Manually capturing and applying SynchronizationContext when completing a Task

问题

I was having a problem with a hanging await (described here). During research I found out that calling SetResult on my TaskCompletionSource actually invokes awaiting continuation in the context of the thread that called SetResult (this is also spelled out in this answer to a somewhat related question). In my case this is a different thread (a thread-pool worker thread) from the one that started the await (an ASP.NET request thread).

While I'm still not sure why this would cause a hang, I decided to try forcing the SetResult into the original context. I stored the value of SynchronizationContext.Current before entering await on the request thread and manually applied it in the worker thread via SynchronizationContext.SetSynchronizationContext just before calling SetResult. This solved the hang and I can now await all my async methods without having to specify ConfigureAwait(false).

My question is: is this a reasonable and correct approach to manually capturing and applying the SynchronizationContext? FWIW, I tried doing a simple Post() with the SetResult delegate first, but that still caused a hang. I'm obviously a bit out of my comfort zone here... Please help me understand what's going on!

回答1:

SetResult is not guaranteed to call anything. Therefore, this is not reliable.

You need to switch the sync context at the point where it is captured. A common pain point here is WebClient which captures the context when starting a web request. So your code would look like this:

SetContext(newContext);
new WebClient().DownloadAsync(...);
SetContext(oldContext);

Restore the old context to not disturb anything.

In other words the problem is in the continuation code, not in the code calling SetResult.

回答2:

To my embarrassment, I had completely overlooked that my HTTP handler was derived from a small base class, which implemented IAsyncHttpHandler in a very questionable way in order to add support for async handlers:

public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
{
    ...
    var task = HandleRequestAsync(...);

    Task.Run(async () => { await task; }).GetAwaiter().GetResult();
    ...
}

I can't even remember why I did this in the first place (it was over a year ago), but it definitely was THE stupid part I was looking for for the last couple of days!

Changing the handler base class to .NET 4.6's HttpTaskAsyncHandler got rid of the hangs. Sorry for wasting everyone's time! :(

来源：https://stackoverflow.com/questions/41011778/manually-capturing-and-applying-synchronizationcontext-when-completing-a-task