Unless you’ve been living under a rock; you know async/await is coming to rust stable. My last post was about implementing a simple protocol using manual futures, and interacting with tokio. It’s only fitting, then, that I update the lib that post was inspired by to async/await and report back on my findings. If you’re curious about my library or you use the window manager i3, it’s available here or on crates under tokio-i3ipc. The version discussed here is currently unpublished, I’m waiting for the syntax to be released on stable and tokio to push their new version.

Background

Suffice to say, much has been said about the foundations of the async ecosystem. The high level overview is that there is a Future trait that provides the method poll that returns the enum Poll::Ready / Poll::Pending . Just like how synchronous io in Rust is built on io::Read / io::Write , the asynchronous world is built on AsyncRead / AsyncWrite , each of which have methods like poll_read or poll_write (respectively) that return a type which can be poll ed. That probably didn’t make any sense unless you were already familiar with the ecosystem, so I’m assuming some previous knowledge here (and completely skipping Pin )!

Protocol refresher

The protocol is i3’s IPC. It is text-based and communicates over a unix socket. The basic format is:

"i3-ipc"<payload len (u32)><message type (u32)><json encoded payload>

Updating hand implemented futures

I originally thought I would just take some of the manual futures I had implemented, update them to the new std::futures format and be done with it. Doing something like that would turn this (0.1 futures):

use futures :: { try_ready , Async , Future , Poll }; use serde :: de :: DeserializeOwned ; use std :: { io as stio , marker :: PhantomData }; use tokio :: io :: { self as tio , AsyncRead }; impl < D , S > Future for I3Msg < D , S > where S : AsyncRead , D : DeserializeOwned , { type Item = MsgResponse < D > ; type Error = stio :: Error ; fn poll ( & mut self ) -> Poll < Self :: Item , Self :: Error > { let mut buf = [ 0_ u8 ; 14 ]; let ( rd , init ) = try_ready ! ( tio :: read_exact ( & mut self . stream , & mut buf ). poll ()); if & init [ 0 .. 6 ] != MAGIC . as_bytes () { panic ! ( "Magic str not received" ); } let payload_len = u32 :: from_ne_bytes ([ init [ 6 ], init [ 7 ], init [ 8 ], init [ 9 ]]) as usize ; let msg_type = u32 :: from_ne_bytes ([ init [ 10 ], init [ 11 ], init [ 12 ], init [ 13 ]]); let mut buf = vec ! [ 0_ u8 ; payload_len ]; let ( _rdr , payload ) = try_ready ! ( tio :: read_exact ( rd , & mut buf ). poll ()); Ok ( Async :: Ready ( MsgResponse { msg_type : msg_type . into (), body : serde_json :: from_slice ( & payload [..]) ? , })) } }

Using a more or less direct translation would lead to (futures-preview):

use futures :: { future :: Future , ready , Poll }; use serde :: de :: DeserializeOwned ; use std :: { io as stio , marker :: PhantomData , pin :: Pin , task :: Context }; use tokio :: io :: { self as tio , AsyncRead , AsyncReadExt }; impl < D , S > Future for I3Msg < D , S > where S : AsyncRead + AsyncReadExt + Unpin , D : DeserializeOwned , { type Output = stio :: Result < MsgResponse < D >> ; fn poll ( mut self : Pin <& mut Self > , ctx : & mut Context < '_ > ) -> Poll < Self :: Output > { let mut init = [ 0_ u8 ; 14 ]; let len_read = ready ! ( & mut self . stream . read_exact ( & mut init ). poll ( ctx )); if & init [ 0 .. 6 ] != MAGIC . as_bytes () { panic ! ( "Magic str not received" ); } let payload_len = u32 :: from_ne_bytes ([ init [ 6 ], init [ 7 ], init [ 8 ], init [ 9 ]]) as usize ; let msg_type = u32 :: from_ne_bytes ([ init [ 10 ], init [ 11 ], init [ 12 ], init [ 13 ]]); let mut buf = vec ! [ 0_ u8 ; payload_len ]; let payload_len = ready ! ( self . stream . read_exact ( & mut buf ). poll ( ctx )); Poll :: Ready ( Ok ( MsgResponse { msg_type : msg_type . into (), body : serde_json :: from_slice ( & buf [..]) ? , })) } }

There’s a couple important things to note here. Future::poll no longer has associated types Item and Error . Instead, there is now a single type Output that if you want to represent something that can fail, should return a Result .

