Suspected race condition in Throttle operator

[malcolms]

Hello, we believe we have identified a race condition in the Throttle operator:

Which library version?

Reproduced in 4.1.0 and 5.0.0

What are the platform(s), environment(s) and related component version(s)?

Reproduced with
Windows 10 20H2
.NET 5.0 (with system.reactive 5.0.0)
.NET Framework 4.8 (with system.reactive 4.1.0)

What is the use case or problem?

If a second event arrives at the Throttle(TimeSpan dueTime) operator at almost exactly the same time as a cached event is due to be propagated, then neither will be emitted. I have reproduced this with Throttle(TimeSpan.Zero) but believe that it could happen with any timespan if a second or subsequent event arrives at an inopportune time.

A review of the code suggests the following interaction can occur between Throttle._.OnNext and Throttle._.Propagate

            public override void OnNext(TSource value)
            {
                ulong currentid;

                lock (_gate)
                {
                    _hasValue = true;
                    _value = value;
                    _id = unchecked(_id + 1);
                    currentid = _id;
                }

                _serialCancelable.Disposable = null;
                _serialCancelable.Disposable = _scheduler.ScheduleAction((@this: this, currentid), _dueTime, static tuple => tuple.@this.Propagate(tuple.currentid));
            }

            private void Propagate(ulong currentid)
            {
                lock (_gate)
                {
                    if (_hasValue && _id == currentid)
                    {
                        ForwardOnNext(_value!);
                    }

                    _hasValue = false;
                }

If two items arrive at OnNext in quick succession:

OnNext(1):

• hasValue = true, value = 1, id = 100 (let's say)
• schedules Propagate(1)

OnNext(2):

• hasValue = true, value = 2, id = 101
• schedules Propagate(2)

Propagate(1):

• hasValue is true but id doesn't match so doesn't forward value
• clears hasValue

Propagate(2):

• id matches but hasValue is false, so doesn't forward value

What is the expected outcome?

An event is emitted if no further events are received within the Throttle dueTime.

What is the actual outcome?

No event is emitted

What is the stacktrace of the exception(s) if any?

N/A

Do you have a code snippet or project that reproduces the problem?

Example .NET 5 console app referencing System.Reactive 5.0.0

using System;
using System.Reactive.Disposables;
using System.Reactive.Linq;

while (true)
{
  var source = Observable.Create<int>(o =>
  {
    o.OnNext(1);
    o.OnNext(2);
    return Disposable.Empty;
  });

  var i = 0;
  try
  {
    while (true)
    {
      i++;

      await source
        .Throttle(TimeSpan.Zero)
        .Timeout(TimeSpan.FromMilliseconds(100))
        .Take(1);
    }
  }
  catch (TimeoutException)
  {
    Console.WriteLine($"Failed after {i} iterations");
  }
}

[danielcweber]

Seems like _hasValue = false; should be within the scope of the 'if', or so I suppose...

[malcolms]

Thank you for fixing this so quickly!