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Futurelock: A subtle risk in async Rust

(rfd.shared.oxide.computer)
421 points bcantrill | 1 comments | 31 Oct 25 16:49 UTC | HN request time: 0.202s | 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

1. qouteall ◴[01 Nov 25 02:30 UTC] No.45778792[source]▶
Simplify: tokio::select! will discard other futures when one future progress.

The discarded futures will never be run again.

Normally when a future is discarded it's dropped. When a future holding lock is dropped, lock is released, but it's passing future borrow to select so the discarded future is not dropped while holding lock.

So it leaves a future that holds a lock that will never run again.