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Futurelock: A subtle risk in async Rust

(rfd.shared.oxide.computer)
427 points bcantrill | 1 comments | 31 Oct 25 16:49 UTC | HN request time: 0.269s | 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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Sytten ◴[31 Oct 25 20:21 UTC] No.45776277[source]▶
I am wondering if there is a larger RFC for Rust to force users to not hold a variable across await points.

In my mind futurelock is similar to keeping a sync lock across an await point. We have nothing right now to force a drop and I think the solution to that problem would help here.

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cogman10 ◴[31 Oct 25 20:37 UTC] No.45776433[source]▶
The ideas that have been batted around is called "async drop" [1]

And it looks like it's still just an unaddressed well known problem [2].

Honestly, once the Mozilla sackening of rust devs happened it seems like the language has been practically rudderless. The RFC system seems almost dead as a lot of the main contributors are no longer working on rust.

This initiative hasn't had motion since 2021. [3]

[1] https://rust-lang.github.io/async-fundamentals-initiative/ro...

[2] https://rust-lang.github.io/async-fundamentals-initiative/

[3] https://github.com/rust-lang/async-fundamentals-initiative

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1. kibwen ◴[01 Nov 25 13:29 UTC] No.45781483[source]▶
While the Mozilla layoffs were a stressful time with a lot of uncertainty involved, in the end it hasn't appeared to have had a deleterious effect on Rust development. Today the activity in the Rust repo is as high as it's ever been (https://github.com/rust-lang/rust/graphs/contributors) and the governance of the project is more organized and healthy than it's ever been (https://blog.rust-lang.org/2025/10/15/announcing-the-new-rus...). The language certainly isn't rudderless, it's just branched out beyond the RFC system (https://blog.rust-lang.org/2025/10/28/project-goals-2025h2/). RFCs are still used for major things as a form of documentation, validation, and community alignment, but doing design up-front in RFCs has turned out to be an extremely difficult process. Instead, it's evolving toward a system where major things get implemented first as experiments, whose design later guides the eventual RFC.