tokio: introduce io::Duplex

[seanmonstar]

DuplexStream provides a bidirectional type that can be used to simulate IO, but over an in-process piece of memory.

I've seen implementations of this rewritten over and over in codebases. They are frequently used in testing situations, where something like tokio_test::io wouldn't work (not trying to assert exact read and write calls, but just join a client and server in a single process).

[davidbarsky]

Compilation failures aside, I'm a big +1 for this change. I've written one of these before and they're super handy.

One thing that could be useful (in the future) is exposing some mechanism to drop writes, pseudo-randomly or whatever.

[davidbarsky]

Not sure how fancy you want to get Sean, but feel free to borrow as much of https://github.com/davidbarsky/stuff/blob/7cf9aa9684da4e3fffc7cdb5556bb65fb7d29af5/src/sim_stream.rs#L1 as you'd like.

[seanmonstar]

Oh cool!

[davidbarsky]

(I do think the naming on your PR is better, however.)

[carllerche]

How does this relate to https://github.com/tokio-rs/tokio/blob/master/tokio-test/src/io.rs

[seanmonstar]

As an update, this should go in tokio::io, and the constructor should be a free function. SomethingStream seems better than Pipe, since they are bidirectional. Maybe MemStream and tokio::io::mem() are less stellar names, though.

[davidbarsky]

@carllerche

How does this relate to https://github.com/tokio-rs/tokio/blob/master/tokio-test/src/io.rs?

MemStream is complimentary to that. The existing mock is great for asserting that certain read/write actions happened, while MemStream-style APIs are really handy for testing interactions between a client and a server together, in memory. Without a MemStream-style API, people (including me and in linkerd2-proxy!) ended up using the OS' loopback interface and port 0 to grab an unused port.

[LucioFranco]

I didn't make my way through the whole thing since I have to head out but super excited for this!!!

[seanmonstar]

Another naming option is Duplex.

tokio::io::duplex() -> (Duplex, Duplex)

[Matthias247]

My super old C# implementation of that was called BufferedPipe: https://github.com/Matthias247/http2dotnet/blob/master/Http2Tests/BufferedPipe.cs

Actually it was only a unidirectional stream, and I simply created 2 of those where it was required.
That could also be an option, since not all IO has bidirectional channels.

You can use this to model a unidirectional OS pipe, or a unidrectional QUIC channel, etc.

[seanmonstar]

Update: renamed to DuplexStream, added a maximum buffer size, constructor is tokio::io::duplex(max_buf_size) -> (DuplexStream, DuplexStream).

I have no idea why CI is blowing up. The tests run locally for me, and if I make the changes it says in CI, it produces warnings locally. Wat?

[taiki-e]

CI failure is rust-lang/rust#73592. I'd suggest to add mut and #[allow(unused_mut)] to warned functions for compatibility with older compilers.

[carllerche]

LGTM. Thanks.