Async fn in trait... not

From the series Updating fasterthanli.me for 2022

Nov 15, 2022 3 min #rust · #async-fn-in-trait · #ftl-infra

Thanks to my sponsors: Marty Penner, David White, Ryan, Brandon Piña, callym, Zeeger Lubsen, prairiewolf, 0lach, Kyle Lacy, Twan Walpot, Julian Schmid, Julien Roncaglia, Mark, Nicholas, Justy, traxys, Alan O'Donnell, Corey Alexander, jatescher, Raphaël Thériault and 254 more

👋 This page was last updated ~3 years ago. Just so you know.

Async fn in trait… not

I was planning on showing the in-progress async_fn_in_trait feature in the context of my website, but it turns out, I can’t!

My website uses two databases: one local SQLite database for content, and a shared Postgres database for user credentials, preferences etc. Migrations are run on startup, and each migration implements one of the following traits:



#[async_trait::async_trait]
pub trait MigrationSqlite {
    fn tag(&self) -> &'static str;
    async fn up(&self, db: &mut Transaction<Sqlite>) -> Result<()>;
}

#[async_trait::async_trait]
pub trait MigrationPostgres {
    fn tag(&self) -> &'static str;
    async fn up(&self, db: &mut Transaction<Postgres>) -> Result<()>;
}

…which can have async functions thanks to the async-trait crate.

Enabling the unstable feature works for the trait definition:



#![allow(incomplete_features)]
#![feature(async_fn_in_trait)]

pub trait MigrationPostgres {
    fn tag(&self) -> &'static str;
    async fn up(&self, db: &mut Transaction<Postgres>) -> Result<()>;
}

…but fails later, because I’m making a Vec of migrations, to pass to:



pub async fn migrate_all_postgres(
    db: &PgPool,
    wanted_list: Vec<Box<dyn MigrationPostgres>>,
) -> Result<()> {
    // (cut)
}

…and traits with async fns (the real thing) aren’t trait-object-safe for now:



$ cargo check
    Checking futile-db v0.1.0 (/home/amos/bearcove/futile/crates/futile-db)
    (cut)
error[E0038]: the trait `MigrationPostgres` cannot be made into an object
   --> crates/futile-db/src/lib.rs:64:47
    |
64  | async fn mk_postgres_pool(migrations: Vec<Box<dyn MigrationPostgres>>) -> Result<sqlx::PgPool> {
    |                                               ^^^^^^^^^^^^^^^^^^^^^ `MigrationPostgres` cannot be made into an object
    |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
   --> crates/futile-db/src/migrations/mod.rs:135:14
    |
133 | pub trait MigrationPostgres {
    |           ----------------- this trait cannot be made into an object...
134 |     fn tag(&self) -> &'static str;
135 |     async fn up(&self, db: &mut Transaction<Postgres>) -> Result<()>;
    |              ^^ ...because method `up` references an `impl Trait` type in its return type
    = help: consider moving `up` to another trait

error[E0038]: the trait `MigrationPostgres` cannot be made into an object
   --> crates/futile-db/src/migrations/mod.rs:71:26
    |
71  |     wanted_list: Vec<Box<dyn MigrationPostgres>>,
    |                          ^^^^^^^^^^^^^^^^^^^^^ `MigrationPostgres` cannot be made into an object
    |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
   --> crates/futile-db/src/migrations/mod.rs:135:14
    |
133 | pub trait MigrationPostgres {
    |           ----------------- this trait cannot be made into an object...
134 |     fn tag(&self) -> &'static str;
135 |     async fn up(&self, db: &mut Transaction<Postgres>) -> Result<()>;
    |              ^^ ...because method `up` references an `impl Trait` type in its return type
    = help: consider moving `up` to another trait

(cut)

There is a very good reason for that limitation, and I hear, to lift it, something like dyn* (which is very hard to search for) is needed.

So, instead, I’ll show them what I did for hring, to get an “owned” version of the AsyncRead / AsyncWrite traits, that is io-uring-friendly:



pub trait ReadOwned {
    async fn read<B: IoBufMut>(&self, buf: B) -> BufResult<usize, B>;
}

pub trait WriteOwned {
    async fn write<B: IoBuf>(&self, buf: B) -> BufResult<usize, B>;

    async fn writev<B: IoBuf>(&self, list: Vec<B>) -> BufResult<usize, Vec<B>> {
        let mut out_list = Vec::with_capacity(list.len());
        let mut list = list.into_iter();
        let mut total = 0;

        while let Some(buf) = list.next() {
            let (res, buf) = self.write(buf).await;
            out_list.push(buf);
            match res {
                Ok(n) => total += n,
                Err(e) => {
                    out_list.extend(list);
                    return (Err(e), out_list);
                }
            }
        }

        (Ok(total), out_list)
    }

    async fn write_all<B: IoBuf>(&self, mut buf: B) -> BufResult<(), B> {
        let mut written = 0;
        let len = buf.bytes_init();
        while written < len {
            let (res, slice) = self.write(buf.slice(written..len)).await;
            buf = slice.into_inner();
            let n = match res {
                Ok(n) => n,
                Err(e) => return (Err(e), buf),
            };
            written += n;
        }
        (Ok(()), buf)
    }
}

pub trait ReadWriteOwned: ReadOwned + WriteOwned {}
impl<T> ReadWriteOwned for T where T: ReadOwned + WriteOwned {}

This all works flawlessly. Here’s the implementation for tokio-uring’s TcpStream type:



impl ReadOwned for TcpStream {
    async fn read<B: IoBufMut>(&self, buf: B) -> BufResult<usize, B> {
        TcpStream::read(self, buf).await
    }
}

impl WriteOwned for TcpStream {
    async fn write<B: IoBuf>(&self, buf: B) -> BufResult<usize, B> {
        TcpStream::write(self, buf).await
    }

    async fn writev<B: IoBuf>(&self, list: Vec<B>) -> BufResult<usize, Vec<B>> {
        TcpStream::writev(self, list).await
    }
}

(It’s trivial, since the trait is closely modelled after tokio-uring’s own types).

Object trait safety is still an issue here, but we can work around it by making most functions take an &impl ReadOwned instead of a &dyn ReadOwned, which makes the function generic (which means it’ll be monomorphized, which in turns means the binary will be larger, but there won’t be dynamic dispatch at all):



/// Returns `None` on EOF, error if partially parsed message.
pub(crate) async fn read_and_parse<Parser, Output>(
    parser: Parser,
    //        👇
    stream: &impl ReadOwned,
    mut buf: RollMut,
    max_len: usize,
) -> eyre::Result<Option<(RollMut, Output)>>
where
    Parser: Fn(Roll) -> IResult<Roll, Output>,
{
    // cut
}

The rest of the hring API is carefully designed to work around that limitation for now, which is… a bit of a balancing act, but it works!

You're reading the Updating fasterthanli.me for 2022 series.

1

2

3

4

5

6

7

8

(JavaScript is required to see this. Or maybe my stuff broke)

Did you know I also make videos? Check them out on PeerTube and also YouTube!

Here's another article just for you:

Some mistakes Rust doesn't catch

Feb 07, 2022

51 min #rust · #golang · #concurrency

I still get excited about programming languages. But these days, it’s not so much because of what they let me do, but rather what they don’t let me do.

Ultimately, what you can with a programming language is seldom limited by the language itself: there’s nothing you can do in C++ that you can’t do in C, given infinite time.

As long as a language is turing-complete and compiles down to assembly, no matter the interface, it’s the same machine you’re talking to. You’re limited by… what your hardware can do, how much memory it has (and how fast it is), what kind of peripherals are plugged into it, and so on.