6 min
Async fn in trait... not
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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!
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Here's another article just for you:
1 hour 3 min
Hi everyone. Has it been two months since I last posted something? Yes it has!
That seems like a nice round duration, so let's break the silence with a few announcements.
I have a new website
If everything goes well, you're on it right now.
Does it feel okay? Take a minute to accustom yourself to your new surroundings. Identify potential sources of fresh water. Gather some supplies with which to fashion a makeshift shelter.