> However, as it turns out, `crossbeam-channel`'s channels don't drop
> remaining messages until all associated senders *and* receivers are
> dropped. This means the exit signal won't be delivered as long as
> there's at least one `HangMonitorRegister` or
> `BackgroundHangMonitorChan` maintaining a copy of the sender. To work
> around this and guarantee a rapid delivery of the exit signal, the
> sender is wrapped in `Arc`, and only the worker thread maintains a
> strong reference, thus ensuring both the sender and receiver are
> dropped as soon as the worker thread exits.
> There's a race condition between the reception of
> `BackgroundHangMonitorControlMsg::Exit` and `MonitoredComponentMsg::
> Register`. When the worker receives `Exit`, it stops receiving
> messages, and any remaining messages (including the
> `MonitoredComponentMsg::Register` we sent) in the channel are dropped.
> Wrapping `exit_signal` with this RAII wrapper ensures the exit signal
> is delivered even in such cases.
This should (hopefully) eliminate the intermittent hang-ups in the test
case `test_hang_monitoring_exit_signal` for good.
Bound sampler buffer, add length feedback message, fix intermittence in test
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