Traditionally the downside of thead-locals has been that usage is constrainted to the LocalKey::with closure with no lifetime escapement, the rationale being that anything beyond this is of an indeterminate lifetime. There is however a way around this limitation: by using a barrier to rendezvous worker threads during runtime shutdown, no tasks will outlive thread local data belonging to any worker thread, and all pointers to thread locals created within an async context and held therein will be of a valid lifetime. Utilizing this barrier mechanism, this crate introduces AsyncLocal::with_async, the async counterpart of LocalKey::with, as well as the unsafe pointer types and safety considerations foundational for using thread local data within an async context.
For best performance, use the Tokio runtime as configured via the tokio::main or tokio::test macro with the crate attribute set to async_local while the barrier-protected-runtime feature is enabled on async-local. Doing so configures the Tokio runtime with a barrier that rendezvous runtime worker threads during shutdown in a way that ensures tasks never outlive thread local data owned by runtime worker threads and obviates the need for Box::leak as a means of lifetime extension.
#[cfg(test)]*
mod tests {
use std::sync::atomic::{AtomicUsize, Ordering};
use async_local::{AsyncLocal, Context};
thread_local! {
static COUNTER: Context<AtomicUsize> = Context::new(AtomicUsize::new(0));
}
#[tokio::test(crate = "async_local", flavor = "multi_thread")]
async fn it_increments() {
COUNTER
.with_async(|counter| {
Box::pin(async move {
counter.fetch_add(1, Ordering::Release);
})
})
.await;
}
}