tokio-rs/tokio · error · io::Error

background task failed

Error message

background task failed

What it means

Runtime error from `tokio::fs::asyncify` (fs/mod.rs:327), which backs `File::open`, `create`, `metadata`, and other blocking-syscall wrappers. If `spawn_blocking(f).await` yields a `JoinError` (the task panicked, was cancelled, or the runtime is shutting down), it is converted to `io::ErrorKind::Other` with this message.

Source

Thrown at tokio/src/fs/mod.rs:327

    pub(crate) use self::open_options::UringOpenOptions;
}

use std::io;

#[cfg(not(test))]
use crate::blocking::spawn_blocking;
#[cfg(test)]
use mocks::spawn_blocking;

pub(crate) async fn asyncify<F, T>(f: F) -> io::Result<T>
where
    F: FnOnce() -> io::Result<T> + Send + 'static,
    T: Send + 'static,
{
    match spawn_blocking(f).await {
        Ok(res) => res,
        Err(_) => Err(io::Error::new(
            io::ErrorKind::Other,
            "background task failed",
        )),
    }
}

View on GitHub (pinned to 625954f365)

Solutions

1. Keep the runtime alive until all `tokio::fs` operations resolve.
2. Do not panic inside closures that run via `asyncify`/`spawn_blocking`.
3. Run all `tokio::fs` calls within the runtime context (`#[tokio::main]` or a runtime you drive).
4. Handle `io::ErrorKind::Other` from `asyncify`-backed calls as a possible shutdown/panic signal and propagate.

Example fix

// before: fs call after the runtime is gone
let f = tokio::fs::File::open("p").await?; // 'background task failed' if runtime dropped

// after: keep fs calls inside the live runtime and handle shutdown
async fn run() -> io::Result<()> {
    let _f = tokio::fs::File::open("p").await.map_err(|e| {
        if e.kind() == io::ErrorKind::Other {
            io::Error::new(e.kind(), "fs operation failed (runtime shutdown?)")
        } else { e }
    })?;
    Ok(())
}

fn is_bg_task_failed(e: &io::Error) -> bool {
    e.kind() == io::ErrorKind::Other && e.to_string() == "background task failed"
}

let f = tokio::fs::File::open("p").await.map_err(|e| {
    if is_bg_task_failed(&e) { anyhow!("fs operation failed: runtime shutdown or blocking task panic") } else { e.into() }
})?;

When it happens

Trigger: The blocking task panicked; the runtime cancelled the blocking task during shutdown; calling `asyncify` (directly or via `tokio::fs::*`) outside/after the runtime lifetime.

Common situations: Runtime shut down while a `tokio::fs` operation was in flight; a blocking syscall closure panicked; using `tokio::fs::*` from a thread that is not inside a Tokio runtime; runtime driver/runtime dropped mid-await.

Related errors

• background task failed
• other file operation is pending, call poll_complete before s
• bytes remaining on stream
• failed to write frame to transport
• frame size too big

AI-assisted analysis of tokio-rs/tokio@625954f365 (2026-08-11). Data as JSON: /api/errors/94bc45e41fe7e162. Report an issue: GitHub.