Extending the personhog test harness

The harness (rust/personhog-test-harness/, see its README for usage) spawns a real personhog stack and asserts one invariant three ways: every acked write is visible afterwards — live via read-your-write probers, at end-of-run via strong reads, and durably via Postgres at the acked version. It has caught real bugs (eviction data loss, unordered lifecycle shutdown, coordinator failover stalls) precisely because chaos scenarios run against those assertions.

When to add a scenario

Add one whenever you fix a personhog bug whose trigger the harness can reproduce — a crash, drain, restart, lag, election, or eviction condition. The fix's PR should show the scenario red before the fix and green after; that run becomes the permanent regression test. Also add one when introducing a new failure mode (a new process to disrupt, a new timing window) or a new correctness property (a new journal check).

Don't add scenarios for behavior already covered by an existing flag combination — compose existing flags instead — and don't add unit-testable logic here: the harness is for whole-stack behavior only.

Where things live

• src/cli.rs — every scenario is driven by GateArgs flags.
• src/scenarios/gate.rs — ChaosEvent enum, chaos_timeline() (offset-sorted events, paired stop/resume events), the dispatch loop, and verification ordering (probers → regressions → strong reads → Postgres quiesce).
• src/stack/mod.rs — stack primitives: spawn/kill/restart leaders (SIGKILL, SIGTERM drain, SIGSTOP/SIGCONT zombie), writer crash/pause, coordinator kill, etcd lease revocation, per-service env. New disruptions are new methods here.
• src/scenarios/merge.rs — the merge lane (--merge-concurrency): MergePersons calls with one or more sources (--merge-sources) against live persons while the other lanes write to them. Merged sources hang off the survivor in the journal tree and are retired from the shared src/pool.rs target pool.
• src/state.rs — the acked-write journal and its verifiers. New invariants go here, and their decision tables get unit tests: a false-negative verifier looks identical to a healthy stack, so e2e runs cannot reveal it.
• README.md — every scenario gets a runnable example; scenarios that stay red because of a known unfixed defect go in the "Known defects" section with their observable signature and fix direction.

Adding a chaos scenario

1. Add the stack primitive if the disruption is new (prefer process signals and etcd operations; never disrupt the shared docker containers — they also serve the dev stack).
2. Add the GateArgs flag, the ChaosEvent variant, and its chaos_timeline() entry. Timed events take an offset; paired disruptions (pause/resume) schedule both entries. State-triggered events (like --kill-handoff-target) poll etcd after their triggering event instead.
3. Guard invalid combinations in gate.rs with an early bail! (for example, coordinator kill requires two routers).
4. Rehearse locally until deterministic — three consecutive green runs is the bar (see the README for build and docker prerequisites). If timing makes a scenario a lottery, make it deterministic (the coordinator election is forced at bring-up for exactly this reason) rather than accepting flakes.
5. Only failed-but-unacked requests are tolerable during chaos; the invariant machinery needs no changes for a new disruption unless you're adding a new property, in which case journal it in state.rs with unit tests.
6. Add the scenario to the personhog-gate CI job in .github/workflows/ci-rust.yml once rehearsed — extend an existing composite run if compatible (keep runs to roughly three disruption kinds so failures stay diagnosable), or add a step. Expected-red scenarios (documenting a known defect) stay out of CI until the defect is fixed.

Validating a fix with the harness

Run the harness binary against binaries built from the fix branch:

cargo build -p personhog-replica -p personhog-router -p personhog-leader -p personhog-writer
personhog-test-harness gate <scenario flags> --bin-dir <fix-worktree>/rust/target/debug

Show the same command red on the base branch and green on the fix in the PR's testing section.