File write just before exit seems racey

[ThinkChaos]

Hi,

I've got an issue with data written to a file right before exiting is lost.
It seems it's a race condition somewhere because, with a lot of tries the data can show up, and adding a small delay solves it consistently.
I'm creating the issue in this repo because I'm not sure which part of the stack is the issue.

Here's a minimal repro (see below for a full crate):

use smol::io::AsyncWriteExt;

fn main() {
    smol::block_on(write_file());
}

async fn write_file() {
    let data = b"Now you see me";
    let mut file = smol::fs::File::create("now-you-dont.txt").await.unwrap();

    file.write_all(data).await.unwrap();
}

Something that seems interesting is that if I use write instead of write_all and output the number of written bytes, it's 14 as expected. Outputting that number using a print adds enough delay for the data to be written, but using the process exit code I get the right number and an empty file.

Here's a repo with a full crate so that all dependencies are pinned: smol-bug-repro.
I tested this using all combinations of:

• rustc
  • 1.76.0 (07dca489a 2024-02-04) (built from a source tarball)
  • 1.77.1 (7cf61ebde 2024-03-27) (built from a source tarball)
• smol
  • 1.3.0
  • 2.0.0

Hopefully you can reproduce, otherwise let me know what I can do to help.

[taiki-e]

This is the intended behavior and is due to the limitations of async rust.

Please read the documentation: https://docs.rs/smol/latest/smol/fs/struct.File.html

NOTE: If writing to a file, make sure to call flush(), sync_data(), or sync_all() before dropping the file or else some written data might get lost!

[ThinkChaos]

Ah right async drop gotcha. Thanks for the quick reply!