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IHost.RunAsync()永远不会返回

c#.net-core-3.0
7
我正在构建一个.NET Core 3.1应用程序，将在Docker容器中运行BackgroundService。虽然我已经实现了BackgroundService的启动和关闭任务，并且当通过SIGTERM触发时服务绝对会关闭，但我发现await host.RunAsync()调用从未完成 - 这意味着我Main()块中剩余的代码没有被执行。

我是否遗漏了一些内容？还是说我不应该期望在后台服务完成停止后RunAsync()调用返回控制权?

（使用最简单的复现方式进行更新...）

    using System;
    using System.Threading;
    using System.Threading.Tasks;
    using Microsoft.Extensions.DependencyInjection;
    using Microsoft.Extensions.Hosting;

    namespace BackgroundServiceTest
    {
        class Program
        {
            static async Task Main(string[] args)
            {
                Console.WriteLine("Main: starting");
                try
                {
                    using var host = CreateHostBuilder(args).Build();

                    Console.WriteLine("Main: Waiting for RunAsync to complete");

                    await host.RunAsync();

                    Console.WriteLine("Main: RunAsync has completed");
                }
                finally
                {
                    Console.WriteLine("Main: stopping");
                }
            }

            public static IHostBuilder CreateHostBuilder(string[] args) =>
                Host.CreateDefaultBuilder(args)
                    .UseConsoleLifetime()
                    .ConfigureServices((hostContext, services) =>
                    {
                        services.AddHostedService<Worker>();

                        // give the service 120 seconds to shut down gracefully before whacking it forcefully
                        services.Configure<HostOptions>(options => options.ShutdownTimeout = TimeSpan.FromSeconds(120));
                    });

        }

        class Worker : BackgroundService
        {
            protected override async Task ExecuteAsync(CancellationToken stoppingToken)
            {
                Console.WriteLine("Worker: ExecuteAsync called...");
                try
                {
                    while (!stoppingToken.IsCancellationRequested)
                    {
                        await Task.Delay(TimeSpan.FromSeconds(10), stoppingToken);
                        Console.WriteLine("Worker: ExecuteAsync is still running...");
                    }
                }
                catch (OperationCanceledException) // will get thrown if TaskDelay() gets cancelled by stoppingToken
                {
                    Console.WriteLine("Worker: OperationCanceledException caught...");
                }
                finally
                {
                    Console.WriteLine("Worker: ExecuteAsync is terminating...");
                }
            }

            public override Task StartAsync(CancellationToken cancellationToken)
            {
                Console.WriteLine("Worker: StartAsync called...");
                return base.StartAsync(cancellationToken);
            }

            public override async Task StopAsync(CancellationToken cancellationToken)
            {
                Console.WriteLine("Worker: StopAsync called...");
                await base.StopAsync(cancellationToken);
            }

            public override void Dispose()
            {
                Console.WriteLine("Worker: Dispose called...");
                base.Dispose();
            }
        }
    }

Dockerfile：

    #See https://aka.ms/containerfastmode to understand how Visual Studio uses this Dockerfile to build your images for faster debugging.

    FROM mcr.microsoft.com/dotnet/core/runtime:3.1-buster-slim AS base
    WORKDIR /app

    FROM mcr.microsoft.com/dotnet/core/sdk:3.1-buster AS build
    WORKDIR /src
    COPY ["BackgroundServiceTest.csproj", "./"]
    RUN dotnet restore "BackgroundServiceTest.csproj"
    COPY . .
    WORKDIR "/src/"
    RUN dotnet build "BackgroundServiceTest.csproj" -c Release -o /app/build

    FROM build AS publish
    RUN dotnet publish "BackgroundServiceTest.csproj" -c Release -o /app/publish

    FROM base AS final
    WORKDIR /app
    COPY --from=publish /app/publish .
    ENTRYPOINT ["dotnet", "BackgroundServiceTest.dll"]

docker-compose.yml:

    version: '3.4'

    services:
      backgroundservicetest:
        image: ${DOCKER_REGISTRY-}backgroundservicetest
        build:
          context: .
          dockerfile: Dockerfile

通过 docker-compose up --build 命令来运行，然后在第二个窗口中运行命令 docker stop -t 90 backgroundservicetest_backgroundservicetest_1

