Two timing tests turn dev gates red under load: the 40ms check-clock cut and the settled-errand temp-dir teardown #1664

What happened

On 2026-09-28, two dev push gates went red in touched packages on tests the merged changes do not reach. A re-run of each failed job went green.

dev@8470cdc39 (#1650), cmd/codeaf, run 36472317002. The test body passed; the temp-dir teardown found something still writing:

--- FAIL: TestASettledErrandLeavesNoNodeOfItsOwnStillRunning (0.65s)
    testing.go:1464: TempDir RemoveAll cleanup: unlinkat /tmp/codeaf-ci/TestASettledErrandLeavesNoNodeOfItsOwnStillRunning2657892707: directory not empty

dev@73ade0354 (#1654), internal/session, run 36474315029. The cut case gives the check a 40 ms window (checkwindow_test.go:432). On a busy runner the window expires before the hanging call is even made, so the report takes the other wording:

--- FAIL: TestACheckTheClockCutNamesTheClockNotTheWork/cut (0.41s)
    checkwindow_test.go:472: the report does not lead with the clock and the call it cut:
        the check ran out of time before a call could be made — it had 40ms
        Wrote greet.go with the greeting.

Locally (8 cores, load ~0.2) on dev@73ade0354, 1 of 60 runs failed. That failure was in the control subcase, which hung for 38 s. The same test passed 30/30 on dev@e7976208e.

Replication

Deterministic (no model). Both tests use stub completers, so no key is needed. Run them under CPU pressure, for example with another package's tests running beside them:

go test -count=40 -run '^TestASettledErrandLeavesNoNodeOfItsOwnStillRunning$' ./cmd/codeaf
go test -count=60 -run '^TestACheckTheClockCutNamesTheClockNotTheWork$' ./internal/session

At idle both usually pass (40/40 and 59/60 here). Under load, expect the two failures quoted above.

Acceptance

• TestACheckTheClockCutNamesTheClockNotTheWork: the cut case no longer depends on a 40 ms wall clock racing the scheduler. For example, inject the check clock, or wait until the hanging call has started before letting the window lapse. The control case must not hang.
• TestASettledErrandLeavesNoNodeOfItsOwnStillRunning: whatever writes into the test's temp dir after the errand settles is waited for (or owned) before teardown, the same repair session: a stopped job reads as ended at once; the log-folder sweep moves to the registry #1641 made for the job log sweep.
• Each passes -count=100 with another heavy package's tests running beside it.