perf(bench): make a multi-hour acceptance run observable and restartable

The Phase-0 gate is 7 passes over the full row set, which is long enough
that capture_output=True was the wrong default: no rows existed anywhere
until a pass ended, so a slow run and a wedged one looked identical, and
killing either threw away everything measured.

Rows now stream to --out-dir/pass-NN.csv with a flush per line, so an
interrupted run keeps what it had. --resume reuses complete pass files
and reruns incomplete ones -- checked against the row count rather than
merely existing, because a partial file silently averaged in as a whole
pass would corrupt the verdict rather than fail it.

Progress is a tqdm bar over rows, not passes; a pass counter would sit
at 1/7 for twenty minutes and report nothing useful. The row total comes
from probing the binary with --reps=0 (0.8s) rather than reimplementing
the sweep in Python, which would drift from the C++ defaults. There is a
plain-stderr fallback when tqdm is absent -- refusing to start a
benchmark over a missing progress dependency is the wrong trade.

bench_runs/ is gitignored: the new default output path would otherwise
land in git status.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/bench_repro_check.py

@@ -9,12 +9,20 @@ 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
@@ -27,11 +35,47 @@ KEY_COLS = ("topology", "size", "work_us", "threads")
 METRIC_COLS = ("items_per_sec", "ns_per_dispatch")
 
 
-def parse_csv(text, metric=None):
+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 text.splitlines():
+    for line in lines:
         line = line.strip()
         if not line or line.startswith("#"):
             continue
@@ -59,36 +103,64 @@ def parse_csv(text, metric=None):
     return rows, metric
 
 
-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)")
+def parse_csv(text, metric=None):
+    return parse_csv_lines(text.splitlines(), metric)
 
-    # 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)
 
-    passes = []
-    metric = args.metric
-    for i in range(args.passes):
-        print(f"pass {i + 1}/{args.passes} ...", file=sys.stderr, flush=True)
-        proc = subprocess.run([args.binary] + extra,
-                              capture_output=True, text=True)
-        if proc.returncode != 0:
-            print(proc.stderr, file=sys.stderr)
-            sys.exit(f"{args.binary} failed with {proc.returncode}")
-        rows, metric = parse_csv(proc.stdout, metric)
-        passes.append(rows)
+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)
@@ -103,20 +175,84 @@ def main():
         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 <= args.tolerance
+        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 +/-{args.tolerance:g}% — "
+        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 +/-{args.tolerance:g}% "
-          f"over {args.passes} passes — Phase-0 gate met.")
+    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())