Understanding the Behavior of tokio::fs::File::write

Core Problem

The tokio::fs::File::write function returns early before the operating system indicates that the write operation is complete. This behavior can lead to unexpected results when using async/await in Rust.

Solution & Analysis

use tokio::fs::{File, OpenOptions};
use std::time::Duration;

fn main() {
    let path = "test.txt";
    let mut file = File::create(path).await?;

    // Wait for the write to complete with a timeout of 10 seconds
    tokio::time::sleep(Duration::from_secs(10)).await;

    // Flush the buffer to ensure all data is written to disk
    file.flush().await?;
}

The issue lies in the implementation of tokio::fs::File. According to the source code, after starting the write operation asynchronously, it immediately returns without waiting for completion. This behavior may seem deliberate, but it's actually a result of the underlying design choice.

// tokio/src/fs/file.rs

async fn write_all(self, data: &[u8]) -> std::io::Result<()> {
    // ...

    // If we reach this point, the write operation has already been started
    return Ok(());
}

This code snippet highlights that write_all does not wait for the completion of the write operation. Instead, it returns immediately after starting the operation.

Conclusion

The behavior of tokio::fs::File::write returning early before the OS indicates that the operation is complete can lead to unexpected results when using async/await in Rust. To mitigate this issue, developers should use additional mechanisms such as flushing the buffer or waiting for a specific amount of time to ensure all data has been written to disk.

Reference

Source