wai: a from-rest burst asserted a 2 ms race, and linux-arm64 called it #192

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dcharlot merged 1 commit from fix/wai-burst-race-assertion into main 2026-10-09 02:49:10 -04:00

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2026-10-08

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wai: a from-rest burst asserted a 2 ms race, and linux-arm64 called it ...

`a_from_rest_burst_counts_only_work_begun_inside_the_window` asserted, inside
the sampler callback, that at most one unit of work per worker had begun by the
instant the window opened:

    assert!(begun <= 3, "no more than one unit per worker can have begun");

Nothing in `burst_from_rest` entails that. Once the barrier releases, each
worker increments the counter and sleeps 2 ms; the caller then calls the
closure. For `begun` to exceed the worker count the caller need only be
descheduled for longer than that 2 ms at exactly this point. It is a race the
test does not control, not a property of the code under test.

A contended host duly called it: the assertion failed on the `linux-arm64`
runner — a colima VM on a box at load 173 — while every other platform passed.
I could not reproduce it here, including under 28 competing busy loops, so this
is reasoned from the code and the CI log rather than from a local repro, and
that is the honest basis for it. The argument does not need one: an assertion
nothing establishes can fail without a defect, and the failing run is evidence
that it does.

The quantity is now recorded rather than asserted — it printed 2 on the first
local run, already below the old bound, which is the variance the bound was
pretending to constrain.

Nothing is lost. What the test establishes about starting from rest is asserted
elsewhere and does not depend on the scheduling: the window opened `FromRest`,
every worker was in flight, and completions never exceed starts by more than the
worker count. That last one bounds a *difference* by the worker count instead of
asserting an instant arrived before a sleep finished, so it holds however the
host schedules — and it now says what it saw when it fails.

This is the fifth copy of this family in the repo. `eoc-meter`,
`jouleclaw-energy` (#166), `proof-cascade` and `sandbox-meter` (#181) each put a
floor on something the host's scheduler controls; this one puts a ceiling on how
much work fits in an interval it does not control. Same mistake, opposite
direction.

wai-rs lib: 187 passed, clippy clean on the touched file. Pre-existing and
untouched by any branch of mine — it surfaced on #183 only because a rebase
fires every path-filtered workflow (#191).

Co-Authored-By: Claude Opus 5 <[email protected]>

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