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Futurelock: A subtle risk in async Rust

(rfd.shared.oxide.computer)
427 points bcantrill | 3 comments | 31 Oct 25 16:49 UTC | HN request time: 0s | source

This RFD describes our distillation of a really gnarly issue that we hit in the Oxide control plane.[0] Not unlike our discovery of the async cancellation issue[1][2][3], this is larger than the issue itself -- and worse, the program that hits futurelock is correct from the programmer's point of view. Fortunately, the surface area here is smaller than that of async cancellation and the conditions required to hit it can be relatively easily mitigated. Still, this is a pretty deep issue -- and something that took some very seasoned Rust hands quite a while to find.

[0] https://github.com/oxidecomputer/omicron/issues/9259

[1] https://rfd.shared.oxide.computer/rfd/397

[2] https://rfd.shared.oxide.computer/rfd/400

[3] https://www.youtube.com/watch?v=zrv5Cy1R7r4

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jacquesm ◴[31 Oct 25 20:43 UTC] No.45776483[source]▶
If any rust designers are lurking about here: what made you decide to go for the async design pattern instead of the actor pattern, which - to me at least - seems so much cleaner and so much harder to get wrong?

Ever since I started using Erlang it felt like I finally found 'the right way' when before then I did a lot of work with sockets and asynchronous worker threads. But even though it usually worked as advertised it had a large number of really nasty pitfalls which the actor model seemed to - effortlessy - step aside.

So I'm seriously wondering what the motivation was. I get why JS uses async, there isn't any other way there, by the time they added async it was too late to change the fundamentals of the language to such a degree. But rust was a clean slate.

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sunshowers ◴[31 Oct 25 20:45 UTC] No.45776498[source]▶
Not a Rust designer, but a big motivation for Rust's async design was wanting it to work on embedded, meaning no malloc and no threads. This unfortunately precludes the vast majority of the design space here, from active futures as seen in JS/C#/Go to the actor model.

You can write code using the actor model with Tokio. But it's not natural to do so.

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1. shepmaster ◴[31 Oct 25 21:05 UTC] No.45776683[source]▶
> But it's not natural to do so.

I tend to write most of my async Rust following the actor model and I find it natural. Alice Rhyl, a prominent Tokio contributor, has written about the specific patterns:

https://ryhl.io/blog/actors-with-tokio/

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2. sunshowers ◴[31 Oct 25 21:06 UTC] No.45776703[source]▶
Oh I do too, and that's one of the recommendations in RFD 400 as well as in my talk. cargo-nextest's runner loop [1] is also structured as two main actors + one for each test. But you have to write it all out and it can get pretty verbose.

[1] https://nexte.st/docs/design/architecture/runner-loop/

3. baq ◴[01 Nov 25 07:55 UTC] No.45779939[source]▶
The ‘Beware of cycles’ section at the end has some striking similarities with futurelock avoidance recommendations from the original article… not sure what to make of this except to say that this stuff is hard?