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port: the dead-press check was passing by luck -- diagnosed and fixed

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Two iterations ago verify-menu-audio's bit-identity assertion began failing and I
filed three suspects in the port. It is none of them. Three IDENTICAL invocations
give two outcomes, 1.207438 s and 1.300317 s, differing by exactly 4096 samples --
one mixing buffer. The recording quantises to whole buffers and a one-buffer shift
moves the length and alignment of everything in it.

The premise -- cross-run bit-determinism -- was never guaranteed. It held while
timing sat away from a buffer boundary, and a larger export moved it onto one. A
test that passes by luck reports the luck running out as a regression in the code,
which is what it did: two iterations of suspects, and the port was never involved.

The fix keeps exact equality and no threshold, allowing the comparison to slide by
whole buffers -- the one degree of freedom the recorder has. Proved it can still
fail: ctrl against walk differs at every alignment.

Distinct from the earlier entries: this check ran and answered the right question,
resting on a property of the environment nothing verified. State what an assertion
assumes about the machine, not only what it checks.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N7FiFFFwbvG2uxdcEh8HyF
This commit is contained in:
Sylpheed port agent
2026-08-30 09:45:39 +00:00
parent 93b823eedf
commit 1cae35b43a
3 changed files with 96 additions and 11 deletions
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14
docs/port/BLOCKED.md
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@@ -677,7 +677,7 @@ what is being withheld rather than implying the question is empty.
---
## `verify-menu-audio`'s dead-press check fails, cause unidentified
## ~~`verify-menu-audio`'s dead-press check fails~~ — ✅ diagnosed and fixed 2026-08-30
*Derived from HANDOFF `9ca1eb5`. Raised 2026-08-30 by the port. **Mine, not
anybody else's** — filed here so it is not lost, not because it needs an answer
@@ -688,9 +688,15 @@ produce a Master bus bit-identical to five waits. It now reports DIFFER across
three consecutive runs: divergence at 0.085 s, 92 % of samples differing, and
recording durations of 1.300 s against 1.207 s where they were previously equal.
🔴 Not diagnosed. Candidates not separated: `formats-pin-2026-08-30`, the
plate-pulse draw path, the static-overlay clock. It is **not** the voice export
change — neither control run plays a voice.
✅ **Diagnosed: none of the candidates.** The recording is not sample-deterministic
across runs and never was. Three **identical** invocations give two outcomes —
1.207438 s and 1.300317 s — differing by **exactly 4096 samples, one mixing
buffer**. A one-buffer shift moves the length and the alignment of everything in
the file, so a byte comparison of two runs fails.
The check now allows whole-buffer alignment and is still **exact** — no threshold,
nothing to tune. Proved it can still fail: `ctrl` against `walk` differs at every
alignment.
⚠️ The check's premise is **cross-run bit-determinism**, which is what made it a
strong assertion with no threshold to tune, and also what makes it brittle:

51
docs/port/DECISIONS.md
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@@ -9,7 +9,7 @@ dies, which is what this file is for.
<!-- INDEX: generated by tools/port/index-decisions -- do not hand-edit -->
134 sections. Search this before re-deriving anything.
135 sections. Search this before re-deriving anything.
* [P0 — the exporter, 2026-08-28](#p0--the-exporter-2026-08-28)
* [P1 — Godot draws the screen, 2026-08-28](#p1--godot-draws-the-screen-2026-08-28)
@@ -145,6 +145,7 @@ dies, which is what this file is for.
* [🔴 Unexplained: `verify-menu-audio`'s dead-press check has started failing](#unexplained-verify-menu-audios-dead-press-check-has-started-failing)
* [External ground truth for every three-chunk region — the movies' own durations](#external-ground-truth-for-every-three-chunk-region--the-movies-own-durations)
* [The menu bed loops at 61.93 s — and my 3.4 s "ugly seam" was mine, not the game's](#the-menu-bed-loops-at-6193-s--and-my-34-s-ugly-seam-was-mine-not-the-games)
* [The dead-press check was passing by luck, and the luck ran out](#the-dead-press-check-was-passing-by-luck-and-the-luck-ran-out)
<!-- /INDEX -->
## P0 — the exporter, 2026-08-28
@@ -7768,3 +7769,51 @@ their table had **no summary line** and mine had a **population that didn't matc
a count from one command earlier**. The defence that covers both is to state the
population and the coverage together — and theirs adds a second: **refuse to read
a table whose summary line is missing.**
## The dead-press check was passing by luck, and the luck ran out
Two iterations ago `verify-menu-audio`'s first assertion — five presses bound to
nothing produce a Master bus **bit-identical** to five waits — began failing. I
filed it undiagnosed and named three candidates: the new pin, the plate-pulse draw
path, the static-overlay clock.
**It is none of them.** Three *identical* invocations of the same command give two
outcomes:
| run | duration |
|---|---|
| 1 | 1.207438 s |
| 2 | **1.300317 s** |
| 3 | 1.207438 s |
The difference is **0.092879 s = exactly 4096 samples**, one mixing buffer. The
recording quantises to whole buffers, and a one-buffer shift moves both the length
and the alignment of everything inside it. So a byte-for-byte comparison of two
separate runs cannot hold.
🔴 **The premise was never guaranteed — it was luck.** It held while the run's
timing sat away from a buffer boundary, and a larger export (three voice streams
where there had been one) moved it onto one. **A test that passes by luck reports
the luck running out as a regression in the code**, which is exactly what it did:
I spent two iterations listing suspects in the port, and the port was never
involved.
⚠️ It also passed for weeks *looking* like the strongest assertion in the harness —
exact equality, no threshold, nothing to tune. That was true and it was resting on
an assumption nothing checked. Strength of the assertion said nothing about
soundness of the premise.
### The fix keeps what mattered
Still **exact** equality and still no threshold; the comparison may now slide by
whole buffers, which is the one degree of freedom the recorder actually has. In
practice it finds `+0` or `+1`.
✅ And it can still fail, which is the part worth proving: `ctrl` against `walk` —
a run that really does contain cues — **differs at every alignment**.
📌 The general form, and it is not the same as the earlier entries: those were
checks nobody ran, or that ran and answered a different question. This one ran,
answered the right question, and rested on a property of the environment that was
never verified and had no reason to be stable. **The thing to state alongside an
assertion is not only what it checks, but what it assumes about the machine.**