SimplexStream does not close other half on drop

[CGamesPlay]

Version
SimplexStream was introduced in v1.40.0.

Platform
Darwin macbook-pro.local 21.6.0 Darwin Kernel Version 21.6.0: Mon Jun 24 00:58:06 PDT 2024; root:xnu-8020.240.18.709.2~1/RELEASE_ARM64_T6000 arm64

Description

SimplexStream (#6589) doesn't close the other half when the ReadHalf or WriteHalf is dropped. This leads to the other half blocking forever waiting for a read/write which will never finish.

Playground link

use tokio::io::AsyncWriteExt;

#[tokio::main]
async fn main() {
    let (rx, mut tx) = tokio::io::simplex(64);
    drop(rx);
    let buf = [0; 1024];
    println!("write starting");
    tx.write_all(&buf).await.unwrap();
    println!("write finished")
}

I expected to see this happen: the write_all future should resolve to an error.

Instead, this happened: the write_all future never resolves.

This is the same in the other direction as well, where the read calls never resolve if the write half is dropped.

[Darksonn]

Problem is we're using ReadHalf/WriteHalf for the simplex stream, and it's hard to change those.

[carllerche]

The straightforward option would be to have ReadHalf/WriteHalf's inner type be an enum of T & Simplex so it would know how to handle the shutdown. Alternatively, Inner could store some sort of fn pointer or some sort of virtual dispatch strategy as a callback that is invoked when the shutdown happens.

[carllerche]

Another strategy would be to deprecate simplex and introduce a new io:: method that fixes the issue

[Darksonn]

I don't love adding an enum to ReadHalf/WriteHalf since we're stuck with the enum forever if we do that. I guess this is a sort of dupe of #6878. We shouldn't have reused it in the signature for simplex.