deadlock on UnixStream pair with ends in separate threads

[dwrensha]

In the following program, I expect the counter ii to grow without bound, as the two threads ping-pong a single byte back and forth:

use futures::{AsyncReadExt, AsyncWriteExt};

fn main() {
    let (mut client_stream, mut server_stream) =
        async_std::os::unix::net::UnixStream::pair().expect("socket pair");

    std::thread::spawn(move || {
        let r: Result<(), Box<dyn std::error::Error>> = futures::executor::block_on(async move {
            loop {
                let mut buf = [0u8];
                server_stream.read_exact(&mut buf[..]).await?;
                server_stream.write_all(&buf).await?;
            }
        });
        r.expect("server");
    });


    let r: Result<(), Box<dyn std::error::Error>> = futures::executor::block_on(async move {
        let mut ii = 0;
        loop {
            dbg!(ii);
            let mut buf = [11u8];
            client_stream.write_all(&buf).await?;
            client_stream.read_exact(&mut buf[..]).await?;
            ii += 1;
        }
    });
    r.expect("client");
}

When I run the program, I find that, contrary to my expectations, it deadlocks after about a second, with ii only climbing to a few thousand.

I'm using the latest nightly on Linux:

$ rustc -vV
rustc 1.39.0-nightly (f0b58fcf0 2019-09-11)
binary: rustc
commit-hash: f0b58fcf03391a91f74224fe38a696d5a5b789d9
commit-date: 2019-09-11
host: x86_64-unknown-linux-gnu
release: 1.39.0-nightly
LLVM version: 9.0

The deadlock also occurs on MacOS.

Thanks for the report!

I can reproduce the problem, have identified the bug, and came up with a fix. Will publish a PR soon!