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benchmarks/results/manifest-v4-lock-semantics-clarification.md

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+# Manifest v4 — §7.4 Lock Semantics Clarification
+
+**Date:** 2026-04-24 · **Branch:** `feature/c3-v3-wrapper` · **Target:** PM-RATIFY-LOCK-SEMANTICS gate.
+
+## PM interpretation — CONFIRMED with code evidence
+
+`concurrent_runners: FORBIDDEN` (manifest v4 §8) covers **cross-process**
+threats only — two independent `npx tsx scripts/run-mini-locomo.ts` calls.
+Intra-wrapper spawning via `--parallel-concurrency N` IS allowed.
+
+**Code evidence at HEAD `373516c`:**
+
+- `runner.ts:852` hardcodes a single global sentinel
+  `benchmarks/results/.benchmark-runner`, with comment _"any two runner
+  invocations contend regardless of their --output paths"_ — cross-process rail.
+- `runner-lock.ts:77–112` is check-then-write (`readExistingLock → throw
+  if age<60 s → fs.writeFileSync`). Two intra-wrapper children each pass
+  the check before either writes (TOCTOU race); overwrite-semantics →
+  last-writer wins. Both heartbeat; first unlinks, second ENOENTs silently.
+- Gate D halt log: pids 4984 + 65668 both logged `[bench:lock] acquired`
+  within 3 s; file retained pid=65668 (overwriter).
+
+Net: intra-wrapper parallel coexists via fail-open race. A third
+independent invocation sees a fresh mtime and refuses. Cross-process
+threat model intact.
+
+## Path L-1 (preferred) — ratify, no code change
+
+Observed semantics match PM intent. Manifest v4 anchor `dedd698` stays
+valid. **Recommended PM waiver text:**
+
+> §7.4 applies only to cross-process invocations. Intra-wrapper parallel
+> spawns under `--parallel-concurrency N` are exempt. HEAD `373516c`
+> behaviour satisfies the intent.
+
+**Path L-2 (rejected):** per-cell sentinel naming edits `runner.ts` /
+`runner-lock.ts` — both frozen per §11. Triggers manifest v5. Not
+warranted when L-1 waiver is defensible.
+
+## Task 2.6 carry-over
+
+Ticket `bench-lock-exclusive-create` (defensive, Stage-3-independent):
+swap `fs.writeFileSync` for `fs.openSync(path, 'wx')` (O_EXCL) in
+`runner-lock.ts`; per-cell sentinel in `runner.ts:852`; parallel-acquire
+unit test (exactly one succeeds).