write_fmt fails inside spawned task

[yoshuawuyts]

It turns out that Arguments is non-Send, which means passing it to a spawned Future may not work because the future itself is Send.

use async_std::task;
use async_std::prelude::*;
use async_std::io;

fn main() {
    task::block_on(async {
        task::spawn(async {
            // This doesn't compile
            io::stderr().write_fmt(format_args!("hello")).await?;
        }).await
    })
}

This is causing some bugs. Posting this for future reference.

As a temporary hack, we could write first to a String and the write that String into the writer.

[yoshuawuyts]

Oh lol, I like that. Sounds good!

[yoshuawuyts]

Going to mark this as "good first issue" so someone can pick this up!

[Michael-J-Ward]

As a temporary hack, we could write first to a String and the write that String into the writer.

Isn't that what it's already doing?

async-std/src/io/write/mod.rs

Lines 239 to 246 in 00d9364

	// In order to not have to implement an async version of `fmt` including private types
	// and all, we convert `Arguments` to a `Result<Vec<u8>>` and pass that to the Future.
	// Doing an owned conversion saves us from juggling references.
	let mut string = String::new();
	let res = std::fmt::write(&mut string, fmt)
	.map(|_| string.into_bytes())
	.map_err(|_| io::Error::new(io::ErrorKind::Other, "formatter error"));
	WriteFmtFuture { writer: self, res: Some(res), buffer: None, amt: 0 }

[Michael-J-Ward]

Or do you mean to change the signature of write_fmt so it takes a String instead of an Arguments like this, which I have working:

use async_std::task;
use async_std::prelude::*;
use async_std::io;

fn main() {
    femme::start(log::LevelFilter::Trace).unwrap();

    task::block_on(async {
        task::spawn(async {
            let error_msg = format!("hello errors");
            io::stderr().write_fmt(error_msg).await;
        }).await
    });
}

[yoshuawuyts]

@Michael-J-Ward dang, you're right. Okay this may not be as straight forward as we'd like :/