docs(decisions): handoff for parallel L0 checks; runner usage names --jobs

Co-Authored-By: Claude Opus 5.5 <[email protected]>

conformance/lib/run_l1_offline.py

@@ -14,7 +14,7 @@ Pure standard library (matches the harness's no-pip-dependency posture); the che
 themselves shell out (render checks to catalog/render.py, whose PyYAML dependency lives
 there; L0 checks to git + PyYAML).
 
-Usage: run_l1_offline.py --junit <out.xml> [--label-prefix P] <test-dir> [<test-dir> ...]
+Usage: run_l1_offline.py --junit <out.xml> [--label-prefix P] [--jobs N] <test-dir> [<test-dir> ...]
 Exit 0 iff every selected offline check passes.
 
 The per-dir check script is whichever of `render-check.sh` / `check.sh` the directory

decisions/2026-11-08-parallel-l0-checks.md

@@ -0,0 +1,116 @@
+---
+date: 2026-11-08
+branch: feat/parallel-l0-checks
+title: L0 offline checks, mutation suites and yamllint run in bounded parallel pools (gate 184s -> 32s locally)
+---
+
+## What changed
+
+The gate's wall time was dominated by serial loops over independent subprocesses. Three of
+them now run in bounded pools; none of them changes a verdict, an exit code, or the order of
+anything printed.
+
+- `conformance/lib/run_l1_offline.py` — the per-check loop (L0 and L1 offline checks) runs in
+  a `ThreadPoolExecutor` of `bash <check>` children. `--jobs N` / env `CONF_OFFLINE_JOBS`,
+  default `min(8, usable CPUs)` (`sched_getaffinity`, so a cgroup cpuset is respected);
+  `--jobs 1` is the old serial runner. Results are consumed in SUBMISSION order, so `cases`,
+  the JUnit, the step summary and the 0/1/3 aggregation are built from exactly the same
+  ordered list as before. Additive output only: one `[offline] running N check(s), jobs=…`
+  line, and a trailing `[N.Ns]` wall time on each per-check status line. Children get
+  `stdin=/dev/null` (parallel children must not share the runner's stdin).
+- `conformance/tests/l0-static/keycloak-{passkey,client-secret}-reconciler/mutations.py` —
+  pass 1 (serial) checks every anchor and builds each mutated source, exactly as before;
+  pass 2 runs the `--selftest` children in a pool (env `MUTATION_JOBS`, default
+  `min(8, CPUs)`, `1` = serial). Verdicts are consumed in `MUTATIONS` order. A timeout
+  (`TimeoutExpired`) still propagates and fails the check; queued mutations are cancelled.
+  An invalid `MUTATION_JOBS` exits 2 (fails the check, never skips it).
+- **Nested-pool budget.** When the outer runner is parallel it exports
+  `MUTATION_JOBS = max(2, min(8, (cpus - jobs + 2) // 2))` unless the caller already set
+  one. Reason: only those two checks fan out; the other `jobs-2` workers are single
+  processes, so the two suites split the CPUs those leave free. Peak processes ≈
+  `(jobs-2) + 2*inner` ≤ cpus whenever cpus ≥ jobs+2 (24 CPUs: 6 + 16 = 22), and ~1.25–1.5×
+  cpus on a small runner (4 CPUs: 2 + 4 = 6). The floor of 2 is deliberate: a 1-wide inner
+  pool would make the 40-mutation passkey suite (~85s serial) the critical path of the stage.
+- `ci/lint.sh` — yamllint runs over `LINT_JOBS` (default `min(8, nproc)`) CONTIGUOUS slices
+  of the sorted file list concurrently; each slice's output is buffered to its own file and
+  printed in slice order, so the text is the same as one serial call and a `file` header is
+  never separated from its `line:col` findings. Stage rc = the highest slice rc; a slice that
+  leaves no rc file counts as 1. Invalid `LINT_JOBS` exits 2.
+
+### Parallel-safety audit (all 25 `l0-static/*/check.sh`, 3 `l1-rendering/*/render-check.sh`)
+
+Grepped every check for fixed `/tmp` paths, `_artifacts`/`ARTIFACTS`/`COVERAGE_OUT`,
+`write_text`/`open(…,"w")`, `mktemp`/`mkdtemp`/`TemporaryDirectory`, sockets/ports, `git`
+writes and `run.sh` invocations, and read what each writer writes to:
+
+- **Scratch state: all private.** Every writer uses its own `mktemp -d` / `mkdtemp` /
+  `TemporaryDirectory` (placeholder-guard-coupling, spec-immutability-enforced,
+  whole-repo-invariants-wired, auditability-proofs-producible, alert-rules-unit-tested and its
+  `platform/monitoring/tests/run.sh`, internal-services-publish-no-public-dns,
+  oauth2-proxy-{api-routes-per-app,cookie-names-distinct}, both keycloak reconcilers, the three
+  L1 render checks). No fixed `/tmp` path anywhere.
+- **Git writes:** only spec-immutability-enforced, and only `git init/add/commit` inside its
+  own `mktemp` repo. Everything else is `git ls-files`/`ls-tree`/`log`/`rev-parse` (read-only).
