EventPipeEventSource blocks until the first event is read

[davmason]

If you follow the sample code on how to use EventPipeEventSource but replace the provider with a provider that doesn't produce any events, it will block forever in the constructor to EventPipeEventSource.

    var providers = new List<EventPipeProvider>()
    {
        new EventPipeProvider("Microsoft-Windows-DotNETRuntime",
            EventLevel.Informational, (long)ClrTraceEventParser.Keywords.GC)
    };

    var client = new DiagnosticsClient(processId);
    using (var session = client.StartEventPipeSession(providers, false))
    {
        var source = new EventPipeEventSource(session.EventStream);

        source.Clr.All += (TraceEvent obj) => {
            Console.WriteLine(obj.EventName);
        }
        try
        {
            source.Process();
        }
        // NOTE: This exception does not currently exist. It is something that needs to be added to TraceEvent.
        catch (EventStreamException e)
        {
            Console.WriteLine("Error encountered while processing events");
            Console.WriteLine(e.ToString());
        }
    }

It will block on this line: https://github.com/microsoft/perfview/blob/cb1943b8ded584e136e140746e5771ca63668c5a/src/TraceEvent/EventPipe/EventPipeEventSource.cs#L68

This seems like a pitfall, and it's not clear what is happening. I've used that code pattern lots before and it's never failed, then all of a sudden it hangs my app and I have to do a lot of digging to understand why. I imagine customers will run in to it as well.

[davmason]

cc @sywhang

[Wojmik]

I have the same issue. Even in first sample code: https://github.com/dotnet/diagnostics/blob/master/documentation/design-docs/diagnostics-client-library.md
If you debug line by line, it can be saw that it hangs for a while on EventPipeEventSource constructor.
In the real application it is very hard even to Dispose, because constructor hangs, so you don't have reference to Dispose on.

[sywhang]

@davmason I think a better place for this issue is the perfview repo since that's where TraceEvent and the implementation of EventPipeEventSource lives. For some reason it seems like I can't transfer this issue to PerfView repo so I just filed microsoft/perfview#1172 to track it there.