The Poll type synonym has changed; gone is the Async type, it’s been replaced by the enum Poll , with variants Ready or Pending .

You may have noticed that read_exact when polled, no longer returns a tuple containing a AsyncRead er and a buffer that was written to. In futures 0.1, you had to do this dance with things that implemented AsyncRead / AsyncWrite and effectively thread instances of the stream through your application. You don’t seem to have to do that anymore, you can call read_exact on stream itself, and you don’t need to get back a new reference to it.

I abandonded this path pretty quickly. For one, it wasn’t bearing much fruit. It’s a little nicer, but writing futures is still sisyphean. Perhaps most importantly, it didn’t improve the API of the end product very much. In trying to understand some of the changes to the Future trait, I spent some time reading the stdlib definitions for Pin, and Unpin. The Pin docs are very good and I recommend re-reading them a few times and experimenting with Pin ‘ing something. Try to make a self-referencing type like they describe in the docs.

Embracing the syntax

I have a synchronous version of this protocol, and after playing around with async/await for a little bit, I realized I could just copy my sync API and add the async keyword and pretty much call it a day.

I deleted all my manually implemented futures and the above turned into:

#[derive(Debug)] pub struct I3 { stream : UnixStream , } impl I3 { async fn _decode_msg ( & mut self ) -> io :: Result < ( u32 , Vec < u8 > ) > { let mut init = [ 0_ u8 ; 14 ]; let _len = self . stream . read_exact ( & mut init ). await ? ; if & init [ 0 .. 6 ] != MAGIC . as_bytes () { panic ! ( "Magic str not received" ); } let payload_len = u32 :: from_ne_bytes ([ init [ 6 ], init [ 7 ], init [ 8 ], init [ 9 ]]) as usize ; let msg_type = u32 :: from_ne_bytes ([ init [ 10 ], init [ 11 ], init [ 12 ], init [ 13 ]]); let mut payload = vec ! [ 0_ u8 ; payload_len ]; let _len_read = self . stream . read_exact ( & mut payload ). await ? ; Ok (( msg_type , payload )) } pub async fn read_msg < D > ( & mut self ) -> io :: Result < MsgResponse < D >> where D : DeserializeOwned , { let ( msg_type , payload ) = self . _decode_msg (). await ? ; Ok ( MsgResponse { msg_type : msg_type . into (), body : serde_json :: from_slice ( & payload [..]) ? , }) } }

I think it’s a huge improvement, both in ease of implementation and the end result of the API it enables. It’s a little less generic, but only because I chose to not parameterize UnixStream above.

That brings me to what I think is the salient thing about this new syntax: it enables a synchronous looking API that is a pleasure to read and write. It gets rid of a lot of the sharp edges in writing futures code (threading streams through combinators, dealing with Poll , and try_ready , etc). Error handling is also, so, so much nicer this time around.

Streams

I haven’t had to manually implement the Stream trait yet, but I do include a codec using tokio’s Decoder trait which enables you to listen to a stream of i3 events from a UnixStream . I haven’t had to modify the code for this at all since pointing to tokio’s master branch. Using it was a bit hairy before:

// code removed let framed = FramedRead :: new ( stream , EventCodec ); let sender = framed . for_each ( move | evt | { // here tx is a futures::mpsc::channel let tx = tx . clone (); tx . send ( evt ) . map ( | _ | ()) . map_err ( | e | io :: Error :: new ( io :: ErrorKind :: BrokenPipe , e )) }) . map_err ( | err | println ! ( "{}" , err )); tokio :: spawn ( sender ); // code removed

tokio’s FramedRead implements the Stream trait, so really all I needed to do was add a method that returned it from my type:

pub fn listen ( self ) -> FramedRead < UnixStream , codec :: EventCodec > { FramedRead :: new ( self . stream , codec :: EventCodec ) }

You can use Stream s in a while let loop

#[tokio::main] async fn main () -> io :: Result < () > { let mut i3 = I3 :: connect (). await ? ; let _resp = i3 . subscribe ([ Subscribe :: Window ]). await ? ; let mut listener = i3 . listen (); while let Some ( event ) = listener . next (). await { println ! ( "{:#?}" , event ? ); } Ok (()) }

The implicit runtimes enabled by proc macros are really a boon to readability.

Conclusion