pmacs/docs/reap-ledger-silent-failures-framing.md

Framing — group cleanup fails silently at four sites

Revision 2. Status: awaiting review round 2. Proposed lane: reap-ledger-silent-failures, worktree ../pmacs-reap-ledger, based on githubsucks/main @ 22df6ab (a reading; re-measure at branch time).

Parked by PR #200's framing §5 and unparked by its evidence. #200 retired the premise that justified the ledger's leniency; it deliberately changed no disposition, and said so. This lane owns what it refused.

Revision history

Revision 1 → 2, after review round 1 (three blocking, two major). All five accepted; all five verified in the code first.

• §0 named the wrong journey step. It claimed step 8, "Open a terminal", while Q#RL5 in the same document says the ledger is unreachable for PTY children. Both cannot be true. The only production group = true caller is compile mode (builtin/runtime/compile.lua:820), so this is step 9, build/test, plus general pipe-process cleanup.
• A fourth site was missed. final_drain_runtime (:2331) probes kill(-pgid, None), treats every error as dead, discards its SIGKILL result and sets its own group_killed flag — and it enforces the ledger deadline while no tick runs. It is now in scope (§1.2a).
• The staging contradicted itself. "Bet 1 ships alone" against an acceptance requiring three failure-path tests, under one-branch / one-PR. A seam with no tests does not even prove it reaches the production calls. §7 now scopes the first PR as seam plus behaviour-preserving tests, which is still diagnosis-only.
• A generic seam is the wrong shape. shutdown() calls self.signal(*id, SIGKILL) before its ledger force-kill, so a single "next kill errno" would be eaten by the wrong call. The seam is now site-directed (§3 Bet 1).
• §1.3 overstated the loop coupling. Early exit needs the ledger empty and any_running() already false; a live managed record keeps the loop going regardless. The precondition is now stated.

Diagnosis first. No disposition change is proposed in this revision. §2 asks whether one is warranted; §5 parks every candidate until the lane's own evidence exists. That ordering is not caution for its own sake — Stage A of the signal lane had three tolerance rules rejected across three revisions, each because it concluded something about one entity from something that was not about that entity, and this is the same shape of problem on the same data structure.

0. Coherence impact (COHERENCE §20)

• Journey step 9, "Build and test" — every compile and grep run, plus general pipe-process cleanup. Not step 8: the ledger arms only for proc.spec.group, which spawn rejects for PTY mode, so no terminal ever reaches it (Q#RL5). The only production group = true caller is compile mode (builtin/runtime/compile.lua:820). No grade change proposed.
• Serves §9 (worker model), failure attribution. The ledger is the one mechanism that can see a survivor nothing else can, and today it cannot report that it failed to.
• Interaction islands: none. Config registry: not adopted. Background-work attribution: unchanged.
• No audited claim in COHERENCE.md changes; under §25 no COHERENCE edit rides this PR.

1. Ground truth (verified at 22df6ab)

1.1 What the ledger is for

tick_reap_ledger (src/process.rs:1472) exists for one case its own doc names: a TERM-ignoring descendant that survived its leader's clean exit with its output redirected. Neither leader state nor reader state can see that survivor — try_wait reports the leader, and the readers see a closed pipe. Only group liveness can (Q#CM3, round-3 finding 1).

That is the blast radius. A silent failure here leaks precisely the process the mechanism exists to catch, and nothing else in the supervisor is looking.

1.2 Three silent failures in the persistent ledger

// (a) any probe error drops the entry
if nix::sys::signal::kill(Pid::from_raw(-*pgid), None).is_err() {
    return false;
}
// (b) the escalating SIGKILL's result is discarded, and the entry is
//     marked killed regardless
if now >= entry.deadline && !entry.killed {
    let _ = nix::sys::signal::kill(Pid::from_raw(-*pgid), Some(Signal::SIGKILL));
    entry.killed = true;
}

(a) retain returning false deletes the entry, so escalation is cancelled. The comment is honest that this is a bounded-growth policy rather than a claim the group is gone — #200 corrected it — but the behaviour is unchanged: an EPERM probe is indistinguishable from ESRCH.

