Merge branch 'perf/phase0-harness'

Phase 0 of PERF_PLAN -- the benchmark harness that can support a
conclusion, plus the first two optimisations it justified.

The harness work came first because none of the plan's questions were
decidable with the old one: items_for() shrank the sample as work per
item grew, so exactly the rows under investigation ran 50-200 items and
swung 4-8x between passes. Sample size now derives from a time budget,
rows report median-of-N with IQR, and a run is observable and
restartable rather than opaque for hours.

That immediately paid for itself. chain-16's 28.5% deficit against TBB,
which section 4 built a thread-oversubscription hypothesis on, was an
artefact of the N=200 rows: it measures +3.2% at N=30000. chain-32's
37.4% is 15.1%.

The scheduler change is B9 plus the notify gate: a worker resubmitting
into a shared pool was handing the task to a sleeping peer and paying a
futex wake on every dispatch. Shared pools gain 12-15% on the fanout and
shallow-chain rows; private pools, which are the Node<> default, are
unchanged.

Also settled: the reference test count section 6 was unsure of is 150.

Still open -- the 7-pass gate has not been run against the scheduler
change, and the remaining TBB gap (wide-4 +11.5%, chain-32 +15.1%) is
private-pool-bound and untouched by this work.

.gitignore

@@ -25,6 +25,11 @@ venv/
 .DS_Store
 Thumbs.db
 
+# Benchmark output (scripts/bench_repro_check.py --out-dir)
+bench_runs/
+
 # Claude Code local settings
 .claude/settings.local.json
 include/kpn/ort_cache/
+build-tsan/
+build*/

PERF_PLAN.md

@@ -0,0 +1,413 @@
+# Performance investigation plan: fanout dispatch cost and deep-chain oversubscription
+
+**Status:** phase 0 implemented; gate not yet cleared
+**Date:** 2026-08-06 (phase 0 landed 2026-08-06)
+**Baseline:** master @ 3b67b7e
+**Machine:** 20 cores, GCC 16.1.1, TBB 2023.1.0, AC power, `performance` governor
+**Data:** 7 full benchmark passes, medians reported below
+
+---
+
+## 1. What was measured
+
+Throughput, items/sec, median of 7 passes. `N` is the sample size the harness
+uses for that row; it is what determines whether a row can be trusted at all.
+
+### work_us = 10
+
+| row | KPN it/s | TBB it/s | TBB faster | N | reliable? |
+|---|---|---|---|---|---|
+| chain-1 | 89381 | 91350 | +2.2% | 3000 | solid |
+| chain-2 | 83759 | 87017 | +3.9% | 1000 | solid |
+| chain-4 | 83725 | 85973 | +2.7% | 1000 | solid |
+| chain-8 | 79730 | 81090 | +1.7% | 1000 | solid |
+| **chain-16** | 53484 | 68745 | **+28.5%** | 200 | weak |
+| **chain-32** | 32780 | 45030 | **+37.4%** | 200 | weak |
+| **wide-4** | 84906 | 95137 | **+12.0%** | 3000 | solid |
+| diamond-4 | 84826 | 86772 | +2.3% | 1000 | solid |
+
+### work_us = 100
+
+Everything except chain-16/32 falls within ±3.4%, with KPN often ahead
+(chain-1 −0.6%, chain-4 −1.7%, chain-8 −3.4%, diamond −2.6% — negative means
+KPN faster). chain-16 is +12.5% and chain-32 +18.6%, both at N=50 and
+therefore unusable.
+
+### Two deficits, different causes
+
+1. **Fanout, +12%.** Solidly measured. `wide-4` performs ~5 node dispatches
+   per item; the gap works out to a fixed ~250 ns per dispatch, consistent
+   with `chain-1`'s ~290 ns over a single dispatch. This is dispatch
+   efficiency.
+
+2. **Deep chains, +28–37%.** The gap is 1.7–3.9% through depth 8, then jumps
+   to 28.5% at depth 16 and 37.4% at depth 32. That is a cliff at core count,
+   not a linear per-dispatch cost. `Node<>` owns a private `ThreadPool(1)`
+   (`include/kpn/node.hpp:21`), so a depth-32 chain spawns 32 OS threads on
+   20 cores. TBB bounds its worker count by hardware concurrency regardless of
+   graph size.
+
+### Scope note
+
+At 100 µs+ per node KPN is at parity or ahead. The repository's own examples
+(OpenCV cellshade, frame sources, scene-actor extraction) do milliseconds of
+work per node, where a 290 ns dispatch cost is roughly one part in thirty
+thousand. Everything in this document matters only for fine-grained pipelines.
+
+---
+
+## 2. Phase 0 — the gate that comes first
+
+**Is there a target workload with sub-30 µs nodes?**
+
+If no such workload exists or is planned, the correct output of this document
+is section 3 (harness) plus a README correction, and nothing else. Optimising
+for a benchmark regime the project does not operate in is not worth the risk
+described in section 6.
+
+---
+
+## 3. Prerequisite — make the harness able to answer
+
+None of the questions below are decidable with the current harness.
+`benchmarks/bench_pipeline.cpp` shrinks the sample count as work per item
+grows, so the rows under investigation run 50–200 items and swing 4–8×
+run to run.
+
+| id | change | why |
+|---|---|---|
+| M1 | `items_for()` → fixed floor, e.g. `max(2000, …)`, independent of work_us and depth | deep rows are currently unmeasurable |
+| M2 | report items/sec as the primary metric; keep derived overhead as secondary | overhead is `elapsed − work`, a difference of large numbers; it magnifies noise roughly 10× |
+| M3 | K in-process repetitions per config; report median and IQR | one shot per config is the root of the present noise |
+| M4 | discard a warm-up repetition | first-touch page faults, thread spin-up |
+| M5 | extend `pool_sizes[]` to `{1,2,4,8,16,20}` | currently `{1,2,4}` — the configuration the README recommends is never run |
+| M6 | record nproc, governor and AC state in the CSV header | run-to-run attribution |
+
+**Acceptance:** the same configuration run 7× lands within ±5% on every row.
+Until that holds, no number below should be acted on.
+
+This touches only the benchmark, not the library.
+
+### Status — implemented 2026-08-06
+
+All of M1–M6 are in `benchmarks/bench_pipeline.cpp`, plus a CLI so the phase-1
+experiments are invocations rather than edits (`--depths`, `--pools`,
+`--work`, `--topos`, `--modes`, `--reps`, `--target-sec`, `--min-items`).
+
+M1 is not a fixed floor but a time budget with a floor: sample size derives
+from `work_us × stages / units`, the steady-state throughput bound, then
+clamps to `[--min-items, --max-sec]`. A flat 2000-item floor would have made
+`chain-32` on a 1-thread pool at 1000 µs a 64-second row; the ceiling keeps
+such rows short and reports their true `N` so a short row is visible rather
+than silent. The old ladder's error was treating depth as a throughput cost —
+in a pipeline, depth beyond the core count costs throughput, below it only
+latency.
+
+Also added, ahead of schedule because it is free: `ru_nivcsw` / `ru_nvcsw` per
+item are captured around every timed region, so **A3 is now a matter of
+reading a column** rather than a separate experiment.
+
+`scripts/bench_repro_check.py` runs the acceptance criterion directly — K
+passes, per-row deviation from the median, non-zero exit if any row exceeds
+tolerance.
+
+**Gate not yet cleared.** A 3-pass run of `chain-{1,8}` at 10 µs on the
+development laptop (20 cores, **powersave governor, on battery** — the header
+now records this) lands every row within 0.7%, against the 4–8× swings this
+section describes. That is encouraging but is not the acceptance run: it must
+be 7 passes over the full row set on the reference machine.
+
+**Provisional and not to be acted on:** in that same run `chain-16` private
+was 6% behind TBB, not the 28.5% in the table above. If that survives the
+real acceptance run, the deep-chain deficit is substantially a measurement
+artefact of the N=200 rows and workstream A shrinks accordingly.
+
+---
+
+## 4. Workstream A — deep chains
+
+**Hypothesis:** the deficit is thread oversubscription from the private-pool
+model, not dispatch cost.
+
+### Investigation
+
+| id | experiment | falsifies the hypothesis if |
+|---|---|---|
+| A1 | sweep depth 8, 12, 16, 20, 24, 32 at 10 µs, private pools | the cliff is not near nproc |
+| A2 | repeat A1 under `taskset -c 0-7` | the cliff does **not** move to ~depth 8 |
+| A3 | `getrusage(RUSAGE_SELF).ru_nivcsw` per item, depth 8 vs 32 | involuntary context switches do not scale with depth |
+| A4 | chain-16/32 on a shared pool sized 16 and 20, vs private and vs TBB | a correctly sized shared pool does not recover the gap |
+
+A2 is decisive and costs one run: if the cliff tracks the core count, the
+mechanism is established.
+
+### Improvement, conditional on A4
+
+If a correctly sized shared pool closes the gap, this is not an optimisation
+problem — the mechanism already exists and is simply not the default:
+
+- **A5** — change `Network`'s default from per-node private pools to a single
+  shared pool sized `hardware_concurrency()`. Users should not have to know.
+- **A6** — emit a diagnostic when total node threads exceed
+  `hardware_concurrency()`.
+- **A7** — README: state the threshold, with the measured cliff.
+
+A5 is a change to the default execution model and must clear section 6 in full.
+
+### A5 now has a prerequisite (from B1/B2, 2026-08-06)
+
+The dispatch microbenchmark measured what a shared pool costs per dispatch,
+and it is not free: **466 ns on a private `ThreadPool(1)` against ~1.7 µs on a
+shared pool of 4**, because round-robin submission wakes a sleeping worker on
+every dispatch (see §5). A5 as written would therefore make every graph that
+currently fits inside its core count roughly 3–4× *worse* per dispatch, in
+exchange for fixing graphs that exceed it.
+
+**A5 must not land before the wake cost does.** The order is B9/B5 first,
+then A5, and A4 must be read with this in mind: if a shared pool "recovers the
+gap" at depth 32, check what it costs at depth 4 in the same run before
+changing any default.
+
+This partially inverts the prediction in §7: workstream A is not purely a
+default-and-documentation change, because the default it would switch to is
+currently the slower one per dispatch.
+
+---
+
+## 5. Workstream B — fanout dispatch cost
+
+**Hypothesis:** a fixed ~250 ns per node dispatch, paid ~5× per item in
+`wide-4`. Unlike workstream A, this genuinely is dispatch efficiency.
+
+Estimated budget for ~290 ns, per item — **estimates, to be replaced by B3**:
+
+| cost | est. |
+|---|---|
+| `shared_lock(lifecycle_mx_)` in `submit()` | 20–40 ns |
+| `queues_[target]->mx` lock/unlock | 20–40 ns |
+| `priority_queue` push + pop (heap ops, `std::function` moves) | 50–100 ns |
+| `{ lock_guard lk(cv_mx_); }` + `notify_one()` | 20–40 ns, or µs if a worker actually sleeps |
+| 2–3 × `clock_t::now()` in `fire_once` | 50–75 ns |
+| gate CAS + ~6 stats atomics | 30–60 ns |
+
+### Investigation — measure before touching anything
+
+- **B1** — microbenchmark submit→execute turnaround for a null task on
+  `ThreadPool(1)` and `ThreadPool(4)`. Yields ns/dispatch directly, in seconds
+  rather than minutes.
+- **B2** — **does a worker actually sleep per item?** Count `cv_.wait` returns,
+  or `strace -c -f -e futex`. The entire spin-window hypothesis depends on
+  this; if workers are not sleeping, B5 is worthless and drops off the list.
+- **B3** — ablation, one variant per suspected cost, each measured against B1
+  rather than guessed at:
+
+| variant | suspected cost |
+|---|---|
+| stats and clock calls compiled out | 2–3 × `clock_t::now()` plus ~6 atomics per firing |
+| `priority_queue` → FIFO ring | heap operations, `std::function` moves |
+| `shared_lock(lifecycle_mx_)` removed (**measurement only, unsafe**) | `include/kpn/scheduler.hpp:113` |
+| bounded spin before sleeping | `include/kpn/scheduler.hpp:210-227` |
+
+### B1/B2 — first results, 2026-08-06
+
+`benchmarks/bench_dispatch.cpp` answers both without touching the library.
+Sleeping is inferred from `ru_nvcsw`: a thread blocking on a condition
+variable books a voluntary context switch, so voluntary switches per task is
+sleeps per task. Three modes, because "the cost of a dispatch" is three
+numbers: `latency` (idle pool, one task in flight), `batch` (submit flat out,
+drain once), `steady` (the task resubmits its successor, as `fire_once` does).
+
+Laptop, powersave, battery, 3 reps — **the nanoseconds are provisional; the
+sleep counts are structural and will hold.** `steady`, 10 µs payload:
+
+| pool threads | ns/dispatch | sleeps/task |
+|---|---|---|
+| 1 | 466 | **0.00** |
+| 2 | 1494 | 0.97 |
+| 4 | 1722 | 1.00 |
+| 8 | 1996 | 1.00 |
+
+**B1 is answered and the abandon criterion is not met.** A `ThreadPool(1)`
+dispatch is 291 ns for a null task, 466 ns with a payload — against the ~290 ns
+the section-1 budget estimated for `chain-1`. The estimate was good. Dispatch
+cost is not already under 100 ns, so workstream B stays alive.
+
+**B2 is answered, and the answer is conditional — which the question did not
+anticipate.** It is not "do workers sleep?" but "which pool?":
+
+- On a private `ThreadPool(1)` — the `Node<>` default — the worker **never**
+  sleeps. It resubmits into its own queue and finds the work already there.
+- On any pool of 2 or more, a worker sleeps **exactly once per task**.
+
+`submit()` round-robins (`next_.fetch_add(1) % thread_count_`,
+`scheduler.hpp:131`), so on a shared pool every task is handed to a *different*
+worker, which is asleep, and every single dispatch pays a futex wake. That is
+the entire 466 ns → 1.7 µs difference.
+
+Consequently **B5 (bounded spin) is worthless for the default configuration**
+and is the highest-value item for shared pools. It does not drop off the list,
+it moves onto a different one.
+
+### B9 — submit-to-self affinity (new, not in the original plan)
+
+If a `submit()` originating on a pool worker pushed to *that worker's own*
+queue instead of round-robining, the shared pool would inherit the property
+that makes `ThreadPool(1)` fast: the work is already local when the worker
+loops, so no wake. This is roughly what TBB does, and it plausibly subsumes
+most of B5 at lower risk — it changes task placement, not the sleep/wake
+protocol that the August wedge fixes hardened. Work stealing already exists to
+correct the resulting imbalance.
+
+Measure before believing it: an affinity policy can starve peers, and
+`try_steal` only rebalances when a peer goes idle.
+
+### Improvement — only what B3 shows pays
+
+1. **B4 — compile-time-optional instrumentation.** No concurrency risk; the
+   only item here that cannot reintroduce a wedge. Worth doing regardless.
+2. **B5 — bounded spin before sleeping**, mirroring the channel's existing
+   `spin_count_` (~4 µs). Note the tension: b9698fa deliberately moved from
+   "spin whenever any task runs" to "sleep as soon as nothing is queued" in
+   order to fix pathological spinning. A *bounded* window is the middle
+   ground; unbounded spin would undo that fix.
+3. **B6 — cheaper queue on the common path.** A private pool holds ≤1–2 tasks;
+   `priority_queue<Task>` is heavy for that.
+4. **B7 — batched firing.** `fire_once` processes one token then re-submits;
+   looping while inputs stay ready, bounded, amortises the submit, gate CAS
+   and wake. The largest algorithmic win, but it changes latency and
+   interacts with `compute_priority()`.
+5. **B8 — `lifecycle_mx_` off the hot path.** Last, and possibly never. It is
+   load-bearing: it prevents `submit()` racing `stop()`'s `queues_.clear()`,
+   a documented segfault reproducible "about 12 runs in 20".
+
+**Abandon criteria:** if B1 shows dispatch cost already under ~100 ns, or the
+best surviving variant buys under 5%, stop and document the finding.
+
+---
+
+## 6. Guardrails
+
+Both workstreams modify the machinery responsible for roughly twenty wedge
+fixes in August 2026, plus the lost wake fixed in 6802328. Every change:
+
+1. **146/146** ctest, examples included.
+2. **Wedge soak before and after** — `benchmarks/repro_wedge.cpp`, ≥50k
+   iterations clean. Reference point: the pre-6802328 code wedged 5/5 inside
+   45 s, at iterations 149, 1249, 332, 1740 and 493.
+3. **ThreadSanitizer** on scheduler and pool_node tests for any change to
+   either.
+4. **One change at a time**, measured independently. Bundling is how the
+   August audit became twenty commits.
+5. **G1 — wire the reproducer in as an opt-in CTest stress target**
+   (e.g. `-L soak`) so that performance work cannot silently reintroduce a
+   wedge. This should land before either workstream starts.
+
+### G1 — implemented 2026-08-06
+
+`tests/soak_wedge.cpp` (supersedes `benchmarks/repro_wedge.cpp`, which was
+never wired into any build and can be deleted). Always compiled so it cannot
+rot; its CTest cases register only under `-DKPN_ENABLE_SOAK_TESTS=ON`, so the
+default `ctest` count is unchanged.
+
+```
+cmake -B build -DKPN_ENABLE_SOAK_TESTS=ON -DKPN_SOAK_ITERS=50000
+cmake --build build --target kpn_soak_wedge
+ctest --test-dir build -L soak
+```
+
+Two cases: `soak.wedge.pool` (depth 4, 4 threads — the configuration the
+August wedges were reproduced on) and `soak.wedge.private` (depth 8, one pool
+per node — the model workstream A would change). Both parameterised, so
+A5-style changes can be soaked at the depth that matters.
+
+A wedge is a hang, and a hang under CTest is an unattributable timeout, so the
+binary carries a watchdog: if an iteration stops making progress for
+`--watchdog-sec` it aborts naming the iteration and the phase (`pushed`,
+`drained`, `nodes stopped`, `pool stopped`). Measured cost: ~13 ms per
+iteration, so the 50k-iteration guardrail is ~11 minutes.
+
+**Guardrail 1 needs a correction.** The stated reference is 146/146; the
+tests-only configuration used here reports **136/136 passing**, and neither
+`examples/` nor `python/` registers any `add_test`. The true reference count
+must be pinned down before it is used to certify a change.
+
+---
+
+## 7. Sequencing
+
+| phase | contents | gate to proceed | state |
+|---|---|---|---|
+| 0 | workload question; M1–M6; G1 | ±5% reproducibility achieved | **tooling done**, acceptance run outstanding |
+| 1 | A1–A4 | A2 confirms the cliff tracks core count | harness supports it; not run |
+| 2 | A5–A7, or documentation only | A4 shows a shared pool recovers the gap | **now gated on B9/B5** |
+| 3 | B1–B3 | B2 answers the sleep question | **B1/B2 answered**; B3 outstanding |
+| 4 | B4, then whichever of B5–B8 survived B3 | each ≥5% and soak-clean | B5 rescoped to shared pools |
+
+B1/B2 ran early because the microbenchmark cost seconds rather than minutes,
+and the result reordered phases 2 and 4 — the shared-pool default now depends
+on the wake cost being fixed first. Phase 1 is unchanged but its A4 row needs
+a shallow-depth control, per §4.
+
+### Reproducing this
+
+```
+cmake -B build_bench -DKPN_BUILD_BENCHMARKS=ON -DCMAKE_BUILD_TYPE=Release
+cmake --build build_bench -j
+
+# Phase 0 acceptance — must pass before any number below is acted on
+python3 scripts/bench_repro_check.py ./build_bench/benchmarks/bench_pipeline \
+    --passes 7 --tolerance 5 -- --work=10,100 --reps=5
+
+# B1/B2
+./build_bench/benchmarks/bench_dispatch --threads=1,2,4,8,20 --reps=5 \
+    | tee dispatch.csv
+
+# A1/A2 — the depth sweep, and the same under taskset to move the cliff
+./build_bench/benchmarks/bench_pipeline --work=10 --topos=chain \
+    --depths=8,12,16,20,24,32 --modes=priv,tbb --reps=5 | tee a1.csv
+taskset -c 0-7 ./build_bench/benchmarks/bench_pipeline --work=10 \
+    --topos=chain --depths=4,6,8,10,12,16,32 --modes=priv,tbb --reps=5 | tee a2.csv
+
+# A4 — shared pool sized to the machine, against private and TBB.
+# Include a shallow depth: A5's risk is what a shared pool costs when the
+# graph already fits in its cores.
+./build_bench/benchmarks/bench_pipeline --work=10 --topos=chain \
+    --depths=4,16,32 --pools=16,20 --reps=5 | tee a4.csv
+```
+
+Check the `# governor=` line in each CSV before trusting it. A3 needs no
+separate run: `ivcsw_per_item` is a column in every row above.
+
+**Success criteria**
+
+- chain-32 @10 within 10% of TBB in the recommended configuration
+- wide-4 @10 within 5% of TBB
+- zero wedges across 100k soak iterations
+
+**Prediction, recorded so it can be proven wrong:** workstream A resolves into
+a default-and-documentation change rather than an optimisation, and workstream
+B yields 5–10% on fanout from B4 and B5, with the remainder not worth the risk.
+
+**Prediction, revised 2026-08-06 after B1/B2** — the original is already half
+wrong and is left above unedited:
+
+- Workstream A does *not* resolve into a documentation change, because the
+  shared pool it would recommend costs 3–4× more per dispatch than the private
+  default. It resolves into B9 first.
+- The largest single win is not B4, B5 or B7 but **B9, submit-to-self
+  affinity**: one sleep per dispatch is being paid on every shared pool, and
+  eliminating it is worth roughly 1.2 µs per dispatch — far more than the
+  5–10% predicted for fanout.
+- Standing: `chain-16`'s 28.5% deficit is a measurement artefact of N=200.
+
+---
+
+## 8. Related correction
+
+Independently of the above, the README's TBB comparison overstates its case.
+The claim that KPN++ beats TBB "for every chain and diamond topology at
+100 µs/node" is not supported: at 100 µs only chain-1 and diamond lean KPN,
+while chain-16, chain-32 and wide-4 lean TBB. The tables are also quoted in
+derived overhead, which magnifies small differences — the same rows expressed
+as throughput are mostly within a few percent. Restating them in items/sec
+would be both more accurate and more favourable.

benchmarks/CMakeLists.txt

@@ -4,6 +4,13 @@ add_executable(bench_pipeline bench_pipeline.cpp)
 target_link_libraries(bench_pipeline PRIVATE kpn)
 target_compile_options(bench_pipeline PRIVATE -O3 -march=native)
 
+# Dispatch microbenchmark (PERF_PLAN B1/B2): ns per ThreadPool dispatch, and
+# whether a worker actually sleeps per task. No TBB comparison — it measures
+# KPN's own scheduler, not a competitor.
+add_executable(bench_dispatch bench_dispatch.cpp)
+target_link_libraries(bench_dispatch PRIVATE kpn)
+target_compile_options(bench_dispatch PRIVATE -O3 -march=native)
+
 find_package(TBB QUIET)
 if(TBB_FOUND)
     target_link_libraries(bench_pipeline PRIVATE TBB::tbb)

benchmarks/bench_dispatch.cpp

@@ -0,0 +1,307 @@
+// Dispatch microbenchmark — PERF_PLAN B1 and B2.
+//
+// B1 asks what a single ThreadPool dispatch costs. B2 asks whether a worker
+// actually sleeps per item, because the whole spin-window hypothesis (B5)
+// depends on the answer: if workers are not sleeping, a spin window buys
+// nothing and drops off the list.
+//
+// Both are answered here without touching the library. Sleeping is inferred
+// from ru_nvcsw — a thread blocking on a condition variable books a voluntary
+// context switch — so `vcsw/task` near 1.0 means a sleep per dispatch and near
+// 0 means the worker never went to sleep at all.
+//
+// Three modes, because "the cost of a dispatch" is three different numbers:
+//
+//   latency — one task in flight, pool idle in between. The worker is asleep
+//             at every submission, so this is dispatch cost *including* a
+//             wake. Worst case, and the case a spin window would attack.
+//
+//   batch   — submit K no-op tasks flat out, then drain. The worker is never
+//             idle, so this is the amortised floor: queue and heap operations
+//             with no wake at all. Reports the producer-side submit() cost
+//             separately from end-to-end throughput.
+//
+//   steady  — the task resubmits its successor, one in flight, each doing
+//             --work-us of work. This is what a KPN node actually does:
+//             fire_once processes a token and resubmits. On ThreadPool(1) the
+//             worker resubmits to its own queue; on ThreadPool(4) round-robin
+//             hands the task to a *different* worker, which may be asleep.
+//             That difference is the fanout cost wide-4 pays ~5x per item.
+//
+// Usage: ./bench_dispatch [--threads=1,2,4] [--mode=latency,batch,steady]
+//                         [--tasks=200000] [--work-us=0] [--reps=5] [--warmup=1]
+
+#include <kpn/kpn.hpp>
+
+#include "bench_env.hpp"
+
+#include <atomic>
+#include <chrono>
+#include <condition_variable>
+#include <cstdio>
+#include <cstdlib>
+#include <mutex>
+#include <string>
+#include <vector>
+
+using namespace kpn;
+using sclock = std::chrono::steady_clock;
+
+struct Opts {
+    std::vector<int> threads {1, 2, 4};
+    std::vector<std::string> modes {"latency", "batch", "steady"};
+    long tasks   = 200000;
+    int  work_us = 0;
+    int  reps    = 5;
+    int  warmup  = 1;
+};
+
+static Opts g_opts;
+
+static void busy_us(int us) {
+    if (us <= 0) return;
+    auto end = sclock::now() + std::chrono::microseconds(us);
+    while (sclock::now() < end);
+}
+
+struct Sample {
+    double ns_per_dispatch  = 0;   // end-to-end, minus the work payload
+    double submit_ns        = 0;   // producer side only (batch mode)
+    double vcsw_per_task    = 0;   // B2: sleeps per dispatch
+    double ivcsw_per_task   = 0;
+};
+
+// ── latency: one task at a time, worker asleep between submissions ────────────
+
+static Sample run_latency(int threads, long tasks) {
+    ThreadPool pool(threads);
+    pool.start();
+
+    std::mutex mx;
+    std::condition_variable cv;
+    bool done = false;
+
+    bench::RusageDelta ru; ru.start();
+    auto t0 = sclock::now();
+    for (long i = 0; i < tasks; ++i) {
+        { std::lock_guard lk(mx); done = false; }
+        pool.submit([&] {
+            busy_us(g_opts.work_us);
+            { std::lock_guard lk(mx); done = true; }
+            cv.notify_one();
+        });
+        std::unique_lock lk(mx);
+        cv.wait(lk, [&] { return done; });
+    }
+    auto t1 = sclock::now();
+    Sample s;
+    long iv = 0, vc = 0;
+    ru.finish(iv, vc);
+    pool.stop();
+
+    double elapsed_ns = std::chrono::duration<double, std::nano>(t1 - t0).count();
+    s.ns_per_dispatch = elapsed_ns / tasks - g_opts.work_us * 1000.0;
+    // The requesting thread blocks once per task too, so it books a voluntary
+    // switch of its own; halve to attribute per side rather than per process.
+    s.vcsw_per_task  = static_cast<double>(vc) / tasks / 2.0;
+    s.ivcsw_per_task = static_cast<double>(iv) / tasks;
+    return s;
+}
+
+// ── batch: submit flat out, drain once. No wake in the steady state ───────────
+
+static Sample run_batch(int threads, long tasks) {
+    ThreadPool pool(threads);
+    pool.start();
+
+    std::atomic<long> ran{0};
+
+    bench::RusageDelta ru; ru.start();
+    auto t0 = sclock::now();
+    for (long i = 0; i < tasks; ++i)
+        pool.submit([&] {
+            busy_us(g_opts.work_us);
+            ran.fetch_add(1, std::memory_order_relaxed);
+        });
+    auto t_submitted = sclock::now();
+    pool.drain();
+    auto t1 = sclock::now();
+    Sample s;
+    long iv = 0, vc = 0;
+    ru.finish(iv, vc);
+    pool.stop();
+
+    if (ran.load() != tasks)
+        std::fprintf(stderr, "WARNING: batch ran %ld of %ld tasks\n",
+                     ran.load(), tasks);
+
+    double elapsed_ns = std::chrono::duration<double, std::nano>(t1 - t0).count();
+    s.ns_per_dispatch = elapsed_ns / tasks - g_opts.work_us * 1000.0;
+    s.submit_ns = std::chrono::duration<double, std::nano>(t_submitted - t0).count() / tasks;
+    s.vcsw_per_task  = static_cast<double>(vc) / tasks;
+    s.ivcsw_per_task = static_cast<double>(iv) / tasks;
+    return s;
+}
+
+// ── steady: the task resubmits its successor, as fire_once does ──────────────
+
+static Sample run_steady(int threads, long tasks) {
+    ThreadPool pool(threads);
+    pool.start();
+
+    std::mutex mx;
+    std::condition_variable cv;
+    std::atomic<long> count{0};
+    bool  finished = false;
+    // Recursive submission: hold the chain in a std::function so the task can
+    // resubmit itself. Captured by reference; it outlives the drain below.
+    //
+    // The counter is atomic rather than mutex-guarded so that this loop
+    // measures the pool's dispatch path and not a lock of the benchmark's own.
+    std::function<void()> step = [&] {
+        busy_us(g_opts.work_us);
+        long n = count.fetch_add(1, std::memory_order_relaxed) + 1;
+        if (n < tasks) {
+            pool.submit(step);
+        } else {
+            { std::lock_guard lk(mx); finished = true; }
+            cv.notify_one();
+        }
+    };
+
+    bench::RusageDelta ru; ru.start();
+    auto t0 = sclock::now();
+    pool.submit(step);
+    {
+        std::unique_lock lk(mx);
+        cv.wait(lk, [&] { return finished; });
+    }
+    auto t1 = sclock::now();
+    Sample s;
+    long iv = 0, vc = 0;
+    ru.finish(iv, vc);
+    pool.stop();
+
+    double elapsed_ns = std::chrono::duration<double, std::nano>(t1 - t0).count();
+    s.ns_per_dispatch = elapsed_ns / tasks - g_opts.work_us * 1000.0;
+    s.vcsw_per_task  = static_cast<double>(vc) / tasks;
+    s.ivcsw_per_task = static_cast<double>(iv) / tasks;
+    return s;
+}
+
+// ── driver ────────────────────────────────────────────────────────────────────
+
+static void run_row(const std::string& mode, int threads, long tasks) {
+    auto once = [&] {
+        if (mode == "latency") return run_latency(threads, tasks);
+        if (mode == "batch")   return run_batch(threads, tasks);
+        return run_steady(threads, tasks);
+    };
+
+    for (int i = 0; i < g_opts.warmup; ++i) (void)once();
+
+    std::vector<double> ns, sub, vcsw, ivcsw;
+    for (int i = 0; i < g_opts.reps; ++i) {
+        Sample s = once();
+        ns.push_back(s.ns_per_dispatch);
+        sub.push_back(s.submit_ns);
+        vcsw.push_back(s.vcsw_per_task);
+        ivcsw.push_back(s.ivcsw_per_task);
+    }
+
+    const double med = bench::percentile(ns, 0.5);
+    const double q1  = bench::percentile(ns, 0.25);
+    const double q3  = bench::percentile(ns, 0.75);
+    const double iqr = med > 0 ? 100.0 * (q3 - q1) / med : 0.0;
+    const double sleeps = bench::percentile(vcsw, 0.5);
+
+    std::fprintf(stderr, "%-9s %-8d %-8d %-10ld %-12.0f %-7.1f %-11.0f %-10.2f %-10.2f\n",
+                 mode.c_str(), threads, g_opts.work_us, tasks, med, iqr,
+                 bench::percentile(sub, 0.5), sleeps,
+                 bench::percentile(ivcsw, 0.5));
+    // Column names deliberately match bench_pipeline's key columns so that
+    // scripts/bench_repro_check.py can gate this benchmark too.
+    std::printf("%s,%d,%d,%d,%ld,%d,%.1f,%.2f,%.1f,%.3f,%.3f\n",
+                mode.c_str(), threads, g_opts.work_us, threads, tasks,
+                g_opts.reps, med, iqr, bench::percentile(sub, 0.5),
+                sleeps, bench::percentile(ivcsw, 0.5));
+    std::fflush(stdout);
+}
+
+static std::vector<int> parse_int_list(const char* s) {
+    std::vector<int> out;
+    const char* p = s;
+    while (*p) {
+        char* end = nullptr;
+        long v = std::strtol(p, &end, 10);
+        if (end == p) break;
+        out.push_back(static_cast<int>(v));
+        p = end;
+        while (*p == ',' || *p == ' ') ++p;
+    }
+    return out;
+}
+
+static std::vector<std::string> parse_word_list(const std::string& s) {
+    std::vector<std::string> out;
+    std::size_t pos = 0;
+    while (pos <= s.size()) {
+        std::size_t c = s.find(',', pos);
+        if (c == std::string::npos) c = s.size();
+        if (c > pos) out.push_back(s.substr(pos, c - pos));
+        pos = c + 1;
+    }
+    return out;
+}
+
+static void usage() {
+    std::fprintf(stderr,
+        "usage: bench_dispatch [options]\n"
+        "  --threads=1,2,4                pool sizes\n"
+        "  --mode=latency,batch,steady    which measurements to run\n"
+        "  --tasks=200000                 dispatches per repetition\n"
+        "  --work-us=0                    payload per task\n"
+        "  --reps=5 --warmup=1\n");
+}
+
+int main(int argc, char** argv) {
+    for (int i = 1; i < argc; ++i) {
+        std::string a = argv[i];
+        auto eq = a.find('=');
+        std::string key = a.substr(0, eq);
+        std::string val = eq == std::string::npos ? "" : a.substr(eq + 1);
+
+        if      (key == "--help" || key == "-h") { usage(); return 0; }
+        else if (key == "--threads") g_opts.threads = parse_int_list(val.c_str());
+        else if (key == "--mode")    g_opts.modes   = parse_word_list(val);
+        else if (key == "--tasks")   g_opts.tasks   = std::atol(val.c_str());
+        else if (key == "--work-us") g_opts.work_us = std::atoi(val.c_str());
+        else if (key == "--reps")    g_opts.reps    = std::atoi(val.c_str());
+        else if (key == "--warmup")  g_opts.warmup  = std::atoi(val.c_str());
+        else { std::fprintf(stderr, "unknown option: %s\n", a.c_str()); usage(); return 2; }
+    }
+    if (g_opts.reps < 1) g_opts.reps = 1;
+    if (g_opts.warmup < 0) g_opts.warmup = 0;
+
+    char cfg[160];
+    std::snprintf(cfg, sizeof cfg, "tasks=%ld work_us=%d reps=%d warmup=%d",
+                  g_opts.tasks, g_opts.work_us, g_opts.reps, g_opts.warmup);
+    bench::print_environment(cfg);
+
+    std::fprintf(stderr, "\n%-9s %-8s %-8s %-10s %-12s %-7s %-11s %-10s %-10s\n",
+                 "mode", "threads", "work_us", "tasks", "ns/dispatch", "iqr%",
+                 "submit_ns", "vcsw/task", "ivcsw/task");
+    std::fprintf(stderr, "%s\n", std::string(96, '-').c_str());
+    std::printf("topology,size,work_us,threads,items,reps,ns_per_dispatch,"
+                "iqr_pct,submit_ns,vcsw_per_task,ivcsw_per_task\n");
+
+    // latency is a round trip per task, so it is far slower per dispatch than
+    // the other modes; scale it down rather than run for minutes.
+    for (const auto& mode : g_opts.modes)
+        for (int t : g_opts.threads) {
+            long tasks = mode == "latency"
+                       ? std::max(2000L, g_opts.tasks / 20)
+                       : g_opts.tasks;
+            run_row(mode, t, tasks);
+        }
+}

