diff --git a/support/rrsh.sh b/support/rrsh.sh
new file mode 100755
index 00000000..83025f2f
--- /dev/null
+++ b/support/rrsh.sh
@@ -0,0 +1,21 @@
+#!/bin/sh
+# abdiff helper: a "remote shell" that emulates an sshd forced-command of
+# `rrsync DIR`.  rsync invokes a remote shell as:
+#     <shell-words...> [ssh-opts] <host> <rsync --server ...>
+# so when used as  -e "sh rrsh.sh <RRSYNC> <DIR>"  rsync calls us as:
+#     sh rrsh.sh <RRSYNC> <DIR> [opts] lh rsync --server ...
+# We hand the server command to rrsync via SSH_ORIGINAL_COMMAND (exactly as
+# sshd would) and exec the restricted wrapper, so abdiff can A/B the rrsync
+# path itself.  Only the pretend hosts "lh"/"localhost" are accepted.
+RRSYNC="$1"; DIR="$2"; shift 2
+while [ $# -gt 0 ]; do
+    case "$1" in
+        -l) shift 2 ;;
+        lh|localhost) shift; break ;;
+        -*) shift ;;
+        *) break ;;
+    esac
+done
+SSH_ORIGINAL_COMMAND="$*"
+export SSH_ORIGINAL_COMMAND
+exec "$RRSYNC" "$DIR"
diff --git a/testsuite/README.md b/testsuite/README.md
index 5c15f5de..4883a5e8 100644
--- a/testsuite/README.md
+++ b/testsuite/README.md
@@ -184,3 +184,53 @@ Each target must be provisioned with the build toolchain its workflow installs
 (autoconf, automake, a C compiler, perl, a python3 markdown module such as
 cmarkgfm or commonmark unless the flags pass `--disable-md2man`, and the dev
 libraries its configure flags enable). A missing piece shows up as `BUILD-FAIL`.
+
+## Differential regression hunting (abdiff.py)
+
+`testsuite/abdiff.py` is a developer tool — **not** a `*_test.py`, so `runtests.py`
+ignores it. It hunts *regressions* by running the **same benign transfer** with
+two rsync binaries (`A` = the build under test, `B` = a baseline) and comparing
+the OUTCOME. The oracle is: for a benign input, a correctness/behaviour change
+between the builds must be **invisible**, so A and B must produce an identical
+result. Any divergence is a regression candidate to investigate and, if real,
+minimize into a `*_test.py`.
+
+It compares exit code, stderr (error markers + normalised text), `--stats`
+"Literal data", the destination tree (content + full metadata: mode/uid/gid/
+mtime/size/symlink target/xattrs/ACLs/hardlink grouping), the `--itemize` list,
+and — with `--cost` — peak process-group RSS (a resource-regression oracle that
+functional comparison misses). A **stability gate** runs each binary several
+times and escalates on a candidate diff; nondeterministic scenarios are
+quarantined `FLAKY`, never reported as regressions.
+
+Run it from the build directory (so `./rsync` and `old_versions/` resolve):
+
+```sh
+testsuite/abdiff.py                       # default: ./rsync vs old_versions/rsync_3.4.1
+testsuite/abdiff.py --sweep all -j5       # broad single pass, 5-way parallel
+testsuite/abdiff.py --loop --timelimit 3600 --cost   # hunt for an hour, resource oracle on
+testsuite/abdiff.py --list --sweep all    # list scenarios without running
+```
+
+Each finding is classed `DIFF` (regression candidate), `ALLOW` (an intentional,
+documented behaviour change listed in the tool's allowlist), `BETTER` (A succeeds
+where B fails), `FLAKY`, or `TIMEOUT`. Findings are printed and appended to a
+per-run `abdiff-log_<TIME>.txt` (and the curated `--findings` log).
+
+Key options: `-j N` parallelism; `--sweep NAME|all`; `--loop` (endless
+random + systematic-combo stream) bounded by `--timelimit SECS`; `--cost`
+(+`--scale N` for the large-tree fixtures); `--repeat N` (stability samples);
+`--rsync-a`/`--rsync-b` the two binaries. Run **as root** to fold in the
+owner/device/specials/fake-super and chroot-daemon sweeps automatically.
+
+Transport lanes (a feature broken only over the wire is invisible to a local
+copy): local, an ssh split (`support/lsh.sh`), a stdio-pipe daemon, a **real TCP
+daemon** (bound port + greeting/handshake, and an auth challenge-response
+variant), and the restricted **rrsync** wrapper (`support/rrsh.sh`). rrsync's
+behaviour ships in the *script*, so pair each binary with its own version's
+rrsync via `--rrsync-a`/`--rrsync-b` (give B's rrsync, e.g. one extracted from
+that release's `support/rrsync`).
+
+Cross-version baselines are the static binaries already in `old_versions/`;
+`old_versions/build_static.sh` builds more from a git tag (and you can grab a
+matching `support/rrsync` from the same tag for the rrsync lane).
diff --git a/testsuite/abdiff.py b/testsuite/abdiff.py
new file mode 100644
index 00000000..06bfa484
--- /dev/null
+++ b/testsuite/abdiff.py
@@ -0,0 +1,2824 @@
+#!/usr/bin/env python3
+"""abdiff.py -- differential A/B regression hunter for rsync.
+
+Runs the same transfer with two rsync binaries (A = the build under test, e.g.
+./rsync; B = a baseline, e.g. old_versions/rsync_3.4.1) and compares the
+OUTCOME: exit code, error output, --stats "Literal data", the destination tree
+(content + full metadata), and the --itemize change list.
+
+Core oracle: for a BENIGN input a correctness/behaviour change between the two
+builds must be invisible, so A and B must produce an identical destination tree
+and both exit 0.  Any divergence is a regression candidate (e.g. a refactor that
+silently changes what a benign `rsync -a` transfers).
+
+This is a developer tool, NOT a runtests.py test (it does not end in _test.py and
+imports nothing from the test harness).  Findings are printed and appended to a
+log; minimize each into a testsuite/*_test.py.
+
+Usage:
+    testsuite/abdiff.py [--rsync-a ./rsync] [--rsync-b old_versions/rsync_3.4.1]
+                      [--sweep options|pathshape|all] [--workdir DIR] [--keep]
+                      [--findings abdiff-findings.txt] [--only NAME] [--list]
+Exit 0 iff no regression candidates were found.
+"""
+
+from __future__ import annotations
+
+import argparse
+import hashlib
+import itertools
+import os
+import random
+import re
+import shutil
+import signal
+import stat
+import subprocess
+import sys
+import threading
+import time
+from concurrent.futures import (FIRST_COMPLETED, ThreadPoolExecutor,
+                                as_completed, wait)
+from pathlib import Path
+
+# ---------------------------------------------------------------------------
+# config / globals (set in main)
+RSYNC_A = "./rsync"
+RSYNC_B = "old_versions/rsync_3.4.1"
+RRSYNC_A = None     # rrsync wrapper paired with A/B (None -> in-tree support/rrsync)
+RRSYNC_B = None
+KEEP = False
+REPEAT = 2   # stability gate: run each binary N times; flaky scenarios quarantined
+CMD_TIMEOUT = 120   # per-subprocess wall-clock guard, seconds (0 = unlimited)
+COST = False        # --cost: also compare peak process-group RSS (resource oracle)
+SCALE_N = 2000      # --scale: element count for the scale-escalation fixtures
+_supports_cache: dict = {}
+_supports_lock = threading.Lock()
+_tls = threading.local()   # per-worker: .measure (bool) + .rss (peak bytes)
+_PAGE = os.sysconf("SC_PAGE_SIZE")
+
+
+def _group_rss(pgid):
+    """Summed RSS (bytes) of every live process in process group pgid -- catches
+    rsync's forked generator/receiver/sender (and ssh/daemon children), which a
+    direct-child measure (/usr/bin/time) would miss."""
+    total = 0
+    for pid in os.listdir("/proc"):
+        if not pid.isdigit():
+            continue
+        try:
+            with open(f"/proc/{pid}/stat") as f:
+                fields = f.read().split()
+            if int(fields[4]) != pgid:        # field 5 (0-idx 4) = pgrp
+                continue
+            with open(f"/proc/{pid}/statm") as f:
+                total += int(f.read().split()[1]) * _PAGE   # resident pages
+        except (OSError, ValueError, IndexError):
+            continue
+    return total
+
+
+def sh(cmd, cwd=None, env=None, timeout=None):
+    """Run cmd capturing stdout/stderr. Runs in its own process group with a
+    wall-clock timeout so a wedged rsync (or its ssh/daemon children) can't hang a
+    worker forever -- on timeout the whole group is killed and rc 124 returned."""
