TPL and the case of the !@#(*@! hung process

time to read 5 min | 814 words

So, here I am writing some really fun code, when I found out that I am running into dead locks in the code. I activate emergency protocols and went into deep debugging mode.

After being really through in figuring out several possible causes, I was still left with what is effectively a WTF @!(*!@ DAMN !(@*#!@* YOU !@*!@( outburst and a sudden longing for something to repeatedly hit.

Eventually, however, I figure out what was going on.

I have the following method: Aggregator.AggregateAsync(), inside which we have a call to the PulseAll method. That method will then go and execute the following code:

   1: public void PulseAll()
   2: {
   3:     Interlocked.Increment(ref state);
   4:     TaskCompletionSource<object> result;
   5:     while (waiters.TryDequeue(out result))
   6:     {
   7:         result.SetResult(null);
   8:     }
   9: }

After that, I return from the method. In another piece of the code (Aggregator.Dispose) I am waiting for the task that is running the AggregateAsync method to complete.

Nothing worked! It took me a while before I figured out that I wanted to check the stack, where I found this:


Basically, I had a dead lock because when I called SetResult on the completion source (which freed the Dispose code to run), I actually switched over to that task and allowed it to run. Still in the same thread, but in a different task, I run through the rest of the code and eventually got to the Aggregator.Dispose(). Now, I could only get to it if it the PulseAll() method was called. But, because we are on the same thread, that task hasn’t been completed yet!

In the end, I “solved” that by introducing a DisposeAsync() method, which allowed us to yield the thread, and then the AggregateAsync task was completed, and then we could move on.

But I am really not very happy about this. Any ideas about proper way to handle async & IDisposable?