控制台输出显示 Worker 关闭并被销毁，但应用程序（显然）在 RunAsync() 返回之前终止。

    Successfully built 3aa605d4798f
    Successfully tagged backgroundservicetest:latest
    Recreating backgroundservicetest_backgroundservicetest_1 ... done
    Attaching to backgroundservicetest_backgroundservicetest_1
    backgroundservicetest_1  | Main: starting
    backgroundservicetest_1  | Main: Waiting for RunAsync to complete
    backgroundservicetest_1  | Worker: StartAsync called...
    backgroundservicetest_1  | Worker: ExecuteAsync called...
    backgroundservicetest_1  | info: Microsoft.Hosting.Lifetime[0]
    backgroundservicetest_1  |       Application started. Press Ctrl+C to shut down.
    backgroundservicetest_1  | info: Microsoft.Hosting.Lifetime[0]
    backgroundservicetest_1  |       Hosting environment: Production
    backgroundservicetest_1  | info: Microsoft.Hosting.Lifetime[0]
    backgroundservicetest_1  |       Content root path: /app
    backgroundservicetest_1  | Worker: ExecuteAsync is still running...
    backgroundservicetest_1  | Worker: ExecuteAsync is still running...
    backgroundservicetest_1  | info: Microsoft.Hosting.Lifetime[0]
    backgroundservicetest_1  |       Application is shutting down...
    backgroundservicetest_1  | Worker: StopAsync called...
    backgroundservicetest_1  | Worker: OperationCanceledException caught...
    backgroundservicetest_1  | Worker: ExecuteAsync is terminating...
    backgroundservicetest_1  | Worker: Dispose called...
    backgroundservicetest_backgroundservicetest_1 exited with code 0
你可以通过按下 CTRL+C 在本地进行测试吗？如果它没有停止，很可能是你的某个资源无法停止。这可能是因为你已经启动了一个前台线程。 - Stefan
@Stefan，BackgroundService按预期完成了关闭。但是Main()中剩余的代码从未执行。 - Mr. T
1
请问您能否分享一下 Worker 的代码？也许有些地方它没有处理取消操作。为了实验，请尝试将其注释掉。 - Guru Stron
2个回答
8

在Github上进行了一次长时间的讨论之后，结果发现一些小的重构就能解决问题。简而言之，.RunAsync()会阻塞直到主机完成并处理主机实例，这将（显然）终止应用程序。

通过更改代码以调用.StartAsync()，然后await host.WaitForShutdownAsync()，可以如预期地将控制返回到Main()中。最后一步是在finally块中处置主机，如下所示:

static async Task Main(string[] args)
{
    Console.WriteLine("Main: starting");
    IHost host = null;
    try
    {
        host = CreateHostBuilder(args).Build();

        Console.WriteLine("Main: Waiting for RunAsync to complete");
        await host.StartAsync();

        await host.WaitForShutdownAsync();

        Console.WriteLine("Main: RunAsync has completed");
    }
    finally
    {
        Console.WriteLine("Main: stopping");

        if (host is IAsyncDisposable d) await d.DisposeAsync();
    }
}
感谢您的跟进。 - Stefan
不确定 .NET 7 是否有所改变。但是这个解决方案不起作用。 - Phil Wright
这在net7上运行。 - ScottDPJ
你不能只是将主机包装在 using 语句中，这样就不需要 finally/dispose 代码吗？ - Kevin Cook
3

应该使用 RunConsoleAsync 来代替 RunAsync。只有 RunConsoleAsync 才监听 Ctrl+C 或 SIGTERM 信号：

RunConsoleAsync 启用控制台支持，构建和启动主机，并等待 Ctrl+C / SIGINT 或 SIGTERM 以关闭。

代码应更改为：

await CreateHostBuilder(args).RunConsoleAsync();

这相当于在构建主机生成器之前调用UseConsoleLifeTime：


var host=CreateHostBuilder(args).UseConsoleLifetime().Build();
...
await host.RunAsync();
1
我已经在主机构建器上调用了.UseConsoleLifetime()。应用程序可以很好地响应SIGTERM - 只是从.RunAsync()中永远不会返回。 - Mr. T
@Mr.T 在哪里？...哦，在那边。你试过在构建之前调用UseConsoleLifeTime()吗？调用的顺序很重要。我不确定源代码在哪里，但如果它尝试使用例如 DI 服务或主机 DI 容器，并且稍后被替换，它将无法工作。 - Panagiotis Kanavos
@Mr.T，它确实如此 - 这段源代码已经移动到另一个仓库，但它表明UseConsoleLifeTime()注册了一个单例的ConsoleLifeTime服务。 - Panagiotis Kanavos
我将代码更改为await CreateHostBuilder(args).RunConsoleAsync() - 行为没有变化。 - Mr. T
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