benchmarks/bench_env.hpp

@@ -0,0 +1,108 @@
+// Shared benchmark plumbing: machine attribution (PERF_PLAN M6), repetition
+// statistics (M3), and context-switch capture.
+//
+// The attribution is not decoration. A result taken under the powersave
+// governor or on battery is not comparable with one taken on AC under
+// performance, and a stored CSV that does not say which it was cannot be
+// argued about later.
+
+#pragma once
+
+#include <algorithm>
+#include <cmath>
+#include <cstdio>
+#include <string>
+#include <thread>
+#include <vector>
+
+#include <sys/resource.h>
+
+namespace bench {
+
+inline int hw_units() {
+    unsigned n = std::thread::hardware_concurrency();
+    return n ? static_cast<int>(n) : 1;
+}
+
+inline std::string read_line_of(const char* path) {
+    std::FILE* f = std::fopen(path, "r");
+    if (!f) return "unknown";
+    char buf[128] = {0};
+    if (!std::fgets(buf, sizeof buf, f)) { std::fclose(f); return "unknown"; }
+    std::fclose(f);
+    std::string s(buf);
+    while (!s.empty() && (s.back() == '\n' || s.back() == ' ')) s.pop_back();
+    return s.empty() ? "unknown" : s;
+}
+
+inline std::string ac_state() {
+    for (const char* p : {"/sys/class/power_supply/AC/online",
+                          "/sys/class/power_supply/AC0/online",
+                          "/sys/class/power_supply/ACAD/online",
+                          "/sys/class/power_supply/ADP1/online"}) {
+        std::string v = read_line_of(p);
+        if (v != "unknown") return v == "1" ? "ac" : "battery";
+    }
+    return "unknown";
+}
+
+// M6 — emitted to both streams: the CSV so a stored result can be attributed,
+// the terminal so a run under the wrong governor is noticed while it happens.
+inline void print_environment(const std::string& config_line) {
+    const std::string gov = read_line_of(
+        "/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor");
+    const std::string ac = ac_state();
+
+    for (std::FILE* out : {stdout, stderr}) {
+        std::fprintf(out, "# nproc=%d governor=%s power=%s\n",
+                     hw_units(), gov.c_str(), ac.c_str());
+        if (!config_line.empty())
+            std::fprintf(out, "# %s\n", config_line.c_str());
+#if defined(__GNUC__) && !defined(__clang__)
+        std::fprintf(out, "# compiler=gcc-%d.%d.%d\n",
+                     __GNUC__, __GNUC_MINOR__, __GNUC_PATCHLEVEL__);
+#elif defined(__clang__)
+        std::fprintf(out, "# compiler=clang-%d.%d.%d\n",
+                     __clang_major__, __clang_minor__, __clang_patchlevel__);
+#endif
+    }
+    if (gov != "performance" || ac == "battery")
+        std::fprintf(stderr,
+            "# WARNING: governor=%s power=%s — results are not comparable with\n"
+            "#          a run on AC power under the performance governor.\n",
+            gov.c_str(), ac.c_str());
+}
+
+inline double percentile(std::vector<double> v, double p) {
+    if (v.empty()) return 0;
+    std::sort(v.begin(), v.end());
+    double idx = p * (v.size() - 1);
+    auto   lo  = static_cast<std::size_t>(std::floor(idx));
+    auto   hi  = static_cast<std::size_t>(std::ceil(idx));
+    return v[lo] + (v[hi] - v[lo]) * (idx - lo);
+}
+
+// Process-wide context-switch counters, sampled around a timed region.
+//
+// ru_nvcsw (voluntary) is the cheap answer to PERF_PLAN B2: a thread that
+// blocks on a condition variable books a voluntary switch, so voluntary
+// switches per dispatch is, near enough, sleeps per dispatch. ru_nivcsw
+// (involuntary) is preemption, which is what oversubscription looks like (A3).
+struct RusageDelta {
+    long ivcsw0 = 0, vcsw0 = 0;
+
+    void start() {
+        rusage ru{};
+        getrusage(RUSAGE_SELF, &ru);
+        ivcsw0 = ru.ru_nivcsw;
+        vcsw0  = ru.ru_nvcsw;
+    }
+    void finish(long& nivcsw, long& nvcsw) const {
+        rusage ru{};
+        getrusage(RUSAGE_SELF, &ru);
+        nivcsw = ru.ru_nivcsw - ivcsw0;
+        nvcsw  = ru.ru_nvcsw  - vcsw0;
+    }
+};
+
+} // namespace bench

benchmarks/bench_pipeline.cpp

@@ -9,24 +9,38 @@
 //   private — each node owns a private ThreadPool(1)  [Node<>]
 //   pool    — all nodes share one ThreadPool(T)        [PoolNode<> + shared pool]
 //
-// Usage: ./bench_pipeline | tee results.csv
+// Each row is run --reps times (plus discarded warm-up runs); the reported
+// figure is the median items/sec, with the inter-quartile spread as a
+// reliability indicator. A row whose iqr_pct is above a few percent is not
+// measuring what it claims to measure.
+//
+// Usage: ./bench_pipeline [options] | tee results.csv
+//        ./bench_pipeline --help
 
 #include <kpn/kpn.hpp>
 
+#include "bench_env.hpp"
+
 #ifdef KPN_BENCH_TBB
 #include <oneapi/tbb/flow_graph.h>
 namespace tbb_flow = oneapi::tbb::flow;
 #endif
 
+#include <algorithm>
 #include <array>
 #include <atomic>
 #include <chrono>
+#include <cmath>
 #include <cstdio>
+#include <cstdlib>
+#include <cstring>
 #include <memory>
 #include <string>
 #include <thread>
 #include <vector>
 
+#include <sys/resource.h>
+
 using namespace kpn;
 using namespace std::chrono_literals;
 using sclock = std::chrono::steady_clock;
@@ -57,31 +71,63 @@ static void push_retry(Channel<int>& ch, int val) {
     }
 }
 
-// ── result ────────────────────────────────────────────────────────────────────
+// ── configuration (M1, M3, M4, M5) ────────────────────────────────────────────
 
-struct Result {
+struct Config {
-    const char* topology;
+    std::vector<int> work_amts {10, 100, 1000};
-    int         size;
+    std::vector<int> pool_sizes{1, 2, 4, 8, 16, 20};   // M5
-    int         work_us;
+    std::vector<int> depths    {1, 2, 4, 8, 16, 32};
-    int         threads;     // 0 = private (1 thread per node), N = shared pool size
+    std::vector<int> widths    {1, 2, 3, 4};
-    double      items_per_sec;
+    int    reps        = 5;      // M3: measured repetitions per row
-    double      overhead_us;
+    int    warmup      = 1;      // M4: discarded repetitions per row
+    double target_sec  = 0.30;   // aimed-for duration of one repetition
+    long   min_items   = 2000;   // M1: floor, independent of work_us and depth
+    double max_sec     = 3.0;    // ceiling; only bites where min_items cannot fit
+    bool   do_chain = true, do_wide = true, do_diamond = true;
+    bool   do_priv  = true, do_pool = true, do_tbb = true;
+};
+
+static Config g_cfg;
+
+// M1 — sample size from a time budget with a hard floor, rather than a
+// hand-tuned ladder that collapsed to 50–200 items on exactly the rows under
+// investigation.
+//
+// `stages` is the number of node firings per item; `units` the number of
+// threads able to run them concurrently. Steady-state throughput of the
+// pipeline is bounded by work_us * stages / units, so that is the per-item
+// cost the sample size is derived from. Depth beyond `units` costs throughput;
+// depth below it costs only latency, which does not scale the run.
+static long pick_items(int work_us, int stages, int units) {
+    units = std::max(1, std::min(units, bench::hw_units()));
+    const double per_item_us =
+        std::max(1.0, static_cast<double>(work_us)) *
+        std::max(1.0, static_cast<double>(stages) / units);
+
+    long want = static_cast<long>(g_cfg.target_sec * 1e6 / per_item_us);
+    long cap  = static_cast<long>(g_cfg.max_sec    * 1e6 / per_item_us);
+
+    want = std::max(want, g_cfg.min_items);
+    // The floor wins unless honouring it would blow the time ceiling by more
+    // than the ceiling allows; such rows are reported with their true N so the
+    // reader can see they are short.
+    if (want > cap) want = std::max(cap, 200L);
+    return want;
+}
+
+// ── one measured repetition ───────────────────────────────────────────────────
+
+struct Sample {
+    double items_per_sec = 0;
+    double overhead_us   = 0;
+    long   nivcsw        = 0;   // involuntary context switches during the run
+    long   nvcsw         = 0;   // voluntary   context switches during the run
 };
 
 // ── chain ─────────────────────────────────────────────────────────────────────
 
-static int items_for(int work_us, int depth = 1) {
+static Sample bench_chain(int depth, int work_us, long N) {
-    int effective = std::max(1, work_us) * std::max(1, depth);
+    const std::size_t CAP = static_cast<std::size_t>(N);
-    if (effective <= 1)     return 5000;
-    if (effective <= 10)    return 3000;
-    if (effective <= 100)   return 1000;
-    if (effective <= 1000)  return 200;
-    return 50;
-}
-
-static Result bench_chain(int depth, int work_us) {
-    const int N   = items_for(work_us, depth);
-    const int CAP = N;
 
     std::vector<std::shared_ptr<Channel<int>>> chs;
     for (int i = 0; i <= depth; ++i)
@@ -98,17 +144,20 @@ static Result bench_chain(int depth, int work_us) {
 
     std::atomic<sclock::time_point> t1;
     std::thread reader([&] {
-        for (int i = 0; i < N; ++i) chs.back()->pop();
+        for (long i = 0; i < N; ++i) chs.back()->pop();
         t1.store(sclock::now(), std::memory_order_release);
     });
 
+    bench::RusageDelta ru; ru.start();
     auto t0 = sclock::now();
     std::thread pusher([&] {
-        for (int i = 0; i < N; ++i) push_retry(*chs[0], i);
+        for (long i = 0; i < N; ++i) push_retry(*chs[0], static_cast<int>(i));
     });
 
     pusher.join();
     reader.join();
+    Sample s;
+    ru.finish(s.nivcsw, s.nvcsw);
     for (auto& n : nodes) n->stop();
 
     double elapsed = std::chrono::duration<double>(
@@ -116,13 +165,13 @@ static Result bench_chain(int depth, int work_us) {
     // Subtract theoretical pipeline fill cost (depth-1)*W so that overhead
     // reflects only framework latency, not the expected pipeline startup time.
     double pipeline_us = static_cast<double>(work_us) * (N + depth - 1);
-    double wus = (elapsed * 1e6 - pipeline_us) / N;
+    s.overhead_us   = (elapsed * 1e6 - pipeline_us) / N;
-    return {"chain", depth, work_us, 0, N / elapsed, wus};
+    s.items_per_sec = N / elapsed;
+    return s;
 }
 
-static Result bench_chain_pool(int depth, int work_us, int pool_threads) {
+static Sample bench_chain_pool(int depth, int work_us, int pool_threads, long N) {
-    const int N   = items_for(work_us, depth);
+    const std::size_t CAP = static_cast<std::size_t>(N);
-    const int CAP = N;
 
     auto pool = std::make_shared<ThreadPool>(pool_threads);
 
@@ -142,33 +191,36 @@ static Result bench_chain_pool(int depth, int work_us, int pool_threads) {
 
     std::atomic<sclock::time_point> t1;
     std::thread reader([&] {
-        for (int i = 0; i < N; ++i) chs.back()->pop();
+        for (long i = 0; i < N; ++i) chs.back()->pop();
         t1.store(sclock::now(), std::memory_order_release);
     });
 
+    bench::RusageDelta ru; ru.start();
     auto t0 = sclock::now();
     std::thread pusher([&] {
-        for (int i = 0; i < N; ++i) push_retry(*chs[0], i);
+        for (long i = 0; i < N; ++i) push_retry(*chs[0], static_cast<int>(i));
     });
 
     pusher.join();
     reader.join();
+    Sample s;
+    ru.finish(s.nivcsw, s.nvcsw);
     for (auto& n : nodes) n->stop();
     pool->stop();
 
     double elapsed = std::chrono::duration<double>(
         t1.load(std::memory_order_acquire) - t0).count();
     double pipeline_us = static_cast<double>(work_us) * (N + depth - 1);
-    double wus = (elapsed * 1e6 - pipeline_us) / N;
+    s.overhead_us   = (elapsed * 1e6 - pipeline_us) / N;
-    return {"chain", depth, work_us, pool_threads, N / elapsed, wus};
+    s.items_per_sec = N / elapsed;
+    return s;
 }
 
 // ── wide (fanout<W>) ──────────────────────────────────────────────────────────
 
 template<std::size_t W>
-static Result bench_wide(int work_us) {
+static Sample bench_wide(int work_us, long N) {
-    const int N   = items_for(work_us);
+    const std::size_t CAP = static_cast<std::size_t>(N);
-    const int CAP = N;
 
     auto src_ch = std::make_shared<Channel<int>>(CAP);
     auto fan    = std::make_unique<FanoutNode<int, W>>(CAP);
@@ -197,33 +249,36 @@ static Result bench_wide(int work_us) {
 
     for (std::size_t w = 0; w < W; ++w) {
         readers[w] = std::thread([&, w] {
-            for (int i = 0; i < N; ++i) sink_chs[w]->pop();
+            for (long i = 0; i < N; ++i) sink_chs[w]->pop();
             if (readers_done.fetch_add(1, std::memory_order_acq_rel) + 1
                     == static_cast<int>(W))
                 t1.store(sclock::now(), std::memory_order_release);
         });
     }
 
+    bench::RusageDelta ru; ru.start();
     auto t0 = sclock::now();
     std::thread pusher([&] {
-        for (int i = 0; i < N; ++i) push_retry(*src_ch, i);
+        for (long i = 0; i < N; ++i) push_retry(*src_ch, static_cast<int>(i));
     });
 
     pusher.join();
     for (auto& r : readers) r.join();
+    Sample s;
+    ru.finish(s.nivcsw, s.nvcsw);
     fan->stop();
     for (auto& n : nodes) n->stop();
 
     double elapsed = std::chrono::duration<double>(
         t1.load(std::memory_order_acquire) - t0).count();
-    double wus = (elapsed * 1e6) / N - static_cast<double>(work_us);
+    s.overhead_us   = (elapsed * 1e6) / N - static_cast<double>(work_us);
-    return {"wide", static_cast<int>(W), work_us, 0, N / elapsed, wus};
+    s.items_per_sec = N / elapsed;
+    return s;
 }
 
 template<std::size_t W>
-static Result bench_wide_pool(int work_us, int pool_threads) {
+static Sample bench_wide_pool(int work_us, int pool_threads, long N) {
-    const int N   = items_for(work_us);
+    const std::size_t CAP = static_cast<std::size_t>(N);
-    const int CAP = N;
 
     auto pool   = std::make_shared<ThreadPool>(pool_threads);
     auto src_ch = std::make_shared<Channel<int>>(CAP);
@@ -254,35 +309,38 @@ static Result bench_wide_pool(int work_us, int pool_threads) {
 
     for (std::size_t w = 0; w < W; ++w) {
         readers[w] = std::thread([&, w] {
-            for (int i = 0; i < N; ++i) sink_chs[w]->pop();
+            for (long i = 0; i < N; ++i) sink_chs[w]->pop();
             if (readers_done.fetch_add(1, std::memory_order_acq_rel) + 1
                     == static_cast<int>(W))
                 t1.store(sclock::now(), std::memory_order_release);
         });
     }
 
+    bench::RusageDelta ru; ru.start();
     auto t0 = sclock::now();
     std::thread pusher([&] {
-        for (int i = 0; i < N; ++i) push_retry(*src_ch, i);
+        for (long i = 0; i < N; ++i) push_retry(*src_ch, static_cast<int>(i));
     });
 
     pusher.join();
     for (auto& r : readers) r.join();
+    Sample s;
+    ru.finish(s.nivcsw, s.nvcsw);
     fan->stop();
     for (auto& n : nodes) n->stop();
     pool->stop();
 
     double elapsed = std::chrono::duration<double>(
         t1.load(std::memory_order_acquire) - t0).count();
-    double wus = (elapsed * 1e6) / N - static_cast<double>(work_us);
+    s.overhead_us   = (elapsed * 1e6) / N - static_cast<double>(work_us);
-    return {"wide", static_cast<int>(W), work_us, pool_threads, N / elapsed, wus};
+    s.items_per_sec = N / elapsed;
+    return s;
 }
 
 // ── diamond ───────────────────────────────────────────────────────────────────
 
-static Result bench_diamond(int work_us) {
+static Sample bench_diamond(int work_us, long N) {
-    const int N   = items_for(work_us, 2);
+    const std::size_t CAP = static_cast<std::size_t>(N);
-    const int CAP = N;
 
     auto src_ch = std::make_shared<Channel<int>>(CAP);
     auto fan    = std::make_unique<FanoutNode<int, 2>>(CAP);
@@ -312,7 +370,7 @@ static Result bench_diamond(int work_us) {
     std::atomic<int> done{0};
     auto make_reader = [&](Channel<int>& ch) {
         return std::thread([&] {
-            for (int i = 0; i < N; ++i) ch.pop();
+            for (long i = 0; i < N; ++i) ch.pop();
             if (done.fetch_add(1, std::memory_order_acq_rel) + 1 == 2)
                 t1.store(sclock::now(), std::memory_order_release);
         });
@@ -320,23 +378,26 @@ static Result bench_diamond(int work_us) {
     auto rL = make_reader(*snkL);
     auto rR = make_reader(*snkR);
 
+    bench::RusageDelta ru; ru.start();
     auto t0 = sclock::now();
     std::thread pusher([&] {
-        for (int i = 0; i < N; ++i) push_retry(*src_ch, i);
+        for (long i = 0; i < N; ++i) push_retry(*src_ch, static_cast<int>(i));
     });
 
     pusher.join(); rL.join(); rR.join();
+    Sample s;
+    ru.finish(s.nivcsw, s.nvcsw);
     fan->stop(); nL->stop(); nR->stop(); nL2->stop(); nR2->stop();
 
     double elapsed = std::chrono::duration<double>(
         t1.load(std::memory_order_acquire) - t0).count();
-    double wus = (elapsed * 1e6) / N - static_cast<double>(work_us);
+    s.overhead_us   = (elapsed * 1e6) / N - static_cast<double>(work_us);
-    return {"diamond", 4, work_us, 0, N / elapsed, wus};
+    s.items_per_sec = N / elapsed;
+    return s;
 }
 
-static Result bench_diamond_pool(int work_us, int pool_threads) {
+static Sample bench_diamond_pool(int work_us, int pool_threads, long N) {
-    const int N   = items_for(work_us, 2);
+    const std::size_t CAP = static_cast<std::size_t>(N);
-    const int CAP = N;
 
     auto pool   = std::make_shared<ThreadPool>(pool_threads);
     auto src_ch = std::make_shared<Channel<int>>(CAP);
@@ -369,7 +430,7 @@ static Result bench_diamond_pool(int work_us, int pool_threads) {
     std::atomic<int> done{0};
     auto make_reader = [&](Channel<int>& ch) {
         return std::thread([&] {
-            for (int i = 0; i < N; ++i) ch.pop();
+            for (long i = 0; i < N; ++i) ch.pop();
             if (done.fetch_add(1, std::memory_order_acq_rel) + 1 == 2)
                 t1.store(sclock::now(), std::memory_order_release);
         });
@@ -377,28 +438,30 @@ static Result bench_diamond_pool(int work_us, int pool_threads) {
     auto rL = make_reader(*snkL);
     auto rR = make_reader(*snkR);
 
+    bench::RusageDelta ru; ru.start();
     auto t0 = sclock::now();
     std::thread pusher([&] {
-        for (int i = 0; i < N; ++i) push_retry(*src_ch, i);
+        for (long i = 0; i < N; ++i) push_retry(*src_ch, static_cast<int>(i));
     });
 
     pusher.join(); rL.join(); rR.join();
+    Sample s;
+    ru.finish(s.nivcsw, s.nvcsw);
     fan->stop();
     nL->stop(); nR->stop(); nL2->stop(); nR2->stop();
     pool->stop();
 
     double elapsed = std::chrono::duration<double>(
         t1.load(std::memory_order_acquire) - t0).count();
-    double wus = (elapsed * 1e6) / N - static_cast<double>(work_us);
+    s.overhead_us   = (elapsed * 1e6) / N - static_cast<double>(work_us);
-    return {"diamond", 4, work_us, pool_threads, N / elapsed, wus};
+    s.items_per_sec = N / elapsed;
+    return s;
 }
 
 // ── TBB flow graph ────────────────────────────────────────────────────────────
 #ifdef KPN_BENCH_TBB
 
-static Result bench_chain_tbb(int depth, int work_us) {
+static Sample bench_chain_tbb(int depth, int work_us, long N) {
-    const int N = items_for(work_us, depth);
-
     tbb_flow::graph g;
     using FN = tbb_flow::function_node<int, int>;
     std::vector<std::unique_ptr<FN>> nodes;
@@ -409,21 +472,23 @@ static Result bench_chain_tbb(int depth, int work_us) {
     for (int i = 0; i + 1 < depth; ++i)
         tbb_flow::make_edge(*nodes[i], *nodes[i + 1]);
 
+    bench::RusageDelta ru; ru.start();
     auto t0 = sclock::now();
-    for (int i = 0; i < N; ++i) nodes[0]->try_put(i);
+    for (long i = 0; i < N; ++i) nodes[0]->try_put(static_cast<int>(i));
     g.wait_for_all();
     auto t1 = sclock::now();
+    Sample s;
+    ru.finish(s.nivcsw, s.nvcsw);
 
     double elapsed     = std::chrono::duration<double>(t1 - t0).count();
     double pipeline_us = static_cast<double>(work_us) * (N + depth - 1);
-    double wus        = (elapsed * 1e6 - pipeline_us) / N;
+    s.overhead_us   = (elapsed * 1e6 - pipeline_us) / N;
-    return {"chain_tbb", depth, work_us, -1, N / elapsed, wus};
+    s.items_per_sec = N / elapsed;
+    return s;
 }
 
 template<std::size_t W>
-static Result bench_wide_tbb(int work_us) {
+static Sample bench_wide_tbb(int work_us, long N) {
-    const int N = items_for(work_us);
-
     tbb_flow::graph g;
     tbb_flow::broadcast_node<int> fan(g);
     using FN = tbb_flow::function_node<int, int>;
@@ -434,19 +499,21 @@ static Result bench_wide_tbb(int work_us) {
         tbb_flow::make_edge(fan, *n);
     }
 
+    bench::RusageDelta ru; ru.start();
     auto t0 = sclock::now();
-    for (int i = 0; i < N; ++i) fan.try_put(i);
+    for (long i = 0; i < N; ++i) fan.try_put(static_cast<int>(i));
     g.wait_for_all();
     auto t1 = sclock::now();
+    Sample s;
+    ru.finish(s.nivcsw, s.nvcsw);
 
     double elapsed = std::chrono::duration<double>(t1 - t0).count();
-    double wus     = (elapsed * 1e6) / N - static_cast<double>(work_us);
+    s.overhead_us   = (elapsed * 1e6) / N - static_cast<double>(work_us);
-    return {"wide_tbb", static_cast<int>(W), work_us, -1, N / elapsed, wus};
+    s.items_per_sec = N / elapsed;
+    return s;
 }
 
-static Result bench_diamond_tbb(int work_us) {
+static Sample bench_diamond_tbb(int work_us, long N) {
-    const int N = items_for(work_us, 2);
-
     tbb_flow::graph g;
     tbb_flow::broadcast_node<int> fan(g);
     using FN = tbb_flow::function_node<int, int>;
@@ -456,71 +523,242 @@ static Result bench_diamond_tbb(int work_us) {
     tbb_flow::make_edge(fan, nL);  tbb_flow::make_edge(fan, nR);
     tbb_flow::make_edge(nL, nL2);  tbb_flow::make_edge(nR, nR2);
 
+    bench::RusageDelta ru; ru.start();
     auto t0 = sclock::now();
-    for (int i = 0; i < N; ++i) fan.try_put(i);
+    for (long i = 0; i < N; ++i) fan.try_put(static_cast<int>(i));
     g.wait_for_all();
     auto t1 = sclock::now();
+    Sample s;
+    ru.finish(s.nivcsw, s.nvcsw);
 
     double elapsed = std::chrono::duration<double>(t1 - t0).count();
-    double wus     = (elapsed * 1e6) / N - static_cast<double>(work_us);
+    s.overhead_us   = (elapsed * 1e6) / N - static_cast<double>(work_us);
-    return {"diamond_tbb", 4, work_us, -1, N / elapsed, wus};
+    s.items_per_sec = N / elapsed;
+    return s;
 }
 #endif // KPN_BENCH_TBB
 
+// ── repetition driver (M2, M3, M4) ────────────────────────────────────────────
+
+using bench::percentile;
+
+// A row: median of `reps` repetitions, after `warmup` discarded ones.
+// M2 — items/sec is the primary figure; derived overhead is secondary,
+// because it is a difference of large numbers and magnifies noise ~10×.
+template<class Fn>
+static void run_row(const char* topology, int size, int work_us, int sched,
+                    long N, Fn&& one_rep) {
+    for (int i = 0; i < g_cfg.warmup; ++i) (void)one_rep();   // M4
+
+    std::vector<double> ips, ovh;
+    long ivcsw = 0, vcsw = 0;
+    for (int i = 0; i < g_cfg.reps; ++i) {
+        Sample s = one_rep();
+        ips.push_back(s.items_per_sec);
+        ovh.push_back(s.overhead_us);
+        ivcsw += s.nivcsw;
+        vcsw  += s.nvcsw;
+    }
+
+    const double med  = percentile(ips, 0.5);
+    const double q1   = percentile(ips, 0.25);
+    const double q3   = percentile(ips, 0.75);
+    const double iqr  = med > 0 ? 100.0 * (q3 - q1) / med : 0.0;
+    const double lo   = *std::min_element(ips.begin(), ips.end());
+    const double hi   = *std::max_element(ips.begin(), ips.end());
+    const double spread = med > 0 ? 100.0 * (hi - lo) / med : 0.0;
+    const double ivcsw_per_item = static_cast<double>(ivcsw) / (double(N) * g_cfg.reps);
+    const double vcsw_per_item  = static_cast<double>(vcsw)  / (double(N) * g_cfg.reps);
+
+    const std::string s = sched  < 0 ? "tbb"
+                        : sched == 0 ? "priv"
+                                     : std::to_string(sched);
+
+    std::fprintf(stderr, "%-10s %-5d %-8d %-6s %-8ld %-12.0f %-7.1f %-7.1f %-9.1f %-8.2f %-8.2f\n",
+                 topology, size, work_us, s.c_str(), N,
+                 med, iqr, spread, percentile(ovh, 0.5), ivcsw_per_item, vcsw_per_item);
+    std::printf("%s,%d,%d,%s,%ld,%d,%.0f,%.0f,%.0f,%.2f,%.2f,%.2f,%.3f,%.3f\n",
+                topology, size, work_us, s.c_str(), N, g_cfg.reps,
+                med, lo, hi, iqr, spread, percentile(ovh, 0.5),
+                ivcsw_per_item, vcsw_per_item);
+    std::fflush(stdout);
+}
+
+// ── argument parsing ──────────────────────────────────────────────────────────
+
+static std::vector<int> parse_int_list(const char* s) {
+    std::vector<int> out;
+    const char* p = s;
+    while (*p) {
+        char* end = nullptr;
+        long v = std::strtol(p, &end, 10);
+        if (end == p) break;
+        out.push_back(static_cast<int>(v));
+        p = end;
+        while (*p == ',' || *p == ' ') ++p;
+    }
+    return out;
+}
+
+static bool has_word(const std::string& csv, const char* word) {
+    return csv.find(word) != std::string::npos;
+}
+
+static void usage() {
+    std::fprintf(stderr,
+        "usage: bench_pipeline [options]\n"
+        "  --work=10,100,1000     per-node busy-work, microseconds\n"
+        "  --depths=1,2,4,8,16,32 chain depths\n"
+        "  --widths=1,2,3,4       fanout widths\n"
+        "  --pools=1,2,4,8,16,20  shared-pool thread counts\n"
+        "  --topos=chain,wide,diamond\n"
+        "  --modes=priv,pool,tbb\n"
+        "  --reps=5               measured repetitions per row\n"
+        "  --warmup=1             discarded repetitions per row\n"
+        "  --target-sec=0.30      aimed-for duration of one repetition\n"
+        "  --min-items=2000       sample-size floor\n"
+        "  --max-sec=3.0          per-repetition ceiling (overrides the floor)\n");
+}
+
+static bool parse_args(int argc, char** argv) {
+    for (int i = 1; i < argc; ++i) {
+        std::string a = argv[i];
+        auto eq = a.find('=');
+        std::string key = a.substr(0, eq);
+        std::string val = eq == std::string::npos ? "" : a.substr(eq + 1);
+
+        if (key == "--help" || key == "-h")   { usage(); std::exit(0); }
+        else if (key == "--work")       g_cfg.work_amts  = parse_int_list(val.c_str());
+        else if (key == "--depths")     g_cfg.depths     = parse_int_list(val.c_str());
+        else if (key == "--widths")     g_cfg.widths     = parse_int_list(val.c_str());
+        else if (key == "--pools")      g_cfg.pool_sizes = parse_int_list(val.c_str());
+        else if (key == "--reps")       g_cfg.reps       = std::atoi(val.c_str());
+        else if (key == "--warmup")     g_cfg.warmup     = std::atoi(val.c_str());
+        else if (key == "--target-sec") g_cfg.target_sec = std::atof(val.c_str());
+        else if (key == "--min-items")  g_cfg.min_items  = std::atol(val.c_str());
+        else if (key == "--max-sec")    g_cfg.max_sec    = std::atof(val.c_str());
+        else if (key == "--topos") {
+            g_cfg.do_chain   = has_word(val, "chain");
+            g_cfg.do_wide    = has_word(val, "wide");
+            g_cfg.do_diamond = has_word(val, "diamond");
+        }
+        else if (key == "--modes") {
+            g_cfg.do_priv = has_word(val, "priv");
+            g_cfg.do_pool = has_word(val, "pool");
+            g_cfg.do_tbb  = has_word(val, "tbb");
+        }
+        else { std::fprintf(stderr, "unknown option: %s\n", a.c_str()); usage(); return false; }
+    }
+    if (g_cfg.reps < 1) g_cfg.reps = 1;
+    if (g_cfg.warmup < 0) g_cfg.warmup = 0;
+    return true;
+}
+
+// `wide` is templated on W, so dispatch the runtime width through a switch.
+template<class F>
+static void with_width(int w, F&& f) {
+    switch (w) {
+        case 1: f(std::integral_constant<std::size_t, 1>{}); break;
+        case 2: f(std::integral_constant<std::size_t, 2>{}); break;
+        case 3: f(std::integral_constant<std::size_t, 3>{}); break;
+        case 4: f(std::integral_constant<std::size_t, 4>{}); break;
+        default:
+            std::fprintf(stderr, "width %d not instantiated (1..4 only)\n", w);
+    }
+}
+
 // ── main ──────────────────────────────────────────────────────────────────────
 
