Tokio (Rust) needs to exit your program safely

When you use Tokio, the popular async library for Rust, your Rust main function becomes asynchronous. This extends to any other library that uses Tokio under the hood, like actix-web.

#[tokio::main]
async fn main() -> Result<()> {
    // Async code ...
    Ok(())
}

It may seem like magic, but Tokio spawns an entire runtime to handle asynchronous code and ensure your program exits successfully. Tokio needs to control the full lifecycle of your application, and interrupting this process can cause your application to panic.

This is exactly the issue I fixed today in Wasm Workers Server. The wws binary was panicking after any errored command:

thread '<unnamed>' panicked at 'cannot access a Thread Local Storage value during or after destruction: AccessError', /rustc/fc594f15669680fa70d255faec3ca3fb507c3405/library/std/src/thread/local.rs:422:26
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5

The issue was in how I handled the errors. When a command failed, I tried to terminate the program early by calling the exit() method. However, Tokio was unable to finish the process safely, causing the entire application to panic.

I'm still digging into the main reason, although that will require a deeper understanding of Tokio. For now, just be sure you leave Tokio to manage your application lifecycle to prevent weird threading errors in your program 😬.