Race condition in the async code when sender drops too quickly

[inetic]

When using the async code and the Sender sends and gets dropped at the wrong time, the Receiver won't receive the sent value. Here is how the race happens:

1. Let's have a Sender tx and a Receiver rx.
2. User calls rx.recv().await, the RecvFut::poll_inner func goes here. So far so good.
3. The next time RecvFut::poll_inner is called, we get to line 377 where we check whether the sender sent something already. At this point the sender has not and thus we do not proceed to the if block (line 378).
4. But now the sending thread is scheduled, the sender sends a value and tx is dropped.
5. The receiving thread wakes up inside RecvFut::poll_inner, on line 379 it determines that the Sender has been destroyed and Poll::Ready(Err(RecvError::Disconnected)) is returned without ever returning the value from the previous step.

I have a solution which I tested and seems to be working fine: instead of testing if self.receiver.shared.is_disconnected() on line 379 we can just check whether the returned value from step #3 is equal to Err(TryRecvTimeoutError::Disconnected). I'll send a PR.

[zesterer]

Closed by #121