+- **Ports:** the reconciler selftests bind `127.0.0.1:0` (kernel-assigned); the passkey
+  selftest also dials `127.0.0.1:1` as a deliberate refused connection. No fixed listener.
+- **Ansible** (dev-nodes-provisioned-as-code): `--syntax-check`/`--list-hosts` only,
+  `retry_files_enabled = False`; its local tmp is per-process.
+- **The one shared writer:** auditability-proofs-producible runs
+  `./run.sh --coverage-only --requirements …`, which REWRITES
+  `conformance/_artifacts/coverage/coverage.{json,md}` (it inherits `CONF_ARTIFACTS`). That was
+  already true serially. No other offline check reads or writes that directory (the only
+  reader is `tests/meta/harness-selfcheck`, not an offline check), and the outer runner
+  writes coverage BEFORE and the JUnit AFTER the pool, so the final state is the same as
+  serial. Not serialised; left as is and recorded here. `lib/status.py` (run by that check)
+  reads `_artifacts/report.junit.xml` as an advisory — written only after the pool, unchanged.
+- whole-repo-invariants-wired calls `run.sh --print-selection` (writes nothing) and the
+  runner itself against synthetic dirs in its own `mktemp`.
+
+Result: no check needed fixing or serialising. A new check that needs shared state must say
+so; the runner's docstring now states that contract.
+
+## What I verified
+
+All in the pinned toolbox (`dev.sh run`, `DEV_KUBECONFIG=/dev/null`, ENGINE shim adding a ro
+mount of the main checkout's `.git`; spec repo resolved as a sibling, spec-immutability
+`pass`), host 24 CPUs, every `dev.sh` call under `flock`.
+
+- **Gate, before** (origin/main `1a33814`, detached scratch worktree + spec mount):
+  `GATE_RC=3`, in-container wall **183.9s**; `l0` stage **168.3s**, `lint` **7.8s**.
+  "Stages that did not run: gitea-settings" (plus the expected on-base
+  touched-set-not-determinable note, since main has no branch commits).
+- **Gate, after** (this branch, commit `a775082`): `GATE_RC=3`, in-container wall **32.1s**;
+  `l0` **21.1s**, `lint` **2.4s**, `l1` 2.3s. "Stages that did not run: gitea-settings" — the
+  only not-run stage. **rc 3, not green.** Caveat: the branch run selected 23 L0 checks (the
+  touched-requirement scoping dropped dev-nodes-provisioned-as-code and
+  placeholder-guard-coupling), the main run 25.
+- **Same-tree L0 comparison** (all 25 l0 dirs, same tree, same container):
+  `CONF_OFFLINE_JOBS=1 MUTATION_JOBS=1` **164.0s / 165.3s** vs default (jobs=8,
+  MUTATION_JOBS=8) **19.2s / 18.7s**. JUnit `cmp`-identical, step summary `cmp`-identical,
+  stderr identical except the timing suffixes and the JUnit path in the last line.
+- **Mutation suites, host:** passkey 83.2s → 10.5s (`MUTATION_JOBS=8`), client-secret
+  11.2s → 2.0s; output of `MUTATION_JOBS=1` and `=8` byte-identical to the pre-change script's,
+  rc 0 all round.
+- **Falsification (not committed):** in a scratch worktree at `a775082`, made the passkey
+  mutation "the protected-subtree screen is never called" a no-op (replacement == anchor).
+  Host: `MUTATION_JOBS=1` and `=8` both rc 1, output byte-identical (with a second, broken
+  anchor also injected: both reported). Toolbox: runner rc 1 both serial and parallel,
+  `L0 FAIL keycloak-passkey-reconciler … 1 of 40 mutation(s) not proven … SURVIVED`; full
+  gate **rc 1**, `stage l0: FAIL`, `MERGE GATE: BLOCKED`. Reverted; scratch worktree removed.
+- **lint.sh, host scratch repo** (31 YAML files, two broken): `LINT_JOBS=1/4/8` all rc 1,
+  stdout identical, findings named `y/f07.yaml` / `2:1 … key-duplicates` etc.; clean tree rc 0;
+  `LINT_JOBS=x` rc 2.
+
+## What I did NOT verify
+
+- **CI timings at the time of writing.** The PR's CI run is measured after push and reported
+  on the PR / handoff message, not here. The CI runner's CPU count is UNVERIFIED — the new
+  `[offline] running … jobs=… cpus=…` line in the CI log is what will show it. On a small
+  runner the speedup will be smaller than locally.
+- **approval-policy** will be red on CI: `ci/**` and `conformance/**` are sensitive paths and
+  need the operator's review. I did not and must not override it.
+- Flakiness under load: the selftests have no wall-clock assertions I found, and 4 toolbox
+  runs + several host runs were all stable, but that is a handful of runs, not a soak.
+- The coverage files under `conformance/_artifacts/coverage/` end in the same state as before
+  only because a single check rewrites them; I did not change that pre-existing quirk.
+- L2/L3 conformance: not run (no cluster in CI, as before).