(b) A failed SIGKILL is recorded as a successful one. The entry then satisfies !entry.killed == false forever and is never retried.

(c) shutdown() has the same discard (:1763-1766), on the path that exists specifically to stop a leak at editor exit:

for (pgid, entry) in &mut self.reap_ledger {
    let _ = nix::sys::signal::kill(Pid::from_raw(-*pgid), Some(Signal::SIGKILL));
    entry.killed = true;
}

Its own comment says it is there because "a pre-deadline ledger ... would be silently discarded at Drop and leak the member". The fix for one silent leak was written with a discarded result of its own.

1.2a A fourth site: the in-drain twin

final_drain_runtime (:2331, called once from :1573) runs the same pattern on the same pgid, while no tick is running:

let group_alive = nix::sys::signal::kill(Pid::from_raw(-ctx.pgid), None).is_ok();
if group_alive && now >= ctx.deadline && !group_killed {
    let _ = nix::sys::signal::kill(Pid::from_raw(-ctx.pgid), Some(Signal::SIGKILL));
    group_killed = true;
}

is_ok() collapses every errno into "dead", exactly as the persistent ledger's is_err() does — and the consequence differs. A false "dead" here makes quiesced true, which sets rt.cancel and cancels the readers, so the failure mode is truncated output rather than a leaked process.

It is not identical to the persistent ledger and the framing does not claim it is. A later tick can retry the ledger entry; this decision is terminal for that drain. It is in scope because it is the same collapse on the same data with its own consequence, not because it is the same bug.

It constrains the seam. final_drain_runtime is a free function taking &RuntimeHandles and Option<GroupDrainCtx> — there is no &mut self to hang a supervisor field on, so the injected outcome has to arrive through GroupDrainCtx, populated at the :1573 call site.

1.3 The shutdown loop's exit condition depends on the silent drop

shutdown()'s final loop (:1773-1775) runs while self.any_running() || !self.reap_ledger.is_empty(), bounded at 2s, and calls tick() — which calls tick_reap_ledger.

So the loop terminates when the ledger empties, and the ledger empties via (a). On ESRCH that is correct: the group is gone. On any other errno the loop exits early, having concluded cleanup finished because the probe failed.

The precondition matters and revision 1 omitted it. The condition is a disjunction: any_running() || !reap_ledger.is_empty(). An early exit therefore needs the ledger empty and any_running() already false — a live managed record keeps the loop running whatever the ledger does. The coupling is real for the case the ledger exists to serve, the leader-exited survivor, and Bet 3 must build exactly that fixture rather than any group.

This coupling is the reason (a) cannot be changed casually: making the probe strict without touching the loop converts a silent early exit into a guaranteed 2-second stall at every editor exit that hits it.

1.4 None of the three has been observed

#200 observed an explicit SIGTERM failing in signal(), not any ledger call. What it established is narrower and still sufficient to open this lane: a group-directed kill computed from the spawn-time pgid == pid assumption returned EPERM while the leader was alive, on macOS, intermittently (run 30553376486).

That retires "EPERM cannot happen for our own children" as a reason to discard an arbitrary group error. It does not show that any of (a), (b) or (c) has fired. This lane must not claim otherwise, and §3's bets are built to find out rather than to assume.

1.5 The ledger's kills have no injection seam

signal() consults forced_kill_errno (Q#PD4), which is how #176 and #200 tested failure paths without provoking real errnos. tick_reap_ledger calls nix directly and consults nothing. Same for shutdown()'s loop.

So all three failure paths are, today, untestable. That is the first thing this lane has to fix, and it is a prerequisite for any disposition change rather than a nicety: a disposition change whose failure path cannot be exercised is a rule nobody can falsify.