+    if timeout is None:
+        timeout = CMD_TIMEOUT
+    p = subprocess.Popen(cmd, cwd=cwd, env=env, stdout=subprocess.PIPE,
+                         stderr=subprocess.PIPE, text=True, start_new_session=True)
+    # cost oracle: while the transfer runs, sample the peak summed RSS of its
+    # whole process group (set per-worker by one_run via _tls.measure).
+    measure = getattr(_tls, "measure", False)
+    stop = peak = poller = None
+    if measure:
+        peak = [0]
+        stop = threading.Event()
+        pgid = os.getpgid(p.pid)
+
+        def _poll():
+            while not stop.is_set():
+                peak[0] = max(peak[0], _group_rss(pgid))
+                stop.wait(0.03)
+        poller = threading.Thread(target=_poll, daemon=True)
+        poller.start()
+    try:
+        out, err = p.communicate(timeout=timeout or None)
+        rc = p.returncode
+    except subprocess.TimeoutExpired:
+        try:
+            os.killpg(os.getpgid(p.pid), signal.SIGKILL)
+        except OSError:
+            pass
+        out, err = p.communicate()
+        rc = 124
+        err = "[abdiff: TIMEOUT]\n" + (err or "")
+    if measure:
+        stop.set()
+        poller.join(timeout=1)
+        _tls.rss = peak[0] or None
+    return subprocess.CompletedProcess(cmd, rc, out or "", err or "")
+
+
+def supports(binary, opt):
+    key = (binary, opt)
+    with _supports_lock:
+        if key in _supports_cache:
+            return _supports_cache[key]
+    r = sh([binary, opt, "--version"])
+    bad = any(m in (r.stderr or "").lower()
+              for m in ("unknown option", "unrecognized option", "no such option"))
+    val = not bad
+    with _supports_lock:
+        _supports_cache[key] = val
+    return val
+
+
+# ---------------------------------------------------------------------------
+# tree snapshot + comparison
+
+ERR_MARKERS = ("rsync error", "failed to open", "rsync: ", "Invalid argument",
+               "No such file", "Operation not permitted", "cannot ")
+
+# Intentional, documented behaviour-change refusals (A errors where B didn't),
+# recorded as ALLOW rather than a silent regression. Each entry is
+# (substring-in-A's-stderr, human note). Populate as deliberate behaviour changes
+# between the two builds are identified.
+ALLOWLIST = []
+
+
+def _xattrs(path):
+    try:
+        names = sorted(os.listxattr(path, follow_symlinks=False))
+    except (OSError, AttributeError):
+        return {}
+    out = {}
+    for n in names:
+        if n.startswith("system.posix_acl_"):
+            continue  # captured via getfacl
+        try:
+            out[n] = os.getxattr(path, n, follow_symlinks=False).hex()
+        except OSError:
+            out[n] = "?"
+    return out
+
+
+def _acl(path, is_dir):
+    r = sh(["getfacl", "-pcEn", path]) if shutil.which("getfacl") else None
+    if not r or r.returncode != 0:
+        return None
+    lines = [ln for ln in r.stdout.splitlines() if ln and not ln.startswith("#")]
+    return "\n".join(sorted(lines)) or None
+
+
+def snapshot(root: Path):
+    """Map rel-path -> attribute dict for every entry under root (root itself
+    excluded). Symlinks/specials are recorded, never followed."""
+    root = Path(root)
+    snap = {}
+    inode_of = {}  # (dev,ino) -> first rel path, for hardlink grouping
+    if not root.exists():
+        return snap
+    stack = [root]
+    while stack:
+        d = stack.pop()
+        try:
+            entries = sorted(os.scandir(d), key=lambda e: e.name)
+        except OSError:
+            continue
+        for e in entries:
+            p = Path(e.path)
+            rel = str(p.relative_to(root))
+            try:
+                st = os.lstat(p)
+            except OSError:
+                snap[rel] = {"type": "GONE"}
+                continue
+            m = st.st_mode
+            a = {
+                "mode": stat.S_IMODE(m),
+                "uid": st.st_uid, "gid": st.st_gid,
+                "mtime": int(st.st_mtime),
+            }
+            if stat.S_ISDIR(m):
+                a["type"] = "d"
+                stack.append(p)
+            elif stat.S_ISLNK(m):
+                a["type"] = "l"
+                a["target"] = os.readlink(p)
+            elif stat.S_ISREG(m):
+                a["type"] = "f"
+                a["size"] = st.st_size
+                a["blocks"] = st.st_blocks  # sparseness
+                # "is this file hardlinked at all" (link-dest / -H) -- a robust
+                # boolean; raw nlink counts are contaminated when A and B share a
+                # --link-dest basis dir, but "copied(1) vs linked(>1)" still
+                # catches a real link-dest/hardlink regression.
+                a["linked"] = st.st_nlink > 1
+                if st.st_nlink > 1:
+                    key = (st.st_dev, st.st_ino)
+                    a["hardlink"] = inode_of.setdefault(key, rel)
+                h = hashlib.sha256()
+                try:
+                    with open(p, "rb") as fh:
+                        for chunk in iter(lambda: fh.read(1 << 20), b""):
+                            h.update(chunk)
+                    a["sha"] = h.hexdigest()
+                except OSError as ex:
+                    a["sha"] = f"ERR:{ex.errno}"
+            elif stat.S_ISFIFO(m):
+                a["type"] = "p"
+            elif stat.S_ISSOCK(m):
+                a["type"] = "s"
+            elif stat.S_ISBLK(m) or stat.S_ISCHR(m):
+                a["type"] = "b" if stat.S_ISBLK(m) else "c"
+                a["rdev"] = (os.major(st.st_rdev), os.minor(st.st_rdev))
+            else:
+                a["type"] = "?"
+            xa = _xattrs(p)
+            if xa:
+                a["xattr"] = xa
+            ac = _acl(p, stat.S_ISDIR(m))
+            if ac:
+                a["acl"] = ac
+            snap[rel] = a
+    return snap
+
+
+# which attrs are meaningful depends on the options used; keep it simple and
+# compare everything, but let callers ignore mtime when -t isn't in play, or
+# for a type whose times rsync intentionally leaves unmanaged (-O dirs, -J
+# symlinks) -> those dest mtimes are creation-time and differ between runs.
+def diff_snapshots(sa, sb, ignore_mtime=False, ignore_mtime_types=()):
+    diffs = []
+    for rel in sorted(set(sa) | set(sb)):
+        a, b = sa.get(rel), sb.get(rel)
+        if a is None:
+            diffs.append(f"  only in B(baseline): {rel} ({b.get('type')})")
+            continue
+        if b is None:
+            diffs.append(f"  only in A(under-test): {rel} ({a.get('type')})")
+            continue
+        for k in sorted(set(a) | set(b)):
+            if k == "mtime" and (ignore_mtime
+                                 or a.get("type") in ignore_mtime_types):
+                continue
+            if a.get(k) != b.get(k):
+                diffs.append(f"  {rel}: {k}  A={a.get(k)!r}  B={b.get(k)!r}")
+    return diffs
+
+
+# ---------------------------------------------------------------------------
+# running a transfer with one binary
+
+def run_xfer(binary, workdir, opts, src_args, dest, cwd=None, pre=None):
+    """Run `binary opts src_args dest` (cwd default=workdir). Returns
+    (rc, stderr, literal_data, itemize)."""
+    cwd = cwd or workdir
+    argv = [binary, "--stats", "-i", *opts, *src_args, dest]
+    return _parse_out(sh(argv, cwd=cwd))
+
+
+def _parse_out(r):
+    literal = None
+    for ln in r.stdout.splitlines():
+        if ln.startswith("Literal data:"):
+            literal = ln.split(":", 1)[1].strip()
+    itemize = "\n".join(sorted(
+        ln for ln in r.stdout.splitlines()
+        if len(ln) > 11 and ln[1] in "fdLDS" and ln[0] in "<>ch.*"
+        and ln[11:].strip() != "./"))  # bench-dependent top-dir time line
+    return r.returncode, (r.stderr or "").strip(), literal, itemize, (r.stdout or "")
+
+
+_NOISE = re.compile(r'bytes/sec|speedup is|^sent .*received |^total size is|'
+                    r'^Number of |^Total |^Literal data:|^Matched data:|'
+                    r'^File list |^total:|^created |^deleting ')
+
+
+def _norm_out(text, wd, dest):
+    """Normalised stdout for A/B compare: canonicalise the workdir/dest paths and
+    drop bench-variant stats lines (keeps itemize/listing/warning lines)."""
