Dealing with readiness race conditions

[hberntsen]

I'm creating a AsyncWrite + AsyncRead wrapper around a custom socket implementation. I'm now facing the problem of race conditions around using SetReadiness.

The following is going on:

1. I do an async read on my socket via https://github.com/tokio-rs/tokio/blob/tokio-0.2.22/tokio/src/io/poll_evented.rs#L391
2. It calls my implementation of read. The implementation determined it is going to return ErrorKind::WouldBlock.
3. In the meantime, another thread disconnects the socket. It calls set_readiness(Ready::all()).
4. The readiness flag is cleared again when read returns.

Normally, when my custom socket disconnects it will call set_readiness(Ready::all()). The function awaiting a read will then try to read but discovers that the socket is disconnected. In this case the code hangs because the poll_read implementation at https://github.com/tokio-rs/tokio/blob/tokio-0.2.22/tokio/src/io/poll_evented.rs#L391 has cleared the readiness flag.

What is the best way to deal with this situation? Shoudn't poll_read clear the readable flag before it calls read and after read is done set the flag again when the result isn't WouldBlock?

cc @carllerche