1.6 What is tested today

liveness_probe_reaps_term_ignoring_survivor_after_leader_exit (:4274), repeated_terminate_does_not_extend_ledger_deadline (:4314), shutdown_force_kills_outstanding_ledger_groups (:4350), leader_exit_reap_bounds_drain_with_pipe_holding_descendant (:4415), and setsid_escapee_is_not_reaped_and_teardown_reclaims_readers (:4450).

Every one exercises the success path. None injects a probe or SIGKILL failure, because §1.5 makes that impossible.

repeated_terminate_does_not_extend_ledger_deadline is also the test that flaked on macOS in #191 — the occurrence that produced §1.4's evidence. It is load-sensitive and unrelated to what this lane changes; noted so a red run on it is not mistaken for this lane's doing.

1.7 Limits of the evidence

• Not reproduced. No ledger probe or SIGKILL has been seen to fail, on any platform.
• The mechanism is not established. Why a group-directed kill can return EPERM against a live owned child is still unknown — #200 narrowed it and explicitly did not solve it.
• Group identity remains unprovable (#200 §1.5). Nothing this lane measures can distinguish the original group from a recycled one; a pidfd covers a process, not a group, and macOS has neither.

2. Questions

• Q#RL1 — Should the probe distinguish ESRCH from other errnos? Unknown, and deliberately not proposed yet. It is the obvious change and it directly trades bounded growth (the stated original reason) for correctness, while §1.3 shows it also changes editor-exit timing. It needs evidence and its own review.
• Q#RL2 — Should a failed escalation be retried, and how many times? Unknown. killed = true on a failed SIGKILL is clearly wrong as bookkeeping; what should replace it is a policy question, not an obvious fix.
• Q#RL3 — How does a background tick report anything? The ledger emits no events and has no status channel. pmacs.error is defined only by a test stub and is dead at 15 call sites, so it is not the answer. This is the lane's real design question.
• Q#RL4 — Does the shutdown() loop need its own termination condition if (a) becomes strict? Almost certainly (§1.3), and that coupling is why the two cannot be changed independently.
• Q#RL5 — Is the ledger reachable for PTY children? No — spec.group is rejected at spawn for PTY mode, so the ledger is a pipe-path mechanism only. Stated because #200 revision 2 got this exact relationship wrong in the opposite direction.

3. Bets

• Bet 1 — the four sites can be made injectable, site by site, without changing behaviour. Not a generic "next kill errno": shutdown() calls self.signal(*id, SIGKILL) before its ledger force-kill, so a single shared one-shot would be consumed by the wrong call and the test would pass while proving nothing.

  The seam is directed and one-shot per site, with four independently addressable outcomes:

  Site	What is injected
  tick_reap_ledger probe	the kill(-pgid, None) result
  tick_reap_ledger escalation	the SIGKILL result
  shutdown() force-kill	the SIGKILL result
  final_drain_runtime probe / kill	both, via GroupDrainCtx (§1.2a)

  Bet 3's coupling test needs two at once — a failed shutdown force-kill and a failed subsequent probe — so the seam must express more than one pending outcome. A typed queue per site, or a per-site slot, either is acceptable; a single global slot is not.

  Fixture cleanup is part of the seam, not an afterthought. An uninjected outcome left armed leaks into the next test in the same binary, and these tests run single-threaded in CI. Each seam is consumed on use and asserted empty at fixture teardown.

  • Falsified if a site cannot take a directed outcome without restructuring the code under test — which would make the test a test of the restructuring. final_drain_runtime is the one at risk, being a free function.

• Bet 2 — each silent failure is demonstrable once injectable. With the seam: an EPERM probe drops an entry whose group is still alive; a failed SIGKILL leaves killed = true and is never retried; and shutdown()'s discard does the same at exit.