+    text = text.replace(str(dest), "DEST").replace(str(wd), "WD")
+    return "\n".join(ln for ln in text.splitlines() if ln and not _NOISE.search(ln))
+
+
+def _norm_err(text, wd, dest):
+    """Normalised stderr TEXT for A/B compare: canonicalise paths, strip the
+    version-dependent role tag ([sender=3.4.x], [client=VERSION]) and at-FILE(LINE)
+    source location (line numbers shift between versions) so only the MESSAGE is
+    compared."""
+    text = text.replace(str(dest), "DEST").replace(str(wd), "WD")
+    text = re.sub(r'\[(?:sender|receiver|generator|client|server'
+                  r'|Sender|Receiver|Generator|Client|Server)=[^\]]*\]',
+                  '[ROLE]', text)
+    text = re.sub(r' at [\w./-]+\(\d+\)', ' at LOC', text)
+    return text.strip()
+
+
+def _wait_port(port, timeout=10.0):
+    import socket as _sock
+    end = time.time() + timeout
+    while time.time() < end:
+        try:
+            _sock.create_connection(("127.0.0.1", port), 0.3).close()
+            return True
+        except OSError:
+            time.sleep(0.05)
+    return False
+
+
+def run_daemon_xfer(binary, wd, module_path, opts, src_args, port, chroot="no"):
+    """Push src_args into a [m] module served by `binary --daemon` over a PRIVATE
+    STDIO PIPE (RSYNC_CONNECT_PROG) -- no TCP port, so no port-bind/startup race
+    (the old TCP path was nondeterministic under load).  `port` only uniquifies
+    the per-invocation config filename.  Same (rc, err, lit, item) as run_xfer."""
+    conf = Path(wd) / f"rsyncd_{port}.conf"
+    Path(module_path).mkdir(parents=True, exist_ok=True)
+    conf.write_text(
+        f"use chroot = {chroot}\n"
+        f"[m]\n  path = {module_path}\n  read only = no\n"
+        f"  hosts allow = 127.0.0.1\n")
+    env = {**os.environ,
+           "RSYNC_CONNECT_PROG": f"{binary} --config={conf} --daemon"}
+    argv = [binary, "--stats", "-i", *opts, *src_args, "rsync://localhost/m/"]
+    return _parse_out(sh(argv, cwd=str(wd), env=env))
+
+
+def run_daemon_pull(binary, wd, served, opts, localdest, port, chroot="no"):
+    """PULL from a read-only [m] module (the daemon SENDER side) over a private
+    stdio pipe into localdest. `served` is the served directory."""
+    conf = Path(wd) / f"rsyncd_{port}.conf"
+    Path(served).mkdir(parents=True, exist_ok=True)
+    Path(localdest).mkdir(parents=True, exist_ok=True)
+    conf.write_text(
+        f"use chroot = {chroot}\n"
+        f"[m]\n  path = {served}\n  read only = yes\n"
+        f"  hosts allow = 127.0.0.1\n")
+    env = {**os.environ,
+           "RSYNC_CONNECT_PROG": f"{binary} --config={conf} --daemon"}
+    argv = [binary, "--stats", "-i", *opts, "rsync://localhost/m/",
+            str(localdest) + "/"]
+    return _parse_out(sh(argv, cwd=str(wd), env=env))
+
+
+# sibling helper scripts live in support/ (abdiff.py itself lives in testsuite/)
+_SUPPORT = Path(__file__).resolve().parent.parent / "support"
+_LSH = str(_SUPPORT / "lsh.sh")
+
+
+def run_ssh_xfer(binary, wd, opts, src_args, dest):
+    """PUSH over a remote-shell split via support/lsh.sh (host 'lh' = no chdir);
+    the remote side runs the same binary via --rsync-path. Real client+server
+    processes + protocol, unlike a both-paths-local copy."""
+    argv = [binary, "--stats", "-i", "-e", f"sh {_LSH}",
+            f"--rsync-path={binary}", *opts, *src_args, f"lh:{dest}/"]
+    return _parse_out(sh(argv, cwd=wd))
+
+
+# --- rrsync lane: route the remote side through the restricted rrsync wrapper --
+_RRSH = str(_SUPPORT / "rrsh.sh")
+_RRSYNC_SRC = _SUPPORT / "rrsync"
+
+
+def _patch_rrsync(binary, rrsync_src, wd):
+    """A copy of `rrsync_src` (the version's rrsync wrapper) whose RSYNC points at
+    `binary`, so the rrsync-launched server is the binary under test. rrsync is
+    SHIPPED PER VERSION, so A and B use their OWN rrsync (the regressions live in
+    the script, not just the binary) -- keyed per (binary, src) to keep them
+    distinct in a shared workdir."""
+    key = abs(hash((binary, str(rrsync_src)))) % 1000000
+    dst = Path(wd) / f"rrsync-{key}"
+    if not dst.exists():
+        txt = Path(rrsync_src).read_text()
+        txt = re.sub(r"^RSYNC = '[^']*'", f"RSYNC = {binary!r}", txt, count=1,
+                     flags=re.M)
+        dst.write_text(txt)
+        dst.chmod(0o755)
+    return dst
+
+
+def run_rrsync_push(binary, rrsync_src, wd, opts, src_args, dest):
+    """PUSH through `rrsync <restricted>` (the dest's parent is the restricted
+    root; the client writes into the <dest-name>/ subdir, so rrsync's subdir
+    restrictions are exercised). Exercises rrsync option/path validation."""
+    dest = Path(dest)
+    rr = _patch_rrsync(binary, rrsync_src, wd)
+    dest.parent.mkdir(parents=True, exist_ok=True)
+    argv = [binary, "--stats", "-i", "-e", f"sh {_RRSH} {rr} {dest.parent}",
+            *opts, *src_args, f"lh:{dest.name}/"]
+    return _parse_out(sh(argv, cwd=str(wd)))
+
+
+def run_rrsync_pull(binary, rrsync_src, wd, served, opts, localdest):
+    """PULL through `rrsync <restricted>` (rrsync's --sender side): the served
+    dir's parent is the restricted root, the client reads the <served-name>/
+    subdir into localdest."""
+    served = Path(served)
+    rr = _patch_rrsync(binary, rrsync_src, wd)
+    Path(localdest).mkdir(parents=True, exist_ok=True)
+    argv = [binary, "--stats", "-i", "-e", f"sh {_RRSH} {rr} {served.parent}",
+            *opts, f"lh:{served.name}/", str(localdest) + "/"]
+    return _parse_out(sh(argv, cwd=str(wd)))
+
+
+# --- real-TCP-daemon lane: a genuine `rsync --daemon` on a bound port ----------
+_port_lock = threading.Lock()
+_next_port = [40000]
+
+
+def _alloc_port():
+    """A free localhost TCP port, bind-probed under a lock so concurrent workers
+    don't collide (the real-daemon path needs a real port, unlike the stdio one)."""
+    import socket as _s
+    with _port_lock:
+        for _ in range(4000):
+            p = _next_port[0]
+            _next_port[0] = 40000 if p >= 60000 else p + 1
+            s = _s.socket(_s.AF_INET, _s.SOCK_STREAM)
+            try:
+                s.setsockopt(_s.SOL_SOCKET, _s.SO_REUSEADDR, 1)
+                s.bind(("127.0.0.1", p))
+                return p
+            except OSError:
+                continue
+            finally:
+                s.close()
+    return _next_port[0]
+
+
+def _tcp_daemon(binary, wd, module_path, opts, src_args, localdest=None,
+                pull=False, chroot="no", auth=False):
+    """Start a real `binary --daemon` on a bound TCP port and run the client over
+    rsync://127.0.0.1:PORT/m/ -- exercises the genuine socket path / greeting /
+    handshake (and, with auth, the challenge-response) that the stdio-pipe daemon
+    lane bypasses."""