-int main() {
+int main(int argc, char** argv) {
-    const int work_amts[]  = {10, 100, 1000};
+    // A rejected option must fail loudly: a harness driver that silently got
-    const int pool_sizes[] = {1, 2, 4};
+    // no CSV back is worse than one that stops.
+    if (!parse_args(argc, argv)) return 2;
 
-    std::fprintf(stderr, "%-12s %-8s %-10s %-8s %-18s %-20s\n",
+    char cfg[192];
-                 "topology", "size", "work_us", "threads", "items/sec", "overhead_us/item");
+    std::snprintf(cfg, sizeof cfg,
-    std::fprintf(stderr, "%s\n", std::string(78, '-').c_str());
+                  "reps=%d warmup=%d target_sec=%.2f min_items=%ld max_sec=%.1f",
-    std::printf("topology,size,work_us,threads,items_per_sec,overhead_us_per_item\n");
+                  g_cfg.reps, g_cfg.warmup, g_cfg.target_sec,
+                  g_cfg.min_items, g_cfg.max_sec);
+    bench::print_environment(cfg);
 
-    auto emit = [](const Result& r) {
+    std::fprintf(stderr, "\n%-10s %-5s %-8s %-6s %-8s %-12s %-7s %-7s %-9s %-8s %-8s\n",
-        std::string sched = r.threads  < 0 ? "tbb"
+                 "topology", "size", "work_us", "sched", "items", "items/sec",
-                          : r.threads == 0 ? "priv"
+                 "iqr%", "range%", "ovh_us", "ivcsw/it", "vcsw/it");
-                          : std::to_string(r.threads);
+    std::fprintf(stderr, "%s\n", std::string(104, '-').c_str());
-        std::fprintf(stderr, "%-12s %-8d %-10d %-8s %-18.0f %-20.1f\n",
+    std::printf("topology,size,work_us,threads,items,reps,items_per_sec,"
-                     r.topology, r.size, r.work_us, sched.c_str(),
+                "items_per_sec_min,items_per_sec_max,iqr_pct,range_pct,"
-                     r.items_per_sec, r.overhead_us);
+                "overhead_us_per_item,ivcsw_per_item,vcsw_per_item\n");
-        std::printf("%s,%d,%d,%s,%.0f,%.2f\n",
-                    r.topology, r.size, r.work_us, sched.c_str(),
-                    r.items_per_sec, r.overhead_us);
-        std::fflush(stdout);
-    };
 
-    for (int w : work_amts) {
+    for (int w : g_cfg.work_amts) {
         g_work_us.store(w, std::memory_order_relaxed);
-        std::fprintf(stderr, "\n── work_us=%-4d private pools ───────────────────────────────────────\n", w);
 
-        for (int d : {1, 2, 4, 8, 16, 32}) emit(bench_chain(d, w));
+        if (g_cfg.do_priv) {
-        emit(bench_wide<1>(w));
+            std::fprintf(stderr, "\n── work_us=%-4d private pools ──────────────────────\n", w);
-        emit(bench_wide<2>(w));
+            if (g_cfg.do_chain)
-        emit(bench_wide<3>(w));
+                for (int d : g_cfg.depths) {
-        emit(bench_wide<4>(w));
+                    long N = pick_items(w, d, d);
-        emit(bench_diamond(w));
+                    run_row("chain", d, w, 0, N, [&] { return bench_chain(d, w, N); });
+                }
+            if (g_cfg.do_wide)
+                for (int wd : g_cfg.widths)
+                    with_width(wd, [&](auto W) {
+                        long N = pick_items(w, W.value, W.value);
+                        run_row("wide", static_cast<int>(W.value), w, 0, N,
+                                [&] { return bench_wide<W.value>(w, N); });
+                    });
+            if (g_cfg.do_diamond) {
+                long N = pick_items(w, 4, 4);
+                run_row("diamond", 4, w, 0, N, [&] { return bench_diamond(w, N); });
+            }
+        }
 
-        for (int pt : pool_sizes) {
+        if (g_cfg.do_pool) {
-            std::fprintf(stderr, "\n── work_us=%-4d shared pool (%d thread%s) ─────────────────────────────\n",
+            for (int pt : g_cfg.pool_sizes) {
+                std::fprintf(stderr, "\n── work_us=%-4d shared pool (%d thread%s) ───────────\n",
                              w, pt, pt == 1 ? "" : "s");
-            for (int d : {1, 2, 4, 8, 16, 32}) emit(bench_chain_pool(d, w, pt));
+                if (g_cfg.do_chain)
-            emit(bench_wide_pool<1>(w, pt));
+                    for (int d : g_cfg.depths) {
-            emit(bench_wide_pool<2>(w, pt));
+                        long N = pick_items(w, d, pt);
-            emit(bench_wide_pool<3>(w, pt));
+                        run_row("chain", d, w, pt, N,
-            emit(bench_wide_pool<4>(w, pt));
+                                [&] { return bench_chain_pool(d, w, pt, N); });
-            emit(bench_diamond_pool(w, pt));
+                    }
+                if (g_cfg.do_wide)
+                    for (int wd : g_cfg.widths)
+                        with_width(wd, [&](auto W) {
+                            long N = pick_items(w, W.value, pt);
+                            run_row("wide", static_cast<int>(W.value), w, pt, N,
+                                    [&] { return bench_wide_pool<W.value>(w, pt, N); });
+                        });
+                if (g_cfg.do_diamond) {
+                    long N = pick_items(w, 4, pt);
+                    run_row("diamond", 4, w, pt, N,
+                            [&] { return bench_diamond_pool(w, pt, N); });
+                }
+            }
         }
 
 #ifdef KPN_BENCH_TBB
-        std::fprintf(stderr, "\n── work_us=%-4d TBB flow graph ──────────────────────────────────────\n", w);
+        if (g_cfg.do_tbb) {
-        for (int d : {1, 2, 4, 8, 16, 32}) emit(bench_chain_tbb(d, w));
+            std::fprintf(stderr, "\n── work_us=%-4d TBB flow graph ─────────────────────\n", w);
-        emit(bench_wide_tbb<1>(w));
+            if (g_cfg.do_chain)
-        emit(bench_wide_tbb<2>(w));
+                for (int d : g_cfg.depths) {
-        emit(bench_wide_tbb<3>(w));
+                    long N = pick_items(w, d, d);
-        emit(bench_wide_tbb<4>(w));
+                    run_row("chain_tbb", d, w, -1, N,
-        emit(bench_diamond_tbb(w));
+                            [&] { return bench_chain_tbb(d, w, N); });
+                }
+            if (g_cfg.do_wide)
+                for (int wd : g_cfg.widths)
+                    with_width(wd, [&](auto W) {
+                        long N = pick_items(w, W.value, W.value);
+                        run_row("wide_tbb", static_cast<int>(W.value), w, -1, N,
+                                [&] { return bench_wide_tbb<W.value>(w, N); });
+                    });
+            if (g_cfg.do_diamond) {
+                long N = pick_items(w, 4, 4);
+                run_row("diamond_tbb", 4, w, -1, N,
+                        [&] { return bench_diamond_tbb(w, N); });
+            }
+        }
 #endif
     }
 }

include/kpn/branch.hpp

@@ -7,12 +7,78 @@
 
 #include <array>
 #include <atomic>
+#include <chrono>
 #include <functional>
 #include <memory>
 #include <thread>
 
 namespace kpn {
 
+// ── Lossless single-output delivery ───────────────────────────────────────────
+//
+// Shared by RouterNode and FilterNode, which each deliver a value to exactly one
+// channel. Both previously did
+//
+//     try { ch->push(val); } catch (const ChannelOverflowError&) {}
+//
+// which discards the value whenever the consumer is behind. 6595e6e made node
+// outputs lossless, 28e0667 stopped them parking a worker, and a8cfe73 did the
+// same for FanoutNode — these two were in none of them, and were the last
+// remaining users of the throwing push() on a data path.
+//
+// A dropped item does not degrade a downstream result, it silently changes one.
+// Worse, a dropped *sentinel* wedges the pipeline outright: EOF is what tells
+// every downstream node to shut down, and there is nothing after it to retry.
+// A filter that passes EOF by predicate but drops it by backpressure is a
+// pipeline that never terminates.
+//
+// So sentinels go out-of-band via push_sentinel (a dedicated slot that consumes
+// no ring capacity and cannot overflow), and everything else is retried until
+// taken. Like FanoutNode and unlike a pool node, these own a private thread, so
+// waiting here costs no scheduler worker and needs no space-callback park.
+// stop_flag_ is rechecked every pass so teardown cannot hang on a full output.
+//
+// `parked` receives the time spent waiting, which the caller charges to blocked
+// rather than exec — a parked node is idle, and charging it to exec reports the
+// node as busy exactly when it is the one being held up.
+//
+// Returns false if stopped with the value undelivered.
+template<typename T>
+bool deliver_one(Channel<T>* ch, T& val, const std::atomic<bool>& stop_flag,
+                 duration_t& parked) {
+    if (is_sentinel_value(val)) {
+        ch->push_sentinel(std::move(val));
+        return true;
+    }
+    const auto park_from = clock_t::now();
+    for (;;) {
+        switch (ch->try_push(val)) {
+            case Channel<T>::PushResult::Taken:
+                parked = duration_t(clock_t::now() - park_from);
+                return true;
+            case Channel<T>::PushResult::Closed:
+                // Nobody is listening any more; the channel has recorded the
+                // drop. Retrying would spin until teardown noticed.
+                parked = duration_t(clock_t::now() - park_from);
+                return false;
+            case Channel<T>::PushResult::Full:
+                break;                       // fall through to the retry logic
+        }
+        if (stop_flag.load(std::memory_order_relaxed)) {
+            // Teardown with work in hand and the output still full. One last
+            // throwing push, purely so the channel's own stats record the
+            // overflow — the point of the lossless path is that a loss is never
+            // invisible, and a silent return here would reintroduce exactly the
+            // hole this function exists to close.
+            try { ch->push(std::move(val)); }
+            catch (const ChannelOverflowError&) {}
+            parked = duration_t(clock_t::now() - park_from);
+            return false;
+        }
+        std::this_thread::sleep_for(std::chrono::microseconds(50));
+    }
+}
+
 // ── RouterNode ────────────────────────────────────────────────────────────────
 //
 // Reads one item and pushes it to exactly one of N output channels, chosen by
@@ -126,15 +192,20 @@ private:
                 auto        t1  = clock_t::now();
                 auto        cpu0 = NodeStats::cpu_now();
 
+                // An out-of-range selector still drops by design (documented on
+                // the class): the item was routed nowhere, not lost to a full
+                // channel. Only the latter is what deliver_one exists to stop.
                 std::size_t idx = selector_(val);
-                if (idx < N && out_channels_[idx]) {
+                duration_t  parked{0};
-                    try { out_channels_[idx]->push(val); }
+                bool        delivered = true;
-                    catch (const ChannelOverflowError&) {}
+                if (idx < N && out_channels_[idx])
-                }
+                    delivered = deliver_one(out_channels_[idx], val, stop_flag_, parked);
 
                 auto cpu1 = NodeStats::cpu_now();
                 auto t2   = clock_t::now();
-                stats_.record_exec(duration_t(t2 - t1), duration_t(t1 - t0), cpu0, cpu1);
+                stats_.record_exec(duration_t(t2 - t1) - parked,
+                                   duration_t(t1 - t0) + parked, cpu0, cpu1);
+                if (!delivered) break;
             } catch (const ChannelClosedError&) {
                 break;
             }
@@ -261,12 +332,19 @@ private:
                 auto t1  = clock_t::now();
                 auto cpu0 = NodeStats::cpu_now();
 
+                // A value the predicate rejects is dropped by design and is not
+                // counted as a processed frame. One it accepts is now delivered
+                // losslessly — including a sentinel, which a filter typically
+                // passes unconditionally so downstream can shut down, and which
+                // the old throwing push discarded whenever the output was full.
                 if (pred_(val) && out_ch_) {
-                    try { out_ch_->push(val); }
+                    duration_t parked{0};
-                    catch (const ChannelOverflowError&) {}
+                    const bool delivered = deliver_one(out_ch_, val, stop_flag_, parked);
                     auto cpu1 = NodeStats::cpu_now();
                     auto t2   = clock_t::now();
-                    stats_.record_exec(duration_t(t2 - t1), duration_t(t1 - t0), cpu0, cpu1);
+                    stats_.record_exec(duration_t(t2 - t1) - parked,
+                                       duration_t(t1 - t0) + parked, cpu0, cpu1);
+                    if (!delivered) break;
                 }
             } catch (const ChannelClosedError&) {
                 break;

include/kpn/channel.hpp

@@ -58,6 +58,18 @@ public:
     ChannelClosedError() : std::runtime_error("channel closed") {}
 };
 
+// Nothing available *right now* on a channel that is still open. Distinct from
+// ChannelClosedError, which means upstream is finished and never coming back.
+//
+// Conflating the two is expensive in one direction only: a consumer that reads
+// "empty" as "closed" stops a live node permanently, and because a stopping
+// node disables its own inputs and outputs, one benign empty read takes the
+// rest of the pipeline with it. The reverse costs nothing.
+class ChannelEmptyError : public std::runtime_error {
+public:
+    ChannelEmptyError() : std::runtime_error("channel empty") {}
+};
+
 // ── CPU pause hint ────────────────────────────────────────────────────────────
 // Signals the CPU that this is a spin-wait loop, improving HT sibling throughput
 // and preventing branch-predictor thrash on x86. Falls back to a compiler barrier.
@@ -124,7 +136,6 @@ public:
             throw ChannelOverflowError(capacity_);
         }
 
-        const bool was_empty = (t == h);
         buf_[t & ring_mask_] = make_storage(std::move(value));
         tail_.store(t + 1, std::memory_order_release);
         stats_.record_push(t - h + 1, data_bytes);
@@ -132,7 +143,8 @@ public:
         wake_.fetch_add(1, std::memory_order_release);
         wake_.notify_one();
 
-        if (was_empty && push_callback_)
+        // Level-triggered, not edge-triggered — see set_push_callback.
+        if (push_callback_)
             push_callback_();
     }
 
@@ -153,23 +165,36 @@ public:
                head_.load(std::memory_order_acquire) < capacity_;
     }
 
-    /// Non-blocking, lossless push. Returns false when the ring is full, having
+    /// Outcome of a non-blocking push.
+    ///
+    /// try_push used to return bool, and returned *true* for a closed channel —
+    /// so "delivered" and "discarded because nobody is listening" were the same
+    /// answer. Both mean "stop trying", which is why the callers were correct,
+    /// but neither they nor the producer's own accounting could tell a value
+    /// that arrived from one that was thrown away. Only the channel's drop
+    /// counter knew.
+    enum class PushResult { Taken, Full, Closed };
+
+    /// Non-blocking, lossless push. Returns Full when the ring is full, having
     /// changed nothing — the caller keeps the value and retries when woken.
-    bool try_push(T& value) {
+    PushResult try_push(T& value) {
-        if (!accepting_.load(std::memory_order_acquire)) { stats_.record_drop(); return true; }
+        if (!accepting_.load(std::memory_order_acquire)) {
+            stats_.record_drop();
+            return PushResult::Closed;
+        }
         const std::size_t t = tail_.load(std::memory_order_relaxed);
         const std::size_t h = head_.load(std::memory_order_acquire);
-        if (t - h >= capacity_) return false;
+        if (t - h >= capacity_) return PushResult::Full;
 
         const std::size_t data_bytes = ChannelDataSize<T>::bytes(value);
-        const bool was_empty = (t == h);
         buf_[t & ring_mask_] = make_storage(std::move(value));
         tail_.store(t + 1, std::memory_order_release);
         stats_.record_push(t - h + 1, data_bytes);
         wake_.fetch_add(1, std::memory_order_release);
         wake_.notify_one();
-        if (was_empty && push_callback_) push_callback_();
+        // Level-triggered, not edge-triggered — see set_push_callback.
-        return true;
+        if (push_callback_) push_callback_();
+        return PushResult::Taken;
     }
 
     // Lossless push with BACKPRESSURE: if the ring is full, wait for the consumer to
@@ -187,13 +212,13 @@ public:
             const std::size_t h = head_.load(std::memory_order_acquire);
             if (t - h < capacity_) {          // space available → normal push
                 const std::size_t data_bytes = ChannelDataSize<T>::bytes(value);
-                const bool was_empty = (t == h);
                 buf_[t & ring_mask_] = make_storage(std::move(value));
                 tail_.store(t + 1, std::memory_order_release);
                 stats_.record_push(t - h + 1, data_bytes);
                 wake_.fetch_add(1, std::memory_order_release);
                 wake_.notify_one();
-                if (was_empty && push_callback_) push_callback_();
+                // Level-triggered, not edge-triggered — see set_push_callback.
+                if (push_callback_) push_callback_();
                 return true;
             }
             // full: yield briefly and retry (consumer will drain)
@@ -214,13 +239,50 @@ public:
     // preserving ordering (EOF arrives after all data pushed before it).
     //
     // Only the sole producer may call it (SPSC contract, same as push()).
-    // Returns false if the channel is already disabled (token discarded —
+    //
-    // teardown is in progress, so the sentinel is moot).
+    // The slot holds exactly one undelivered token. A second offered before the
-    bool push_sentinel(T value) {
+    // first is taken is refused, not queued and not overwritten: two control
+    // tokens on one channel means the stream ended twice, which is a caller
+    // protocol error rather than backpressure, and silently coalescing them
+    // would hide it.
+    /// Outcome of offering a sentinel. SlotBusy is a protocol error, not
+    /// backpressure: it means a second control token was offered while the
+    /// first was still undelivered, and a channel carries at most one.
+    enum class SentinelResult { Taken, Closed, SlotBusy };
+
+    /// Non-consuming form. `value` is left untouched unless the result is
+    /// Taken, so a refused token is still the caller's to report.
+    SentinelResult try_push_sentinel(T& value) {
         if (!accepting_.load(std::memory_order_acquire)) {
             stats_.record_drop();
-            return false;
+            return SentinelResult::Closed;
         }
+        // Refuse rather than overwrite. Overwriting lost the first token
+        // silently, and worse, wrote eof_value_ while the consumer could be
+        // moving the previous one out of it — a data race on the storage, which
+        // for a shared_ptr payload is a torn refcount rather than a stale read.
+        //
+        // Checking here is what makes the slot a correct SPSC handshake: the
+        // producer is the only writer of eof_value_ and the only one that sets
+        // has_eof_, the consumer is the only one that clears it, so observing
+        // false here means the consumer has finished with the storage and will
+        // not touch it again until this store publishes the next token.
+        //
+        // Not counted as a drop, and this is the important part. A source that
+        // has reached the end of its input keeps being polled and keeps
+        // returning EOF — that is the normal steady state, not an error — so a
+        // token arriving while one is already pending is a *re-offer*, and
+        // refusing it loses nothing: the pending token carries the same
+        // meaning and is already on its way. Counting it as a drop made a
+        // clean run report data loss and exit non-zero.
+        //
+        // The cost of that choice, stated plainly: a genuinely distinct second
+        // token would also be refused silently, and the channel cannot tell the
+        // two apart. Re-offering is the case that actually occurs here, and the
+        // delivery guarantee that matters — the first token arrives — holds
+        // either way.
+        if (has_eof_.load(std::memory_order_acquire))
+            return SentinelResult::SlotBusy;
         eof_value_ = make_storage(std::move(value));
         has_eof_.store(true, std::memory_order_release);
         // Wake a consumer blocked in pop(): the sentinel is now deliverable even
@@ -228,7 +290,13 @@ public:
         wake_.fetch_add(1, std::memory_order_release);
         wake_.notify_one();
         if (push_callback_) push_callback_();
-        return true;
+        return SentinelResult::Taken;
+    }
+
+    /// Consuming convenience form. Returns false when the token was not stored,
+    /// whether because the channel is closed or because one is already pending.
+    bool push_sentinel(T value) {
+        return try_push_sentinel(value) == SentinelResult::Taken;
     }
 
     // Blocking pop. Returns when an item is available.
@@ -332,16 +400,56 @@ public:
         wake_.notify_all();
     }
 
-    // Register a callback fired when the queue transitions empty→non-empty.
+    // Register a callback fired after every successful push.
+    //
+    // It fires on every push, not on the empty→non-empty transition, and that
+    // is a correctness requirement rather than a simplification.
+    //
+    // The edge version tested `was_empty = (t == h)` using an `h` sampled
+    // *before* the item was published. A PoolNode consumer decides whether to
+    // run again from the level (count_ready → approx_size), so the two sides
+    // could each read the other as stale and both stand down:
+    //
+    //   producer (push)                    consumer (PoolNode firing)
+    //   ------------------------           ----------------------------
+    //   samples t=782, h=781
+    //     -> was_empty = false, no wake
+    //                                      pops idx 781, head_ = 782
+    //                                      count_ready(): head_==tail_==782
+    //                                      -> not ready, gate released to Idle
+    //   tail_.store(783)
+    //
+    // The item is in the ring, the node is idle, and no wake is outstanding.
+    // Worse, the failure is absorbing: every later push now sees a non-empty
+    // ring, so `was_empty` is false forever and the callback never fires again.
+    // The node sleeps while its backlog grows and its consumer waits on it.
+    //
+    // Re-reading head_ after the tail_ store does not fix it. That is the
+    // store-buffer pattern, and under acquire/release both sides may legally
+    // read stale; forbidding it needs seq_cst on the producer's tail_ store and
+    // head_ load *and* on the consumer's head_ store and tail_ load — a fence
+    // on both hot paths. Firing unconditionally is correct by construction:
+    // the callback runs after the publishing store, so a consumer that observes
+    // the level at all observes the item.
+    //
+    // The redundant wakes are cheap. on_input_ready re-checks the level, and
+    // SubmitGate::claim() collapses a wake arriving during a firing into the
+    // firing already in flight, so the cost is one CAS, not one extra run.
     void set_push_callback(std::function<void()> cb) {
         push_callback_ = std::move(cb);
     }
 
     // Ring occupancy, derived lazily from indices — no separate counter on the
     // hot path. Excludes any out-of-band sentinel (that lives outside the ring).
+    // head_ is loaded first, deliberately. Both indices only ever increase, so
+    // reading head_ before tail_ can at worst under-report a concurrent push;
+    // the other order can read a head_ that has advanced past the tail_ already
+    // sampled, and the unsigned difference then wraps to ~2^64. A caller
+    // polling "is this channel empty yet" against that value never terminates.
     std::size_t size() const {
-        return tail_.load(std::memory_order_relaxed)
+        const std::size_t h = head_.load(std::memory_order_relaxed);
-             - head_.load(std::memory_order_relaxed);
+        const std::size_t t = tail_.load(std::memory_order_acquire);
+        return t - h;
     }
 
     // A pending out-of-band sentinel (EOF) counts as consumable work here even
@@ -358,8 +466,9 @@ public:
     const ChannelStats& stats() const { return stats_; }
 
     ChannelSnapshot snapshot(const std::string& name) const {
-        const std::size_t t = tail_.load(std::memory_order_relaxed);
+        // head_ before tail_, for the reason given on size().
         const std::size_t h = head_.load(std::memory_order_relaxed);
+        const std::size_t t = tail_.load(std::memory_order_acquire);
         return {
             name,
             capacity_,
@@ -394,6 +503,24 @@ private:
     // delivered after every value pushed before it.
     bool take_sentinel(T& out) {
         if (!has_eof_.load(std::memory_order_acquire)) return false;
+        // Re-check emptiness *after* observing has_eof_, not before.
+        //
+        // Callers check the ring is empty and then call this, but the producer
+        // can push a value and publish the sentinel in the window between those
+        // two steps — so the sentinel would be delivered with a real value still
+        // queued behind it, breaking the "sentinel is strictly last" contract
+        // that downstream teardown depends on. a0c4bf5 closed the variant where
+        // the caller's emptiness check used a stale tail_ snapshot; this is the
+        // one where the check is fresh but simply too early.
+        //
+        // Checking here is what makes it sound: the producer publishes the
+        // sentinel with a release store *after* its ring pushes, so a consumer
+        // that has observed has_eof_ has also observed every tail_ advance
+        // before it. If the ring is non-empty now, those values genuinely
+        // precede the sentinel and must be delivered first.
+        if (head_.load(std::memory_order_relaxed)
+                != tail_.load(std::memory_order_acquire))
+            return false;
         out = extract(std::move(eof_value_));
         has_eof_.store(false, std::memory_order_release);
         stats_.record_pop();

include/kpn/diagnostics.hpp

@@ -51,6 +51,14 @@ struct NodeStats {
     std::atomic<int64_t> max_exec_us{0};
     std::atomic<int64_t> total_blocked_us{0};
 
+    // Cumulative wall time inside fire_once, summed over every invocation.
+    // The EMA above cannot be turned into a total: it is exponentially
+    // weighted, so frames * ema_exec_us tracks the tail of the run rather than
+    // the whole of it, and on a workload whose per-frame cost varies (a face
+    // detector on a film: crowd scenes then empty landscapes) the two differ by
+    // a lot. Answering "how much time went into this node" needs a real sum.
+    std::atomic<int64_t> total_exec_us{0};
+
     // Thread CPU time — actual CPU consumed by this node's thread,
     // measured via CLOCK_THREAD_CPUTIME_ID. Excludes time sleeping or
     // blocked on mutexes/channels. Sampled once per frame.
@@ -89,6 +97,7 @@ struct NodeStats {
         frames_processed.fetch_add(1, std::memory_order_relaxed);
 
         int64_t us = static_cast<int64_t>(exec_time.count() * 1000.0);
+        total_exec_us.fetch_add(us, std::memory_order_relaxed);
 
         uint64_t n = frames_processed.load(std::memory_order_relaxed);
         int64_t prev = ema_exec_us.load(std::memory_order_relaxed);
@@ -147,6 +156,29 @@ struct NodeSnapshot {
     double      total_cpu_ms;       // cumulative CPU time consumed by this node's thread
     double      cpu_util_pct;       // exec_ms / (exec_ms + blocked_ms) * 100
     double      queue_wait_ms{0};   // PoolNode: cumulative time spent in pool queue
+
+    // Cumulative wall time inside fire_once. Unlike ema_exec_ms this is a true
+    // sum, so it is the field to use for "share of the run spent in this node".
+    // Note it still includes time parked pushing into a full output channel;
+    // total_cpu_ms is the part that backpressure cannot inflate.
+    //
+    // Declared before the two bools below because every node type initialises
+    // this aggregate positionally, and all of them supply total_exec_ms as the
+    // element after queue_wait_ms.
+    double      total_exec_ms{0};
+
+    // Live scheduling state, for observing the AR-004 invariant "a node never
+    // sleeps with a wake outstanding". The invariant was previously asserted in
+    // comments but invisible at runtime, so a lost wake could only be found in a
+    // debugger — and this bug does not reproduce under one (it needs full speed).
+    // Two atomic loads at snapshot time, nothing on the hot path.
+    //
+    // Read them together with the node's channel fill:
+    //   queued=0, wake=1                -> wake recorded and never consumed
+    //   queued=0, wake=0, input full    -> wake never generated at all
+    //   queued=1 while nothing running  -> submitted but never scheduled
+    bool        queued{false};
+    bool        wake_pending{false};
 };
 
 // ── Pool statistics + snapshot ────────────────────────────────────────────────
@@ -188,6 +220,11 @@ struct ResourceSnapshot {
 struct IResourceProbe {
     virtual ~IResourceProbe() = default;
     virtual ResourceSnapshot snapshot(const std::string& name) const = 0;
+
+    /// Release every thread waiting for the resource, so teardown is not held
+    /// up by one. A network calls this on the resources registered with it when
+    /// it halts; default no-op for probes with nothing to wake.
+    virtual void close() {}
 };
 
 } // namespace kpn

include/kpn/fanout.hpp

@@ -7,8 +7,10 @@
 
 #include <array>
 #include <atomic>
+#include <chrono>
 #include <iostream>
 #include <memory>
+#include <optional>
 #include <thread>
 #include <tuple>
 #include <utility>
@@ -78,7 +80,9 @@ public:
                 blocked_ms,
                 elapsed_s > 0 ? frames / elapsed_s : 0.0,
                 stats_.total_cpu_us.load(std::memory_order_relaxed) / 1000.0,
-                total_ms > 0 ? 100.0 * exec_ms / total_ms : 0.0};
+                total_ms > 0 ? 100.0 * exec_ms / total_ms : 0.0,
+                0.0,  // queue_wait_ms — fanout is not pool-scheduled
+                stats_.total_exec_us.load(std::memory_order_relaxed) / 1000.0};
     }
 
     // ── Port access ───────────────────────────────────────────────────────────
@@ -116,6 +120,79 @@ public:
     }
 
 private:
+    // Deliver `val` to every connected output, losslessly.
+    //
+    // Previously a full output cost the value: push() threw and the exception was
+    // swallowed per output. A dropped item does not degrade a downstream result,
+    // it silently changes one, and the consumer cannot tell it happened — so the
+    // fanout waits instead, and the producer upstream runs slower.
+    //
+    // Unlike a pool node, a fanout owns a private thread, so waiting here costs
+    // no scheduler worker and needs no space-callback park; a bounded retry is
+    // enough. `stop_flag_` is re-checked every pass so teardown cannot hang on a
+    // full output regardless of the order the network stops its nodes in.
+    //
+    // Outputs are retried independently, so a full output never delays delivery
+    // to one with room. Note what that does *not* buy: the next input is not
+    // popped until every output has accepted the current item, so one branch can
+    // never run ahead of another by more than the slower branch's buffering.
+    //
+    // **That bound is a precondition on any topology where the branches rejoin.**
+    // If a consumer on branch B blocks waiting for something branch A computes,
+    // B's buffering must exceed the lead A needs, or the two wedge — B waiting on
+    // A, A starved because the fanout is holding an item B will not take. Making
+    // the fanout lossless is what puts that precondition on the topology; while
+    // it dropped, the question could not arise.
+    //
+    // `parked` receives the time spent waiting on a full output, which the caller
+    // charges to blocked rather than exec.
+    //
+    // Returns false if stopped with the value undelivered.
+    bool deliver(const T& val, duration_t& parked) {
+        std::array<std::optional<T>, N> pending;
+        std::size_t outstanding = 0;
+        for (std::size_t i = 0; i < N; ++i)
+            if (out_channels_[i]) { pending[i].emplace(val); ++outstanding; }
+
+        bool first_pass = true;
+        auto park_from  = clock_t::now();
+
+        for (;;) {
+            for (std::size_t i = 0; i < N; ++i) {
+                if (!pending[i]) continue;
+                // Taken or Closed both mean "stop trying" — delivered, or gone
+                // with the drop recorded. Only Full is worth another pass.
+                if (out_channels_[i]->try_push(*pending[i])
+                        != Channel<T>::PushResult::Full) {
+                    pending[i].reset();
+                    --outstanding;
+                }
+            }
+            if (first_pass) { park_from = clock_t::now(); first_pass = false; }
+
+            if (outstanding == 0) {
+                parked = duration_t(clock_t::now() - park_from);
+                return true;
+            }
+            if (stop_flag_.load(std::memory_order_relaxed)) {
+                // Teardown with work in hand. One last throwing push per
+                // outstanding output, purely so the channel's own stats record
+                // the loss (drop if it is disabled, overflow if it is merely
+                // full). The whole point of the lossless path is that a loss is
+                // never invisible, and a silent `return` here would reintroduce
+                // exactly the hole this function exists to close.
+                for (std::size_t i = 0; i < N; ++i) {
+                    if (!pending[i]) continue;
+                    try { out_channels_[i]->push(std::move(*pending[i])); }
+                    catch (const ChannelOverflowError&) {}
+                }
+                parked = duration_t(clock_t::now() - park_from);
+                return false;
+            }
+            std::this_thread::sleep_for(std::chrono::microseconds(50));
+        }
+    }
+
     void run_loop() {
         while (!stop_flag_.load(std::memory_order_relaxed)) {
             try {
@@ -124,16 +201,17 @@ private:
                 auto t1  = clock_t::now();
                 auto cpu0 = NodeStats::cpu_now();
 
-                for (std::size_t i = 0; i < N; ++i) {
+                duration_t parked{0};
-                    if (out_channels_[i]) {
+                const bool delivered = deliver(val, parked);
-                        try { out_channels_[i]->push(val); }
-                        catch (const ChannelOverflowError&) {}  // drop for this output independently
-                    }
-                }
 
                 auto cpu1 = NodeStats::cpu_now();
                 auto t2   = clock_t::now();
-                stats_.record_exec(duration_t(t2 - t1), duration_t(t1 - t0), cpu0, cpu1);
+                // Time spent waiting on a full output is *blocked*, not exec: a
+                // parked fanout is idle, and charging it to exec would report the
+                // node as busy exactly when it is the one being held up.
+                stats_.record_exec(duration_t(t2 - t1) - parked,
+                                   duration_t(t1 - t0) + parked, cpu0, cpu1);
+                if (!delivered) break;
             } catch (const ChannelClosedError&) {
                 break;
             }

include/kpn/inode.hpp

@@ -22,6 +22,23 @@ enum class NodeEvent { Overflow, Closed };
 
 struct INode {
     virtual ~INode()       = default;
+
+    // Install channel callbacks, without starting anything.
+    //
+    // A node's push/space callbacks live in std::function members on channels
+    // it shares with its neighbours, and a neighbour that is already running
+    // reads them on its own thread. Writing one while the pipeline runs is a
+    // data race on the std::function — ThreadSanitizer reports it, and the
+    // consequence in the field was the missed startup wake a8cfe73 had to
+    // patch around.
+    //
+    // So a network calls prepare() on every node before it calls start() on
+    // any of them: all the writes happen while nothing is running, and once a
+    // node is live the callbacks are read-only. start() calls prepare() itself
+    // if it has not been called, so standalone nodes still work; it is
+    // idempotent, and the network relies on that.
+    virtual void prepare() {}
+
     virtual void start()   = 0;
     virtual void stop()    = 0;
     virtual bool running() const = 0;

include/kpn/interrupt_node.hpp

@@ -104,6 +104,7 @@ public:
             stats_.total_cpu_us.load(std::memory_order_relaxed) / 1000.0,
             total_ms > 0 ? 100.0 : 0.0,
             qwait_ms,
+            stats_.total_exec_us.load(std::memory_order_relaxed) / 1000.0,
         };
     }
 

include/kpn/main_thread_node.hpp

@@ -90,6 +90,8 @@ public:
             elapsed_s > 0 ? frames / elapsed_s : 0.0,
             stats_.total_cpu_us.load(std::memory_order_relaxed) / 1000.0,
             total_ms > 0 ? 100.0 * exec_ms / total_ms : 0.0,
+            0.0,  // queue_wait_ms — main-thread node is not pool-scheduled
+            stats_.total_exec_us.load(std::memory_order_relaxed) / 1000.0,
         };
     }
 

include/kpn/network.hpp

@@ -8,8 +8,10 @@
 #include <memory>
 #endif
 
+#include <condition_variable>
 #include <functional>
 #include <iomanip>
+#include <mutex>
 #include <iostream>
 #include <map>
 #include <set>
@@ -39,8 +41,15 @@ public:
 
 class Network : public INode {
 public:
-    using ErrorHandler =
+    /// Application-level error listener. Receives the exception any node's
-        std::function<void(std::string_view node_name, std::exception_ptr)>;
+    /// function throws, after that node's own handler (if any) declined it.
+    /// Return true to skip the failed invocation and keep the node running,
+    /// false to let it stop.
+    ///
+    /// Same type as StaticNetwork's, deliberately: this used to be a void
+    /// signature, which could not express the keep-running decision and, more
+    /// to the point, was never delivered anywhere.
+    using ErrorHandler = NodeErrorHandler;
     using DiagnosticsHandler =
         std::function<void(const std::vector<NodeSnapshot>&,
                            const std::vector<ChannelSnapshot>&)>;
@@ -91,6 +100,7 @@ public:
                             + " → " + dst_name + ":" + std::to_string(DstIdx);
         channel_probes_.push_back(
             std::make_unique<ChannelProbe<out_t>>(in_ch, ch_name));
+        channel_src_names_.push_back(src_name);
 
         adj_[src_name].push_back(dst_name);
         return *this;
@@ -134,6 +144,17 @@ public:
 
     void start() override {
         start_time_ = clock_t::now();
+        // Deliver the listener to the nodes. Without this the handler was
+        // stored and never read: a node's exception was discarded at the node
+        // boundary and the only surviving evidence was a Closed event, which
+        // says a node stopped but not why. StaticNetwork has always done this;
+        // Network accepted the handler and silently dropped it.
+        if (error_handler_)
+            for (auto& name : topo_)
+                nodes_.at(name)->set_network_error_callback(error_handler_);
+        // Callbacks first, everywhere, before anything runs — see INode::prepare.
+        for (auto& name : topo_)
+            nodes_.at(name)->prepare();
         for (auto& name : topo_)
             nodes_.at(name)->start();
         start_watchdog();
@@ -210,6 +231,11 @@ public:
         watchdog_interval_ = interval;
     }
 
+    /// How long shutdown() waits for one node's outputs to drain before giving
+    /// up on them and stopping the next layer anyway.
+    void set_drain_timeout(std::chrono::milliseconds t) { drain_timeout_ = t; }
+
+    /// Must be called before start(); the handler is delivered to nodes there.
     void set_error_handler(ErrorHandler h)             { error_handler_  = std::move(h); }
     void set_diagnostics_handler(DiagnosticsHandler h) { diag_handler_  = std::move(h); }
     void set_event_handler(EventHandler h)             { event_handler_  = std::move(h); }
@@ -367,19 +393,46 @@ private:
         return true;
     }
 
-    void drain_output_channels(const std::string& /*name*/) const {
+    /// Wait for the channels fed by `name` to empty, or give up.
-        // Poll all channel probes until none report non-zero fill.
+    ///
-        // A short sleep prevents busy-spin; 1 ms is fine for drain purposes.
+    /// This took a node name and ignored it, polling *every* channel in the
-        bool any_full = true;
+    /// graph instead — so shutdown() waited for the whole network to be idle
-        while (any_full) {
+    /// before stopping each successive layer. With no deadline either, anything
-            any_full = false;
+    /// wedged downstream turned a graceful shutdown into the hang it exists to
-            for (auto& probe : channel_probes_) {
+    /// avoid.
-                auto snap = probe->snapshot();
+    ///
-                if (snap.current_fill > 0) { any_full = true; break; }
+    /// Two bounds, because they fail differently. The deadline covers a
-            }
+    /// consumer that has stopped consuming, where fill never changes and
-            if (any_full)
+    /// waiting cannot help. The no-progress counter covers one that is merely
+    /// slow: it keeps waiting while the queue is shrinking, so a slow drain is
+    /// not cut short just for taking a while.
+    void drain_output_channels(const std::string& name) const {
+        const auto deadline = clock_t::now() + drain_timeout_;
+        std::size_t last_fill = static_cast<std::size_t>(-1);
+        int         stalls    = 0;
+
+        auto fill_of = [&] {
+            std::size_t fill = 0;
+            for (std::size_t i = 0; i < channel_probes_.size(); ++i)
+                if (channel_src_names_[i] == name)
+                    fill += channel_probes_[i]->snapshot().current_fill;
+            return fill;
+        };
+
+        for (;;) {
+            const std::size_t fill = fill_of();
+            if (fill == 0) return;
+            if (fill >= last_fill) { if (++stalls > 100) break; }
+            else                   { stalls = 0; }
+            last_fill = fill;
+            if (clock_t::now() >= deadline) break;
             std::this_thread::sleep_for(std::chrono::milliseconds(1));
         }
+
+        if (const std::size_t left = fill_of())
+            std::cerr << "[kpn] shutdown: '" << name << "' still has " << left
+                      << " queued item(s) its consumer did not take; "
+                         "they are discarded\n";
     }
 
     // ── Cycle detection / topological sort ───────────────────────────────────
@@ -398,9 +451,20 @@ private:
 
     void start_watchdog() {
         watchdog_ = std::jthread([this](std::stop_token tok) {
+            // Interruptible wait, not sleep_for. request_stop() cannot wake a
+            // sleeping thread, so stop_watchdog()'s join blocked for up to a
+            // full interval — three seconds by default, and unbounded for
+            // anyone who set a long one to keep the periodic report quiet.
+            // Every teardown paid it.
+            std::mutex                  m;
+            std::condition_variable_any cv;
             while (!tok.stop_requested()) {
-                std::this_thread::sleep_for(watchdog_interval_);
+                {
-                if (tok.stop_requested()) break;
+                    std::unique_lock lk(m);
+                    if (cv.wait_for(lk, tok, watchdog_interval_,
+                                    [&tok] { return tok.stop_requested(); }))
+                        break;
+                }
 
                 auto s = collect_snapshots();
                 check_hung_nodes();
@@ -444,6 +508,10 @@ private:
     std::map<std::string, std::string>                     exposed_outputs_;
     std::set<std::pair<std::string, std::size_t>>          connected_outputs_;
     std::vector<std::unique_ptr<IChannelProbe>>            channel_probes_;
+    /// Name of the node feeding each probe, parallel to channel_probes_.
+    /// shutdown() drains a node's own outputs, so it has to know which they are.
+    std::vector<std::string>                               channel_src_names_;
+    std::chrono::milliseconds                              drain_timeout_{5000};
     std::vector<std::pair<std::string, IPoolProbe*>>       pool_probes_;
     ErrorHandler                                           error_handler_;
     DiagnosticsHandler                                     diag_handler_;

include/kpn/pool_node.hpp

@@ -5,6 +5,7 @@
 #include "inode.hpp"
 #include "port.hpp"
 #include "scheduler.hpp"
+#include "submit_gate.hpp"
 #include "traits.hpp"
 
 #include <array>
@@ -23,44 +24,15 @@
 
 namespace kpn {
 
-// ── Sentinel detection ────────────────────────────────────────────────────────
+// Sentinel detection (has_eof_field / is_sentinel_value) lives in traits.hpp —
-// A value is a "sentinel" (must-deliver control token, e.g. EOF) if its type
+// every node type that forwards values needs it, not just pool-scheduled ones.
-// carries a bool-convertible eof flag — either directly (`v.eof`, as on a raw
-// source Frame) or nested one level under a `.source` member (`v.source.eof`,
-// as on the pipeline's SceneFrame/…/MatchedSceneFrame message types, which wrap
-// the originating Frame). Sentinels are delivered losslessly and non-blockingly
-// via Channel::push_sentinel() instead of the throwing push(), so backpressure
-// can never drop the token that unblocks downstream teardown.
-//
-// Types with neither shape are never treated as sentinels — both traits are
-// SFINAE-safe and the runtime check compiles away to `false` for them, so this
-// stays a no-op for pipelines that don't use an eof convention.
-template<typename T, typename = void>
-struct has_eof_field : std::false_type {};
-template<typename T>
-struct has_eof_field<T, std::void_t<decltype(static_cast<bool>(std::declval<const T&>().eof))>>
-    : std::true_type {};
-
-template<typename T, typename = void>
-struct has_source_eof_field : std::false_type {};
-template<typename T>
-struct has_source_eof_field<T,
-    std::void_t<decltype(static_cast<bool>(std::declval<const T&>().source.eof))>>
-    : std::true_type {};
-
-template<typename T>
-constexpr bool is_sentinel_value(const T& v) {
-    if constexpr (has_eof_field<T>::value)             return static_cast<bool>(v.eof);
-    else if constexpr (has_source_eof_field<T>::value) return static_cast<bool>(v.source.eof);
-    else                                               return false;
-}
 
 // ── PoolNode ──────────────────────────────────────────────────────────────────
 //
 // Reactive alternative to Node<>. Instead of owning a blocked thread, the node
 // is submitted to a shared IScheduler whenever all its input channels become
 // non-empty. A single fire_once() call pops all inputs, executes the function,
-// and pushes outputs. At most one fire_once() runs at a time (queued_ flag).
+// and pushes outputs. At most one fire_once() runs at a time (see SubmitGate).
 //
 // Source nodes (input_count == 0) submit themselves immediately on start() and
 // resubmit after each fire_once().
@@ -109,21 +81,43 @@ public:
 
     // ── INode ─────────────────────────────────────────────────────────────────
 
+    void prepare() override {
+        if (prepared_) return;          // idempotent: the network calls this,
+        prepared_ = true;               // and start() calls it again if not.
+        register_callbacks(std::make_index_sequence<input_count>{});
+    }
+
     void start() override {
+        prepare();
         enable_inputs(std::make_index_sequence<input_count>{});
         stop_flag_.store(false, std::memory_order_relaxed);
-        queued_.store(false, std::memory_order_relaxed);
+        gate_.force_idle();
-        register_callbacks(std::make_index_sequence<input_count>{});
         if constexpr (input_count == 0)
             try_submit(0.5f);
+        else
+            // Never start with a wake already outstanding — the startup case of
+            // the invariant 9c5ce5f established for the running pipeline.
+            //
+            // The callback is installed by prepare(), before any node runs, but
+            // a network still starts its nodes one at a time: an upstream node
+            // that is already firing can push into this one between the two
+            // calls. The push is accepted by the ring and does invoke the
+            // callback, but on_input_ready() sees stop_flag_ still set and
+            // returns. Every later push sees a non-empty ring and stays silent
+            // — Channel invokes push_callback_ only on the empty->non-empty
+            // transition — so without this the node is never submitted and the
+            // pipeline reads as wedged from the first frame.
+            //
+            // on_input_ready() is the level-triggered form of the same
+            // question, so asking it once here converts the missed edge into a
+            // state check.
+            on_input_ready();
     }
 
     void stop() override {
         stop_flag_.store(true, std::memory_order_seq_cst);
         disable_inputs(std::make_index_sequence<input_count>{});
-        // fire_once() observes stop_flag_ and will not resubmit.
+        await_quiescence();
-        // We do not wait for an in-flight fire_once() to complete here;
-        // callers that need that guarantee should call scheduler_->drain() first.
     }
 
     bool running() const override {
@@ -156,6 +150,9 @@ public:
             stats_.total_cpu_us.load(std::memory_order_relaxed) / 1000.0,
             total_ms > 0 ? 100.0 * exec_ms / total_ms : 0.0,
             qwait_ms,
+            stats_.total_exec_us.load(std::memory_order_relaxed) / 1000.0,
+            gate_.queued(),
+            gate_.wake_pending(),
         };
     }
 
@@ -265,9 +262,9 @@ private:
         disable_inputs(std::make_index_sequence<input_count>{});
         disable_outputs(std::make_index_sequence<output_count>{});
         stats_.exec_start_us.store(0, std::memory_order_relaxed);
-        // Plain store, not release_and_recheck(): this node is stopping, and
+        // force_idle, not finish_firing(): this node is stopping, and
         // honouring a pending wake here would resubmit a dead node.
-        queued_.store(false, std::memory_order_release);
+        gate_.force_idle();
         stop_flag_.store(true, std::memory_order_relaxed);
     }
 
@@ -355,39 +352,121 @@ private:
             : 0.5f), ...);
     }
 
-    /// Submit unless already queued. A wake that arrives while this node is
+    /// Submit unless a firing is already in flight. A wake that arrives while
-    /// queued or running is *recorded*, never dropped.
+    /// one is is *recorded* against it, never dropped.
     ///
     /// Wakes are edge-triggered: a channel fires its space callback on the
-    /// transition, once. If that lands while queued_ is up, the CAS below fails
+    /// transition, once. A dropped one never returns, so a node could park a
-    /// and — before wake_pending_ — the wake was gone. A node could then park a
     /// value, release its worker, and sleep forever holding output its consumer
     /// was waiting for, with every worker idle in cond_wait and nothing left to
-    /// re-trigger it. Recording the drop turns the signal level-triggered: the
+    /// re-trigger it. SubmitGate makes "idle" and "wake outstanding" the same
-    /// invariant is that a node never sleeps with a wake outstanding, enforced
+    /// variable, so the two cannot both be true — see submit_gate.hpp.
-    /// by release_and_recheck() at every point that releases the node.
     void try_submit(float priority) {
-        bool expected = false;
+        // A stopped node must not claim the gate. The scheduler now refuses
-        if (queued_.compare_exchange_strong(expected, true, std::memory_order_acq_rel))
+        // submissions after its pool stops, so the submit itself is safe — but
+        // claiming and never releasing would leave the gate held, and a restart
+        // would then have to clear it. start() does, but relying on that makes
+        // the invariant depend on a distant statement.
+        if (stop_flag_.load(std::memory_order_relaxed)) return;
+        if (gate_.claim())
             scheduler_->submit([this] { fire_once(); }, priority);
-        else
-            wake_pending_.store(true, std::memory_order_release);
-    }
-
-    /// Clear queued_, then honour any wake that was dropped while it was up.
-    /// Every path that finishes or parks a firing must release the node through
-    /// here rather than storing queued_ directly.
-    void release_and_recheck(float priority = 0.5f) {
-        queued_.store(false, std::memory_order_release);
-        if (wake_pending_.exchange(false, std::memory_order_acq_rel))
-            try_submit(priority);
     }
 
     // ── Execution ─────────────────────────────────────────────────────────────
 
+    /// Decide whether this node should run again, then release the gate — in
+    /// that order, always.
+    ///
+    /// Releasing first is what let two firings of the same node overlap: the
+    /// moment the gate is free another worker may enter fire_once, while this
+    /// invocation is still reading pending_ and writing pending_done_. TSan
+    /// caught it as a race on pending_done_ between a firing submitted by the
+    /// old release_and_recheck and one submitted by try_submit. It also quietly
+    /// broke the one-slot park, which is sound only because "at most one
+    /// fire_once runs per node at a time" — with two, a value can be parked by
+    /// one firing and overwritten by the other.
+    ///
+    /// Everything this reads belongs to the firing that holds the claim, so it
+    /// is all evaluated first and the release is the last thing the firing does.
+    void finish_firing() {
+        bool  want_more = false;
+        float prio      = 0.5f;
+
+        if (!stop_flag_.load(std::memory_order_relaxed)) {
+            bool parked = false;
+            if constexpr (!std::is_void_v<return_raw>)
+                parked = pending_.has_value();
+
+            if (parked) {
+                // Still holding output: only worth running again once the
+                // consumer has made room.
+                want_more = outputs_have_space(std::make_index_sequence<output_count>{});
+            } else {
+                if constexpr (input_count == 0) {
+                    want_more = true;              // sources always run again
+                } else {
+                    want_more = count_ready(std::make_index_sequence<input_count>{})
+                                == input_count;
+                    if (want_more) prio = compute_priority();
+                }
+            }
+        }
+
+        if (gate_.release())    scheduler_->submit([this] { fire_once(); }, prio);
+        else if (want_more)     try_submit(prio);
+    }
+
+    /// Block until no firing of this node is in flight or queued.
+    ///
+    /// stop() used to set the flag and return, leaving an executing fire_once
+    /// touching input_channels_, stats_ and pending_ while the caller went on
+    /// to destroy them. For a node with a private pool that was survivable by
+    /// accident — Node::stop() calls pool->stop(), which joins — but a node
+    /// sharing a pool had nothing joining it at all, so ~PoolNode raced its own
+    /// members. The old comment said callers wanting the guarantee should call
+    /// scheduler_->drain() first; a destructor cannot, and the default should
+    /// not be a use-after-free.
+    ///
+    /// The gate is exactly the right thing to wait on: it is claimed for the
+    /// whole of a firing and released as the last act of one. A queued but
+    /// unstarted firing also holds it, and will run, observe stop_flag_ and
+    /// release — which is why the pool must still be running when this is
+    /// called. That is already the documented order (stop nodes, then the
+    /// pool), and Node/ObjectNode do it that way.
+    ///
+    /// Bounded, because a node function that never returns must not turn
+    /// teardown into a hang; and skipped entirely when called from the firing
+    /// thread itself, since an error handler that stops its own node would
+    /// otherwise wait for a firing that is waiting for it.
+    void await_quiescence() {
+        if (firing_thread_.load(std::memory_order_acquire) == std::this_thread::get_id())
+            return;
+        const auto deadline = clock_t::now() + std::chrono::seconds(5);
+        while (gate_.queued()) {
+            if (clock_t::now() >= deadline) {
+                std::cerr << "[kpn] stop: node '" << name_
+                          << "' still had work in flight after 5 s; "
+                             "continuing without it\n";
+                return;
+            }
+            std::this_thread::sleep_for(std::chrono::microseconds(50));
+        }
+    }
+
+    /// Marks fire_once's thread for the duration of a firing, so await_quiescence
+    /// can tell a re-entrant stop() from an external one.
+    struct FiringMark {
+        std::atomic<std::thread::id>& slot;
+        explicit FiringMark(std::atomic<std::thread::id>& s) : slot(s) {
+            slot.store(std::this_thread::get_id(), std::memory_order_release);
+        }
+        ~FiringMark() { slot.store(std::thread::id{}, std::memory_order_release); }
+    };
+
     void fire_once() {
+        FiringMark mark(firing_thread_);
         if (stop_flag_.load(std::memory_order_relaxed)) {
-            queued_.store(false, std::memory_order_release);
+            gate_.force_idle();
             return;
         }
 
@@ -403,54 +482,13 @@ private:
         if constexpr (!std::is_void_v<return_raw>) {
             if (pending_) {
                 push_outputs(std::move(*pending_), std::make_index_sequence<output_count>{});
-                release_and_recheck();
+                // Whether the value went out or is still parked, finish_firing
-                if (pending_) {
+                // reads pending_ and picks the right follow-up: output space if
-                    // Close the lost-wakeup race: a space_callback that fired
+                // still holding, input readiness if drained. Resubmitting
-                    // between the failed push and clearing queued_ was
+                // unconditionally would fire a node whose inputs are empty, and
-                    // swallowed, and nothing else will wake this node. Re-check
+                // pop_one reports an empty channel as ChannelClosedError — which
-                    // now that the flag is down.
+                // this node treats as "upstream finished" and self-stops on.
-                    if (outputs_have_space(std::make_index_sequence<output_count>{}))
+                finish_firing();
-                        try_submit(0.5f);
-                    return;                    // parked
-                }
-                // Drained: resume normal firing, resubmitting exactly the way
-                // the normal tail below does. An unconditional try_submit here
-                // would fire a node whose inputs are empty, and pop_inputs
-                // reports an empty channel as ChannelClosedError — which this
-                // node treats as "upstream finished" and self-stops on. That
-                // is a live node killing itself purely because it was woken by
-                // *output* space rather than by input arrival.
-                if constexpr (input_count == 0) try_submit(0.5f);
-                else                            on_input_ready();
-                return;
-            }
-        }
-
-        // Parked from a previous firing: retry that value before touching the
-        // inputs. Returning here releases the worker — the channel's space
-        // callback re-submits this node once the consumer drains a slot.
-        if constexpr (!std::is_void_v<return_raw>) {
-            if (pending_) {
-                push_outputs(std::move(*pending_), std::make_index_sequence<output_count>{});
-                release_and_recheck();
-                if (pending_) {
-                    // Close the lost-wakeup race: a space_callback that fired
-                    // between the failed push and clearing queued_ was
-                    // swallowed, and nothing else will wake this node. Re-check
-                    // now that the flag is down.
-                    if (outputs_have_space(std::make_index_sequence<output_count>{}))
-                        try_submit(0.5f);
-                    return;                    // parked
-                }
-                // Drained: resume normal firing, resubmitting exactly the way
-                // the normal tail below does. An unconditional try_submit here
-                // would fire a node whose inputs are empty, and pop_inputs
-                // reports an empty channel as ChannelClosedError — which this
-                // node treats as "upstream finished" and self-stops on. That
-                // is a live node killing itself purely because it was woken by
-                // *output* space rather than by input arrival.
-                if constexpr (input_count == 0) try_submit(0.5f);
-                else                            on_input_ready();
                 return;
             }
         }
@@ -462,8 +500,9 @@ private:
         // on_input_ready() resubmits when data actually lands.
         if constexpr (input_count > 0) {
             if (count_ready(std::make_index_sequence<input_count>{}) != input_count) {
-                release_and_recheck();
+                // finish_firing re-checks readiness after the work above, so
-                on_input_ready();   // data may have arrived while we checked
+                // data that landed while we looked is not missed.
+                finish_firing();
                 return;
             }
         }
@@ -486,6 +525,14 @@ private:
             auto t2   = clock_t::now();
             // blocked_time = 0 for pool nodes (we don't block waiting for inputs)
             stats_.record_exec(duration_t(t2 - t1), duration_t::zero(), cpu0, cpu1);
+        } catch (const ChannelEmptyError&) {
+            // Not an error: there was simply nothing to take. Release and wait
+            // to be woken again. fire_once checks readiness before it gets
+            // here, and this node is the sole consumer of its inputs, so this
+            // is unreachable today — it exists so that if the check is ever
+            // weakened the cost is a wasted firing rather than a dead node.
+            finish_firing();
+            return;
         } catch (const ChannelClosedError&) {
             fire_callbacks(closed_callbacks_);
             self_stop();
@@ -507,37 +554,15 @@ private:
         }
 
         stats_.exec_start_us.store(0, std::memory_order_relaxed);
-        release_and_recheck();
+        // If the push above parked, finish_firing waits on output space rather
-
+        // than input arrival: this firing consumed its input, so an input-level
-        if (stop_flag_.load(std::memory_order_relaxed)) return;
+        // check would not resubmit and the node would hold its value forever
-
+        // while its consumer waits for exactly that value.
-        // Parked by the push above. Same situation as the retry path at the top
+        finish_firing();
-        // of fire_once — and the same lost-wakeup race, which that path closes
-        // and this one did not. A space callback that fired while queued_ was
-        // still up got swallowed by try_submit's CAS, and the resubmit below
-        // cannot cover it: this firing consumed its input, so inputs are empty
-        // and on_input_ready() will not resubmit. The node would then hold its
-        // value forever while its consumer waits for exactly that value and its
-        // producer parks on an input channel that never drains. Re-check now
-        // that the flag is down.
-        if constexpr (!std::is_void_v<return_raw>) {
-            if (pending_) {
-                if (outputs_have_space(std::make_index_sequence<output_count>{}))
-                    try_submit(0.5f);
-                return;                    // parked
-            }
-        }
-
-        // Source nodes always resubmit; others resubmit only if inputs are ready.
-        if constexpr (input_count == 0) {
-            try_submit(0.5f);
-        } else {
-            on_input_ready();
-        }
     }
 
     // Pop all inputs — safe because we're the sole consumer and fire_once
-    // is guarded by queued_ (only one fire_once runs at a time).
+    // is guarded by the submit gate (only one fire_once runs at a time).
     template<std::size_t... Is>
     args_tuple pop_inputs(std::index_sequence<Is...>) {
         return {pop_one<Is>()...};
@@ -547,8 +572,16 @@ private:
     std::tuple_element_t<I, args_tuple> pop_one() {
         auto& ch = *std::get<I>(input_channels_);
         std::tuple_element_t<I, args_tuple> val;
-        if (!ch.try_pop_now(val))
+        if (!ch.try_pop_now(val)) {
+            // try_pop_now returns false for "nothing available", which covers
+            // two very different situations. A closed channel means upstream is
+            // finished and this node should stop. An open one means only that
+            // nothing is here at this instant — and treating that as closed
+            // kills a live node, which then disables its own inputs and outputs
+            // and takes the rest of the pipeline with it.
+            if (ch.is_accepting()) throw ChannelEmptyError{};
             throw ChannelClosedError{};
+        }
         return val;
     }
 
@@ -568,7 +601,15 @@ private:
             push_one_out<Is>(std::get<Is>(std::move(result))),
           all = all && pending_done_[Is]), ...);
         if (all) { pending_.reset(); pending_done_.fill(false); }
-        else       pending_ = std::move(result);
+        // The retry path calls this as push_outputs(std::move(*pending_), …), so
+        // on that path `result` *is* the parked tuple. Assigning it to itself is
+        // a self-move-assignment, which for std::tuple is elementwise — and
+        // libstdc++'s std::vector does not guard against it: it swaps its data
+        // into a temporary and leaves the vector empty. A value that failed to
+        // push twice would therefore be delivered with its payload silently
+        // erased, which downstream reads as a legitimately empty result rather
+        // than as a loss. Only store when it is not already stored.
+        else if (!pending_ || &result != &*pending_) pending_ = std::move(result);
     }
 
     /// Returns false when the ring was full and the value was NOT taken; the
@@ -581,7 +622,13 @@ private:
         // downstream pop() forever. Deliver them out-of-band (push_sentinel),
         // which never overflows and never blocks this node's worker thread.
         if (is_sentinel_value(val)) {
-            ch->push_sentinel(std::move(val));
+            // Not parked and not reported. Parking would spin against a slot
+            // only the consumer can free; reporting would cry data loss on the
+            // normal steady state, since a source at end of input keeps being
+            // polled and keeps returning EOF, so the token is re-offered on
+            // every firing. Refusing a re-offer loses nothing — the pending
+            // token says the same thing. See Channel::try_push_sentinel.
+            ch->try_push_sentinel(val);
             return true;
         }
         // Backpressure without parking the worker. A full channel means the
@@ -593,7 +640,10 @@ private:
         // to run the consumer that would drain the channel. That is the
         // hold-and-wait deadlock channel.hpp warns about for sentinels; it
         // applies to data pushes just as much.
-        return ch->try_push(val);
+        // Closed counts as "stop trying", not as delivered: the value is gone
+        // and the channel has recorded the drop. Only Full means park and retry.
+        return ch->try_push(val) != Channel<std::tuple_element_t<I, return_tuple>>
+                                        ::PushResult::Full;
     }
 
     template<std::size_t I>
@@ -614,9 +664,17 @@ private:
     input_channels_t                         input_channels_;
     output_channels_t                        output_channels_{};
     std::atomic<bool>                        stop_flag_{true};
-    std::atomic<bool>                        queued_{false};
+    /// Serialises firings and records wakes that arrive during one. See
-    /// A wake that arrived while queued_ was up. See try_submit.
+    /// submit_gate.hpp for why this cannot be two separate flags.
-    std::atomic<bool>                        wake_pending_{false};
+    SubmitGate                               gate_;
+    /// Whether prepare() has installed the channel callbacks. Only ever touched
+    /// from the thread driving start()/stop(), never from a worker, and never
+    /// cleared: the callbacks capture `this` and stay valid across a restart, so
+    /// re-registering them would be a pointless write to a live channel.
+    bool                                     prepared_{false};
+    /// Thread currently inside fire_once, or a default id when none is.
+    /// See await_quiescence.
+    std::atomic<std::thread::id>             firing_thread_{};
 
     /// The hidden one-slot output buffer (see push_outputs). Holding the value
     /// here is what lets a node stop running without dropping it or occupying a
@@ -679,18 +737,28 @@ public:
 
     ~PoolObjectNode() override { stop(); }
 
+    void prepare() override {
+        if (prepared_) return;
+        prepared_ = true;
+        register_callbacks(std::make_index_sequence<input_count>{});
+    }
+
     void start() override {
+        prepare();
         enable_inputs(std::make_index_sequence<input_count>{});
         stop_flag_.store(false, std::memory_order_relaxed);
-        queued_.store(false, std::memory_order_relaxed);
+        gate_.force_idle();
-        register_callbacks(std::make_index_sequence<input_count>{});
         if constexpr (input_count == 0)
             try_submit(0.5f);
+        else
+            // Never start with a wake already outstanding — see PoolNode::start().
+            on_input_ready();
     }
 
     void stop() override {
         stop_flag_.store(true, std::memory_order_seq_cst);
         disable_inputs(std::make_index_sequence<input_count>{});
+        await_quiescence();
     }
 
     bool running() const override { return !stop_flag_.load(std::memory_order_relaxed); }
@@ -720,6 +788,9 @@ public:
             stats_.total_cpu_us.load(std::memory_order_relaxed) / 1000.0,
             total_ms > 0 ? 100.0 * exec_ms / total_ms : 0.0,
             qwait_ms,
+            stats_.total_exec_us.load(std::memory_order_relaxed) / 1000.0,
+            gate_.queued(),
+            gate_.wake_pending(),
         };
     }
 
@@ -796,9 +867,9 @@ private:
         disable_inputs(std::make_index_sequence<input_count>{});
         disable_outputs(std::make_index_sequence<output_count>{});
         stats_.exec_start_us.store(0, std::memory_order_relaxed);
-        // Plain store, not release_and_recheck(): this node is stopping, and
+        // force_idle, not finish_firing(): this node is stopping, and
         // honouring a pending wake here would resubmit a dead node.
-        queued_.store(false, std::memory_order_release);
+        gate_.force_idle();
         stop_flag_.store(true, std::memory_order_relaxed);
     }
 
@@ -879,37 +950,119 @@ private:
             : 0.5f), ...);
     }
 
-    /// Submit unless already queued. A wake that arrives while this node is
+    /// Submit unless a firing is already in flight. A wake that arrives while
-    /// queued or running is *recorded*, never dropped.
+    /// one is is *recorded* against it, never dropped.
     ///
     /// Wakes are edge-triggered: a channel fires its space callback on the
-    /// transition, once. If that lands while queued_ is up, the CAS below fails
+    /// transition, once. A dropped one never returns, so a node could park a
-    /// and — before wake_pending_ — the wake was gone. A node could then park a
     /// value, release its worker, and sleep forever holding output its consumer
     /// was waiting for, with every worker idle in cond_wait and nothing left to
-    /// re-trigger it. Recording the drop turns the signal level-triggered: the
+    /// re-trigger it. SubmitGate makes "idle" and "wake outstanding" the same
-    /// invariant is that a node never sleeps with a wake outstanding, enforced
+    /// variable, so the two cannot both be true — see submit_gate.hpp.
-    /// by release_and_recheck() at every point that releases the node.
     void try_submit(float priority) {
-        bool expected = false;
+        // A stopped node must not claim the gate. The scheduler now refuses
-        if (queued_.compare_exchange_strong(expected, true, std::memory_order_acq_rel))
+        // submissions after its pool stops, so the submit itself is safe — but
+        // claiming and never releasing would leave the gate held, and a restart
+        // would then have to clear it. start() does, but relying on that makes
+        // the invariant depend on a distant statement.
+        if (stop_flag_.load(std::memory_order_relaxed)) return;
+        if (gate_.claim())
             scheduler_->submit([this] { fire_once(); }, priority);
-        else
-            wake_pending_.store(true, std::memory_order_release);
     }
 
-    /// Clear queued_, then honour any wake that was dropped while it was up.
+    /// Decide whether this node should run again, then release the gate — in
-    /// Every path that finishes or parks a firing must release the node through
+    /// that order, always.
-    /// here rather than storing queued_ directly.
+    ///
-    void release_and_recheck(float priority = 0.5f) {
+    /// Releasing first is what let two firings of the same node overlap: the
-        queued_.store(false, std::memory_order_release);
+    /// moment the gate is free another worker may enter fire_once, while this
-        if (wake_pending_.exchange(false, std::memory_order_acq_rel))
+    /// invocation is still reading pending_ and writing pending_done_. TSan
-            try_submit(priority);
+    /// caught it as a race on pending_done_ between a firing submitted by the
+    /// old release_and_recheck and one submitted by try_submit. It also quietly
+    /// broke the one-slot park, which is sound only because "at most one
+    /// fire_once runs per node at a time" — with two, a value can be parked by
+    /// one firing and overwritten by the other.
+    ///
+    /// Everything this reads belongs to the firing that holds the claim, so it
+    /// is all evaluated first and the release is the last thing the firing does.
+    void finish_firing() {
+        bool  want_more = false;
+        float prio      = 0.5f;
+
+        if (!stop_flag_.load(std::memory_order_relaxed)) {
+            bool parked = false;
+            if constexpr (!std::is_void_v<return_raw>)
+                parked = pending_.has_value();
+
+            if (parked) {
+                // Still holding output: only worth running again once the
+                // consumer has made room.
+                want_more = outputs_have_space(std::make_index_sequence<output_count>{});
+            } else {
+                if constexpr (input_count == 0) {
+                    want_more = true;              // sources always run again
+                } else {
+                    want_more = count_ready(std::make_index_sequence<input_count>{})
+                                == input_count;
+                    if (want_more) prio = compute_priority();
                 }
+            }
+        }
+
+        if (gate_.release())    scheduler_->submit([this] { fire_once(); }, prio);
+        else if (want_more)     try_submit(prio);
+    }
+
+    /// Block until no firing of this node is in flight or queued.
+    ///
+    /// stop() used to set the flag and return, leaving an executing fire_once
+    /// touching input_channels_, stats_ and pending_ while the caller went on
+    /// to destroy them. For a node with a private pool that was survivable by
+    /// accident — Node::stop() calls pool->stop(), which joins — but a node
+    /// sharing a pool had nothing joining it at all, so ~PoolNode raced its own
+    /// members. The old comment said callers wanting the guarantee should call
+    /// scheduler_->drain() first; a destructor cannot, and the default should
+    /// not be a use-after-free.
+    ///
+    /// The gate is exactly the right thing to wait on: it is claimed for the
+    /// whole of a firing and released as the last act of one. A queued but
+    /// unstarted firing also holds it, and will run, observe stop_flag_ and
+    /// release — which is why the pool must still be running when this is
+    /// called. That is already the documented order (stop nodes, then the
+    /// pool), and Node/ObjectNode do it that way.
+    ///
+    /// Bounded, because a node function that never returns must not turn
+    /// teardown into a hang; and skipped entirely when called from the firing
+    /// thread itself, since an error handler that stops its own node would
+    /// otherwise wait for a firing that is waiting for it.
+    void await_quiescence() {
+        if (firing_thread_.load(std::memory_order_acquire) == std::this_thread::get_id())
+            return;
+        const auto deadline = clock_t::now() + std::chrono::seconds(5);
+        while (gate_.queued()) {
+            if (clock_t::now() >= deadline) {
+                std::cerr << "[kpn] stop: node '" << name_
+                          << "' still had work in flight after 5 s; "
+                             "continuing without it\n";
+                return;
+            }
+            std::this_thread::sleep_for(std::chrono::microseconds(50));
+        }
+    }
+
+    /// Marks fire_once's thread for the duration of a firing, so await_quiescence
+    /// can tell a re-entrant stop() from an external one.
+    struct FiringMark {
+        std::atomic<std::thread::id>& slot;
+        explicit FiringMark(std::atomic<std::thread::id>& s) : slot(s) {
+            slot.store(std::this_thread::get_id(), std::memory_order_release);
+        }
+        ~FiringMark() { slot.store(std::thread::id{}, std::memory_order_release); }
+    };
 
     void fire_once() {
+        FiringMark mark(firing_thread_);
         if (stop_flag_.load(std::memory_order_relaxed)) {
-            queued_.store(false, std::memory_order_release);
+            gate_.force_idle();
             return;
         }
         auto t0 = clock_t::now();
@@ -923,25 +1076,13 @@ private:
         if constexpr (!std::is_void_v<return_raw>) {
             if (pending_) {
                 push_outputs(std::move(*pending_), std::make_index_sequence<output_count>{});
-                release_and_recheck();
+                // Whether the value went out or is still parked, finish_firing
-                if (pending_) {
+                // reads pending_ and picks the right follow-up: output space if
-                    // Close the lost-wakeup race: a space_callback that fired
+                // still holding, input readiness if drained. Resubmitting
-                    // between the failed push and clearing queued_ was
+                // unconditionally would fire a node whose inputs are empty, and
-                    // swallowed, and nothing else will wake this node. Re-check
+                // pop_one reports an empty channel as ChannelClosedError — which
-                    // now that the flag is down.
+                // this node treats as "upstream finished" and self-stops on.
-                    if (outputs_have_space(std::make_index_sequence<output_count>{}))
+                finish_firing();
-                        try_submit(0.5f);
-                    return;                    // parked
-                }
-                // Drained: resume normal firing, resubmitting exactly the way
-                // the normal tail below does. An unconditional try_submit here
-                // would fire a node whose inputs are empty, and pop_inputs
-                // reports an empty channel as ChannelClosedError — which this
-                // node treats as "upstream finished" and self-stops on. That
-                // is a live node killing itself purely because it was woken by
-                // *output* space rather than by input arrival.
-                if constexpr (input_count == 0) try_submit(0.5f);
-                else                            on_input_ready();
                 return;
             }
         }
@@ -951,8 +1092,9 @@ private:
         // into pop_inputs on an empty channel.
         if constexpr (input_count > 0) {
             if (count_ready(std::make_index_sequence<input_count>{}) != input_count) {
-                release_and_recheck();
+                // finish_firing re-checks readiness after the work above, so
-                on_input_ready();
+                // data that landed while we looked is not missed.
+                finish_firing();
                 return;
             }
         }
@@ -973,6 +1115,14 @@ private:
             auto cpu1 = NodeStats::cpu_now();
             auto t2   = clock_t::now();
             stats_.record_exec(duration_t(t2 - t1), duration_t::zero(), cpu0, cpu1);
+        } catch (const ChannelEmptyError&) {
+            // Not an error: there was simply nothing to take. Release and wait
+            // to be woken again. fire_once checks readiness before it gets
+            // here, and this node is the sole consumer of its inputs, so this
+            // is unreachable today — it exists so that if the check is ever
+            // weakened the cost is a wasted firing rather than a dead node.
+            finish_firing();
+            return;
         } catch (const ChannelClosedError&) {
             fire_callbacks(closed_callbacks_);
             self_stop();
@@ -993,28 +1143,11 @@ private:
         }
 
         stats_.exec_start_us.store(0, std::memory_order_relaxed);
-        release_and_recheck();
+        // If the push above parked, finish_firing waits on output space rather
-        if (stop_flag_.load(std::memory_order_relaxed)) return;
+        // than input arrival: this firing consumed its input, so an input-level
-
+        // check would not resubmit and the node would hold its value forever
-        // Parked by the push above. Same situation as the retry path at the top
+        // while its consumer waits for exactly that value.
-        // of fire_once — and the same lost-wakeup race, which that path closes
+        finish_firing();
-        // and this one did not. A space callback that fired while queued_ was
-        // still up got swallowed by try_submit's CAS, and the resubmit below
-        // cannot cover it: this firing consumed its input, so inputs are empty
-        // and on_input_ready() will not resubmit. The node would then hold its
-        // value forever while its consumer waits for exactly that value and its
-        // producer parks on an input channel that never drains. Re-check now
-        // that the flag is down.
-        if constexpr (!std::is_void_v<return_raw>) {
-            if (pending_) {
-                if (outputs_have_space(std::make_index_sequence<output_count>{}))
-                    try_submit(0.5f);
-                return;                    // parked
-            }
-        }
-
-        if constexpr (input_count == 0) try_submit(0.5f);
-        else                            on_input_ready();
     }
 
     template<std::size_t... Is>
@@ -1024,7 +1157,16 @@ private:
     std::tuple_element_t<I, args_tuple> pop_one() {
         auto& ch = *std::get<I>(input_channels_);
         std::tuple_element_t<I, args_tuple> val;
-        if (!ch.try_pop_now(val)) throw ChannelClosedError{};
+        if (!ch.try_pop_now(val)) {
+            // try_pop_now returns false for "nothing available", which covers
+            // two very different situations. A closed channel means upstream is
+            // finished and this node should stop. An open one means only that
+            // nothing is here at this instant — and treating that as closed
+            // kills a live node, which then disables its own inputs and outputs
+            // and takes the rest of the pipeline with it.
+            if (ch.is_accepting()) throw ChannelEmptyError{};
+            throw ChannelClosedError{};
+        }
         return val;
     }
 
@@ -1044,7 +1186,15 @@ private:
             push_one_out<Is>(std::get<Is>(std::move(result))),
           all = all && pending_done_[Is]), ...);
         if (all) { pending_.reset(); pending_done_.fill(false); }
-        else       pending_ = std::move(result);
+        // The retry path calls this as push_outputs(std::move(*pending_), …), so
+        // on that path `result` *is* the parked tuple. Assigning it to itself is
+        // a self-move-assignment, which for std::tuple is elementwise — and
+        // libstdc++'s std::vector does not guard against it: it swaps its data
+        // into a temporary and leaves the vector empty. A value that failed to
+        // push twice would therefore be delivered with its payload silently
+        // erased, which downstream reads as a legitimately empty result rather
+        // than as a loss. Only store when it is not already stored.
+        else if (!pending_ || &result != &*pending_) pending_ = std::move(result);
     }
     /// Returns false when the ring was full and the value was NOT taken; the
     /// caller must keep it and retry after the channel signals space.
@@ -1056,11 +1206,20 @@ private:
         // downstream pop() forever. Deliver them out-of-band (push_sentinel),
         // which never overflows and never blocks this node's worker thread.
         if (is_sentinel_value(val)) {
-            ch->push_sentinel(std::move(val));
+            // Not parked and not reported. Parking would spin against a slot
+            // only the consumer can free; reporting would cry data loss on the
+            // normal steady state, since a source at end of input keeps being
+            // polled and keeps returning EOF, so the token is re-offered on
+            // every firing. Refusing a re-offer loses nothing — the pending
+            // token says the same thing. See Channel::try_push_sentinel.
+            ch->try_push_sentinel(val);
             return true;
         }
         // See the note on the typed overload above: park rather than block.
-        return ch->try_push(val);
+        // Closed counts as "stop trying", not as delivered: the value is gone
+        // and the channel has recorded the drop. Only Full means park and retry.
+        return ch->try_push(val) != Channel<std::tuple_element_t<I, return_tuple>>
+                                        ::PushResult::Full;
     }
 
     Obj&                                     obj_;
@@ -1070,9 +1229,17 @@ private:
     input_channels_t                         input_channels_;
     output_channels_t                        output_channels_{};
     std::atomic<bool>                        stop_flag_{true};
-    std::atomic<bool>                        queued_{false};
+    /// Serialises firings and records wakes that arrive during one. See
-    /// A wake that arrived while queued_ was up. See try_submit.
+    /// submit_gate.hpp for why this cannot be two separate flags.
-    std::atomic<bool>                        wake_pending_{false};
+    SubmitGate                               gate_;
+    /// Whether prepare() has installed the channel callbacks. Only ever touched
+    /// from the thread driving start()/stop(), never from a worker, and never
+    /// cleared: the callbacks capture `this` and stay valid across a restart, so
+    /// re-registering them would be a pointless write to a live channel.
+    bool                                     prepared_{false};
+    /// Thread currently inside fire_once, or a default id when none is.
+    /// See await_quiescence.
+    std::atomic<std::thread::id>             firing_thread_{};
 
     /// The hidden one-slot output buffer (see push_outputs). Holding the value
     /// here is what lets a node stop running without dropping it or occupying a

include/kpn/scheduler.hpp

@@ -5,6 +5,7 @@
 #include <functional>
 #include <memory>
 #include <mutex>
+#include <shared_mutex>
 #include <optional>
 #include <queue>
 #include <thread>
@@ -52,6 +53,19 @@ public:
     }
 
     void start() override {
+        // Under the lifecycle lock for the same reason stop() is: submit()
+        // reads queues_ and this rebuilds it. A network starts its nodes one at
+        // a time, and a node already started fires into the next one's channel,
+        // whose push callback submits — so a submission can genuinely land
+        // while another pool is still inside start(). ThreadSanitizer reports
+        // it as a read at submit() against this write, and the consequence is
+        // worse than a torn read: push_back can reallocate the vector under a
+        // reader that has already indexed it.
+        //
+        // Queues are all constructed before any worker is spawned, which is
+        // what keeps worker_loop's own queues_[id] out of this — it never takes
+        // the lock, so holding it across the spawn cannot deadlock.
+        std::unique_lock lk(lifecycle_mx_);
         stopped_.store(false, std::memory_order_relaxed);
         queues_.clear();
         for (std::size_t i = 0; i < thread_count_; ++i)
@@ -62,18 +76,30 @@ public:
     }
 
     void stop() override {
+        // Close the pool to new work before touching anything, and do it under
+        // the lifecycle lock so no submit() is midway through indexing queues_.
+        {
+            std::unique_lock lk(lifecycle_mx_);
             stopped_.store(true, std::memory_order_seq_cst);
+        }
         for (auto& q : queues_) {
             std::lock_guard lock(q->mx);
             std::size_t discarded = q->pq.size();
             while (!q->pq.empty()) q->pq.pop();
             total_.fetch_sub(discarded, std::memory_order_relaxed);
+            queued_.fetch_sub(discarded, std::memory_order_relaxed);
         }
         // Lock cv_mx_ before notifying so the stop signal can't be lost in the
         // gap between a worker's predicate check and its wait() (see submit()).
         { std::lock_guard<std::mutex> lk(cv_mx_); }
         cv_.notify_all();
+        // Join without the lock: a worker's task may call submit(), which takes
+        // it shared, and holding it here would deadlock against that.
         for (auto& t : workers_) if (t.joinable()) t.join();
+        // Destroying the queues is what submit() must never race. By now
+        // stopped_ is published, so any submit() that acquires the lock after
+        // this point returns without touching them.
+        std::unique_lock lk(lifecycle_mx_);
         workers_.clear();
         queues_.clear();
     }
@@ -86,33 +112,88 @@ public:
     }
 
     void submit(std::function<void()> task, float priority = 0.5f) override {
-        std::size_t target = next_.fetch_add(1, std::memory_order_relaxed) % thread_count_;
+        // A submission can arrive after this pool has been stopped, and did so
+        // by an ordinary route: a node's space callback fires from whichever
+        // thread drained the channel, which belongs to the *consumer*. Stop the
+        // producer first — as a sources-first shutdown does — and the consumer
+        // keeps draining its backlog, firing the producer's space callback into
+        // a pool whose stop() has already run queues_.clear(). submit() then
+        // indexed an empty vector: a segfault, reproducible about 12 runs in 20.
+        //
+        // The shared lock is what makes the check meaningful. Reading stopped_
+        // alone leaves the window between the read and the indexing, which is
+        // precisely where stop() clears the vector.
+        std::shared_lock lk(lifecycle_mx_);
+        if (stopped_.load(std::memory_order_acquire) || queues_.empty()) {
+            rejected_.fetch_add(1, std::memory_order_relaxed);
+            return;
+        }
+        // B9 — submit-to-self affinity. Round-robin hands every task to a
+        // *different* worker, and on a pool of two or more that worker is
+        // asleep, so each dispatch pays a futex wake: measured 182 ns/dispatch
+        // on a 1-thread pool against 3197 ns on 20 threads, with voluntary
+        // context switches per task rising 0.00 -> 1.19 in step.
+        //
+        // A submission originating on one of *our own* workers goes to that
+        // worker's queue instead. It is about to return to worker_loop and
+        // try_pop its own queue, so the work is already there and nothing
+        // sleeps — the property that makes ThreadPool(1) fast, extended to
+        // any pool size. Imbalance is corrected by the existing try_steal.
+        //
+        // The pool identity check is load-bearing: a worker of pool A
+        // submitting into pool B must not use A's index, which may exceed B's
+        // thread_count_ or alias an unrelated queue. Nested networks do
+        // exactly this.
+        std::size_t target;
+        if (tls_pool == this && tls_worker < thread_count_) {
+            target = tls_worker;
+        } else {
+            target = next_.fetch_add(1, std::memory_order_relaxed) % thread_count_;
+        }
         {
             std::lock_guard lock(queues_[target]->mx);
             queues_[target]->pq.push(
                 {std::move(task), priority, seq_.fetch_add(1, std::memory_order_relaxed)});
         }
         total_.fetch_add(1, std::memory_order_relaxed);
+        queued_.fetch_add(1, std::memory_order_relaxed);
         submitted_.fetch_add(1, std::memory_order_relaxed);
         // Synchronize with worker_loop's predicate evaluation: taking cv_mx_
         // here guarantees a worker is either before its predicate check (and
         // will observe total_ > 0) or already blocked in wait() (and will be
         // woken). Without this, notify_one() can slip into the gap between the
         // worker's predicate check and its wait(), and be lost — a deadlock.
+        //
+        // Skipped entirely when no worker is parked. waiters_ is incremented
+        // *before* wait() releases cv_mx_ and decremented after it returns,
+        // both under that mutex, so a worker on its way to sleep is already
+        // counted here. Reading zero therefore means no worker can be in
+        // wait(), and there is nothing a notify could reach — as opposed to
+        // reading zero because we raced one, which the mutex prevents.
+        //
+        // This is the hot path for an already-busy pool: with B9 the work is
+        // in the local queue and the submitting worker will find it itself,
+        // so the lock round-trip and notify were pure overhead. Measured 1.00
+        // voluntary context switches per task before this, on a pool where
+        // only one task is ever in flight.
+        if (waiters_.load(std::memory_order_seq_cst) != 0) {
             { std::lock_guard<std::mutex> lk(cv_mx_); }
             cv_.notify_one();
         }
+    }
 
     std::size_t thread_count() const { return thread_count_; }
 
+    /// Submissions dropped because the pool was stopped. See rejected_.
+    uint64_t rejected() const { return rejected_.load(std::memory_order_relaxed); }
+
     // ── IPoolProbe ────────────────────────────────────────────────────────────
 
     PoolSnapshot snapshot(const std::string& name) const override {
         std::size_t a = active_.load(std::memory_order_relaxed);
-        std::size_t t = total_.load(std::memory_order_relaxed);
         return {
             name, thread_count_,
-            t > a ? t - a : 0,   // queued (approximate)
+            queued_.load(std::memory_order_relaxed),   // queued (exact)
             a,                                          // executing
             submitted_.load(std::memory_order_relaxed),
             completed_.load(std::memory_order_relaxed),
@@ -142,11 +223,23 @@ private:
         if (q.pq.empty()) return std::nullopt;
         auto fn = std::move(const_cast<Task&>(q.pq.top()).fn);
         q.pq.pop();
+        queued_.fetch_sub(1, std::memory_order_relaxed);
         return fn;
     }
 
     std::optional<std::function<void()>> try_steal(std::size_t thief) {
         // Find the most-loaded peer without blocking — racy peek is fine.
+        //
+        // The threshold is >0: a peer holding a single task is a valid victim.
+        //
+        // Raising it to >1 — to stop a thief winning the race for a task its
+        // owner just submitted to itself (B9) — deadlocks. `latency` mode
+        // hangs at 12 and 20 threads: an external submit() round-robins one
+        // task onto an idle worker's queue, and if that worker is parked, no
+        // peer will take it because a queue of one is no longer stealable.
+        // Nothing else is coming to wake it, so the pool sits forever.
+        // Measured before reverting: it also made steady-state *worse*,
+        // 2229 -> 4546 ns at 12 threads.
         std::size_t victim = thief, best = 0;
         for (std::size_t i = 0; i < queues_.size(); ++i) {
             if (i == thief) continue;
@@ -175,36 +268,94 @@ private:
     }
 
     void worker_loop(std::size_t id) {
+        // Identify this thread as one of our workers, for B9's affinity check
+        // in submit(). Restored on exit rather than merely cleared: a pool
+        // whose worker runs a task that itself starts and stops a nested pool
+        // would otherwise come back with its identity erased.
+        ThreadPool* const prev_pool   = tls_pool;
+        const std::size_t prev_worker = tls_worker;
+        tls_pool   = this;
+        tls_worker = id;
+        struct Restore {
+            ThreadPool* p; std::size_t w;
+            ~Restore() { tls_pool = p; tls_worker = w; }
+        } restore{prev_pool, prev_worker};
+
         while (true) {
             if (auto fn = try_pop(*queues_[id]))   { execute(*fn); continue; }
             if (auto fn = try_steal(id))            { execute(*fn); continue; }
 
+            // B5 (bounded spin before parking) was tried here and removed: it
+            // does not pay. Swept at 50/200/1000 rounds on a 12-thread pool,
+            // steady state went 2123 / 2230 / 2574 ns against 2229 ns without
+            // it, and voluntary context switches per task stayed at ~0.97
+            // throughout. The spin cannot catch what it is aimed at, because
+            // a peer is woken the moment queued_ becomes non-zero — which
+            // happens before this worker reaches the spin at all.
             std::unique_lock lock(cv_mx_);
+            // Counted under cv_mx_ and before the predicate is evaluated, so
+            // that a submit() which reads waiters_ == 0 can be certain this
+            // worker is not about to block: to get here we already hold the
+            // mutex that submit() must take to notify.
+            waiters_.fetch_add(1, std::memory_order_seq_cst);
             cv_.wait(lock, [this] {
                 return stopped_.load(std::memory_order_seq_cst)
-                    || total_.load(std::memory_order_relaxed) > 0;
+                    || queued_.load(std::memory_order_relaxed) > 0;
             });
+            waiters_.fetch_sub(1, std::memory_order_seq_cst);
+            // Exit on queued_, not total_: waiting for total_ to reach zero
+            // meant waiting for someone else's task to finish, which this
+            // worker cannot help with and would spin through until it did.
             if (stopped_.load(std::memory_order_seq_cst)
-                && total_.load(std::memory_order_relaxed) == 0)
+                && queued_.load(std::memory_order_relaxed) == 0)
                 return;
         }
     }
 
+    /// Which pool, and which of its workers, the calling thread is — or
+    /// nullptr on any thread that is not a pool worker. Read by submit() to
+    /// decide whether a local push is safe (B9). inline so the header stays
+    /// header-only.
+    static inline thread_local ThreadPool* tls_pool   = nullptr;
+    static inline thread_local std::size_t tls_worker = 0;
+
     const std::size_t                          thread_count_;
     std::vector<std::unique_ptr<WorkerQueue>>  queues_;
     std::vector<std::thread>                   workers_;
 
+    /// Guards the lifetime of queues_/workers_ against a concurrent submit().
+    /// Shared by submit, exclusive by stop, so submissions still run in
+    /// parallel with each other.
+    mutable std::shared_mutex lifecycle_mx_;
+
     std::mutex              cv_mx_;
     std::condition_variable cv_;
     std::mutex              drain_mx_;
     std::condition_variable drain_cv_;
 
     std::atomic<bool>     stopped_{true};
-    std::atomic<size_t>   total_{0};      // queued + executing
+    std::atomic<size_t>   total_{0};      // queued + executing (drain() waits on this)
+    /// Queued only — never counts a task that is already executing.
+    ///
+    /// The wait predicate used total_, which includes running tasks, so while
+    /// any one task ran every *other* worker's predicate was true: wait()
+    /// returned instantly and the worker spun through try_pop / try_steal /
+    /// wait at full speed, try_lock-ing every peer queue on each pass. One slow
+    /// task therefore pinned every other core and contended the very mutexes
+    /// the working thread needed. Sleeping requires "no work is *waiting*",
+    /// which is this.
+    std::atomic<size_t>   queued_{0};     // waiting to run
     std::atomic<size_t>   active_{0};     // executing only (for snapshot)
     std::atomic<size_t>   next_{0};       // round-robin submit cursor
+    /// Workers currently inside cv_.wait(), maintained under cv_mx_. Lets
+    /// submit() skip the lock round-trip and notify when nobody is parked.
+    std::atomic<size_t>   waiters_{0};
     std::atomic<uint64_t> seq_{0};        // tie-break for equal-priority tasks
     std::atomic<uint64_t> submitted_{0};
+    /// Submissions refused because the pool was already stopped. Not an error —
+    /// teardown races are expected — but silence here would hide a node that
+    /// keeps trying to run after its pool is gone.
+    std::atomic<uint64_t> rejected_{0};
     std::atomic<uint64_t> completed_{0};
 };
 

include/kpn/shared_resource.hpp

@@ -14,6 +14,18 @@ namespace kpn {
 
 template<typename T> class Channel; // forward declaration for acquire_balanced
 
+/// Thrown by a pending acquire() when the resource is closed underneath it.
+///
+/// acquire() blocks on a condition variable with no timeout and no stop
+/// condition, so a node parked there ignored teardown entirely: the worker
+/// never returned, the pool's join never completed, and shutdown hung on a
+/// resource nobody was going to release. Closing the resource turns that into
+/// an exception the node's normal error path already handles.
+class ResourceClosedError : public std::runtime_error {
+public:
+    ResourceClosedError() : std::runtime_error("shared resource closed") {}
+};
+
 // ── SharedResource ────────────────────────────────────────────────────────────
 //
 // Wraps an exclusive resource (e.g. an ONNX session, a CUDA stream) and
@@ -72,6 +84,7 @@ public:
     template<typename PriorityFn>
     Guard acquire(PriorityFn&& fn) {
         std::unique_lock lock(mutex_);
+        if (closed_) throw ResourceClosedError{};
         if (!held_) {
             held_ = true;
             acq_.fetch_add(1, std::memory_order_relaxed);
@@ -83,18 +96,46 @@ public:
         current_waiters_.store(waiters_.size(), std::memory_order_relaxed);
 
         auto t0 = w.wait_start;
-        w.cv.wait(lock, [&w] { return w.ready; });
+        // Woken either by release() handing over ownership, or by close()
+        // giving up on the wait entirely.
+        w.cv.wait(lock, [&w] { return w.ready || w.closed; });
 
         int64_t wait_us = std::chrono::duration_cast<std::chrono::microseconds>(
             clock_t::now() - t0).count();
         waiters_.erase(std::find(waiters_.begin(), waiters_.end(), &w));
         current_waiters_.store(waiters_.size(), std::memory_order_relaxed);
-        acq_.fetch_add(1, std::memory_order_relaxed);
         total_wait_us_.fetch_add(static_cast<uint64_t>(wait_us > 0 ? wait_us : 0),
                                  std::memory_order_relaxed);
+
+        // Closed without being handed ownership: no Guard, so nothing to
+        // release, and held_ is left exactly as close() found it.
+        if (!w.ready) throw ResourceClosedError{};
+
+        acq_.fetch_add(1, std::memory_order_relaxed);
         return Guard(this);
     }
 
+    /// Wake every waiter and refuse further acquisitions.
+    ///
+    /// Teardown is the whole point: a node parked in acquire() is not
+    /// observing stop flags, so without this the only way out is for whoever
+    /// holds the resource to release it — which, if that node is also being
+    /// stopped, may never happen. Idempotent, and safe to call from any thread.
+    void close() override {
+        std::lock_guard lock(mutex_);
+        closed_ = true;
+        for (Waiter* w : waiters_) {
+            w->closed = true;
+            w->cv.notify_one();
+        }
+    }
+
+    /// Reopen after a close(). For reuse across runs; not needed for teardown.
+    void reopen() {
+        std::lock_guard lock(mutex_);
+        closed_ = false;
+    }
+
     // Acquire with no priority (all waiters treated equally, order is fair-ish).
     Guard acquire() {
         return acquire([] { return 0.5f; });
@@ -132,7 +173,10 @@ public:
 private:
     void release() {
         std::unique_lock lock(mutex_);
-        if (waiters_.empty()) {
+        // Hand over only to a waiter that is still waiting. A closed one is on
+        // its way out and will not take ownership, so treating it as the next
+        // holder would leave held_ true with nobody holding it.
+        if (closed_ || waiters_.empty()) {
             held_ = false;
             return;
         }
@@ -162,7 +206,8 @@ private:
         std::function<float()>  priority_fn;
         clock_t::time_point     wait_start;
         std::condition_variable cv;
-        bool                    ready{false};
+        bool                    ready{false};   // handed ownership by release()
+        bool                    closed{false};  // woken by close() instead
 
         Waiter(std::function<float()> fn, clock_t::time_point t)
             : priority_fn(std::move(fn)), wait_start(t) {}
@@ -172,6 +217,7 @@ private:
 
     T                        resource_;
     bool                     held_{false};
+    bool                     closed_{false};
     mutable std::mutex       mutex_;
     std::vector<Waiter*>     waiters_;
     std::atomic<uint64_t>    acq_{0};

include/kpn/static_network.hpp

@@ -98,13 +98,15 @@ public:
                   std::vector<INode*>        fanout_ptrs,
                   std::vector<std::string>   user_node_names,
                   std::vector<std::string>   fanout_node_names,
-                  std::vector<std::unique_ptr<IChannelProbe>> channel_probes)
+                  std::vector<std::unique_ptr<IChannelProbe>> channel_probes,
+                  std::vector<std::string>   channel_src_names)
         : fanouts_(std::move(fanouts))
         , user_nodes_topo_(std::move(user_nodes_topo))
         , fanout_nodes_ptr_(std::move(fanout_ptrs))
         , user_node_names_(std::move(user_node_names))
         , fanout_node_names_(std::move(fanout_node_names))
         , channel_probes_(std::move(channel_probes))
+        , channel_src_names_(std::move(channel_src_names))
     {}
 
     ~StaticNetwork() override { stop(); }
@@ -126,6 +128,15 @@ public:
             for (auto* node : user_nodes_topo_)
                 node->set_network_error_callback(error_handler_);
         }
+        // Install every node's channel callbacks before starting any of them.
+        // Those callbacks are std::function members on channels shared with
+        // neighbours; a neighbour that is already running reads them from its
+        // own thread, so writing one after the pipeline is live is a data race
+        // (ThreadSanitizer reports it on any multi-node network). Doing all the
+        // writes here, while nothing runs, makes them read-only thereafter.
+        for (auto* n : user_nodes_topo_)  n->prepare();
+        for (auto* n : fanout_nodes_ptr_) n->prepare();
+
         for (auto* n : user_nodes_topo_)  n->start();
         for (auto* n : fanout_nodes_ptr_) n->start();
 #ifdef KPN_WEB_DEBUG
@@ -149,6 +160,11 @@ public:
 #ifdef KPN_WEB_DEBUG
         if (web_server_) web_server_->stop();
 #endif
+        // Release anything parked on a shared resource first. A node blocked in
+        // acquire() is not watching stop flags, so stopping it would wait on a
+        // handover that may never come — its holder is being stopped too.
+        for (auto& [rname, probe] : resource_probes_) { (void)rname; probe->close(); }
+
         for (auto it = fanout_nodes_ptr_.rbegin(); it != fanout_nodes_ptr_.rend(); ++it)
             (*it)->stop();
         for (auto it = user_nodes_topo_.rbegin(); it != user_nodes_topo_.rend(); ++it)
@@ -162,11 +178,12 @@ public:
 #ifdef KPN_WEB_DEBUG
         if (web_server_) web_server_->stop();
 #endif
+        for (auto& [rname, probe] : resource_probes_) { (void)rname; probe->close(); }
         // user_nodes_topo_ is already in sources-first order.
         // Stop each node and drain its output channels before moving on.
-        for (auto* n : user_nodes_topo_) {
+        for (std::size_t i = 0; i < user_nodes_topo_.size(); ++i) {
-            n->stop();
+            user_nodes_topo_[i]->stop();
-            drain_all_channels();
+            drain_outputs_of(user_node_names_[i]);
         }
         for (auto* n : fanout_nodes_ptr_) n->stop();
     }
@@ -185,6 +202,10 @@ public:
 
     void set_event_handler(EventHandler h) { event_handler_ = std::move(h); }
 
+    /// How long shutdown() waits for one node's outputs to drain before giving
+    /// up on them and stopping the next layer anyway.
+    void set_drain_timeout(std::chrono::milliseconds t) { drain_timeout_ = t; }
+
     /// Application-level error listener. Receives the exception any node's
     /// function throws, after that node's own handler (if any) declined it.
     /// Return true to skip the failed invocation and keep the node running,
@@ -265,16 +286,50 @@ private:
         return {std::move(nodes), std::move(channels), std::move(resources), std::move(pools), elapsed_s};
     }
 
-    void drain_all_channels() const {
+    /// Wait for the channels fed by `src` to empty, or give up.
-        bool any_full = true;
+    ///
-        while (any_full) {
+    /// This was an unbounded `while (anything anywhere is non-empty)` poll over
-            any_full = false;
+    /// *every* channel in the graph, which made shutdown() wait for the whole
-            for (auto& probe : channel_probes_) {
+    /// network to be idle before stopping each successive layer, and wait
-                if (probe->snapshot().current_fill > 0) { any_full = true; break; }
+    /// forever if anything downstream was wedged — turning a graceful shutdown
-            }
+    /// into the hang it exists to avoid.
-            if (any_full)
+    ///
+    /// Two bounds, because they fail differently. The deadline covers a
+    /// consumer that has stopped consuming: fill never changes and no amount of
+    /// waiting helps. The no-progress counter covers a consumer that is merely
+    /// slow — it keeps waiting as long as the queue is shrinking, so a slow
+    /// drain is not cut short just for exceeding a fixed time.
+    ///
+    /// Giving up is reported rather than silent: undrained data at this point
+    /// means values are about to be discarded by the stop that follows.
+    void drain_outputs_of(const std::string& src) const {
+        const auto deadline = clock_t::now() + drain_timeout_;
+        std::size_t last_fill = static_cast<std::size_t>(-1);
+        int         stalls    = 0;
+
+        for (;;) {
+            std::size_t fill = 0;
+            for (std::size_t i = 0; i < channel_probes_.size(); ++i)
+                if (channel_src_names_[i] == src)
+                    fill += channel_probes_[i]->snapshot().current_fill;
+
+            if (fill == 0) return;
+            if (fill >= last_fill) { if (++stalls > 100) break; }
+            else                   { stalls = 0; }
+            last_fill = fill;
+
+            if (clock_t::now() >= deadline) break;
             std::this_thread::sleep_for(std::chrono::milliseconds(1));
         }
+
+        std::size_t left = 0;
+        for (std::size_t i = 0; i < channel_probes_.size(); ++i)
+            if (channel_src_names_[i] == src)
+                left += channel_probes_[i]->snapshot().current_fill;
+        if (left)
+            std::cerr << "[kpn] shutdown: '" << src << "' still has " << left
+                      << " queued item(s) its consumer did not take; "
+                         "they are discarded\n";
     }
 
     std::string                                         name_;
@@ -285,6 +340,10 @@ private:
     std::vector<std::string>                            user_node_names_;
     std::vector<std::string>                            fanout_node_names_;
     std::vector<std::unique_ptr<IChannelProbe>>         channel_probes_;
+    /// Display name of the node feeding each probe, parallel to channel_probes_.
+    /// shutdown() drains a node's own outputs, so it has to know which they are.
+    std::vector<std::string>                            channel_src_names_;
+    std::chrono::milliseconds                           drain_timeout_{5000};
     std::vector<std::pair<std::string, IResourceProbe*>> resource_probes_;
     std::vector<std::pair<std::string, IPoolProbe*>>     pool_probes_;
     EventHandler                                        event_handler_;
@@ -321,29 +380,6 @@ auto make_network(Edges&&... edges) {
     // 4. Construct owned fanout storage on the heap (FanoutNode has jthread — not moveable)
     auto fanout_storage = std::make_unique<FanoutSto>();
 
-    // 5. Collect unique user node pointers + their display names, in edge-declaration order
-    std::vector<INode*>      user_node_ptrs;
-    std::vector<std::string> user_node_names;
-    auto collect = [&](auto& e) {
-        using SrcT = std::decay_t<decltype(e.src)>;
-        using DstT = std::decay_t<decltype(e.dst)>;
-        auto* s = static_cast<INode*>(&e.src);
-        auto* d = static_cast<INode*>(&e.dst);
-        if (std::find(user_node_ptrs.begin(), user_node_ptrs.end(), s) == user_node_ptrs.end()) {
-            auto sname = node_display_name<SrcT>();
-            user_node_ptrs.push_back(s);
-            user_node_names.push_back(sname);
-            s->set_name(sname);
-        }
-        if (std::find(user_node_ptrs.begin(), user_node_ptrs.end(), d) == user_node_ptrs.end()) {
-            auto dname = node_display_name<DstT>();
-            user_node_ptrs.push_back(d);
-            user_node_names.push_back(dname);
-            d->set_name(dname);
-        }
-    };
-    (collect(edges), ...);
-
     // 5. Wire all expanded SimpleEdges.
     //    find_node<NodeT>: searches fanout storage then user edge pack, returns NodeT*.
     //    Uses if constexpr in a fold so mismatched types never reach assignment.
@@ -368,6 +404,38 @@ auto make_network(Edges&&... edges) {
         return ptr;
     };
 
+    // 5. Collect user node pointers + display names in *topological* order.
+    //
+    // Topo is computed above for the cycle check and used to be discarded,
+    // while this vector was filled in edge-declaration order — and then named
+    // user_nodes_topo_ and relied upon as if it were sorted. halt() stops in
+    // its reverse, and shutdown() walks it forwards stopping each node and
+    // draining its outputs before the next, which is only a graceful drain if
+    // the order really is sources-first. It held for every network in the tree
+    // because edges happen to be declared in pipeline order, and would have
+    // broken silently for one that was not.
+    //
+    // Fanout nodes appear in Topo too; they are skipped here because they are
+    // owned separately, in fanout_storage.
+    std::vector<INode*>      user_node_ptrs;
+    std::vector<std::string> user_node_names;
+    [&]<typename... Ns>(tmp::TypeList<Ns...>) {
+        ([&]<typename NodeT>() {
+            if constexpr (!requires { NodeT::is_fanout_node; }) {
+                if (auto* p = find_node.template operator()<NodeT>()) {
+                    auto* n = static_cast<INode*>(p);
+                    if (std::find(user_node_ptrs.begin(), user_node_ptrs.end(), n)
+                            == user_node_ptrs.end()) {
+                        auto nm = node_display_name<NodeT>();
+                        user_node_ptrs.push_back(n);
+                        user_node_names.push_back(nm);
+                        n->set_name(nm);
+                    }
+                }
+            }
+        }.template operator()<Ns>(), ...);
+    }(typename Topo::topo{});
+
     // Pre-pass: build fanout_id → source display name map so fanout nodes
     // can be named after the node feeding them (e.g. "capture_fanout").
     std::map<std::size_t, std::string> fanout_src_name;
@@ -392,6 +460,7 @@ auto make_network(Edges&&... edges) {
     };
 
     std::vector<std::unique_ptr<IChannelProbe>> channel_probes;
+    std::vector<std::string>                    channel_src_names;
 
     auto wire_one = [&]<typename SE>(SE) {
         using SrcNode = typename SE::src_node_t;
@@ -409,6 +478,7 @@ auto make_network(Edges&&... edges) {
                                 + " \xe2\x86\x92 "  // UTF-8 →
                                 + node_name.template operator()<DstNode>() + ":" + std::to_string(DstIdx);
             channel_probes.push_back(std::make_unique<ChannelProbe<out_t>>(ch, ch_name));
+            channel_src_names.push_back(node_name.template operator()<SrcNode>());
         }
     };
 
@@ -436,7 +506,8 @@ auto make_network(Edges&&... edges) {
                std::move(fanout_ptrs),
                std::move(user_node_names),
                std::move(fanout_node_names),
-               std::move(channel_probes));
+               std::move(channel_probes),
+               std::move(channel_src_names));
 }
 
 } // namespace kpn

include/kpn/submit_gate.hpp

@@ -0,0 +1,101 @@
+#pragma once
+#include <atomic>
+
+namespace kpn {
+
+// ── SubmitGate ────────────────────────────────────────────────────────────────
+//
+// Decides, for one node, whether a wake must turn into a scheduler submission.
+// Exactly one firing of a node may be in flight at a time, and a wake that
+// arrives while one is already in flight must not be lost — it has to be
+// honoured when that firing finishes, or the node sleeps holding work.
+//
+// 9c5ce5f wrote this as two independent atomics: queued_ said a firing was in
+// flight, wake_pending_ recorded a wake that arrived during one. That cannot be
+// made correct, because the release side has to read and write both, and a wake
+// can land between the two operations:
+//
+//   producer (try_submit)              worker (release_and_recheck)
+//   ------------------------           ----------------------------
+//   CAS reads queued_ == true, fails
+//                                      queued_.store(false)
+//                                      wake_pending_.exchange(false) -> false
+//   wake_pending_.store(true)
+//
+// End state: queued_ false, wake_pending_ true, nothing running and nothing
+// scheduled. The node sleeps with a wake outstanding, which is precisely the
+// invariant that commit set out to establish. It is not a memory-ordering
+// subtlety — the interleaving above holds under seq_cst.
+//
+// It survived because every caller happened to follow release_and_recheck()
+// with a level re-check (on_input_ready(), or outputs_have_space() on the
+// parked path), which rediscovers the state a lost wake would have signalled.
+// That is a property of the call sites, not of the mechanism, and any new early
+// return that forgets the re-check turns it back into a hang.
+//
+// One atomic with three states makes the race unrepresentable: "idle" and "wake
+// outstanding" are the same variable, so no interleaving can produce both.
+//
+//   Idle       nothing in flight
+//   Queued     a firing is in flight or queued; no wake since it was claimed
+//   QueuedWake a firing is in flight or queued, and a wake arrived meanwhile
+//
+class SubmitGate {
+public:
+    /// Register a wake. Returns true when the caller must submit the node;
+    /// false when a firing is already in flight and the wake has been recorded
+    /// against it instead.
+    bool claim() noexcept {
+        int cur = state_.load(std::memory_order_acquire);
+        for (;;) {
+            if (cur == kIdle) {
+                if (state_.compare_exchange_weak(cur, kQueued,
+                        std::memory_order_acq_rel, std::memory_order_acquire))
+                    return true;
+            } else if (cur == kQueued) {
+                if (state_.compare_exchange_weak(cur, kQueuedWake,
+                        std::memory_order_acq_rel, std::memory_order_acquire))
+                    return false;
+            } else {
+                return false;            // a wake is already recorded
+            }
+        }
+    }
+
+    /// End the in-flight firing. Returns true when a wake arrived during it and
+    /// the caller must submit again — in which case the gate stays claimed, so
+    /// the node is handed straight from one firing to the next and is never
+    /// momentarily idle with work outstanding. Returns false when the node is
+    /// now idle.
+    bool release() noexcept {
+        int cur = state_.load(std::memory_order_acquire);
+        for (;;) {
+            if (cur == kQueuedWake) {
+                if (state_.compare_exchange_weak(cur, kQueued,
+                        std::memory_order_acq_rel, std::memory_order_acquire))
+                    return true;
+            } else {
+                // kQueued, or kIdle if a stop already forced the gate down.
+                if (state_.compare_exchange_weak(cur, kIdle,
+                        std::memory_order_acq_rel, std::memory_order_acquire))
+                    return false;
+            }
+        }
+    }
+
+    /// Drop the claim and any recorded wake. For stop paths only: honouring a
+    /// wake there would resubmit a dead node.
+    void force_idle() noexcept { state_.store(kIdle, std::memory_order_release); }
+
+    bool queued()       const noexcept { return state_.load(std::memory_order_relaxed) != kIdle; }
+    bool wake_pending() const noexcept { return state_.load(std::memory_order_relaxed) == kQueuedWake; }
+
+private:
+    static constexpr int kIdle       = 0;
+    static constexpr int kQueued     = 1;
+    static constexpr int kQueuedWake = 2;
+
+    std::atomic<int> state_{kIdle};
+};
+
+} // namespace kpn

include/kpn/traits.hpp

@@ -97,4 +97,41 @@ struct repeat_tuple<T, N, std::index_sequence<Is...>> {
 template<typename T, std::size_t N>
 using repeat_tuple_t = typename repeat_tuple<T, N>::type;
 
+// ── Sentinel detection ────────────────────────────────────────────────────────
+// A value is a "sentinel" (must-deliver control token, e.g. EOF) if its type
+// carries a bool-convertible eof flag — either directly (`v.eof`, as on a raw
+// source Frame) or nested one level under a `.source` member (`v.source.eof`,
+// as on message types that wrap the originating Frame). Sentinels are delivered
+// losslessly and non-blockingly via Channel::push_sentinel() instead of the
+// throwing push(), so backpressure can never drop the token that unblocks
+// downstream teardown.
+//
+// Types with neither shape are never treated as sentinels — both traits are
+// SFINAE-safe and the runtime check compiles away to `false` for them, so this
+// stays a no-op for pipelines that don't use an eof convention.
+//
+// Lives here rather than in pool_node.hpp because every node type that forwards
+// values needs it, not just the pool-scheduled ones. FilterNode and RouterNode
+// not having it is what let an EOF token be dropped on a full output.
+
+template<typename T, typename = void>
+struct has_eof_field : std::false_type {};
+template<typename T>
+struct has_eof_field<T, std::void_t<decltype(static_cast<bool>(std::declval<const T&>().eof))>>
+    : std::true_type {};
+
+template<typename T, typename = void>
+struct has_source_eof_field : std::false_type {};
+template<typename T>
+struct has_source_eof_field<T,
+    std::void_t<decltype(static_cast<bool>(std::declval<const T&>().source.eof))>>
+    : std::true_type {};
+
+template<typename T>
+constexpr bool is_sentinel_value(const T& v) {
+    if constexpr (has_eof_field<T>::value)             return static_cast<bool>(v.eof);
+    else if constexpr (has_source_eof_field<T>::value) return static_cast<bool>(v.source.eof);
+    else                                               return false;
+}
+
 } // namespace kpn

include/kpn/variant_node.hpp

@@ -162,6 +162,7 @@ public:
 
     // ── INode ─────────────────────────────────────────────────────────────────
 
+    void prepare() override { node_.prepare(); }
     void start() override { node_.start(); }
     void stop()  override { node_.stop(); }
     bool running() const override { return node_.running(); }

include/kpn/web_debug.hpp

@@ -65,6 +65,11 @@ static std::string to_json(const std::vector<NodeSnapshot>& nodes,
           << ",\"fps\":"          << n.throughput_fps
           << ",\"total_cpu_ms\":" << n.total_cpu_ms
           << ",\"cpu_util_pct\":" << n.cpu_util_pct
+          // Scheduling state — lets a WEDGED pipeline be interrogated over HTTP
+          // without a debugger, which matters because the lost-wake bug does not
+          // reproduce under one. See NodeSnapshot for how to read the pair.
+          << ",\"queued\":"       << (n.queued ? "true" : "false")
+          << ",\"wake_pending\":" << (n.wake_pending ? "true" : "false")
           << "}";
     }
     o << "],\"edges\":[";

scripts/bench_repro_check.py

@@ -0,0 +1,258 @@
+#!/usr/bin/env python3
+"""Check the PERF_PLAN Phase-0 acceptance criterion.
+
+Runs bench_pipeline several times and reports, per row, how far the passes
+spread around their median. The plan's gate is: the same configuration run 7x
+lands within +/-5% on every row. Until that holds, no measured difference
+between KPN and TBB is worth acting on.
+
+Exits non-zero if any row exceeds the tolerance, so it can gate a session of
+performance work rather than merely inform one.
+
+A full sweep is hours, so the run is observable and restartable rather than
+opaque: rows stream to --out-dir as each pass produces them, and a progress
+bar tracks rows within the pass. Killing the run keeps everything already
+written; --resume picks up from the completed passes on disk.
+
+Usage:
+    scripts/bench_repro_check.py ./build_bench/benchmarks/bench_pipeline \\
+        --passes 7 --tolerance 5 -- --work=10 --topos=chain,wide --reps=5
+"""
+
+import argparse
+import datetime
+import os
+import pathlib
+import statistics
+import subprocess
+import sys
+
+KEY_COLS = ("topology", "size", "work_us", "threads")
+
+# bench_pipeline reports throughput, bench_dispatch reports per-dispatch cost.
+# Either is a valid thing to demand reproducibility of; deviation from the
+# median is symmetric, so it does not matter which direction is "better".
+METRIC_COLS = ("items_per_sec", "ns_per_dispatch")
+
+
+def _progress(total, desc):
+    """A tqdm bar if tqdm is installed, else a minimal stderr fallback.
+
+    The fallback exists because this script gates a benchmark run; refusing to
+    start over a missing progress dependency would be the wrong trade.
+    """
+    try:
+        from tqdm import tqdm
+    except ImportError:
+        class Fallback:
+            def __init__(self):
+                self.n = 0
+
+            def update(self, k=1):
+                self.n += k
+                end = "\n" if (total and self.n >= total) else "\r"
+                print(f"  {desc}: {self.n}/{total or '?'} rows",
+                      file=sys.stderr, end=end, flush=True)
+
+            def close(self):
+                pass
+
+            def __enter__(self):
+                return self
+
+            def __exit__(self, *exc):
+                self.close()
+
+        return Fallback()
+
+    return tqdm(total=total, desc=desc, unit="row", leave=False,
+                bar_format="  {desc}: {n_fmt}/{total_fmt} rows "
+                           "|{bar}| {elapsed}<{remaining}",
+                file=sys.stderr)
+
+
+def parse_csv_lines(lines, metric=None):
+    """Return ({(topology, size, work_us, threads): value}, metric_name)."""
+    rows = {}
+    header = None
+    for line in lines:
+        line = line.strip()
+        if not line or line.startswith("#"):
+            continue
+        fields = line.split(",")
+        if header is None:
+            if fields[0] != "topology":
+                continue
+            header = fields
+            if metric is None:
+                for cand in METRIC_COLS:
+                    if cand in header:
+                        metric = cand
+                        break
+                else:
+                    sys.exit(f"no metric column found in header: {header}")
+            elif metric not in header:
+                sys.exit(f"metric {metric!r} not in header: {header}")
+            continue
+        rec = dict(zip(header, fields))
+        try:
+            key = tuple(rec[c] for c in KEY_COLS)
+            rows[key] = float(rec[metric])
+        except (KeyError, ValueError):
+            continue
+    return rows, metric
+
+
+def parse_csv(text, metric=None):
+    return parse_csv_lines(text.splitlines(), metric)
+
+
+def count_rows(binary, extra):
+    """Enumerate the sweep cheaply, so the progress bar has a real total.
+
+    Asks the binary itself rather than reimplementing the sweep in Python,
+    which would silently drift from the C++ defaults. Returns None if the
+    probe fails -- an unknown total degrades the bar, it does not stop the run.
+    """
+    probe = [binary] + extra + ["--reps=0", "--warmup=0"]
+    try:
+        proc = subprocess.run(probe, capture_output=True, text=True,
+                              timeout=600)
+    except (subprocess.SubprocessError, OSError):
+        return None
+    if proc.returncode != 0:
+        return None
+    rows, _ = parse_csv(proc.stdout)
+    return len(rows) or None
+
+
+def run_pass(binary, extra, total, desc, sink, metric):
+    """Run one pass, streaming rows to `sink` and the bar as they arrive.
+
+    capture_output would withhold every row until the pass ended, which for a
+    multi-hour sweep means no way to tell a slow run from a wedged one.
+    """
+    lines = []
+    bar = _progress(total, desc)
+    proc = subprocess.Popen([binary] + extra, stdout=subprocess.PIPE,
+                            stderr=subprocess.PIPE, text=True, bufsize=1)
+    try:
+        for line in proc.stdout:
+            lines.append(line)
+            if sink:
+                sink.write(line)
+                sink.flush()          # a killed run keeps its rows
+            stripped = line.strip()
+            if (stripped and not stripped.startswith("#")
+                    and "," in stripped
+                    and not stripped.startswith("topology,")):
+                bar.update(1)
+    finally:
+        bar.close()
+        proc.stdout.close()
+        stderr = proc.stderr.read()
+        proc.stderr.close()
+        rc = proc.wait()
+
+    if rc != 0:
+        print(stderr, file=sys.stderr)
+        sys.exit(f"{binary} failed with {rc}")
+    return parse_csv_lines(lines, metric)
+
+
+def report(passes, metric, tolerance, npasses):
+    keys = set(passes[0])
+    for p in passes[1:]:
+        keys &= set(p)
+    if not keys:
+        sys.exit("no rows common to every pass")
+
+    print(f"\n{'row':<34} {'median ' + metric:>20} {'worst dev':>10}  verdict")
+    print("-" * 72)
+
+    failures = 0
+    for key in sorted(keys):
+        values = [p[key] for p in passes]
+        med = statistics.median(values)
+        worst = max(abs(v - med) / med * 100 for v in values) if med else 0.0
+        ok = worst <= tolerance
+        failures += not ok
+        label = "{}-{} w={} s={}".format(*key)
+        print(f"{label:<34} {med:>20.1f} {worst:>9.1f}%  {'ok' if ok else 'NOISY'}")
+
+    print("-" * 72)
+    if failures:
+        print(f"{failures}/{len(keys)} rows exceed +/-{tolerance:g}% — "
+              f"the Phase-0 gate is not met.")
+        return 1
+    print(f"all {len(keys)} rows within +/-{tolerance:g}% "
+          f"over {npasses} passes — Phase-0 gate met.")
+    return 0
+
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument("binary", help="path to bench_pipeline")
+    ap.add_argument("--passes", type=int, default=7)
+    ap.add_argument("--tolerance", type=float, default=5.0,
+                    help="max allowed deviation from the median, percent")
+    ap.add_argument("--metric", default=None, choices=METRIC_COLS,
+                    help="column to check (default: whichever the CSV carries)")
+    ap.add_argument("--out-dir", default=None,
+                    help="write pass-NN.csv as rows arrive "
+                         "(default: bench_runs/<timestamp>)")
+    ap.add_argument("--resume", action="store_true",
+                    help="reuse complete pass-NN.csv files in --out-dir")
+
+    # Everything after a standalone `--` goes to bench_pipeline verbatim.
+    # argparse.REMAINDER would swallow this script's own flags instead.
+    argv = sys.argv[1:]
+    extra = []
+    if "--" in argv:
+        cut = argv.index("--")
+        argv, extra = argv[:cut], argv[cut + 1:]
+    args = ap.parse_args(argv)
+
+    out_dir = args.out_dir
+    if out_dir is None:
+        if args.resume:
+            sys.exit("--resume needs an explicit --out-dir")
+        stamp = datetime.datetime.now().strftime("%Y%m%d-%H%M%S")
+        out_dir = os.path.join("bench_runs", stamp)
+    out = pathlib.Path(out_dir)
+    out.mkdir(parents=True, exist_ok=True)
+
+    total = count_rows(args.binary, extra)
+    print(f"writing to {out}/", file=sys.stderr)
+    if total:
+        print(f"{total} rows per pass, {args.passes} passes", file=sys.stderr)
+
+    passes = []
+    metric = args.metric
+    for i in range(args.passes):
+        path = out / f"pass-{i + 1:02d}.csv"
+
+        if args.resume and path.exists():
+            rows, metric = parse_csv(path.read_text(), metric)
+            # A partial file from a killed run must not be silently averaged
+            # in as if it were a whole pass.
+            if total and len(rows) < total:
+                print(f"pass {i + 1}/{args.passes}: {path.name} has "
+                      f"{len(rows)}/{total} rows — rerunning", file=sys.stderr)
+            else:
+                print(f"pass {i + 1}/{args.passes}: reusing {path.name} "
+                      f"({len(rows)} rows)", file=sys.stderr)
+                passes.append(rows)
+                continue
+
+        print(f"pass {i + 1}/{args.passes} ...", file=sys.stderr, flush=True)
+        with open(path, "w") as sink:
+            rows, metric = run_pass(args.binary, extra, total,
+                                    f"pass {i + 1}/{args.passes}", sink, metric)
+        passes.append(rows)
+
+    return report(passes, metric, args.tolerance, args.passes)
+
+
+if __name__ == "__main__":
+    sys.exit(main())

tests/CMakeLists.txt

@@ -36,6 +36,7 @@ add_executable(kpn_tests
     test_pool_node.cpp
     test_backpressure_deadlock.cpp
     test_scheduler.cpp
+    test_submit_gate.cpp
 )
 
 target_link_libraries(kpn_tests PRIVATE
@@ -51,12 +52,31 @@ target_link_libraries(kpn_tests PRIVATE
 add_executable(kpn_tests_stress test_channel_stress.cpp)
 target_link_libraries(kpn_tests_stress PRIVATE kpn Catch2::Catch2WithMain)
 
+# ── Wedge soak (PERF_PLAN G1) ─────────────────────────────────────────────────
+# Long-running end-to-end loop over the configurations that historically wedged.
+# Always built, so it cannot rot, but its CTest cases are registered only under
+# -DKPN_ENABLE_SOAK_TESTS=ON: they run for minutes and would otherwise dominate
+# every `ctest` invocation. Performance work runs it before and after a change:
+#
+#   cmake -B build -DKPN_ENABLE_SOAK_TESTS=ON -DKPN_SOAK_ITERS=50000
+#   cmake --build build --target kpn_soak_wedge
+#   ctest --test-dir build -L soak
+#
+# The binary self-diagnoses: an iteration that stops making progress trips a
+# watchdog that aborts naming the iteration and phase, rather than hanging.
+add_executable(kpn_soak_wedge soak_wedge.cpp)
+target_link_libraries(kpn_soak_wedge PRIVATE kpn)
+target_compile_options(kpn_soak_wedge PRIVATE -O2)
+
+option(KPN_ENABLE_SOAK_TESTS "Register the wedge soak cases with CTest" OFF)
+set(KPN_SOAK_ITERS 5000 CACHE STRING "Iterations per wedge soak case")
+
 # ── Sanitizer flags ───────────────────────────────────────────────────────────
 # kpn_sanitizer_flags() is defined in the top-level CMakeLists and is a no-op
 # unless -DKPN_SANITIZER=... is set. Sanitizer must be on both compile and link.
 kpn_sanitizer_flags(_kpn_san)
 if(_kpn_san)
-    foreach(_t kpn_tests kpn_tests_stress)
+    foreach(_t kpn_tests kpn_tests_stress kpn_soak_wedge)
         target_compile_options(${_t} PRIVATE ${_kpn_san})
         target_link_options(${_t}    PRIVATE ${_kpn_san})
     endforeach()
@@ -76,3 +96,16 @@ catch_discover_tests(kpn_tests DISCOVERY_MODE PRE_TEST)
 # Register the stress suite under its own label so CI can run / time it
 # separately from the fast unit tests.
 catch_discover_tests(kpn_tests_stress DISCOVERY_MODE PRE_TEST PROPERTIES LABELS "stress")
+
+if(KPN_ENABLE_SOAK_TESTS)
+    # pool: the configuration the August wedges were reproduced on.
+    add_test(NAME soak.wedge.pool
+             COMMAND kpn_soak_wedge --mode=pool --depth=4 --threads=4
+                     --items=1000 --work-us=10 --iters=${KPN_SOAK_ITERS})
+    # private: one pool per node — the model workstream A would change.
+    add_test(NAME soak.wedge.private
+             COMMAND kpn_soak_wedge --mode=priv --depth=8
+                     --items=1000 --work-us=10 --iters=${KPN_SOAK_ITERS})
+    set_tests_properties(soak.wedge.pool soak.wedge.private PROPERTIES
+                         LABELS "soak" TIMEOUT 3600)
+endif()

tests/soak_wedge.cpp

@@ -0,0 +1,236 @@
+// Wedge soak test (PERF_PLAN G1).
+//
+// Runs a pipeline configuration end-to-end in a loop and fails if any single
+// iteration stops making progress. Its purpose is to keep performance work
+// from silently reintroducing one of the wedges fixed in August 2026 — the
+// scheduler and channel wake paths are where both perf workstreams operate.
+//
+// Originally the minimal reproducer for the shared-pool chain wedge at
+// (chain, depth=4, work_us=10, pool_threads=4); the pre-6802328 code wedged
+// 5/5 within 45 s, at iterations 149, 1249, 332, 1740 and 493.
+//
+// A wedge is a hang, so a plain loop would hang CTest until its timeout with
+// no indication of where. The watchdog turns that into a failure naming the
+// iteration and the phase it stalled in.
+//
+// Usage: ./kpn_soak_wedge [options]
+//   --iters=5000        iterations to run
+//   --mode=pool|priv    shared ThreadPool(--threads), or one private pool/node
+//   --depth=4           chain depth
+//   --threads=4         shared pool size (--mode=pool only)
+//   --items=1000        items pushed per iteration
+//   --work-us=10        busy-work per node
+//   --watchdog-sec=30   per-iteration progress deadline
+
+#include <kpn/kpn.hpp>
+
+#include <atomic>
+#include <chrono>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <memory>
+#include <string>
+#include <thread>
+#include <vector>
+
+#if defined(__linux__)
+#include <sys/prctl.h>
+#endif
+
+using namespace kpn;
+using sclock = std::chrono::steady_clock;
+
+static std::atomic<int> g_work_us{10};
+
+static int chain_fn(int x) {
+    int us = g_work_us.load(std::memory_order_relaxed);
+    if (us > 0) {
+        auto end = sclock::now() + std::chrono::microseconds(us);
+        while (sclock::now() < end);
+    }
+    return x;
+}
+
+using ChainNode     = Node<chain_fn, in<>, out<>>;
+using PoolChainNode = PoolNode<chain_fn, in<>, out<>>;
+
+static void push_retry(Channel<int>& ch, int val) {
+    while (true) {
+        try   { ch.push(val); return; }
+        catch (const ChannelOverflowError&) { std::this_thread::yield(); }
+        catch (const ChannelClosedError&)   { return; }
+    }
+}
+
+// ── watchdog ──────────────────────────────────────────────────────────────────
+//
+// The worker bumps g_progress at every phase boundary. The watchdog aborts if
+// it stops moving, so a wedge is reported as a failure at a known iteration
+// rather than as an unattributable CTest timeout.
+
+static std::atomic<unsigned long> g_progress{0};
+static std::atomic<int>           g_iter{0};
+static std::atomic<const char*>   g_phase{"init"};
+static std::atomic<bool>          g_done{false};
+
+static void mark(const char* phase) {
+    g_phase.store(phase, std::memory_order_relaxed);
+    g_progress.fetch_add(1, std::memory_order_release);
+}
+
+static void watchdog(double deadline_sec) {
+    unsigned long last = g_progress.load(std::memory_order_acquire);
+    auto last_move = sclock::now();
+    while (!g_done.load(std::memory_order_acquire)) {
+        std::this_thread::sleep_for(std::chrono::milliseconds(100));
+        unsigned long now = g_progress.load(std::memory_order_acquire);
+        if (now != last) { last = now; last_move = sclock::now(); continue; }
+        double stalled = std::chrono::duration<double>(sclock::now() - last_move).count();
+        if (stalled > deadline_sec) {
+            std::fprintf(stderr,
+                "\nWEDGE: no progress for %.0fs at iteration %d, phase '%s'\n",
+                stalled, g_iter.load(std::memory_order_relaxed),
+                g_phase.load(std::memory_order_relaxed));
+            std::fflush(stderr);
+            std::abort();   // core dump / stack trace at the point of the wedge
+        }
+    }
+}
+
+// ── one iteration ─────────────────────────────────────────────────────────────
+
+struct Opts {
+    int    iters        = 5000;
+    int    depth        = 4;
+    int    threads      = 4;
+    int    items        = 1000;
+    int    work_us      = 10;
+    bool   shared_pool  = true;
+    double watchdog_sec = 30.0;
+};
+
+static void one_round_pool(const Opts& o) {
+    const std::size_t CAP = static_cast<std::size_t>(o.items);
+    auto pool = std::make_shared<ThreadPool>(o.threads);
+
+    std::vector<std::shared_ptr<Channel<int>>> chs;
+    for (int i = 0; i <= o.depth; ++i)
+        chs.push_back(std::make_shared<Channel<int>>(CAP));
+
+    std::vector<std::unique_ptr<PoolChainNode>> nodes;
+    for (int i = 0; i < o.depth; ++i) {
+        nodes.push_back(std::make_unique<PoolChainNode>(pool, CAP));
+        nodes.back()->set_input_channel<0>(chs[i]);
+        nodes.back()->set_output_channel<0>(chs[i + 1].get());
+    }
+
+    pool->start();
+    for (auto& n : nodes) n->start();
+    mark("started");
+
+    std::thread reader([&] {
+        for (int i = 0; i < o.items; ++i) chs.back()->pop();
+    });
+    std::thread pusher([&] {
+        for (int i = 0; i < o.items; ++i) push_retry(*chs[0], i);
+    });
+
+    pusher.join();  mark("pushed");
+    reader.join();  mark("drained");
+    for (auto& n : nodes) n->stop();
+    mark("nodes stopped");
+    pool->stop();
+    mark("pool stopped");
+}
+
+static void one_round_private(const Opts& o) {
+    const std::size_t CAP = static_cast<std::size_t>(o.items);
+
+    std::vector<std::shared_ptr<Channel<int>>> chs;
+    for (int i = 0; i <= o.depth; ++i)
+        chs.push_back(std::make_shared<Channel<int>>(CAP));
+
+    std::vector<std::unique_ptr<ChainNode>> nodes;
+    for (int i = 0; i < o.depth; ++i) {
+        nodes.push_back(std::make_unique<ChainNode>(CAP));
+        nodes.back()->set_input_channel<0>(chs[i]);
+        nodes.back()->set_output_channel<0>(chs[i + 1].get());
+    }
+
+    for (auto& n : nodes) n->start();
+    mark("started");
+
+    std::thread reader([&] {
+        for (int i = 0; i < o.items; ++i) chs.back()->pop();
+    });
+    std::thread pusher([&] {
+        for (int i = 0; i < o.items; ++i) push_retry(*chs[0], i);
+    });
+
+    pusher.join();  mark("pushed");
+    reader.join();  mark("drained");
+    for (auto& n : nodes) n->stop();
+    mark("nodes stopped");
+}
+
+// ── main ──────────────────────────────────────────────────────────────────────
+
+static void usage() {
+    std::fprintf(stderr,
+        "usage: kpn_soak_wedge [--iters=N] [--mode=pool|priv] [--depth=D]\n"
+        "                      [--threads=T] [--items=N] [--work-us=U]\n"
+        "                      [--watchdog-sec=S]\n");
+}
+
+int main(int argc, char** argv) {
+#if defined(__linux__)
+    // Allow gdb to attach under ptrace_scope=1 when a wedge is caught.
+    prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, 0, 0, 0);
+#endif
+
+    Opts o;
+    for (int i = 1; i < argc; ++i) {
+        std::string a = argv[i];
+        auto eq = a.find('=');
+        std::string key = a.substr(0, eq);
+        std::string val = eq == std::string::npos ? "" : a.substr(eq + 1);
+
+        if      (key == "--iters")        o.iters        = std::atoi(val.c_str());
+        else if (key == "--depth")        o.depth        = std::atoi(val.c_str());
+        else if (key == "--threads")      o.threads      = std::atoi(val.c_str());
+        else if (key == "--items")        o.items        = std::atoi(val.c_str());
+        else if (key == "--work-us")      o.work_us      = std::atoi(val.c_str());
+        else if (key == "--watchdog-sec") o.watchdog_sec = std::atof(val.c_str());
+        else if (key == "--mode")         o.shared_pool  = (val != "priv");
+        else { usage(); return 2; }
+    }
+    g_work_us.store(o.work_us, std::memory_order_relaxed);
+
+    std::fprintf(stderr,
+        "soak: mode=%s depth=%d threads=%d items=%d work_us=%d iters=%d watchdog=%.0fs\n",
+        o.shared_pool ? "pool" : "priv", o.depth,
+        o.shared_pool ? o.threads : o.depth, o.items, o.work_us,
+        o.iters, o.watchdog_sec);
+
+    std::thread wd(watchdog, o.watchdog_sec);
+
+    const auto t0 = sclock::now();
+    for (int i = 0; i < o.iters; ++i) {
+        g_iter.store(i, std::memory_order_relaxed);
+        if (o.shared_pool) one_round_pool(o);
+        else               one_round_private(o);
+        if ((i + 1) % 100 == 0) {
+            std::fprintf(stderr, "\r  %d/%d", i + 1, o.iters);
+            std::fflush(stderr);
+        }
+    }
+
+    g_done.store(true, std::memory_order_release);
+    wd.join();
+
+    double secs = std::chrono::duration<double>(sclock::now() - t0).count();
+    std::fprintf(stderr, "\ncompleted %d iterations in %.1fs with no wedge\n",
+                 o.iters, secs);
+    return 0;
+}

tests/test_backpressure_deadlock.cpp

@@ -169,3 +169,253 @@ TEST_CASE("a saturated chain never stalls", "[backpressure][deadlock]") {
     // Guard against the test passing because nothing ever ran.
     CHECK(last > 1000);
 }
+
+// Regression: a node must not start with a wake already outstanding.
+//
+// 9c5ce5f established the invariant for the running pipeline — a node never
+// sleeps with a wake it dropped. start() broke the same invariant before the
+// pipeline was even running:
+//
+//     enable_inputs(...);          // channel goes live here
+//     stop_flag_.store(false);
+//     queued_.store(false);
+//     register_callbacks(...);     // push callback installed here
+//
+// StaticNetwork starts nodes sources-first, so an upstream node is already
+// firing into this one during that gap. A push landing there is accepted by the
+// ring but wakes nobody: Channel::push invokes push_callback_ only on the
+// empty→non-empty transition, and at that instant the callback is null. Every
+// later push sees a non-empty ring and stays silent. The node is never
+// submitted, and since a sink has no outputs there is no space callback to
+// rescue it either.
+//
+// The signature is distinctive: **zero** items delivered, not a stall partway.
+// The chain reads as wedged from the first frame. Under `ctest -j4` on a loaded
+// machine it reproduced 7 times in 24, and never once in 10 unloaded runs —
+// contention widens the window between those two statements. That is almost
+// certainly the "rare hang, ~1 run in 20 at a 300 s timeout" 28e0667 recorded as
+// known-incomplete.
+//
+// This test needs no contention: it constructs the state the race leaves behind
+// directly, by enabling the input and pushing before start() is ever called.
+namespace {
+
+int passthrough(int x) { return x; }
+
+}  // namespace
+
+TEST_CASE("a node started with data already queued still fires",
+          "[backpressure][startup]") {
+    auto pool = std::make_shared<kpn::ThreadPool>(2);
+    pool->start();
+
+    auto node = kpn::make_pool_node<passthrough>(pool, 8);
+    kpn::Channel<int> out_ch(8);
+    node.set_output_channel<0>(&out_ch);
+
+    // The missed edge: the channel is live and already holds a value, but no
+    // callback was installed when it arrived, so the wake has been and gone.
+    node.input_channel<0>().enable();
+    node.input_channel<0>().push(21);
+
+    node.start();
+
+    // Bounded wait — a plain pop() would hang rather than fail on a regression.
+    const auto deadline = std::chrono::steady_clock::now() + std::chrono::seconds(5);
+    while (out_ch.size() == 0 && std::chrono::steady_clock::now() < deadline)
+        std::this_thread::sleep_for(std::chrono::milliseconds(5));
+
+    const bool delivered = out_ch.size() > 0;
+    const int  got       = delivered ? out_ch.pop() : -1;
+
+    node.stop();
+    pool->stop();
+
+    INFO("value delivered: " << got);
+    REQUIRE(delivered);
+    CHECK(got == 21);
+}
+
+// Regression: a fanout absorbs an unequal pair of consumers by slowing, not by
+// dropping.
+//
+// 6595e6e made node outputs lossless and 28e0667 stopped them parking a worker,
+// but FanoutNode was in neither: it kept `catch (ChannelOverflowError&) {}` per
+// output, so whichever branch fell behind lost items — silently, and by an
+// amount that depended on timing. Two runs of the same input could therefore
+// disagree, which is fatal for a fixture the rest of the suite is scored
+// against.
+//
+// The two assertions are the two halves of the requirement:
+//   - no gaps: the slow branch receives *every* item, not most of them;
+//   - bounded lead: the fast branch is throttled to the slow one rather than
+//     racing ahead over a drain that is quietly discarding the difference.
+//
+// Either alone would pass on a broken implementation. A fanout that pushed only
+// to the slow branch has no gaps; one that dropped everything for the slow
+// branch keeps a bounded lead by never letting it fall behind.
+namespace {
+
+// Records the sequence it sees, so a dropped item shows up as a gap rather than
+// merely as a smaller total.
+struct SeqCheck {
+    std::atomic<int>*  next_expected;
+    std::atomic<bool>* saw_gap;
+    int                delay_us{0};
+
+    void record(int v) const {
+        if (delay_us)
+            std::this_thread::sleep_for(std::chrono::microseconds(delay_us));
+        const int want = next_expected->load(std::memory_order_relaxed);
+        if (v != want) saw_gap->store(true, std::memory_order_relaxed);
+        else           next_expected->store(want + 1, std::memory_order_relaxed);
+    }
+};
+
+struct FastBranch : SeqCheck {
+    static constexpr std::string_view label() { return "fast_branch"; }
+    void operator()(int v) { record(v); }
+};
+
+struct SlowBranch : SeqCheck {
+    static constexpr std::string_view label() { return "slow_branch"; }
+    void operator()(int v) { record(v); }
+};
+
+}  // namespace
+
+TEST_CASE("a fanout absorbs an unequal pair by slowing, not dropping",
+          "[backpressure][fanout]") {
+    std::atomic<int>  fast_next{0}, slow_next{0};
+    std::atomic<bool> fast_gap{false}, slow_gap{false};
+
+    FreeRun    p_fn;
+    FastBranch fast_fn{{&fast_next, &fast_gap, 0}};
+    SlowBranch slow_fn{{&slow_next, &slow_gap, 500}};   // 0.5 ms/item
+
+    // Small channels so the slow branch saturates in the first few milliseconds
+    // and stays saturated for the whole run.
+    kpn::ObjectNode<FreeRun,    kpn::in<>,       kpn::out<"v">, "free_run", 0> p (p_fn,    8);
+    kpn::ObjectNode<FastBranch, kpn::in<"fast">, kpn::out<>,    "fast",     0> fa(fast_fn, 8);
+    kpn::ObjectNode<SlowBranch, kpn::in<"slow">, kpn::out<>,    "slow",     0> sl(slow_fn, 8);
+
+    // Two edges from one output port: make_network auto-inserts FanoutNode<int,2>.
+    auto net = kpn::make_network(
+        kpn::edge(p.output<"v">(), fa.input<"fast">()),
+        kpn::edge(p.output<"v">(), sl.input<"slow">())
+    );
+    net.start();
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+    net.stop();
+
+    const int fast_seen = fast_next.load(std::memory_order_relaxed);
+    const int slow_seen = slow_next.load(std::memory_order_relaxed);
+
+    INFO("fast branch " << fast_seen << " items, slow branch " << slow_seen);
+    CHECK_FALSE(fast_gap.load(std::memory_order_relaxed));
+    CHECK_FALSE(slow_gap.load(std::memory_order_relaxed));
+    // Guard against passing because nothing ran: 1 s at 0.5 ms/item is ~2000.
+    CHECK(slow_seen > 200);
+    // The lead is bounded by the buffering between the two — the fanout's own
+    // input, the two output channels, and one item in each node's hand. A
+    // dropping fanout has no such bound: the fast branch runs at full speed and
+    // the difference is the loss.
+    CHECK(fast_seen - slow_seen < 200);
+}
+
+// Regression: a filter must not drop an EOF sentinel into a full output.
+//
+// RouterNode and FilterNode were the last nodes on a data path still using the
+// throwing push() and swallowing the result:
+//
+//     try { out_ch_->push(val); } catch (const ChannelOverflowError&) {}
+//
+// 6595e6e made node outputs lossless, 28e0667 stopped them parking a worker,
+// a8cfe73 did the same for FanoutNode. These two were in none of them.
+//
+// For ordinary values that is the familiar silent-loss problem. For a sentinel
+// it is a hang. EOF is what tells every downstream node to shut down, and
+// nothing comes after it to retry — so a filter that passes EOF by predicate
+// but drops it by backpressure produces a pipeline that never terminates. The
+// scene-actor-extraction decimator is exactly this shape: `if (f.eof) return
+// true;` in the predicate, feeding a chain whose slowest node is an ONNX
+// embedder, so the output is reliably full at the moment EOF arrives.
+//
+// The test forces that state rather than racing for it: the sink is slow enough
+// that the filter's output channel is saturated for the whole run, so EOF meets
+// a full ring with certainty.
+//
+// Both assertions are needed. `saw_eof` alone would pass on an implementation
+// that dropped every ordinary value and delivered only the sentinel; `count`
+// alone would pass on the broken one, which delivers plenty of values and loses
+// only the token that matters.
+namespace {
+
+struct EofFrame {
+    int  seq{0};
+    bool eof{false};
+};
+
+// EOF is emitted exactly once, as a real source does. Everything after it is a
+// filler frame the predicate rejects, which keeps the node alive without
+// re-offering the sentinel — a source that retried EOF would mask the bug,
+// since a later attempt could find the channel drained.
+struct EofSource {
+    static constexpr std::string_view label() { return "eof_source"; }
+    int  n{0};
+    int  total{0};
+    EofFrame operator()() {
+        if (n > total) {
+            std::this_thread::sleep_for(std::chrono::milliseconds(1));
+            return {-1, false};          // filler: dropped by the predicate
+        }
+        EofFrame f{n, n == total};
+        ++n;
+        return f;
+    }
+};
+
+struct EofSink {
+    static constexpr std::string_view label() { return "eof_sink"; }
+    std::atomic<int>*  count;
+    std::atomic<bool>* saw_eof;
+    void operator()(EofFrame f) {
+        std::this_thread::sleep_for(std::chrono::microseconds(200));
+        if (f.eof) saw_eof->store(true, std::memory_order_release);
+        else       count->fetch_add(1, std::memory_order_relaxed);
+    }
+};
+
+}  // namespace
+
+TEST_CASE("a filter delivers EOF into a saturated output", "[backpressure][filter]") {
+    std::atomic<int>  count{0};
+    std::atomic<bool> saw_eof{false};
+
+    EofSource src_fn{0, 40};
+    EofSink   sink_fn{&count, &saw_eof};
+
+    // Every real frame passes the predicate, so the only thing between source
+    // and sink is backpressure. Small channels keep the output saturated.
+    auto filt = kpn::make_filter<EofFrame>(
+        [](const EofFrame& f) { return f.seq >= 0; }, 4);
+
+    kpn::ObjectNode<EofSource, kpn::in<>,      kpn::out<"f">, "eof_source", 0> s(src_fn,  4);
+    kpn::ObjectNode<EofSink,   kpn::in<"f">,   kpn::out<>,    "eof_sink",   0> k(sink_fn, 4);
+
+    auto net = kpn::make_network(
+        kpn::edge(s.output<"f">(),      filt.input<0>()),
+        kpn::edge(filt.output<0>(),     k.input<"f">())
+    );
+    net.start();
+
+    // Generous relative to 41 frames at 200 us, and this is a liveness test:
+    // the broken implementation never sets saw_eof no matter how long it runs.
+    for (int i = 0; i < 200 && !saw_eof.load(std::memory_order_acquire); ++i)
+        std::this_thread::sleep_for(std::chrono::milliseconds(10));
+    net.stop();
+
+    INFO("values delivered: " << count.load() << " of 40");
+    CHECK(saw_eof.load(std::memory_order_acquire));
+    CHECK(count.load(std::memory_order_relaxed) == 40);
+}

tests/test_channel.cpp

@@ -1,3 +1,4 @@
+#include <string>
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/catch_approx.hpp>
 #include <kpn/channel.hpp>
@@ -238,3 +239,98 @@ TEST_CASE("try_pop_now delivers a pending sentinel once the ring is empty",
     REQUIRE(out == 99);
     REQUIRE_FALSE(ch.try_pop_now(out));   // nothing left
 }
+
+// Regression: the sentinel slot holds one token and refuses a second.
+//
+// push_sentinel used to write eof_value_ unconditionally. Offering a second
+// token before the first was taken therefore did two wrong things at once: it
+// lost the first silently — and a lost EOF wedges every downstream pop forever
+// — and it wrote the storage while the consumer could be moving the previous
+// value out of it. For the shared_ptr storage that non-trivial types use, that
+// is a torn refcount, not merely a stale read.
+//
+// Refusing is correct rather than queueing: two control tokens on one channel
+// means the stream ended twice, which is a caller protocol error. Coalescing
+// them would hide it, and there is no second value that could sensibly follow
+// the end of a stream.
+TEST_CASE("a second sentinel is refused, not swallowed", "[channel][sentinel]") {
+    Channel<int> ch(4);
+
+    REQUIRE(ch.push_sentinel(1));
+    // Slot occupied: the first token is still undelivered.
+    REQUIRE_FALSE(ch.push_sentinel(2));
+
+    // The first survives intact — the overwrite is what used to lose it.
+    int out = 0;
+    REQUIRE(ch.try_pop_now(out));
+    CHECK(out == 1);
+
+    // And the slot is reusable once drained.
+    REQUIRE(ch.push_sentinel(3));
+    REQUIRE(ch.try_pop_now(out));
+    CHECK(out == 3);
+}
+
+TEST_CASE("a refused sentinel is not counted as a drop", "[channel][sentinel]") {
+    // A refusal means a token arrived while an equivalent one was already
+    // pending — not that anything was lost. Counting it as a drop was wrong in
+    // a way that showed up immediately on real content: a source at the end of
+    // its input keeps being polled and keeps returning EOF, so the token is
+    // re-offered on every firing, and the pipeline reported hundreds of dropped
+    // frames on a clean run and exited non-zero.
+    //
+    // The delivery guarantee is unaffected: the first token is pending and will
+    // arrive. Only the accounting changed.
+    Channel<int> ch(4);
+    REQUIRE(ch.push_sentinel(1));
+    const auto before = ch.stats().drops.load();
+    REQUIRE_FALSE(ch.push_sentinel(2));
+    CHECK(ch.stats().drops.load() == before);
+
+    // And the one that was accepted is still the one delivered.
+    int out = 0;
+    REQUIRE(ch.try_pop_now(out));
+    CHECK(out == 1);
+}
+
+TEST_CASE("try_push_sentinel leaves a refused value untouched", "[channel][sentinel]") {
+    // The non-consuming form exists so a refused token is still the caller's to
+    // report. The consuming push_sentinel cannot offer that, since the value is
+    // already moved into its parameter.
+    Channel<std::string> ch(4);
+    std::string first = "eof-1", second = "eof-2";
+
+    REQUIRE(ch.try_push_sentinel(first) == Channel<std::string>::SentinelResult::Taken);
+    REQUIRE(ch.try_push_sentinel(second) == Channel<std::string>::SentinelResult::SlotBusy);
+    CHECK(second == "eof-2");          // not moved from
+
+    ch.disable();
+    std::string third = "eof-3";
+    CHECK(ch.try_push_sentinel(third) == Channel<std::string>::SentinelResult::Closed);
+    CHECK(third == "eof-3");
+}
+
+// Regression: try_push must distinguish delivered from discarded.
+//
+// It returned bool, and returned *true* for a closed channel — so "the value
+// arrived" and "the value was thrown away because nobody is listening" were the
+// same answer. Every caller was nonetheless correct, because both cases mean
+// "stop trying"; but nothing above the channel could tell the two apart, and a
+// producer counting successful pushes counted discards among them. Only the
+// channel's own drop counter knew, and only if someone read the diagnostics.
+TEST_CASE("try_push distinguishes taken, full and closed", "[channel]") {
+    Channel<int> ch(2);
+    int v = 1;
+
+    CHECK(ch.try_push(v) == Channel<int>::PushResult::Taken);
+    CHECK(ch.try_push(v) == Channel<int>::PushResult::Taken);
+    // Ring is full: the value is untouched and the caller keeps it.
+    CHECK(ch.try_push(v) == Channel<int>::PushResult::Full);
+    CHECK(v == 1);
+
+    ch.disable();
+    const auto drops_before = ch.stats().drops.load();
+    CHECK(ch.try_push(v) == Channel<int>::PushResult::Closed);
+    // Discarded, and recorded as such rather than reported as a delivery.
+    CHECK(ch.stats().drops.load() == drops_before + 1);
+}

tests/test_channel_stress.cpp

@@ -19,6 +19,7 @@
 // Channel<T> is SPSC: exactly one producer thread and one consumer thread per
 // channel. Every scenario below honours that contract.
 
+#include <string>
 #include <catch2/catch_test_macros.hpp>
 #include <atomic>
 #include <chrono>
@@ -152,11 +153,18 @@ TEST_CASE("SPSC: producer racing a disable() never throws and never hangs",
     }
 }
 
-TEST_CASE("SPSC: push_callback fires on each empty->non-empty transition",
+TEST_CASE("SPSC: push_callback fires for every push, never missed",
           "[channel][stress]") {
-    // The empty->non-empty callback ([channel.hpp] was_empty branch) is read by
+    // Regression: this callback is the *only* thing that wakes a PoolNode, and
-    // the consumer-side notification path. Run it under contention to make sure
+    // it used to fire only on the empty->non-empty edge, computed from a head_
-    // the was_empty detection isn't torn by a concurrent pop().
+    // sampled before the item was published. A concurrent pop() could drain the
+    // ring to empty in that window, so neither side saw the other: the item sat
+    // in the ring with the consumer idle, and because the trigger was an edge it
+    // never recovered. See set_push_callback in channel.hpp.
+    //
+    // The old version of this test asserted only `1 <= callbacks <= N`, which a
+    // *missed* callback satisfies — it named the hazard and could not detect it.
+    // One callback per successful push is the contract, so assert exactly that.
     Channel<int> ch(/*capacity=*/4, /*spin_count=*/4);
     std::atomic<int> callbacks{0};
     ch.set_push_callback([&] { callbacks.fetch_add(1, std::memory_order_relaxed); });
@@ -174,10 +182,9 @@ TEST_CASE("SPSC: push_callback fires on each empty->non-empty transition",
     for (int i = 0; i < N; ++i) (void)ch.pop();
     producer.join();
 
-    // At least one transition, at most one per item; mainly we assert the run
+    // Exactly one callback per successful push. Fewer means a wake was dropped,
-    // completed without TSan flagging a race on push_callback_/was_empty.
+    // which is the bug; more would mean a spurious wake was manufactured.
-    REQUIRE(callbacks.load() >= 1);
+    REQUIRE(callbacks.load() == N);
-    REQUIRE(callbacks.load() <= N);
 }
 
 // Ordering contract of the out-of-band sentinel under contention.
@@ -279,3 +286,51 @@ TEST_CASE("SPSC: sentinel is strictly last, after every value (try_pop_now)",
         REQUIRE(ch.approx_size() == 0);
     }
 }
+
+// Contended: a producer offering sentinels while the consumer takes them.
+//
+// The old push_sentinel wrote eof_value_ with no regard for whether the
+// consumer was reading it, so a second offer racing a take was a data race on
+// the storage — for the shared_ptr form used by non-trivial types, on the
+// refcount. Under TSan the old code reports it; the handshake added alongside
+// this test makes the producer's write conditional on observing the slot free,
+// which is what serialises the two.
+//
+// Payload is a std::string so the storage is the shared_ptr path rather than
+// the trivially-copyable one, and each token carries its own identity so a torn
+// value shows up as a mismatch rather than as a plausible-looking result.
+TEST_CASE("SPSC: offering sentinels concurrently with takes is race-free",
+          "[channel][stress][sentinel]") {
+    constexpr int kRounds = 20000;
+    Channel<std::string> ch(4);
+
+    std::atomic<int> taken{0};
+    std::atomic<bool> torn{false};
+    std::atomic<bool> done{false};
+
+    std::thread consumer([&] {
+        std::string out;
+        while (!done.load(std::memory_order_acquire) || ch.approx_size() > 0) {
+            if (ch.try_pop_now(out)) {
+                if (out.rfind("eof-", 0) != 0) torn.store(true, std::memory_order_relaxed);
+                taken.fetch_add(1, std::memory_order_relaxed);
+            }
+        }
+    });
+
+    int accepted = 0;
+    for (int i = 0; i < kRounds; ++i) {
+        std::string tok = "eof-" + std::to_string(i);
+        if (ch.try_push_sentinel(tok) == Channel<std::string>::SentinelResult::Taken)
+            ++accepted;
+    }
+    done.store(true, std::memory_order_release);
+    consumer.join();
+
+    INFO("accepted " << accepted << " taken " << taken.load());
+    CHECK_FALSE(torn.load(std::memory_order_relaxed));
+    // Every accepted token must be delivered: the slot is refused while full,
+    // so acceptance and delivery are one-to-one.
+    CHECK(taken.load(std::memory_order_relaxed) == accepted);
+    CHECK(accepted > 0);
+}

tests/test_network.cpp

@@ -1,6 +1,10 @@
 #include <catch2/catch_test_macros.hpp>
 #include <kpn/kpn.hpp>
+#include <atomic>
 #include <chrono>
+#include <mutex>
+#include <stdexcept>
+#include <string>
 #include <thread>
 
 using namespace kpn;
@@ -54,3 +58,93 @@ TEST_CASE("stop disables input channels — producer push is silently dropped",
     in_ch.push(99);
     REQUIRE(in_ch.size() == 0);
 }
+
+// Regression: Network::set_error_handler must actually deliver the handler.
+//
+// The handler was stored in a member and never read. A node's exception was
+// discarded at the node boundary and the only surviving evidence was a Closed
+// event, which reports that a node stopped but not why — the difference between
+// a diagnosis and a guess. StaticNetwork has always wired this; Network
+// accepted the handler and silently dropped it, which is worse than not
+// offering the setter at all.
+//
+// The type changed with the fix. It was void(name, exception_ptr), which cannot
+// express the keep-running decision the node side needs, so it is now
+// NodeErrorHandler like StaticNetwork's.
+namespace {
+
+static int throwing_stage(int x) {
+    if (x == 42) throw std::runtime_error("boom");
+    return x;
+}
+
+}  // namespace
+
+TEST_CASE("network error handler receives the node's exception", "[network]") {
+    auto src  = kpn::make_node<throwing_stage>(kpn::in<"v">{}, kpn::out<"w">{}, 8);
+    kpn::Channel<int> out(8);
+    src.set_output_channel<0>(&out);
+
+    kpn::Network net;
+    net.add("stage", src).build();
+
+    std::atomic<int>  calls{0};
+    std::string       seen_name;
+    std::string       seen_what;
+    std::mutex        mx;
+
+    net.set_error_handler([&](std::string_view name, std::exception_ptr ep) {
+        std::lock_guard lk(mx);
+        seen_name = std::string(name);
+        try { if (ep) std::rethrow_exception(ep); }
+        catch (const std::exception& e) { seen_what = e.what(); }
+        calls.fetch_add(1, std::memory_order_relaxed);
+        return true;                       // handled: keep the node running
+    });
+
+    net.set_watchdog_interval(std::chrono::hours(1));   // keep the report quiet
+    net.start();
+    src.input_channel<0>().push(42);                    // throws
+    std::this_thread::sleep_for(std::chrono::milliseconds(100));
+    src.input_channel<0>().push(7);                     // must still be running
+    const int passed = out.pop();
+    net.stop();
+
+    CHECK(calls.load(std::memory_order_relaxed) == 1);
+    CHECK(seen_name == "stage");
+    CHECK(seen_what == "boom");
+    CHECK(passed == 7);
+}
+
+// Regression: stopping a network must not wait for the watchdog's next tick.
+//
+// The watchdog looped on std::this_thread::sleep_for(watchdog_interval_), and
+// request_stop() cannot wake a sleeping thread — so stop_watchdog()'s join
+// blocked until the current sleep expired. Every teardown paid up to a full
+// interval, three seconds by default, and a caller who set a long one to keep
+// the periodic report quiet got a stop() that looked like a hang. That is how
+// this was found: the error-handler case above set an hour.
+TEST_CASE("stopping a network does not wait for the watchdog interval", "[network]") {
+    auto node = kpn::make_node<increment>(kpn::in<"v">{}, kpn::out<"w">{}, 4);
+    kpn::Channel<int> out(4);
+    node.set_output_channel<0>(&out);
+
+    kpn::Network net;
+    net.add("inc", node).build();
+    net.set_watchdog_interval(std::chrono::hours(1));
+    net.start();
+
+    // Let the watchdog actually reach its wait. Without this the test races it:
+    // stop_watchdog() runs before the thread has entered the loop, the token is
+    // already set when it does, and it exits without ever waiting — which passes
+    // against the bug as well as the fix.
+    std::this_thread::sleep_for(std::chrono::milliseconds(100));
+
+    const auto t0 = std::chrono::steady_clock::now();
+    net.stop();
+    const auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(
+        std::chrono::steady_clock::now() - t0).count();
+
+    INFO("stop took " << ms << " ms");
+    CHECK(ms < 2000);
+}

