StatsdClient should implement RefUnwindSafe

[Diggsey]

All Sync types should also implement RefUnwindSafe as the former implies the latter.

The StatsdClient currently does not implement RefUnwindSafe because it stores two trait objects:

    sink: Arc<MetricSink + Sync + Send>,
    errors: Arc<Fn(MetricError) -> () + Sync + Send>,

The RefUnwindSafe bound should be added to both of these:

    sink: Arc<MetricSink + Sync + Send + RefUnwindSafe>,
    errors: Arc<Fn(MetricError) -> () + Sync + Send + RefUnwindSafe>,

And then the client will automatically become RefUnwindSafe itself.

[56quarters]

Thanks for the issue! I'll start looking into this.

Seems like it may be a little tricky since QueuingMetricSink makes use of a CondVar(1) and a Crossbeam MsQueue(2) which in turn uses UnsafeCell. It'll be interesting to see what comes from the issue you opened in rust-lang/rust#54768

[56quarters]

The two places that prevent the client from being panic safe are both in the QueuingMetricSink.

I've replaced the use of CondVar with an atomic and thread::yield_now() - 36e809d93b64e38ab64ba94ab221a3dc071396a1^...HEAD

The other place that prevents QueuingMetricSink from being panic safe is the use of a crossbeam::MsQueue. I believe it's safe to wrap this in AssertUnwindSafe based on the issue you opened rust-lang/rust#54768. I'm currently in the process of writing a few more tests to increase my comfort with the change.