+    port = _alloc_port()
+    conf = Path(wd) / f"tcpd_{port}.conf"
+    Path(module_path).mkdir(parents=True, exist_ok=True)
+    authlines = ""
+    env = dict(os.environ)
+    if auth:
+        sp = Path(wd) / f"tcpd_{port}.secrets"
+        sp.write_text("abuser:abpass\n")
+        sp.chmod(0o600)
+        authlines = f"  auth users = abuser\n  secrets file = {sp}\n"
+        env["RSYNC_PASSWORD"] = "abpass"
+    conf.write_text(
+        f"use chroot = {chroot}\nport = {port}\n"
+        f"log file = {wd}/tcpd_{port}.log\npid file = {wd}/tcpd_{port}.pid\n"
+        f"[m]\n  path = {module_path}\n  read only = {'yes' if pull else 'no'}\n"
+        f"  hosts allow = 127.0.0.1\n{authlines}")
+    proc = subprocess.Popen(
+        [binary, "--daemon", "--no-detach", f"--config={conf}",
+         f"--port={port}", "--address=127.0.0.1"],
+        stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
+        start_new_session=True)
+    try:
+        if not _wait_port(port):
+            return (99, "tcp daemon failed to start", None, "", "")
+        user = "abuser@" if auth else ""
+        url = f"rsync://{user}127.0.0.1:{port}/m/"
+        if pull:
+            Path(localdest).mkdir(parents=True, exist_ok=True)
+            argv = [binary, "--stats", "-i", *opts, url, str(localdest) + "/"]
+        else:
+            argv = [binary, "--stats", "-i", *opts, *src_args, url]
+        return _parse_out(sh(argv, cwd=str(wd), env=env))
+    finally:
+        proc.terminate()
+        try:
+            proc.wait(timeout=5)
+        except subprocess.TimeoutExpired:
+            proc.kill()
+
+
+# ---------------------------------------------------------------------------
+# fixtures
+
+def _write(p: Path, data: bytes):
+    p.parent.mkdir(parents=True, exist_ok=True)
+    p.write_bytes(data)
+
+
+def build_kitchen(src: Path):
+    """A benign 'kitchen-sink' tree: regular/empty/large/sparse files, nested
+    and empty dirs, in-tree + dangling symlinks, a dir-symlink, a hardlink pair,
+    odd modes, and a user.* xattr. No attacker paths."""
+    src.mkdir(parents=True, exist_ok=True)
+    _write(src / "empty", b"")
+    _write(src / "small.txt", b"hello world\n")
+    _write(src / "data.bin", bytes((i * 7) & 0xFF for i in range(200000)))
+    _write(src / "dir/a.txt", b"a" * 100)
+    _write(src / "dir/sub/b.txt", b"b" * 100)
+    (src / "emptydir").mkdir(exist_ok=True)
+    # sparse file: hole + data
+    with open(src / "sparse.bin", "wb") as f:
+        f.seek(1 << 20)
+        f.write(b"END")
+    # hardlink pair
+    _write(src / "hl_a", b"hardlinked\n")
+    try:
+        os.link(src / "hl_a", src / "hl_b")
+    except OSError:
+        pass
+    # symlinks
+    os.symlink("small.txt", src / "rel_link")          # in-tree relative
+    os.symlink("dir", src / "dir_link")                # dir symlink
+    os.symlink("nonexistent", src / "dangling")        # dangling
+    # odd modes
+    os.chmod(src / "small.txt", 0o4755)                # setuid
+    os.chmod(src / "dir", 0o2775)                       # setgid dir
+    # xattr (best effort)
+    try:
+        os.setxattr(src / "data.bin", "user.abtest", b"v1")
+    except OSError:
+        pass
+
+
+def build_relfile(src: Path):
+    """Minimal nested file for path-shape tests."""
+    _write(src / "sub/deep/file", b"relative content\n")
+    os.symlink("file", src / "sub/deep/link")
+
+
+# ---------------------------------------------------------------------------
+# scenarios
+
+class Scenario:
+    def __init__(self, name, setup, opts, src_args, dest="destX/",
+                 cwd_is_workdir=True, pre_dest=None, abspath=False,
+                 dest_prep=None, snap_dest=None, dest_arg=None, daemon=None,
+                 ssh=False):
+        self.name = name
+        self.setup = setup            # fn(src_dir)
+        self.opts = opts              # list[str]
+        self.src_args = src_args      # fn(workdir)->list[str] OR list[str]
+        self.dest = dest
+        self.cwd_is_workdir = cwd_is_workdir
+        self.pre_dest = pre_dest      # fn(dest_dir) to pre-populate (delete/update)
+        self.abspath = abspath
+        # dest_prep(dest_path): create the dest specially (e.g. as a symlink to a
+        # real dir) instead of letting rsync create it. snap_dest(dest_path)->Path
+        # picks what to snapshot (e.g. the symlink's real target).
+        self.dest_prep = dest_prep
+        self.snap_dest = snap_dest
+        # dest_arg(dest_base)->str: the actual rsync destination argument (e.g.
+        # write THROUGH an in-tree dir-symlink: dest_base/link/). Default is
+        # dest_base + "/".
+        self.dest_arg = dest_arg
+        # daemon: None for a local transfer, or {"chroot": "no"|"yes"} to PUSH
+        # src_args into a [m] daemon module whose path is the dest dir.
+        self.daemon = daemon
+        # ssh: True to PUSH over a remote-shell split (support/lsh.sh, host "lh")
+        # -- separate client+server processes, real protocol + arg passing.
+        self.ssh = ssh
+        # rrsync: None, or {"pull": bool} to route through the restricted rrsync
+        # wrapper (support/rrsync) as an sshd forced-command would -- exercises
+        # rrsync's own option/path validation. ssh/daemon-style transport.
+        self.rrsync = None
+
+
+def _liftable(scn):
+    """A benign push-into-dest scenario whose transport can be swapped for free:
+    no dest_prep/dest_arg (those need local dest-path semantics), not already a
+    daemon/ssh scenario, and a list src_args that ends by pushing src/ -> dest."""
+    return (not scn.dest_prep and not scn.dest_arg and not scn.abspath
+            and scn.daemon is None and not scn.ssh and scn.rrsync is None
+            and isinstance(scn.src_args, list) and scn.src_args
+            and scn.src_args[-1] in ("src/", "src"))
+
+
+def _clone_transport(scn, mode):
+    import copy
+    c = copy.copy(scn)
+    c.name = f"{scn.name}@{mode}"
+    if mode == "ssh":
+        c.ssh = True
+    elif mode == "daemon":
+        c.daemon = {"chroot": "no"}
+    return c
+
+
+def lift_transports(scns, modes=("ssh", "daemon")):
+    """Make transport an ORTHOGONAL axis: keep each local scenario and, for the
+    liftable ones, also run it over ssh and a daemon module. This is where the
+    daemon/ssh-only regression family hides -- a feature broken only over the
+    wire is invisible to a local-only sweep."""
+    out = []
+    for scn in scns:
+        out.append(scn)
+        if _liftable(scn):
+            out += [_clone_transport(scn, m) for m in modes]
+    return out
+
+
+def options_sweep():
+    """-a plus one option at a time, over the kitchen-sink, relative trailing
+    slash source -> dest. The bread-and-butter single-option regression check."""
+    base = ["-a"]
+    variants = [
+        ["-a"], ["-aH"], ["-aHS"], ["-a", "--sparse"], ["-a", "--inplace"],
+        ["-a", "-A"], ["-a", "-X"], ["-a", "-AX"], ["-a", "-U"], ["-a", "-N"],
+        ["-a", "-l"], ["-a", "-L"], ["-a", "-k"], ["-a", "-K"],
+        ["-a", "--copy-unsafe-links"], ["-a", "--safe-links"],
+        ["-a", "--munge-links"], ["-a", "-z"], ["-a", "--compress-choice=zstd"],
+        ["-a", "--compress-choice=zlib"], ["-a", "-c"],
+        ["-a", "--checksum-choice=md5"], ["-a", "-W"], ["-a", "--no-whole-file"],
+        ["-a", "-O"], ["-a", "-J"], ["-a", "--numeric-ids"], ["-a", "-E"],
+        ["-a", "--no-inc-recursive"], ["-a", "--fake-super"],
+        ["-a", "--chmod=u+rwx"], ["-rlptD"], ["-rtz"],
+        ["-a", "-B", "1024"], ["-a", "--max-size=1000"], ["-a", "--min-size=50"],
+        ["-a", "--exclude=*.bin"], ["-a", "-C"], ["-a", "--prune-empty-dirs"],
+    ]
+    scns = []
+    for v in variants:
+        nm = "opt:" + "_".join(x.lstrip("-") for x in v if x != "-a") or "opt:a"
+        scns.append(Scenario("opt:" + "+".join(v), build_kitchen, v,
+                             ["src/"], "dest/"))
+    return scns
+
+
+def pathshape_sweep():
+    """The --relative class: same content under many source-path shapes."""