  • Falsified if any of the three turns out to be unreachable in practice — for instance if some earlier guard makes the entry already absent. That would be a genuinely good outcome and would shrink this lane rather than embarrass it.

• Bet 3 — the shutdown coupling is real and measurable. A test shows the final loop exiting early when the probe errors, rather than running to its 2s bound.

  • The fixture must have any_running() already false (§1.3): a leader that has exited leaving a group survivor. Any other shape tests the disjunction's other arm and proves nothing about the coupling.
  • Falsified if the loop still runs to its bound with the ledger emptied and no managed record live — in which case §1.3 overstates the coupling and the two changes can be separated after all.

• Bet 4 — a failure here is reportable at all. Q#RL3 has no answer yet. This bet is a scouting obligation, not a design: find every channel a background tick could use, and say plainly if none exists.

  • Falsified if the only available channel is one already known dead (pmacs.error) — in which case reporting becomes its own lane and this one ships instrumentation plus tests without it.

4. Acceptance

1. A directed, multi-outcome injection seam covering all four sites of §3 Bet 1, on Q#PD4's terms: result only, everything else production code. Consumed on use, asserted empty at teardown.
2. A test per silent failure, each asserting the observable consequence — entry dropped while the group lives; killed set after a failed kill; the same at shutdown; readers cancelled after a false "dead" in the drain — rather than that a function was called.
3. Each new test falsified by an actual revert, both directions recorded in the PR body.
4. The shutdown() coupling of §1.3 pinned by a test, whichever way Bet 3 resolves.
5. No disposition change. retain still drops on any error; killed is still set unconditionally. This lane makes the failures visible and testable; changing them is §5's.
6. Q#RL3 answered in the PR body with the channels actually found, or an explicit statement that none exists.
7. docs/agent-handoff.md records the three silent paths and that none has been observed — the distinction #200 had to make twice.

5. Parked

• Q#RL1's strict-ESRCH probe, Q#RL2's retry policy, and any other disposition change. Each needs this lane's evidence.
• Reporting, if Bet 4 finds no channel.
• Retargeting to the measured pgid, and any EPERM/ESRCH tolerance rule in signal() — still parked from #200, unchanged.
• signal_target's read-then-kill of tcgetpgrp on the PTY path — Stage A's "most likely real fix site", still unframed, still not this.
• Why a group-directed kill can return EPERM against a live owned child. The mechanism is unknown; this lane instruments the consequences, not the cause.

6. Gates

Standard suite, each its own step with a real exit status and nothing after the command that could mask it: cargo fmt --check; cargo clippy --workspace --all-targets -- -D warnings; cargo test --lib; cargo test --lib --features crdt; compile_mode_acceptance; terminal_copy_mode_acceptance (both feature configurations); cargo test --test m4_acceptance -- --skip basedpyright; PMACS_REQUIRE_GPU=1 cargo test -p pmacs-gpu; git diff --check.

All five bootstrap-storage variables controlled locally — XDG_CONFIG_HOME, XDG_DATA_HOME, XDG_STATE_HOME, XDG_CACHE_HOME, PMACS_STATE_HOME — per the ambient-root framing merged as #201. Isolating only the config root leaves the data-root write path open.

Process-supervisor tests are load-sensitive; the PR body records repetition counts rather than a single green.

7. Branch plan

One branch, one PR: the seam together with the tests that exercise it. Revision 1 said "Bet 1 ships alone", which contradicted an acceptance requiring failure-path tests under one-branch/one-PR — and was wrong on its own terms, because a seam with no tests does not even demonstrate that it reaches the intended production calls.

This is still diagnosis-only: every test pins current behaviour, including the behaviour that is wrong. Nothing in this PR changes what the supervisor does.

If Bet 2 finds a path unreachable, the lane shrinks and says so rather than manufacturing a failure to justify itself. If Q#RL3 finds no reporting channel, reporting becomes its own lane and this PR ships instrumentation plus tests without it.