Tokio kicked off a research effort on structured concurrency. Some great ideas were proposed, but we're still pretty much at square one and don't have readily available solutions. 4/29https://github.com/tokio-rs/tokio/issues/1879 …
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But here's the catch: you can only forget() tasks that resolve to (), which means you can't accidentally forget() a task that resolves to a Result or some other type. Now spawn(process(stream)) doesn't even compile - problem solved! 15/29
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The compiler now requires us to unwrap results in spawned tasks. But it'd be nice to have some API sugar here that is not as verbose as this... 16/29pic.twitter.com/YB7eLhDSgh
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What if there were unwrap() and expect() methods on Task<Result<T,E>> that transform it into Task<T> and panic on error? Those methods spawn a new task that simply unwraps the result and returns the success value. 17/29pic.twitter.com/7eQfRDqSN8
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Now we can do spawn(process(stream)).unwrap().forget(), which is pretty nice! This is what the entire TCP echo server looks like in my new runtime I'm working on. 18/29pic.twitter.com/H9Z2Nm9TOJ
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Finally, what about panics? Tokio silently ignores panics, meaning they might accidentally slip through. In particular, if an assertion fails in a task spawned from tokio within a unit test, the test will pass when it should fail! 19/29pic.twitter.com/3Mf9WskU4I
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Async-std crashes on panics. If an assertion fails in a task spawned from async-std within a unit test, the whole process crashes. No panics can slip through. 20/29pic.twitter.com/PTOTXiuOaE
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Crashing the entire process immediately is a bit annoying. If the whole process crashes, the test suite will not display the nicely formatted report of failed unit tests at the end. 21/29
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We need a panic handling strategy that is harsher than tokio's and gentler than async-std's. Why don't we propagate panics into the executor? 22/29
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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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