+    scns = []
+
+    def absfile(wd):
+        return [str(Path(wd) / "src/sub/deep/file")]
+
+    scns += [
+        Scenario("path:rel-dir-slash", build_relfile, ["-a"], ["src/"], "dest/"),
+        Scenario("path:rel-dir-noslash", build_relfile, ["-a"], ["src"], "dest/"),
+        Scenario("path:rel-file", build_relfile, ["-a"], ["src/sub/deep/file"], "dest/"),
+        Scenario("path:abs-file", build_relfile, ["-a"], absfile, "dest/"),
+        Scenario("path:abs-dir", build_relfile, ["-a"],
+                 lambda wd: [str(Path(wd) / "src") + "/"], "dest/"),
+        Scenario("path:R-rel-file", build_relfile, ["-aR"], ["src/sub/deep/file"], "dest/"),
+        Scenario("path:R-abs-file", build_relfile, ["-aR"], absfile, "dest/"),
+        Scenario("path:R-dot", build_relfile, ["-aR"], ["./src/sub/deep/file"], "dest/"),
+        Scenario("path:R-rel-dir", build_relfile, ["-aR"], ["src/sub/"], "dest/"),
+        Scenario("path:R-noimplied", build_relfile, ["-aR", "--no-implied-dirs"],
+                 ["src/sub/deep/file"], "dest/"),
+        Scenario("path:multi-src", build_relfile, ["-a"],
+                 ["src/sub/deep/file", "src/sub/deep/link"], "dest/"),
+        Scenario("path:link-as-src", build_relfile, ["-a"], ["src/sub/deep/link"], "dest/"),
+        Scenario("path:L-link-as-src", build_relfile, ["-aL"], ["src/sub/deep/link"], "dest/"),
+    ]
+    return scns
+
+
+T_OLD = 1000000000   # fixed timestamps so pre-state is identical for A and B
+T_NEW = 1700000000
+
+
+def _ut(p, t=T_NEW):
+    os.utime(p, (t, t))
+
+
+def build_recvtree(src: Path):
+    """Small, interpretable source tree for receiver/stateful scenarios."""
+    _write(src / "file1.txt", b"NEW content line\n" * 3)
+    _write(src / "dir/file2.txt", b"data2\n")
+    _write(src / "big.bin", bytes((i * 3) & 0xFF for i in range(60000)))
+    os.symlink("file1.txt", src / "slink")
+    _write(src / "hl1", b"hard\n")
+    try:
+        os.link(src / "hl1", src / "hl2")
+    except OSError:
+        pass
+    for f in ("file1.txt", "dir/file2.txt", "big.bin", "hl1", "hl2"):
+        _ut(src / f)
+    _ut(src / "dir")
+    _ut(src)
+
+
+def setup_with_basis(src: Path):
+    """src plus an identical sibling basis/ (for --link-dest/--copy-dest/etc.)."""
+    build_recvtree(src)
+    basis = src.parent / "basis"
+    build_recvtree(basis)
+
+
+def stale_dest(dest: Path):
+    """Pre-populate a dest as an OLDER state: file1 differs (older mtime),
+    big.bin differs (older), an extra obsolete file, file2 already current."""
+    _write(dest / "file1.txt", b"OLD content\n")
+    _ut(dest / "file1.txt", T_OLD)
+    _write(dest / "dir/file2.txt", b"data2\n")
+    _ut(dest / "dir/file2.txt")
+    _ut(dest / "dir")
+    _write(dest / "big.bin", bytes((i * 5) & 0xFF for i in range(60000)))
+    _ut(dest / "big.bin", T_OLD)
+    _write(dest / "obsolete.txt", b"remove me\n")
+    _ut(dest / "obsolete.txt", T_OLD)
+
+
+# ===========================================================================
+# Domain-knowledge-driven scenario generation.
+# "Edges of interest": equivalence-class boundary representatives, not volume
+# (empty-dir vs 1-file matters; 10 vs 11 doesn't; mode 0 vs 0400 vs 0200 matters;
+# 100 files of one mode don't). Each option is modelled by its precondition (the
+# dest/src state that makes it active) and, for options taking a dir, whether the
+# aux location sits INSIDE or OUTSIDE the tree.
+# ===========================================================================
+
+# interesting permission edges (no-perm / read / write / exec / special bits)
+MODES = [0o000, 0o400, 0o200, 0o644, 0o755, 0o4755, 0o2755, 0o1777]
+# size edges around rsync's block boundary (BLOCK_SIZE 700): empty/1B/under/at/over/multi
+SIZES = [0, 1, 699, 700, 701, 100003]
+
+
+def _mk_reg(p, n, mode=0o644, t=T_NEW, fill=7):
+    p.parent.mkdir(parents=True, exist_ok=True)
+    with open(p, "wb") as f:
+        f.write(bytes((i * fill) & 0xFF for i in range(n)))
+    os.chmod(p, mode)
+    _ut(p, t)
+
+
+def _mk_sparse(p, hole=1 << 20, tail=b"end"):
+    p.parent.mkdir(parents=True, exist_ok=True)
+    with open(p, "wb") as f:
+        f.seek(hole)
+        f.write(tail)
+    _ut(p)
+
+
+def _mk_fifo(p):
+    p.parent.mkdir(parents=True, exist_ok=True)
+    os.mkfifo(p)
+
+
+def _mk_sock(p):
+    import socket as _s
+    p.parent.mkdir(parents=True, exist_ok=True)
+    s = _s.socket(_s.AF_UNIX)
+    try:
+        s.bind(str(p))
+    finally:
+        s.close()
+
+
+def mode_sweep():
+    """One representative file per interesting permission edge (+ a setgid/sticky
+    dir), plain -a and -a --chmod. Boundary values, not 100 files of one mode."""
+    s = []
+    for m in MODES:
+        def setup(src, m=m):
+            _mk_reg(src / "f", 64, mode=m)
+            os.mkdir(src / "d")
+            os.chmod(src / "d", 0o2755 if m == 0o2755 else
+                     (0o1777 if m == 0o1777 else 0o755))
+            _ut(src / "d")
+        s.append(Scenario(f"mode:{m:04o}", setup, ["-a"], ["src/"], "dest/"))
+    s.append(Scenario("mode:chmod-Dg-Fo", lambda src: _mk_reg(src / "f", 8),
+                      ["-a", "--chmod=D2755,F644"], ["src/"], "dest/"))
+    return s
+
+
+def size_sweep():
+    """One file per size edge around the block boundary, plain and --inplace
+    (delta path), -c (whole-file checksum), -S on a sparse file."""
+    s = []
+    for n in SIZES:
+        s.append(Scenario(f"size:{n}", lambda src, n=n: _mk_reg(src / "f", n),
+                          ["-a"], ["src/"], "dest/"))
+        s.append(Scenario(f"size:{n}+inplace",
+                          lambda src, n=n: _mk_reg(src / "f", n),
+                          ["-a", "--inplace", "--no-whole-file"], ["src/"], "dest/"))
+    s.append(Scenario("size:sparse", lambda src: _mk_sparse(src / "sp.bin"),
+                      ["-aS"], ["src/"], "dest/"))
+    s.append(Scenario("size:sparse-inplace", lambda src: _mk_sparse(src / "sp.bin"),
+                      ["-aS", "--inplace"], ["src/"], "dest/"))
+    return s
+
+
+def filetype_sweep():
+    """One representative per file type/symlink shape, each with the option(s)
+    that actually exercise it. Specials/devices are in priv_sweep (root)."""
