThreadLocal(ref System.Threading.Tasks.Task currentTaskSlot)
                        System.Private.CoreLib.dll!System.Threading.ThreadPoolWorkQueue.Dispatch()

Not Flagged     1496    10  Worker Thread   Worker Thread   Microsoft.AspNetCore.Routing.dll!Microsoft.AspNetCore.Builder.RouterMiddleware.Invoke
                        Microsoft.AspNetCore.Routing.dll!Microsoft.AspNetCore.Builder.RouterMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext httpContext)
                        ShibbolethAttributes.Service.dll!RoleManager.Service.Middleware.ApiKeyHandlerMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 38
                        Microsoft.AspNetCore.Diagnostics.dll!Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context)
                        System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncMethodBuilderCore.Startd__7>(ref Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.d__7 stateMachine)
                        Microsoft.AspNetCore.Diagnostics.dll!Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context)
                        Microsoft.AspNetCore.Hosting.dll!Microsoft.AspNetCore.Hosting.Internal.HostingApplication.ProcessRequestAsync(Microsoft.AspNetCore.Hosting.Internal.HostingApplication.Context context)
                        Microsoft.AspNetCore.TestHost.dll!Microsoft.AspNetCore.TestHost.TestServer.ApplicationWrapper.ProcessRequestAsync(Microsoft.AspNetCore.Hosting.Internal.HostingApplication.Context context)
                        Microsoft.AspNetCore.TestHost.dll!Microsoft.AspNetCore.TestHost.HttpContextBuilder.SendAsync.AnonymousMethod__0()
                        System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncMethodBuilderCore.Startc__DisplayClass10_0.b__0>d stateMachine)
                        Microsoft.AspNetCore.TestHost.dll!Microsoft.AspNetCore.TestHost.HttpContextBuilder.SendAsync.AnonymousMethod__0()
                        System.Private.CoreLib.dll!System.Threading.Tasks.Task.InnerInvoke()
                        System.Private.CoreLib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state)
                        System.Private.CoreLib.dll!System.Threading.Tasks.Task.ExecuteWithThreadLocal(ref System.Threading.Tasks.Task currentTaskSlot)
                        System.Private.CoreLib.dll!System.Threading.ThreadPoolWorkQueue.Dispatch()

Answer 1

（结论总结，针对以后的任何观看者）。

您没有在任何地方进行阻止，因此我能想到的唯一可能的解释是，至少有一个请求没有完成。

（如果您正在同步等待Task完成，那么我很可能会陷入僵局，因为您没有使用ConfigureAwait(false)，但是因为您只await个任务，我很确定这不是原因）。

鉴于您的请求在单独运行时成功完成，这意味着在并行运行多个请求时可能存在一些并发问题，这可能与_httpClient或服务器有关请求是针对（如果它们是针对真实服务器运行的）。

鉴于您的“任务”列表没有显示任何有趣的内容，我倾向于认为其中一个请求已同步阻止了调用该线程的线程。

仅查看其中一个请求是否已被同步阻止。打开“线程”窗口，然后依次双击每个线程。大多数不会做任何事情，但是至少有一个可能正在运行您的代码，或者正在运行从您的代码中调用的方法。查看调用堆栈以尝试了解发生了什么。您可以双击调用堆栈中的条目以在每个点检查作用域中的变量。

等待Task.WhenAll（）似乎处于死锁状态

1 个答案: