Seed tokio with the world rng #245
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In preparation for making tokio's rng seed configurable, introduce a new `rt::Config` struct that holds configuration for creating tokio runtimes and pass that around instead of the bare `enable_io` bool.
If the `tokio_unstable` flag is set, seed host and client runtimes using the world rng. This makes the simulation more deterministic. In particular, the order in which tokio polls `select!` branches is determined by this seed.
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mcches
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Nov 3, 2025
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The tokio runtime builder provides rng_seed as a way to make "certain parts" of the runtime deterministic. We can use it to make turmoil simulations more deterministic. Turmoil already allows specifying a custom seeded world rng through
Builder::build_with_rng. We just have to make sure that rng is used to seed the tokio runtime.I've taken the approach of introducing a new
rt::Configstruct, to avoid having to juggle two config arguments (enable_ioandrng) in multiple places. Though the juggling wouldn't be too bad either, so happy to drop thert::Configstruct if that's preferred.Caveats
tokio::runtime::Builder::rng_seedis only available under thetokio_unstableconfig. Nothing we can do about that.Rt::no_softwareconstructor. I think for now that's fine since no-software runtimes are only used for timekeeping currently. Once we start using it for more, we'll need to reconsider.