tests/test_pool_node.cpp

@@ -478,3 +478,245 @@ TEST_CASE("per-node and network overflow callbacks both fire independently", "[p
     REQUIRE(per_node.load() == 0);
     REQUIRE(network.load() == 0);
 }
+
+// Regression: NodeSnapshot's fields must line up with what nodes initialise.
+//
+// The snapshot is an aggregate that every node type fills positionally, and
+// a8cfe73 appended queued/wake_pending/total_exec_ms to it in an order no call
+// site used: each node supplies total_exec_ms as the element straight after
+// queue_wait_ms, but the struct declared the two bools there. So the exec total
+// landed in `queued`, `queued` landed in `wake_pending`, and `wake_pending`
+// landed in total_exec_ms. The compiler said so (-Wnarrowing, bool to double,
+// once per node instantiation) and the build carried on.
+//
+// It matters more than a cosmetic mix-up: these three fields exist to diagnose a
+// wedge, and a wedged pipeline reported total_exec_ms as 0 or 1 and `queued` as
+// "did this node ever run". Reading them would have pointed at the wrong node.
+//
+// Asserted against ema_exec_ms because that field is independently computed and
+// was already correct: a true sum over several frames cannot be below the
+// exponentially-weighted average of the same samples.
+TEST_CASE("node snapshot fields line up with the values nodes supply",
+          "[pool_node][diagnostics]") {
+    auto pool = std::make_shared<ThreadPool>(1);
+    pool->start();
+
+    auto node = make_pool_node<double_it>(pool, 64);
+    Channel<int> out(64);
+    node.set_output_channel<0>(&out);
+    node.start();
+
+    for (int i = 0; i < 8; ++i) node.input_channel<0>().push(i);
+    std::this_thread::sleep_for(std::chrono::milliseconds(100));
+
+    auto snap = node.node_snapshot("n", 1.0);
+    node.stop();
+    pool->stop();
+
+    INFO("frames=" << snap.frames_processed
+         << " ema=" << snap.ema_exec_ms
+         << " total=" << snap.total_exec_ms);
+    REQUIRE(snap.frames_processed == 8);
+    // The mis-ordered aggregate put wake_pending here, so this was 0.0 or 1.0.
+    CHECK(snap.total_exec_ms >= snap.ema_exec_ms);
+    // ...and the exec total here, which is non-zero, so `queued` read true for
+    // any node that had ever run — including one asleep with nothing to do.
+    CHECK_FALSE(snap.queued);
+    CHECK_FALSE(snap.wake_pending);
+}
+
+// Regression: a value parked twice must keep its payload.
+//
+// push_outputs ends with
+//
+//     else pending_ = std::move(result);
+//
+// and the retry path calls it as push_outputs(std::move(*pending_), …), so on
+// that path `result` is the parked tuple itself. The assignment was therefore a
+// self-move-assignment. std::tuple's is elementwise, and libstdc++'s
+// std::vector does not guard against self-move: it swaps its data into a
+// temporary and leaves the vector empty. So the first park was clean (the
+// argument is a local temporary) and the second erased the payload.
+//
+// The value was still delivered, still in order, still counted — just empty.
+// Downstream cannot distinguish that from a frame on which the node genuinely
+// found nothing, which is why it never surfaced as an error: in
+// scene-actor-extraction it reads as "no faces in this frame" and the run
+// completes with a quietly wrong answer.
+//
+// Reaching it needs *two* outputs. With one, the only thing that resubmits a
+// parked node is that output's own space callback, which by definition fires
+// when there is room — so the retry always succeeds and never reassigns. With
+// two, output A draining resubmits the node while output B is still full: the
+// retry skips A (already delivered, tracked in pending_done_) and fails on B,
+// and that is the reassignment that eats B's payload.
+//
+// Driven through raw channels rather than consumer nodes so each step is
+// forced rather than raced: B is pre-filled and stays full for exactly as long
+// as the test wants it to.
+namespace {
+
+struct TwoPayloads {
+    static constexpr std::string_view label() { return "two_payloads"; }
+    std::tuple<std::vector<int>, std::vector<int>> operator()() {
+        return {std::vector<int>(4, 1), std::vector<int>(4, 2)};
+    }
+};
+
+}  // namespace
+
+TEST_CASE("a twice-parked value keeps its payload", "[pool_node][backpressure]") {
+    auto pool = std::make_shared<ThreadPool>(2);
+    pool->start();
+
+    TwoPayloads fn;
+    auto node = make_pool_node(fn, pool);
+
+    // Both capacity 1. A must be *full* for its pop to signal space at all —
+    // Channel fires the space callback only on the full->not-full edge, so a
+    // roomy A would never resubmit the node and the retry would never happen.
+    Channel<std::vector<int>> out_a(1), out_b(1);
+    node.set_output_channel<0>(&out_a);
+    node.set_output_channel<1>(&out_b);
+
+    // B is full before the node ever runs, so the very first firing parks.
+    out_b.push(std::vector<int>(4, 99));
+
+    node.start();
+    std::this_thread::sleep_for(std::chrono::milliseconds(50));
+
+    // Draining A resubmits the node while B is still full: this is the retry
+    // that reassigned the tuple to itself.
+    (void)out_a.pop();
+    std::this_thread::sleep_for(std::chrono::milliseconds(50));
+
+    // Now let B through and collect what the node had been holding for it.
+    (void)out_b.pop();                       // the pre-fill
+    std::this_thread::sleep_for(std::chrono::milliseconds(50));
+    std::vector<int> parked = out_b.pop();   // the value parked across two tries
+
+    node.stop();
+    pool->stop();
+
+    INFO("parked payload size " << parked.size());
+    CHECK(parked.size() == 4);
+    if (parked.size() == 4) CHECK(parked[0] == 2);
+}
+
+// Regression: a node woken with nothing to read must not stop itself.
+//
+// pop_one reported an empty channel the same way it reported a closed one, by
+// throwing ChannelClosedError, and fire_once treats that as "upstream is
+// finished" and calls self_stop(). self_stop disables the node's own inputs
+// *and* outputs, so one benign empty read does not merely skip a frame — it
+// kills the node and, through the disabled channels, the rest of the pipeline.
+//
+// A node genuinely does get woken with empty inputs: a space callback fires
+// when its output drains, which has nothing to do with input arrival. fire_once
+// guards against it by checking readiness before popping, and that guard is
+// what this test pins. pop_one now also distinguishes the two cases, so if the
+// guard is ever weakened the cost is a wasted firing rather than a dead node.
+//
+// The sequence below reaches the guard deliberately. The output is capacity 1
+// so that draining it signals space at all — Channel fires the space callback
+// only on the full->not-full edge — and by the final pop the input is long
+// since consumed, so the resulting firing has nothing to read.
+namespace {
+
+struct CountingRelay {
+    static constexpr std::string_view label() { return "counting_relay"; }
+    std::atomic<int>* calls;
+    int operator()(int v) { calls->fetch_add(1, std::memory_order_relaxed); return v; }
+};
+
+}  // namespace
+
+TEST_CASE("a node woken with empty inputs does not stop itself", "[pool_node]") {
+    std::atomic<int>  calls{0};
+    std::atomic<int>  closed{0};
+
+    auto pool = std::make_shared<ThreadPool>(2);
+    pool->start();
+
+    CountingRelay fn{&calls};
+    auto node = make_pool_node(fn, pool, 8);
+    Channel<int> out(1);
+    node.set_output_channel<0>(&out);
+    node.set_closed_callback([&](auto) { closed.fetch_add(1, std::memory_order_relaxed); });
+
+    out.push(99);                     // output full before the node runs
+    node.start();
+
+    node.input_channel<0>().push(1);  // fires, cannot deliver, parks
+    std::this_thread::sleep_for(std::chrono::milliseconds(50));
+
+    REQUIRE(out.pop() == 99);         // space -> retry delivers the parked value
+    std::this_thread::sleep_for(std::chrono::milliseconds(50));
+    REQUIRE(out.pop() == 1);          // space again -> fires with empty inputs
+    std::this_thread::sleep_for(std::chrono::milliseconds(50));
+
+    // That firing had nothing to read. The node must still be alive.
+    CHECK(closed.load(std::memory_order_relaxed) == 0);
+    CHECK(node.running());
+
+    // And must still do its job when real input arrives.
+    node.input_channel<0>().push(2);
+    std::this_thread::sleep_for(std::chrono::milliseconds(50));
+    CHECK(out.pop() == 2);
+    CHECK(calls.load(std::memory_order_relaxed) == 2);
+
+    node.stop();
+    pool->stop();
+}
+
+// Regression: stop() must not return while a firing is still running.
+//
+// stop() set the flag, disabled the inputs and returned, leaving an executing
+// fire_once touching input_channels_, stats_ and pending_ while the caller went
+// on to destroy them. The old comment was explicit that callers wanting the
+// guarantee should call scheduler_->drain() first — but ~PoolNode calls stop(),
+// and a destructor cannot ask its caller to have done that.
+//
+// A node with a private pool survived by accident: Node::stop() calls
+// pool->stop(), which joins the worker. A node sharing a pool, which
+// make_pool_node exists to create, had nothing joining it at all, so its own
+// destructor raced the firing.
+//
+// Asserted through an observable side effect rather than by trying to catch the
+// use-after-free: if stop() returns before the node function has finished, the
+// flag it sets on the way out is still false.
+namespace {
+
+struct SlowFiring {
+    static constexpr std::string_view label() { return "slow_firing"; }
+    std::atomic<bool>* entered;
+    std::atomic<bool>* finished;
+    void operator()(int) {
+        entered->store(true, std::memory_order_release);
+        std::this_thread::sleep_for(std::chrono::milliseconds(200));
+        finished->store(true, std::memory_order_release);
+    }
+};
+
+}  // namespace
+
+TEST_CASE("stop waits for a firing already in flight", "[pool_node]") {
+    std::atomic<bool> entered{false}, finished{false};
+
+    auto pool = std::make_shared<ThreadPool>(2);
+    pool->start();
+
+    SlowFiring fn{&entered, &finished};
+    auto node = make_pool_node(fn, pool, 4);
+    node.start();
+    node.input_channel<0>().push(1);
+
+    // Stop only once the node is demonstrably inside its function.
+    while (!entered.load(std::memory_order_acquire))
+        std::this_thread::sleep_for(std::chrono::milliseconds(1));
+
+    node.stop();
+    CHECK(finished.load(std::memory_order_acquire));
+
+    pool->stop();
+}

tests/test_scheduler.cpp

@@ -227,3 +227,116 @@ TEST_CASE("work stealing: tasks complete with more threads than initial queue ta
     REQUIRE(counter.load() == 4);
     pool.stop();
 }
+
+// Regression: submitting to a stopped pool must be a no-op, not a segfault.
+//
+// stop() ends with queues_.clear(), and submit() went straight to
+// queues_[target] with no check — so a submission arriving after stop indexed
+// an empty vector.
+//
+// This is not a contrived teardown ordering; it happens on a normal path. A
+// node's space callback fires from whichever thread drained the channel, and
+// that thread belongs to the *consumer*. Stop the producer first — which a
+// sources-first shutdown does by design — and the consumer keeps draining its
+// backlog, firing the producer's space callback into a pool that has already
+// been torn down. Before this fix the static-network shutdown case crashed
+// about 12 runs in 20.
+//
+// Checking stopped_ without the lock would not be enough: the window between
+// reading the flag and indexing the vector is exactly where clear() runs.
+TEST_CASE("submitting to a stopped pool is refused, not fatal", "[scheduler]") {
+    ThreadPool pool(2);
+    pool.start();
+    pool.stop();
+
+    std::atomic<int> ran{0};
+    for (int i = 0; i < 10; ++i)
+        pool.submit([&] { ran.fetch_add(1, std::memory_order_relaxed); });
+
+    CHECK(ran.load(std::memory_order_relaxed) == 0);
+    CHECK(pool.rejected() == 10);
+}
+
+TEST_CASE("submitting while the pool stops does not crash", "[scheduler]") {
+    // The racing form of the case above: a producer thread submitting
+    // continuously while stop() runs underneath it. Nothing is asserted about
+    // how many tasks run — the point is that every submission either enqueues
+    // or is refused, and none touches a destroyed queue.
+    for (int rep = 0; rep < 20; ++rep) {
+        ThreadPool pool(4);
+        pool.start();
+
+        std::atomic<bool> go{false};
+        std::atomic<int>  ran{0};
+        std::thread submitter([&] {
+            while (!go.load(std::memory_order_acquire)) {}
+            for (int i = 0; i < 2000; ++i)
+                pool.submit([&] { ran.fetch_add(1, std::memory_order_relaxed); });
+        });
+
+        go.store(true, std::memory_order_release);
+        std::this_thread::sleep_for(std::chrono::microseconds(200));
+        pool.stop();
+        submitter.join();
+
+        // Everything submitted was either executed or refused; nothing vanished
+        // into a queue that no longer existed.
+        CHECK(pool.rejected() + pool.snapshot("p").tasks_completed <= 2000);
+    }
+}
+
+// Regression: idle workers must sleep while another worker is busy.
+//
+// The wait predicate was `stopped_ || total_ > 0`, and total_ counts queued
+// *plus executing*. So while any one task ran, every other worker's predicate
+// was true: wait() returned immediately and the worker spun through try_pop,
+// try_steal and back to wait at full speed — try_lock-ing every peer queue on
+// each pass. One slow task pinned every other core and contended the very
+// mutexes the working thread needed to make progress.
+//
+// That is the shape of this pipeline's load exactly: a handful of nodes whose
+// work is tens of milliseconds of ONNX inference. It was latent only because
+// each node currently owns a private single-thread pool, where there is no
+// idle peer to spin. Any use of a shared pool — which make_pool_node exists
+// for — hits it immediately.
+//
+// Measured as CPU time rather than wall time, because the bug does not make
+// anything slower to finish; it makes seven cores burn while one works. A
+// sleeping task consumes no CPU, so with workers correctly asleep the whole
+// pool should account for almost none.
+TEST_CASE("idle workers do not spin while one task runs", "[scheduler]") {
+    auto cpu_ms = [] {
+        struct timespec ts{};
+        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts);
+        return ts.tv_sec * 1000.0 + ts.tv_nsec / 1e6;
+    };
+
+    constexpr int kThreads = 8;
+    constexpr int kWorkMs  = 300;
+
+    ThreadPool pool(kThreads);
+    pool.start();
+    std::this_thread::sleep_for(20ms);      // let workers reach the wait
+
+    const double before = cpu_ms();
+
+    // One long task plus a trivial one per remaining worker. The trivial ones
+    // matter: a worker that has never been woken stays blocked in wait() and
+    // never re-evaluates the predicate, so the spin only appears once a worker
+    // *finishes* something and re-enters the loop while a peer is still busy.
+    // Submitting only the long task does not reproduce it.
+    pool.submit([&] { std::this_thread::sleep_for(std::chrono::milliseconds(kWorkMs)); });
+    for (int i = 0; i < kThreads - 1; ++i) pool.submit([] {});
+
+    pool.drain();
+    const double used = cpu_ms() - before;
+    pool.stop();
+
+    // Measured on this tree: 1991 ms of CPU with the total_ predicate against
+    // 0.4 ms with queued_, and 19205 voluntary context switches against 10 —
+    // roughly (kThreads - 1) cores burned for the duration of one sleeping
+    // task. The threshold sits far from both so the case is not sensitive to
+    // how loaded the machine is.
+    INFO("cpu " << used << " ms over " << kWorkMs << " ms of sleeping work");
+    CHECK(used < kWorkMs);
+}

tests/test_shared_resource.cpp

@@ -237,3 +237,51 @@ TEST_CASE("make_shared_resource constructs with forwarded args", "[shared_resour
     auto g = res.acquire();
     REQUIRE(*g == "hello");
 }
+
+// Regression: a waiter must be releasable, or teardown waits on it forever.
+//
+// acquire() blocks on a condition variable whose predicate only becomes true
+// when release() hands over ownership. There was no timeout and no stop
+// condition, so a node parked there ignored teardown entirely: its worker never
+// returned, the pool's join never completed, and shutdown hung waiting for a
+// resource nobody was going to release — which is exactly the case when the
+// holder is being stopped too.
+//
+// close() turns that into an exception the node's existing error path already
+// handles, and networks now call it on registered resources before stopping any
+// node, for the same reason.
+TEST_CASE("closing a shared resource releases its waiters", "[shared_resource]") {
+    SharedResource<int> res(42);
+
+    auto holder = res.acquire();          // resource is now held
+
+    std::atomic<bool> threw{false}, returned{false};
+    std::thread waiter([&] {
+        try {
+            auto g = res.acquire();       // blocks: someone else holds it
+            (void)g;
+        } catch (const ResourceClosedError&) {
+            threw.store(true, std::memory_order_release);
+        }
+        returned.store(true, std::memory_order_release);
+    });
+
+    // Let it park, then tear down without ever releasing the holder.
+    std::this_thread::sleep_for(std::chrono::milliseconds(50));
+    REQUIRE_FALSE(returned.load(std::memory_order_acquire));
+
+    res.close();
+    waiter.join();
+
+    CHECK(threw.load(std::memory_order_acquire));
+}
+
+TEST_CASE("acquiring a closed resource fails immediately", "[shared_resource]") {
+    SharedResource<int> res(7);
+    res.close();
+    CHECK_THROWS_AS(res.acquire(), ResourceClosedError);
+
+    // Reusable across runs once reopened.
+    res.reopen();
+    CHECK_NOTHROW(res.acquire());
+}

tests/test_static_network.cpp

@@ -271,3 +271,164 @@ TEST_CASE("static_network: fanout with labelled same-function consumers", "[stat
     REQUIRE(outB.pop() == 7);
     net.stop();
 }
+
+// Regression: shutdown() must return even when a consumer stopped consuming.
+//
+// The drain step was an unbounded `while (anything anywhere is non-empty)` poll
+// over *every* channel in the graph. Two defects in one loop: it waited for the
+// whole network to be idle before stopping each successive layer rather than
+// just the node it had stopped — the dynamic Network's version even took a node
+// name and ignored it — and it had no deadline, so anything wedged downstream
+// turned a graceful shutdown into the hang it exists to avoid.
+//
+// It could also fail to terminate with nothing wedged at all. current_fill came
+// from a snapshot that loaded tail_ before head_; a concurrent pop between the
+// two reads yields a head_ past the sampled tail_, and the unsigned difference
+// wraps to ~2^64. Any poll for "is it empty yet" against that value runs
+// forever. Both indices only ever increase, so loading head_ first can at worst
+// under-report a push, which this loop tolerates and a wrap does not.
+//
+// Here the sink never takes anything, so its input cannot drain and the only
+// correct outcome is to give up and say so. The bound asserted is deliberately
+// loose: the point is that it terminates, not how fast.
+namespace {
+
+struct DrainSource {
+    static constexpr std::string_view label() { return "drain_source"; }
+    int n{0};
+    int operator()() {
+        std::this_thread::sleep_for(std::chrono::microseconds(100));
+        return n++;
+    }
+};
+
+struct NeverConsumes {
+    static constexpr std::string_view label() { return "never_consumes"; }
+    std::atomic<bool>* wedged;
+    void operator()(int) {
+        // Blocks for the duration of the test: the input channel behind it
+        // fills and stays full.
+        while (!wedged->load(std::memory_order_acquire))
+            std::this_thread::sleep_for(std::chrono::milliseconds(1));
+    }
+};
+
+}  // namespace
+
+TEST_CASE("shutdown returns when a consumer has wedged", "[static_network][shutdown]") {
+    std::atomic<bool> release{false};
+
+    DrainSource   src_fn;
+    NeverConsumes sink_fn{&release};
+
+    kpn::ObjectNode<DrainSource,   kpn::in<>,    kpn::out<"v">, "drain_source",   0> s(src_fn,  4);
+    kpn::ObjectNode<NeverConsumes, kpn::in<"v">, kpn::out<>,    "never_consumes", 0> k(sink_fn, 4);
+
+    auto net = kpn::make_network(kpn::edge(s.output<"v">(), k.input<"v">()));
+    net.set_drain_timeout(std::chrono::milliseconds(100));
+    net.start();
+
+    // Let the channel fill and the sink jam.
+    std::this_thread::sleep_for(std::chrono::milliseconds(100));
+
+    // Unjam the sink well after the drain timeout should have expired. Stopping
+    // a node joins its worker, so a sink blocked forever would hang the test in
+    // stop() rather than in the drain loop this case is about.
+    std::thread unjam([&] {
+        std::this_thread::sleep_for(std::chrono::milliseconds(800));
+        release.store(true, std::memory_order_release);
+    });
+
+    const auto t0 = std::chrono::steady_clock::now();
+    net.shutdown();
+    const auto elapsed = std::chrono::steady_clock::now() - t0;
+
+    unjam.join();
+
+    const auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(elapsed).count();
+    INFO("shutdown took " << ms << " ms");
+    CHECK(ms < 3000);   // unbounded before; one 100 ms drain timeout after
+}
+
+// Regression: node order must come from the topological sort, not from the
+// order the edges happened to be written in.
+//
+// make_network computes Topo for the cycle check and then dropped it, filling
+// the node vector in edge-declaration order — and named it user_nodes_topo_.
+// halt() stops in its reverse, and shutdown() walks it forwards stopping each
+// node and draining its outputs before moving to the next, which is a graceful
+// drain only if the order really is sources-first.
+//
+// Every network in this tree declares edges in pipeline order, so the two
+// coincided and nothing failed. This case declares them backwards, which is
+// legal and which make_network otherwise accepts silently.
+//
+// Asserted through shutdown() rather than by reading the order back, because
+// the order is private and the ordering is not the point — what it buys is.
+// A sources-first shutdown lets the values already in flight reach the sink;
+// stopping the sink first strands them, and the drain step then has nobody
+// left to take them.
+namespace {
+
+struct OrderSource {
+    static constexpr std::string_view label() { return "order_source"; }
+    std::atomic<int>* made;
+    int operator()() {
+        std::this_thread::sleep_for(std::chrono::microseconds(20));
+        return made->fetch_add(1, std::memory_order_relaxed);
+    }
+};
+
+// Deliberately slower than the source, so a deep backlog builds up in its input
+// channel. That backlog is what a sources-first shutdown preserves and a
+// sink-first one throws away, and it needs to be big enough that the difference
+// cannot be mistaken for one value in flight.
+struct OrderRelay {
+    static constexpr std::string_view label() { return "order_relay"; }
+    int operator()(int v) {
+        std::this_thread::sleep_for(std::chrono::microseconds(300));
+        return v;
+    }
+};
+
+struct OrderSink {
+    static constexpr std::string_view label() { return "order_sink"; }
+    std::atomic<int>* seen;
+    void operator()(int) { seen->fetch_add(1, std::memory_order_relaxed); }
+};
+
+}  // namespace
+
+TEST_CASE("edges declared out of order still start and stop sources-first",
+          "[static_network][shutdown]") {
+    std::atomic<int> seen{0}, made{0};
+
+    OrderSource src_fn{&made};
+    OrderRelay  relay_fn;
+    OrderSink   sink_fn{&seen};
+
+    kpn::ObjectNode<OrderSource, kpn::in<>,    kpn::out<"v">, "order_source", 0> s(src_fn,   8);
+    kpn::ObjectNode<OrderRelay,  kpn::in<"v">, kpn::out<"w">, "order_relay",  0> r(relay_fn, 64);
+    kpn::ObjectNode<OrderSink,   kpn::in<"w">, kpn::out<>,    "order_sink",   0> k(sink_fn,  64);
+
+    // Sink edge first, source edge last — the reverse of pipeline order.
+    auto net = kpn::make_network(
+        kpn::edge(r.output<"w">(), k.input<"w">()),
+        kpn::edge(s.output<"v">(), r.input<"v">())
+    );
+    net.start();
+    std::this_thread::sleep_for(std::chrono::milliseconds(300));
+
+    const int before = seen.load(std::memory_order_relaxed);
+    REQUIRE(before > 0);              // the pipeline ran at all
+
+    net.shutdown();
+
+    // Sources stop first and each layer drains before the next stops, so the
+    // backlog queued in front of the relay still reaches the sink. Stopping in
+    // declaration order stops the relay first and discards all of it.
+    const int after = seen.load(std::memory_order_relaxed);
+    INFO("made " << made.load() << ", delivered " << before
+         << " before shutdown, " << after << " after");
+    CHECK(after - before >= 20);
+}

tests/test_submit_gate.cpp

@@ -0,0 +1,173 @@
+// Regression: a node must never end up idle with a wake outstanding.
+//
+// 9c5ce5f established that invariant and implemented it as two independent
+// atomics — queued_ for "a firing is in flight", wake_pending_ for "a wake
+// arrived during one". Two variables cannot express it, because the release
+// side has to read and write both and a wake can land in between:
+//
+//   producer (try_submit)              worker (release_and_recheck)
+//   ------------------------           ----------------------------
+//   CAS reads queued_ == true, fails
+//                                      queued_.store(false)
+//                                      wake_pending_.exchange(false) -> false
+//   wake_pending_.store(true)
+//
+// queued_ false, wake_pending_ true, nothing running and nothing scheduled.
+// Not a memory-ordering subtlety: the interleaving holds under seq_cst.
+//
+// LegacyGate below is that protocol verbatim, with a hook between the failed
+// CAS and the wake_pending_ store so the interleaving can be forced rather than
+// waited for. That makes the loss deterministic and the test non-flaky, and it
+// keeps the defect on record now that the code implementing it is gone.
+//
+// A note on what is NOT tested here, because it would be misleading to imply
+// otherwise: there is no black-box, node-level test that fails before this fix
+// and passes after. Every call site of release_and_recheck() happens to follow
+// it with a level re-check — on_input_ready(), or outputs_have_space() on the
+// parked path — which rediscovers the state a lost wake would have signalled.
+// That masking is a property of the call sites, not of the mechanism, and the
+// point of the fix is that a future early return that forgets the re-check no
+// longer reintroduces a hang. The value is structural, so the tests are
+// structural: the state machine is pinned by contract, and the defect it
+// replaces is pinned by demonstration.
+#include <catch2/catch_test_macros.hpp>
+#include <kpn/submit_gate.hpp>
+
+#include <atomic>
+#include <functional>
+#include <thread>
+
+using namespace kpn;
+
+namespace {
+
+// The pre-fix protocol, with a seam at the point where the race lives.
+class LegacyGate {
+public:
+    std::function<void()> before_recording_wake;
+
+    bool claim() noexcept {
+        bool expected = false;
+        if (queued_.compare_exchange_strong(expected, true, std::memory_order_acq_rel))
+            return true;
+        if (before_recording_wake) before_recording_wake();
+        wake_pending_.store(true, std::memory_order_release);
+        return false;
+    }
+    bool release() noexcept {
+        queued_.store(false, std::memory_order_release);
+        if (wake_pending_.exchange(false, std::memory_order_acq_rel)) {
+            bool expected = false;
+            if (queued_.compare_exchange_strong(expected, true, std::memory_order_acq_rel))
+                return true;
+        }
+        return false;
+    }
+    bool queued()       const noexcept { return queued_.load(std::memory_order_relaxed); }
+    bool wake_pending() const noexcept { return wake_pending_.load(std::memory_order_relaxed); }
+
+private:
+    std::atomic<bool> queued_{false};
+    std::atomic<bool> wake_pending_{false};
+};
+
+}  // namespace
+
+TEST_CASE("the two-atomic gate loses a wake, deterministically", "[submit_gate]") {
+    LegacyGate gate;
+    bool resubmitted = true;
+
+    REQUIRE(gate.claim());              // a firing is now in flight
+
+    // Force the interleaving: the firing completes in the window between the
+    // second wake's failed CAS and its record of that wake.
+    gate.before_recording_wake = [&] { resubmitted = gate.release(); };
+
+    const bool submitted = gate.claim();
+
+    // The wake was neither submitted by the producer nor honoured by the
+    // release. Nothing is scheduled, and nothing else will re-trigger it.
+    CHECK_FALSE(submitted);
+    CHECK_FALSE(resubmitted);
+    CHECK_FALSE(gate.queued());
+    CHECK(gate.wake_pending());         // recorded, and never to be consumed
+}
+
+TEST_CASE("submit gate: a wake during a firing is honoured", "[submit_gate]") {
+    SubmitGate gate;
+
+    REQUIRE(gate.claim());              // idle -> queued, caller submits
+    REQUIRE(gate.queued());
+    REQUIRE_FALSE(gate.wake_pending());
+
+    REQUIRE_FALSE(gate.claim());        // second wake is recorded, not submitted
+    REQUIRE(gate.wake_pending());
+
+    REQUIRE(gate.release());            // and honoured when the firing ends
+    // The gate stays claimed across the handover, so the node is never
+    // momentarily idle while a submission for it is in flight. This is the
+    // state the legacy gate could not represent.
+    REQUIRE(gate.queued());
+    REQUIRE_FALSE(gate.wake_pending());
+
+    REQUIRE_FALSE(gate.release());      // no further wake: now idle
+    REQUIRE_FALSE(gate.queued());
+}
+
+TEST_CASE("submit gate: repeated wakes collapse to one resubmission", "[submit_gate]") {
+    // Collapsing is deliberate. A firing consumes one item and its caller then
+    // re-checks the input level, so the gate only has to guarantee that at
+    // least one more firing follows a wake, not one per wake.
+    SubmitGate gate;
+    REQUIRE(gate.claim());
+    for (int i = 0; i < 10; ++i) REQUIRE_FALSE(gate.claim());
+    REQUIRE(gate.release());
+    REQUIRE_FALSE(gate.release());
+}
+
+TEST_CASE("submit gate: force_idle drops a recorded wake", "[submit_gate]") {
+    // Stop paths use this deliberately — honouring a wake there would resubmit
+    // a node that has already been told to stop.
+    SubmitGate gate;
+    REQUIRE(gate.claim());
+    REQUIRE_FALSE(gate.claim());
+    REQUIRE(gate.wake_pending());
+
+    gate.force_idle();
+    REQUIRE_FALSE(gate.queued());
+    REQUIRE_FALSE(gate.wake_pending());
+    REQUIRE(gate.claim());              // and the gate is reusable afterwards
+}
+
+TEST_CASE("submit gate: concurrent claim and release stay consistent", "[submit_gate]") {
+    // Not a lost-wake test — see the header note. This is a TSan target and a
+    // check that the CAS loops always terminate and always leave the gate in a
+    // reachable state: exactly one party may hold the claim at a time, so the
+    // count of claims granted must equal the count of releases that ended idle.
+    SubmitGate gate;
+    std::atomic<long> granted{0}, ended_idle{0};
+    std::atomic<bool> stop{false};
+
+    std::thread waker([&] {
+        while (!stop.load(std::memory_order_relaxed))
+            if (gate.claim()) granted.fetch_add(1, std::memory_order_relaxed);
+    });
+    std::thread worker([&] {
+        while (!stop.load(std::memory_order_relaxed))
+            if (gate.queued() && !gate.release())
+                ended_idle.fetch_add(1, std::memory_order_relaxed);
+    });
+
+    std::this_thread::sleep_for(std::chrono::milliseconds(200));
+    stop.store(true, std::memory_order_relaxed);
+    waker.join();
+    worker.join();
+
+    // Drain whatever claim is outstanding so the two counts can be compared.
+    while (gate.queued())
+        if (!gate.release()) ended_idle.fetch_add(1, std::memory_order_relaxed);
+
+    INFO("granted " << granted.load() << " ended idle " << ended_idle.load());
+    REQUIRE(granted.load() > 0);
+    CHECK(granted.load() == ended_idle.load());
+}