Deadlock in GroupByUntil when using with TaskPoolScheduler

[mattpodwysocki]

Deadlock in GroupByUntil when using with TaskPoolScheduler

[mattpodwysocki]

CraigGidney wrote Dec 22, 2013 at 1:23 PM

I peeked at the GroupByUntil source and it's holding locks while sending items:

 ...
                    return;
                }

                lock (base._observer)
                    base._observer.OnNext(group);

                var md = new SingleAssignmentDisposable();
...

I bet that has something to do with it. I copied the relevant source into a project, put some logging around the acquisition of those locks and lo-and-behold:
...
Writing 350
(GroupByUntil near line 160) Requesting writer lock with hash 53969920 on thread 9
(GroupByUntil near line 160) Acquired writer lock with hash 53969920 on thread 9
(GroupByUntil near line 160) Started sending OnNext notification on thread 9
(GroupByUntil near line 220) Requesting _writer lock with hash 53969920 on thread 10
(GroupByUntil near line 220) Acquired _writer lock with hash 55330791 on thread 12
(GroupByUntil near line 220) Started sending OnCompleted notification on thread 12
(GroupByUntil near line 220) Finished sending OnCompleted notification on thread 12
(GroupByUntil near line 220) Released _writer lock with hash 55330791 on thread 12
[program deadlocks here]

See how thread 9 starts to send OnNext but never finishes? It's probably waiting for thread 10 to do something, but thread 10 is waiting for the lock thread 9 has. Thread 12 seems to have gone fine, though.

[bartdesmet]

This has been fixed for the upcoming Rx.NET v2.x release.