+    def base(src):
+        _mk_reg(src / "anchor", 16)
+
+    def f_emptydir(src):
+        base(src)
+        os.makedirs(src / "empty")
+        _ut(src / "empty")
+
+    def f_intree(src):
+        base(src)
+        _mk_reg(src / "real", 20)
+        os.symlink("real", src / "lnk")          # in-tree relative
+
+    def f_dirlink(src):
+        base(src)
+        _mk_reg(src / "rd/inner", 20)
+        os.symlink("rd", src / "dl")             # symlink to a dir
+
+    def f_abslink(src):
+        base(src)
+        os.symlink("/etc/hostname", src / "abs")  # out-of-tree absolute
+
+    def f_dangling(src):
+        base(src)
+        os.symlink("nonexistent", src / "dead")
+
+    def f_chain(src):
+        base(src)
+        _mk_reg(src / "real", 20)
+        os.symlink("real", src / "l1")
+        os.symlink("l1", src / "l2")             # symlink chain
+
+    def f_hardlinks(src):
+        _mk_reg(src / "a", 40)
+        os.link(src / "a", src / "b")
+        os.link(src / "a", src / "c")
+        base(src)
+
+    def f_fifo(src):
+        base(src)
+        _mk_fifo(src / "pipe")
+
+    def f_sock(src):
+        base(src)
+        _mk_sock(src / "sock")
+
+    return [
+        Scenario("ft:emptydir", f_emptydir, ["-a"], ["src/"], "dest/"),
+        Scenario("ft:intree-l", f_intree, ["-a"], ["src/"], "dest/"),
+        Scenario("ft:intree-L", f_intree, ["-aL"], ["src/"], "dest/"),
+        Scenario("ft:dirlink-l", f_dirlink, ["-a"], ["src/"], "dest/"),
+        Scenario("ft:dirlink-k", f_dirlink, ["-ak"], ["src/"], "dest/"),
+        Scenario("ft:dirlink-L", f_dirlink, ["-aL"], ["src/"], "dest/"),
+        Scenario("ft:abslink-l", f_abslink, ["-a"], ["src/"], "dest/"),
+        Scenario("ft:abslink-L", f_abslink, ["-aL"], ["src/"], "dest/"),
+        Scenario("ft:abslink-safe", f_abslink, ["-a", "--safe-links"], ["src/"], "dest/"),
+        Scenario("ft:abslink-munge", f_abslink, ["-a", "--munge-links"], ["src/"], "dest/"),
+        Scenario("ft:dangling", f_dangling, ["-a"], ["src/"], "dest/"),
+        Scenario("ft:chain-l", f_chain, ["-a"], ["src/"], "dest/"),
+        Scenario("ft:chain-L", f_chain, ["-aL"], ["src/"], "dest/"),
+        Scenario("ft:hardlinks", f_hardlinks, ["-aH"], ["src/"], "dest/"),
+        Scenario("ft:fifo", f_fifo, ["-a"], ["src/"], "dest/"),
+        Scenario("ft:sock", f_sock, ["-a"], ["src/"], "dest/"),
+    ]
+
+
+# --- preconditions: dest/src state that makes an option actually ACTIVE -------
+def _pc_tree(src):
+    _mk_reg(src / "f", 100)
+    _mk_reg(src / "dir/g", 50)
+    _ut(src / "dir")
+    _ut(src)
+
+
+def _setup_samemeta(src):
+    _mk_reg(src / "f", 100, t=T_NEW, fill=7)
+    _mk_reg(src / "keep", 20, t=T_NEW, fill=7)
+
+
+def _pre_samemeta(dest):                 # same size+mtime, DIFFERENT content
+    _mk_reg(dest / "f", 100, t=T_NEW, fill=200)
+    _mk_reg(dest / "keep", 20, t=T_NEW, fill=200)
+
+
+def _setup_older(src):
+    _mk_reg(src / "f", 100, t=T_OLD, fill=7)
+
+
+def _pre_newer(dest):                    # dest newer + different (for -u)
+    _mk_reg(dest / "f", 60, t=T_NEW, fill=200)
+
+
+def _setup_mixed(src):                   # an existing file + a new file
+    _mk_reg(src / "exist", 30, t=T_NEW, fill=7)
+    _mk_reg(src / "newfile", 30, t=T_NEW, fill=7)
+
+
+def _pre_existing(dest):                 # only "exist" present (older, different)
+    _mk_reg(dest / "exist", 99, t=T_OLD, fill=200)
+
+
+def _setup_sizes(src):
+    _mk_reg(src / "small", 50)
+    _mk_reg(src / "big", 5000)
+
+
+def _setup_emptydirs(src):
+    os.makedirs(src / "empty/sub")
+    _mk_reg(src / "keep/f", 10)
+    _ut(src / "keep")
+    _ut(src / "empty/sub")
+    _ut(src / "empty")
+
+
+def _setup_append(src):
+    _mk_reg(src / "f", 200, t=T_NEW, fill=7)
+
+
+def _pre_append(dest):                   # dest is a shorter prefix (for --append)
+    with open(dest / "f", "wb") as fh:
+        fh.write(bytes((i * 7) & 0xFF for i in range(80)))
+    _ut(dest / "f", T_OLD)
+
+
+def _setup_kdest(src):
+    _mk_reg(src / "dir/f1", 10)
+    _mk_reg(src / "dir/f2", 10)
+    _mk_reg(src / "top", 5)
+
+
+def _prep_kdest(dest):                   # dest has realdir + dir->realdir symlink
+    dest.mkdir(parents=True, exist_ok=True)
+    (dest / "realdir").mkdir(exist_ok=True)
+    if not (dest / "dir").is_symlink():
+        os.symlink("realdir", dest / "dir")
+
+
+def selection_sweep():
+    """File-selection / timestamp / size options at the boundary that makes the
+    comparison non-trivial (same size+mtime but different content, newer dest,
+    existing-vs-new, size limits)."""
+    plain = "src/"
+    s = [
+        Scenario("sel:checksum", _setup_samemeta, ["-a", "-c"], [plain], "dest/", pre_dest=_pre_samemeta),
+        Scenario("sel:ignore-times", _setup_samemeta, ["-a", "-I"], [plain], "dest/", pre_dest=_pre_samemeta),
+        Scenario("sel:size-only", _setup_samemeta, ["-a", "--size-only"], [plain], "dest/", pre_dest=_pre_samemeta),
+        Scenario("sel:quickcheck", _setup_samemeta, ["-a"], [plain], "dest/", pre_dest=_pre_samemeta),
+        Scenario("sel:update", _setup_older, ["-a", "-u"], [plain], "dest/", pre_dest=_pre_newer),
+        Scenario("sel:modify-window", _setup_samemeta, ["-a", "--modify-window=2"], [plain], "dest/", pre_dest=_pre_samemeta),
+        Scenario("sel:existing", _setup_mixed, ["-a", "--existing"], [plain], "dest/", pre_dest=_pre_existing),
+        Scenario("sel:ignore-existing", _setup_mixed, ["-a", "--ignore-existing"], [plain], "dest/", pre_dest=_pre_existing),
+        Scenario("sel:max-size", _setup_sizes, ["-a", "--max-size=1000"], [plain], "dest/"),
+        Scenario("sel:min-size", _setup_sizes, ["-a", "--min-size=1000"], [plain], "dest/"),
+        Scenario("sel:times-only", _pc_tree, ["-rlpt"], [plain], "dest/"),
+        Scenario("sel:atimes", _pc_tree, ["-a", "--atimes"], [plain], "dest/"),
+        Scenario("sel:crtimes", _pc_tree, ["-a", "--crtimes"], [plain], "dest/"),
+        Scenario("sel:open-noatime", _pc_tree, ["-a", "--open-noatime"], [plain], "dest/"),
+    ]
+    return lift_transports(s)
+
+
+def behavior_sweep():
+    """Behaviour options at their active preconditions (overwrite/backup/delete/
+    inplace/append/keep-dirlinks-dest/prune-empty/mkpath/dirs)."""
