When a task is cancelled, it gets immediately woken and the next time the executor takes it out from the task queue, it will be simply dropped. Automatic cancellation is one of the core tenets of structured concurrency. 12/29
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In the case of async-std, it's not really obvious where the executor exactly is. Its thread pool is running in the background and we can only crash the process or perhaps let the user specify a custom panic handler. 23/29
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In tokio, the executor is the tokio Runtime instance, so instead of ignoring errors it'd make sense for Runtime::block_on() to propagate panics from tasks. 24/29pic.twitter.com/EF6m8hvgWK
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For now, suppose we had a very simple single-threaded runtime invoked by a function called run(), which propagates panics upwards. Don't think too much about it because I will tweet more about runtimes later... 25/29pic.twitter.com/qJaXb9eKbc
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In unit tests, panic propagation does the right thing by default, and it doesn't crash the whole test suite so we get a nice report at the end. 26/29pic.twitter.com/cQoG6uwvyo
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When run() propagates panics, it's up to the user to handle them however they wish. Panics can be ignored, logged, or simply left to continue unwinding. 27/29
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In summary: 28/29 1. Tasks are cancelled when dropped. 2. Tasks can't get accidentally dropped because we get compiler warnings. 3. Errors in tasks cannot get silently lost because we get compiler errors. 4. Unwrapping errors is easy. 5. Panics are propagated into the executor.
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That's all! This design isn't the "holy grail" of structured concurrency by any means, but it gets us very far with little effort and eliminates a lot of common pitfalls in async Rust. 29/29
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