feat: add DeterministicRng to make a runtime deterministic#1689
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drmingdrmer merged 1 commit intodatabendlabs:mainfrom Mar 19, 2026
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feat: add DeterministicRng to make a runtime deterministic#1689drmingdrmer merged 1 commit intodatabendlabs:mainfrom
drmingdrmer merged 1 commit intodatabendlabs:mainfrom
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Replaces the global AtomicU64 seed with per-instance seed stored in task-local storage. Each DeterministicRng instance owns its seed counter, set via set_seed() and propagated into block_on() via sync_scope. Seed derivation uses two SplitMix64 bijective mixers with different additive constants (golden ratio vs sqrt(2)) to prevent collisions in the spawn tree: child.seed = f(parent.seed), parent.seed = g(parent.seed). This ensures parent, child, and sibling tasks all get independent deterministic RNG sequences. Suite::test_all() is now sync — creates its own runtime internally, simplifying callers to just Suite::<Rt>::test_all(). Changes: - Add DeterministicRng<RT> with per-instance Cell<u64> seed - Add splitmix64() mixer, derive_spawn_seed(), advance_seed() - spawn() wraps child future in DETSIM_SEED.scope() with derived seed - Add DetsimSuite with 4 determinism tests (self-contained block_on) - Change Suite::test_all() from async to sync - Change ThreadLocalRng bound from RngCore to Rng - Part of: databendlabs#639
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Changelog
feat: add DeterministicRng to make a runtime deterministic
Replaces the global AtomicU64 seed with per-instance seed stored in
task-local storage. Each DeterministicRng instance owns its seed counter,
set via set_seed() and propagated into block_on() via sync_scope.
Seed derivation uses two SplitMix64 bijective mixers with different
additive constants (golden ratio vs sqrt(2)) to prevent collisions
in the spawn tree: child.seed = f(parent.seed), parent.seed = g(parent.seed).
This ensures parent, child, and sibling tasks all get independent
deterministic RNG sequences.
Suite::test_all() is now sync — creates its own runtime internally,
simplifying callers to just Suite::
Changes:
Add DeterministicRng
Add splitmix64() mixer, derive_spawn_seed(), advance_seed()
spawn() wraps child future in DETSIM_SEED.scope() with derived seed
Add DetsimSuite with 4 determinism tests (self-contained block_on)
Change Suite::test_all() from async to sync
Change ThreadLocalRng bound from RngCore to Rng
Part of: Build deterministic tests based on turmoil #639
This change is