+    return [
+        Scenario("beh:backup", build_recvtree, ["-ab"], ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("beh:delete", build_recvtree, ["-a", "--delete"], ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("beh:delete-before", build_recvtree, ["-a", "--delete-before"], ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("beh:delete-after", build_recvtree, ["-a", "--delete-after"], ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("beh:delete-delay", build_recvtree, ["-a", "--delete-delay"], ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("beh:delete-excluded", build_recvtree,
+                 ["-a", "--delete", "--delete-excluded", "--exclude=obsolete.txt"],
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("beh:prune-empty", _setup_emptydirs, ["-a", "-m"], ["src/"], "dest/"),
+        Scenario("beh:inplace", build_recvtree, ["-a", "--inplace"], ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("beh:append", _setup_append, ["-a", "--append"], ["src/"], "dest/", pre_dest=_pre_append),
+        Scenario("beh:numeric-ids", build_recvtree, ["-a", "--numeric-ids"], ["src/"], "dest/"),
+        Scenario("beh:delay-updates", build_recvtree, ["-a", "--delay-updates"], ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("beh:dirs", _pc_tree, ["-dlpt"], ["src/"], "dest/"),
+        Scenario("beh:mkpath", _pc_tree, ["-a", "--mkpath"], ["src/"], "dest/",
+                 dest_arg=lambda d: str(d) + "/made/sub/"),
+        Scenario("beh:keep-dirlinks-dest", _setup_kdest, ["-aK"], ["src/"], "dest/",
+                 dest_prep=_prep_kdest, snap_dest=lambda d: d / "realdir"),
+    ]
+
+
+def _auxp(dest, where, name):
+    """Aux path INSIDE the dest tree, or OUTSIDE it (sibling under the workdir)."""
+    dest = Path(dest)
+    return str((dest / name) if where == "inside" else (dest.parent / f"aux_{name}"))
+
+
+def placement_sweep():
+    """Options that take a DIR/path, each with the aux location INSIDE vs OUTSIDE
+    the destination tree -- the edge most likely to expose confinement/escape
+    regressions.  backup/temp/partial-dir + link/compare/copy-dest."""
+    s = []
+    for where in ("inside", "outside"):
+        s.append(Scenario(f"place:backup-dir-{where}", build_recvtree,
+                 (lambda wd, dest, w=where: ["-ab", f"--backup-dir={_auxp(dest, w, 'bak')}"]),
+                 ["src/"], "dest/", pre_dest=stale_dest))
+        # temp-dir must exist -> pre-create dest (empty) + the temp dir
+        def _prep_temp(dest, w=where):
+            dest.mkdir(parents=True, exist_ok=True)
+            os.makedirs(_auxp(dest, w, "tmp"), exist_ok=True)
+        s.append(Scenario(f"place:temp-dir-{where}", build_recvtree,
+                 (lambda wd, dest, w=where: ["-a", f"--temp-dir={_auxp(dest, w, 'tmp')}"]),
+                 ["src/"], "dest/", dest_prep=_prep_temp))
+        s.append(Scenario(f"place:partial-dir-{where}", build_recvtree,
+                 (lambda wd, dest, w=where: ["-a", "--partial", f"--partial-dir={_auxp(dest, w, 'part')}"]),
+                 ["src/"], "dest/"))
+    # alt-dest basis (a prior identical copy at wd/basis): absolute vs relative path
+    for opt in ("link-dest", "compare-dest", "copy-dest"):
+        s.append(Scenario(f"place:{opt}-abs", setup_with_basis,
+                 (lambda wd, dest, o=opt: ["-a", f"--{o}={wd}/basis"]), ["src/"], "dest/"))
+        s.append(Scenario(f"place:{opt}-rel", setup_with_basis,
+                 (lambda wd, dest, o=opt: ["-a", f"--{o}=../basis"]), ["src/"], "dest/"))
+    return lift_transports(s)
+
+
+def wire_sweep():
+    """Protocol / wire / algorithm options (checksum & compress choice, old/
+    secluded args, iconv, odd block sizes)."""
+    bt = build_recvtree
+    return [
+        Scenario("wire:cc-md5", bt, ["-a", "--checksum-choice=md5"], ["src/"], "dest/"),
+        Scenario("wire:cc-md4", bt, ["-a", "--checksum-choice=md4"], ["src/"], "dest/"),
+        Scenario("wire:cc-xxh64", bt, ["-a", "--checksum-choice=xxh64"], ["src/"], "dest/"),
+        Scenario("wire:zc-zstd", bt, ["-a", "-z", "--compress-choice=zstd"], ["src/"], "dest/"),
+        Scenario("wire:zc-zlib", bt, ["-a", "-z", "--compress-choice=zlib"], ["src/"], "dest/"),
+        Scenario("wire:zc-zlibx", bt, ["-a", "-z", "--compress-choice=zlibx"], ["src/"], "dest/"),
+        Scenario("wire:old-args", bt, ["-a", "--old-args"], ["src/"], "dest/"),
+        Scenario("wire:secluded-args", bt, ["-a", "-s"], ["src/"], "dest/"),
+        Scenario("wire:iconv", bt, ["-a", "--iconv=utf8,latin1"], ["src/"], "dest/"),
+        Scenario("wire:block-1024", bt, ["-a", "-B", "1024"], ["src/"], "dest/"),
+        Scenario("wire:block-999", bt, ["-a", "-B", "999"], ["src/"], "dest/"),
+    ]
+
+
+# module-level file-type fixtures (also used by the pairwise sweep)
+def _ft_intree(src):
+    _mk_reg(src / "real", 20)
+    os.symlink("real", src / "lnk")
+    _mk_reg(src / "anchor", 10)
+
+
+def _ft_dirlink(src):
+    _mk_reg(src / "rd/inner", 20)
+    os.symlink("rd", src / "dl")
+    _mk_reg(src / "anchor", 10)
+    _ut(src / "rd")
+
+
+def _ft_hardlinks(src):
+    _mk_reg(src / "a", 40)
+    os.link(src / "a", src / "b")
+    os.link(src / "a", src / "c")
+    _mk_reg(src / "anchor", 10)
+
+
+def _ft_sparse(src):
+    _mk_sparse(src / "sp.bin")
+    _mk_reg(src / "anchor", 10)
+
+
+def pairwise_sweep():
+    """Guided pairwise: curated feature interactions (domain knowledge about where
+    two options collide), plus an auto option x file-type covering set."""
+    s = [
+        Scenario("pair:delete+dirlink-dest", _setup_kdest, ["-aK", "--delete"],
+                 ["src/"], "dest/", dest_prep=_prep_kdest, snap_dest=lambda d: d / "realdir"),
+        Scenario("pair:backup-inside+delete", build_recvtree,
+                 (lambda wd, dest: ["-ab", "--delete", f"--backup-dir={_auxp(dest, 'inside', 'bak')}"]),
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("pair:inplace+sparse", _ft_sparse,
+                 ["-aS", "--inplace", "--no-whole-file"], ["src/"], "dest/"),
+        Scenario("pair:link-dest+hardlinks", setup_with_basis,
+                 (lambda wd, dest: ["-aH", f"--link-dest={wd}/basis"]), ["src/"], "dest/"),
+        Scenario("pair:copy-links+dirlink", _ft_dirlink, ["-aL"], ["src/"], "dest/"),
+        Scenario("pair:keepdirlinks+delete-excluded", _setup_kdest,
+                 ["-aK", "--delete", "--delete-excluded", "--exclude=top"],
+                 ["src/"], "dest/", dest_prep=_prep_kdest, snap_dest=lambda d: d / "realdir"),
+        Scenario("pair:sparse+whole-file", _ft_sparse, ["-aS", "-W"], ["src/"], "dest/"),
+        Scenario("pair:partial-inside+delete", build_recvtree,
+                 (lambda wd, dest: ["-a", "--delete", "--partial",
+                                    f"--partial-dir={_auxp(dest, 'inside', 'part')}"]),
+                 ["src/"], "dest/", pre_dest=stale_dest),
+    ]
+    # auto option x file-type covering set (each option relevant to several types)
+    fts = [("intree", _ft_intree), ("dirlink", _ft_dirlink),
+           ("hardlinks", _ft_hardlinks), ("sparse", _ft_sparse)]
+    opts = ["-c", "-z", "-b", "--inplace", "-H", "-L", "-k", "--checksum-choice=md5"]
+    for ftn, ftfn in fts:
+        for o in opts:
+            tag = o.lstrip("-").split("=")[0]
+            s.append(Scenario(f"pair:{tag}x{ftn}", ftfn, ["-a", o], ["src/"], "dest/"))
+    return s
+
+
+def recv_sweep():
+    """Receiver/generator + stateful scenarios: existing-dest update/delete,
+    backup, dest-variants (link/compare/copy-dest), inplace, temp-dir. These
+    exercise the receiver's existing-dest / basis / temp-dir paths that the
+    single-transfer-into-empty-dest sweeps don't reach."""
