Struct Governor 

pub struct Governor { /* private fields */ }

The adaptive concurrency governor — the inline thread::scope closure that used to live in [crate::pipeline::chunked::exec::run_chunked_parallel] turned into a self-contained, testable abstraction.

Why a struct (not just functions): the decision policy ([GovernorState]) and the sample cadence (sample/poll intervals, RIVET_GOVERNOR_INTERVAL_MS env override) are runtime-coupled — the poll interval must be clamped to the sample interval, and the decision state is mutated across ticks. Bundling them into one type makes the “what to test, what to fake” boundary obvious: the source is the dependency, the runner-side side effects are a callback.

Implementations

impl Governor

pub fn new(start: usize, floor: usize, ceiling: usize) -> Self

Build a governor that starts at start, clamped into [floor, ceiling], and uses the env-tunable sample cadence (RIVET_GOVERNOR_INTERVAL_MS; falls back to [GOVERNOR_SAMPLE_INTERVAL_MS]). The poll interval is clamped to the sample interval so a tiny override (used in deterministic live tests) actually polls that fast, instead of being capped at the default [GOVERNOR_POLL_MS].

pub fn tick(&mut self, sample: Option<u64>) -> Option<(usize, usize)>

Pure decision step: fold one sample into the state. Returns Some((from, to)) on a parallelism transition, None otherwise. Mirrors [GovernorState::observe]; exposed at the Governor surface so tests can drive the policy without entering run.

pub fn run<S, Stop, Decide>( &mut self, source: &mut S, stop: Stop, on_decision: Decide, )

where S: PressureSource + ?Sized, Stop: Fn() -> bool, Decide: FnMut(usize, usize),

Drive the sample loop until stop returns true. On every parallelism transition the on_decision(from, to) callback fires — the runner binds it to its semaphore-resize + log + decision-log machinery. Polls every poll_interval, samples every sample_interval. The stop predicate is re-checked after each poll sleep so a finished run exits within one poll quantum.