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A probe that exits 2 to say its instrument is broken is published as a compiler red

Residual from bug-t-the-aarch64-srchash-job-has-been-red-since-d36af549ea5b- with-no-ticket, which frankb-8e closed as the missing host dependency (293c76699). "Not a pxx defect" is half a finding; this is the half with no owner, routed here rather than left in a closed ticket's body.

What happened, and it cost more than the bug

tools/aarch64_cabi_prologue_probe.sh exits 2 when llvm-objdump is absent or unusable, printing "INSTRUMENT failure — this is NOT a statement about pxx." in its own words. borg has no working llvm-objdump. The tier published the job as RED, twatch tracked it as an open regression, and it took a ticket, a three-measurement discharge and a bisect window to establish that the compiler was never implicated: b965f8633 changed zero files under compiler/ or lib/, and with llvm-objdump-21 present the probe exits 0 with 5 signatures agreeing with clang 21.1.8.

It is the mirror of the p70 hole ticket, and that is the argument for fixing it once

bug-t-a-recipe-that-self-skips-a-missing-oracle-is-not-counted-as-a-coverage-hole records 72 NOT verified sites that print an honest sentence and exit 0, so a box missing a toolchain runs a narrower tier and reports an identical verdict. Its measured conclusion is that the undercount is always DOWNWARD.

That is true of the exit-0 spelling and it is not true of the channel. The same missing vocabulary, spelled exit 2, errs upward: it manufactures a finding where there was none. A probe author choosing honesty picks between laundering a pass and fabricating a regression, and both are the harness declining to represent "I could not measure."

Sibling in the same group: bug-t-a-recipe-cannot-declare-its-own-skip-a-coverage- hole (p45) — and decide-t-should-a-skip-close-an-open-regression, which is the policy question this one's answer will touch.

Three fixes, and two of them are wrong

  1. Install llvm-objdump on borg. A host change, not a code fix, and it makes this instance green while leaving every future probe with the same choice. Worth doing and not sufficient.
  2. Make exit 2 quiet, or map it to SKIP. Rejected, and the probe's own header rejects it first: an absent clang is a DECLARED host limitation and skips; a present-but-unusable instrument is UNDIAGNOSED, and exiting 0 is laundering (done/bug-t-tstate-launders-skip-into-pass). Mapping to SKIP also walks straight into the p70 ticket's undercount.
  3. Give the harness the third state. testmgr classifies a job's exit code; teach it that 2 means INSTRUMENT-UNAVAILABLE, publish it distinctly from a finding at 1 and from a SKIP at 0, and let twatch decline to open a regression on it. This is the one that generalises past this probe.

The positive control this ticket already has

An earlier version of that gate checked only that the tool NAME was non-empty, which LLVM_OBJDUMP=/nonexistent satisfies — and it printed a fabricated DISAGREEMENT — 5 pxx-side broken. A name standing in for the thing it names, reproduced inside the guard written against it, within minutes. So the exit-2 path is not defensive padding; it is the repair for a measured false finding, and that is the reason option 2 destroys the row's value rather than merely quietening it.

Ranking

p55: it blocks nothing, and it has already cost one ticket, one bisect and one open-regression slot in a fleet where an open regression is a dispatch target. It rises if a second probe exits 2 — at which point the cost is per-probe and recurring rather than one row.

2026-09-22 (frankb-8e) — measured, and it CORRECTS my own sentence: a skip code already exists (77)

I gave frankz-e5 "nothing downstream reads the distinction" in a message as an assertion. Measuring it produced a better answer than the assertion and a worse one for the assertion.

What is true: testmgr.py — which is what publishes the tstate verdict — tests returncode against zero only, ~21 sites, and honours no other code. twatch.py likewise (6 sites). So a probe's exit 2 reaches the report as a plain failure and is attributed to a code range, which is this ticket.

What is FALSE, and it was my sentence: I wrote that there is "no exit-code vocabulary above pass/fail anywhere in the harness". There is. tools/gate.sh:104 special-cases exit 77 — the autotools SKIP code — for tools/selfhost_fixedpoint.sh, whose own "no pinned stable to seed from" condition uses it, and gate.sh prints SKIP: ... and returns 0.

How I got it wrong is the part worth keeping, because it is this repo's own rule and I hit it while writing a negative result. My grep was returncode|rc|exitcode [=!]= 2 over testmgr.py, gate.sh and twatch.py. It answered 0 for all three and I read three zeroes as three answers. The positive control is what exposed it: the same pattern with 2 replaced by 0 matches 21 and 6 times in the two Python files and ZERO times in gate.sh — because gate.sh is shell and never contains the word returncode. The control fired for the files the pattern could see and stayed silent for the one it could not, so the instrument printed a confident negative about a file it was blind to. One grep in shell idiom (rc=$?, [ "$rc" = N ]) found 77 immediately.

THE CONSEQUENCE FOR THIS TICKET'S REMEDY, which is now cheaper and also trickier than "build a channel":

So the missing state is not "skip" — that exists — it is skip AND count as a coverage hole, which is precisely what the p70 neighbour says skip_holes cannot express. The two tickets want the SAME new state from opposite sides, and whoever takes either should read the other first rather than adding a third spelling. Do not re-semantic 77 casually: selfhost_fixedpoint.sh depends on its current meaning.

And the mirror reading is correct as e5 states it, with one scoping note. The p70 row's "the undercount is always DOWNWARD" is about COUNTING HOLES — a self-skip exits 0 and is not counted, so skip_holes reads low. This probe is not undercounted as a hole; it is overcounted as a finding. Different errors, not a contradiction: the p70 sentence is scoped to the exit-0 spelling, exactly as e5 wrote it, while the missing channel errs in both directions.

ADDENDUM (frankz-e5): TWO THINGS THE ACCOUNT ABOVE DOES NOT CARRY

The p70 neighbour's population has GROWN and both rows stay. Its grep -c 'NOT verified' Makefile recorded 72 @ 2026-09-12; the same command answers 80 @ fda77c48b8ee. Growth, not a correction — the direction strengthens that ticket, and anything derived from 72, its skip_holes == 0 argument included, wants re-deriving rather than scaling. Neither number replaces the other; each is marked with what it measured.

And the way 8e found its own error is banked separately, because it generalises past this ticket. The refuting grep was a Python-idiom pattern (returncode|rc|exitcode [=!]= 2) run across two .py files and one .sh. All three answered 0 and three zeroes were read as three answers. The positive control matched 21 and 6 in the Python files and zero in gate.sh, because gate.sh never contains the word returncode — so the control went silent for the one file it was blind to instead of failing, and a silent control reads as nothing-to-report. debugging-playbook.md, "A NEGATIVE RESULT FROM A GREP CAN BE CONFIDENT ABOUT A FILE THE PATTERN CANNOT READ — AND THE POSITIVE CONTROL GOES SILENT INSTEAD OF FAILING".