on_cancel

Register a callable and return a CancellationSubscription . cancel! invokes each registered callback once, outside the token lock, on its calling task. If already cancelled, invoke f() before returning. Keep callbacks prompt: signal an event or stop an owned process; do not wait for operation completion. Callback failures are collected after all callbacks run and raised together.

on_cancel(f, token::CancellationToken)

Closing a subscription prevents later invocation and joins a callback already running on another task. Close subscriptions when their operation retires.