Poller's callbacks can be called even after it is stopped

[mrakgr]

Environment

NetMQ Version:    4.0.1.2-pre
Operating System: Windows 10
.NET Version:     .NET Core 3.1

Expected behaviour

Calling Stop renders it inactive.

Actual behaviour

The poller continues working for one more iteration. Unlike for my previous issues, this happens on the first iteration.

Steps to reproduce the behaviour

Run the last example in the UI. What happens is that the two of the workers get printed twice.

In particular, in...

use __ = req.SendReady.Subscribe(fun _ -> 
    req.SendFrame("Hi Boss")
    let msg = req.ReceiveFrameString()
    if msg.ToString() = msg_fired then poller.Stop(); log <| sprintf "Completed %d tasks." !total
    else Thread.Sleep(rnd.Next(1,500)); incr total
    )

...that log gets triggered twice.

This behavior took me completely by surprise. For all I know though, this might be intentional to give a few more messages a chance to be processed. Which case is it?

[stale[bot]]

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