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c# - 如果在“等待”之后引发任务,则吞下从任务引发的异常

发布于 2020-03-27 10:24:03

我正在使用.net的HostBuilder编写后台服务。我有一个名为MyService的类,该类实现BackgroundService ExecuteAsync方法,并且在那里遇到了一些奇怪的行为。在方法内部,我等待某个任务,并且吞没了等待之后引发的任何异常,但是在等待终止过程之前引发的异常。

我在各种论坛(堆栈溢出,msdn,中等)中都在线查看,但是找不到这种行为的解释。

public class MyService : BackgroundService
    {
        protected override async Task ExecuteAsync(CancellationToken stoppingToken)
        {
            await Task.Delay(500, stoppingToken);
            throw new Exception("oy vey"); // this exception will be swallowed
        }
    }

public class MyService : BackgroundService
    {
        protected override async Task ExecuteAsync(CancellationToken stoppingToken)
        {
            throw new Exception("oy vey"); // this exception will terminate the process
            await Task.Delay(500, stoppingToken);
        }
    }

我希望这两个异常都会终止该过程

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提问者
TheDotFestClub
被浏览
149
Panagiotis Kanavos 2019-07-03 22:44

TL;DR;

Don't let exceptions get out of ExecuteAsync. Handle them, hide them or request an application shutdown explicitly.

Don't wait too long before starting the first asynchronous operation in there either

Explanation

This has little to do with await itself. Exceptions thrown after it will bubble up to the caller. It's the caller that handles them, or not.

ExecuteAsync is a method called by BackgroundService which means any exception raised by the method will be handled by BackgroundService. That code is :

    public virtual Task StartAsync(CancellationToken cancellationToken)
    {
        // Store the task we're executing
        _executingTask = ExecuteAsync(_stoppingCts.Token);

        // If the task is completed then return it, this will bubble cancellation and failure to the caller
        if (_executingTask.IsCompleted)
        {
            return _executingTask;
        }

        // Otherwise it's running
        return Task.CompletedTask;
    }

Nothing awaits the returned task, so nothing is going to throw here. The check for IsCompleted is an optimization that avoids creating the async infrastructure if the task is already complete.

The task won't be checked again until StopAsync is called. That's when any exceptions will be thrown.

    public virtual async Task StopAsync(CancellationToken cancellationToken)
    {
        // Stop called without start
        if (_executingTask == null)
        {
            return;
        }

        try
        {
            // Signal cancellation to the executing method
            _stoppingCts.Cancel();
        }
        finally
        {
            // Wait until the task completes or the stop token triggers
            await Task.WhenAny(_executingTask, Task.Delay(Timeout.Infinite, cancellationToken));
        }

    }

From Service to Host

In turn, the StartAsync method of each service is called by the StartAsync method of the Host implementation. The code reveals what's going on :

    public async Task StartAsync(CancellationToken cancellationToken = default)
    {
        _logger.Starting();

        await _hostLifetime.WaitForStartAsync(cancellationToken);

        cancellationToken.ThrowIfCancellationRequested();
        _hostedServices = Services.GetService<IEnumerable<IHostedService>>();

        foreach (var hostedService in _hostedServices)
        {
            // Fire IHostedService.Start
            await hostedService.StartAsync(cancellationToken).ConfigureAwait(false);
        }

        // Fire IHostApplicationLifetime.Started
        _applicationLifetime?.NotifyStarted();

        _logger.Started();
    }

The interesting part is :

        foreach (var hostedService in _hostedServices)
        {
            // Fire IHostedService.Start
            await hostedService.StartAsync(cancellationToken).ConfigureAwait(false);
        }

All the code up to the first real asynchronous operation runs on the original thread. When the first asynchronous operation is encountered, the original thread is released. Everything after the await will resume once that task completes.

From Host to Main()

The RunAsync() method used in Main() to start the hosted services actually calls the Host's StartAsync but not StopAsync :

    public static async Task RunAsync(this IHost host, CancellationToken token = default)
    {
        try
        {
            await host.StartAsync(token);

            await host.WaitForShutdownAsync(token);
        }
        finally
        {
#if DISPOSE_ASYNC
            if (host is IAsyncDisposable asyncDisposable)
            {
                await asyncDisposable.DisposeAsync();
            }
            else
#endif
            {
                host.Dispose();
            }

        }
    }

This means that any exceptions thrown inside the chain from RunAsync to just before the first async operation will bubble up to the Main() call that starts the hosted services :

await host.RunAsync();

or

await host.RunConsoleAsync();

This means that everything up to the first real await in the list of BackgroundService objects runs on the original thread. Anything thrown there will bring down the application unless handled. Since the IHost.RunAsync() or IHost.StartAsync() are called in Main(), that's where the try/catch blocks should be placed.

This also means that putting slow code before the first real asynchronous operation could delay the entire application.

Everything after that first asynchronous operation will keep running on a threadpool thread. That's why exceptions thrown after that first operation won't bubble up until either the hosted services shut down by calling IHost.StopAsync or any orphaned tasks get GCd

Conclusion

Don't let exceptions escape ExecuteAsync. Catch them and handle them appropriately. The options are :

  • Log and "ignore" them. This will live the BackgroundService inoperative until either the user or some other event calls for an application shutdown. Exiting ExecuteAsync doesn't cause the application to exit.
  • Retry the operation. That's probably the most common option of a simple service.
  • In a queued or timed service, discard the message or event that faulted and move to the next one. That's probably the most resilient option. The faulty message can be inspected, moved to a "dead letter" queue, retried etc.
  • Explicitly ask for a shutdown. To do that, add the IHostedApplicationLifetTime interface as a dependency and call StopAsync from the catch block. This will call StopAsync on all other background services too

Documentation

托管服务和行为BackgroundService中描述实现与IHostedService和BackgroundService类微服务后台任务,并在ASP.NET核心托管服务后台任务

该文档没有解释如果其中一项服务抛出该怎么办。他们通过明确的错误处理演示了特定的使用方案。排队的后台服务示例将丢弃导致故障的消息,并转到下一个消息:

    while (!cancellationToken.IsCancellationRequested)
    {
        var workItem = await TaskQueue.DequeueAsync(cancellationToken);

        try
        {
            await workItem(cancellationToken);
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, 
               $"Error occurred executing {nameof(workItem)}.");
        }
    }