+    s = []
+    s += [
+        Scenario("recv:update", build_recvtree, ["-a"], ["src/"], "dest/",
+                 pre_dest=stale_dest),
+        Scenario("recv:update-W", build_recvtree, ["-a", "-W"], ["src/"], "dest/",
+                 pre_dest=stale_dest),
+        Scenario("recv:update-delete", build_recvtree, ["-a", "--delete"],
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("recv:delete-during", build_recvtree, ["-a", "--delete-during"],
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("recv:delete-after", build_recvtree, ["-a", "--delete-after"],
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("recv:delete-delay", build_recvtree, ["-a", "--delete-delay"],
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("recv:backup", build_recvtree, ["-a", "-b"], ["src/"], "dest/",
+                 pre_dest=stale_dest),
+        Scenario("recv:backup-dir", build_recvtree, ["-a", "-b", "--backup-dir=bak"],
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("recv:backup-suffix", build_recvtree, ["-a", "-b", "--suffix=.old"],
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("recv:inplace", build_recvtree, ["-a", "--inplace", "--no-whole-file"],
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("recv:delay-updates", build_recvtree, ["-a", "--delay-updates"],
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("recv:temp-dir", build_recvtree, ["-a", "--temp-dir=tmpd"],
+                 ["src/"], "dest/", pre_dest=lambda d: (stale_dest(d), (d / "tmpd").mkdir(exist_ok=True))),
+        Scenario("recv:partial-dir", build_recvtree, ["-a", "--partial-dir=.part"],
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("recv:fuzzy", build_recvtree, ["-a", "--fuzzy"], ["src/"], "dest/",
+                 pre_dest=stale_dest),
+        # dest-variant basis lookups (basis = identical sibling dir)
+        Scenario("recv:link-dest-rel", setup_with_basis, ["-a", "--link-dest=../basis"],
+                 ["src/"], "dest/"),
+        Scenario("recv:link-dest-abs", setup_with_basis, ["-a"],
+                 lambda wd: ["--link-dest=" + str(Path(wd) / "basis"), "src/"], "dest/"),
+        Scenario("recv:compare-dest-rel", setup_with_basis,
+                 ["-a", "--compare-dest=../basis"], ["src/"], "dest/"),
+        Scenario("recv:copy-dest-rel", setup_with_basis,
+                 ["-a", "--copy-dest=../basis"], ["src/"], "dest/"),
+    ]
+    return lift_transports(s)
+
+
+def destshape_sweep():
+    """Destination path shapes (symlinked dest dir, --mkpath) that stress the
+    receiver's destination-path handling."""
+    def symlinked_dest(dest: Path):
+        real = Path(str(dest) + "_real")
+        real.mkdir(parents=True, exist_ok=True)
+        if not dest.is_symlink():
+            os.symlink(real.name, dest)          # dest -> dest_X_real (in-tree)
+
+    def real_of(dest: Path):
+        return Path(str(dest) + "_real")
+
+    def via_symlink_parent(dest: Path):
+        # dest = .../dest_X ; make its PARENT route through an in-tree symlink:
+        # realbase/, link->realbase, and rsync writes to link/<dest_X name>
+        real = Path(str(dest) + "_rb")
+        real.mkdir(parents=True, exist_ok=True)
+        link = Path(str(dest) + "_lnk")
+        if not link.is_symlink():
+            os.symlink(real.name, link)
+        return link
+
+    s = [
+        Scenario("dest:symlinked-dir", build_recvtree, ["-a"], ["src/"], "dest/",
+                 dest_prep=symlinked_dest, snap_dest=real_of),
+        Scenario("dest:mkpath", build_recvtree, ["-a", "--mkpath"], ["src/"],
+                 "dest/new/deep/", snap_dest=lambda d: d),
+    ]
+    return s
+
+
+def name_sweep():
+    """Unusual but benign filenames (arg-handling / secluded-args)."""
+    names = ["a space", "two  spaces", "café_ünïcode", "semi;colon",
+             "dollar$sign", "paren(s)", "quote'name", "amp&and", "back\\slash",
+             "newline\nname", "tab\tname", "trailing ", "leaddash"]
+
+    def setup(src: Path):
+        for i, n in enumerate(names):
+            _write(src / n, f"content {i}\n".encode())
+        # a leading-dash file (separate so it can't be mistaken for an option)
+        _write(src / "-leadingdash.txt", b"dash\n")
+        os.symlink("a space", src / "link to spaced")
+
+    return [Scenario("name:weird", setup, ["-a"], ["src/"], "dest/")]
+
+
+def filesfrom_sweep():
+    """--files-from with relative & absolute name lists, and --from0.
+
+    The list files live in the workdir, which is also rsync's cwd, so the
+    --files-from arg is a BARE filename (cwd-relative) -- referencing it via
+    str(wd) double-resolves and silently fails when --workdir is relative, which
+    makes the whole scenario a vacuous pass. The absolute list uses src.resolve()
+    so the "/" transfer-root case works regardless of workdir."""
+    def setup(src: Path):
+        build_recvtree(src)
+        wd = src.parent
+        asrc = src.resolve()
+        (wd / "list_rel.txt").write_text("file1.txt\ndir/file2.txt\nslink\n")
+        (wd / "list_abs.txt").write_text(
+            f"{asrc}/file1.txt\n{asrc}/dir/file2.txt\n")
+        (wd / "list0.txt").write_bytes(b"file1.txt\0dir/file2.txt\0")
+
+    s = [
+        Scenario("ff:rel", setup, ["-a"],
+                 ["--files-from=list_rel.txt", "src/"], "dest/"),
+        Scenario("ff:rel-R", setup, ["-aR"],
+                 ["--files-from=list_rel.txt", "src/"], "dest/"),
+        Scenario("ff:abs", setup, ["-a"],
+                 ["--files-from=list_abs.txt", "/"], "dest/"),
+        Scenario("ff:from0", setup, ["-a", "--from0"],
+                 ["--files-from=list0.txt", "src/"], "dest/"),
+    ]
+    return lift_transports(s)
+
+
+def build_privtree(src: Path):
+    """Root-only fixture: owned files, special perms, FIFO, devices. Falls back
+    gracefully to what the euid can create."""
+    build_recvtree(src)
+    os.chmod(src / "file1.txt", 0o4755)   # setuid
+    os.chmod(src / "dir", 0o2755)         # setgid
+    try:
+        os.mkfifo(src / "fifo")
+    except OSError:
+        pass
+    if os.geteuid() == 0:
+        try:
+            os.mknod(src / "chardev", stat.S_IFCHR | 0o644, os.makedev(1, 3))
+            os.mknod(src / "blockdev", stat.S_IFBLK | 0o644, os.makedev(7, 0))
+        except OSError:
+            pass
+        # chown a file to a different uid/gid if any exists
+        try:
+            os.chown(src / "dir/file2.txt", 1, 1)
+        except OSError:
+            pass
+
+
+def priv_sweep():
+    """Run as root (sudo): owner/group, devices/specials, fake-super."""
+    return [
+        Scenario("priv:archive", build_privtree, ["-a"], ["src/"], "dest/"),
+        Scenario("priv:devices", build_privtree, ["-aD"], ["src/"], "dest/"),
+        Scenario("priv:HD", build_privtree, ["-aHD"], ["src/"], "dest/"),
+        Scenario("priv:numeric-ids", build_privtree, ["-a", "--numeric-ids"],
+                 ["src/"], "dest/"),
+        Scenario("priv:fake-super", build_privtree, ["-a", "--fake-super"],
+                 ["src/"], "dest/"),
+        Scenario("priv:specials", build_privtree, ["-a", "--specials"],
+                 ["src/"], "dest/"),
+        Scenario("priv:acls-xattrs", build_privtree, ["-aAX"], ["src/"], "dest/"),
+    ]
+
+
+def intree_sweep():
+    """Traverse an IN-TREE dir-symlink as a path component (source via a
+    dir-symlink, dest through a dir-symlink, keep-dirlinks, alt-basis via a
+    dir-symlink) -- a path-handling shape that varies across builds/platforms."""
+    def src_dirlink(src: Path):
+        _write(src / "real/f1", b"in real\n")
+        _write(src / "real/sub/f2", b"deep\n")
+        os.symlink("real", src / "link")
+
+    def dirtree(src: Path):
+        _write(src / "dir/f1", b"one\n")
+        _write(src / "dir/f2", b"two\n")
+        _write(src / "top.txt", b"top\n")
+
+    def dest_dirlink_prep(dest: Path):
+        dest.mkdir(parents=True, exist_ok=True)
+        (dest / "real").mkdir(exist_ok=True)
+        if not (dest / "link").is_symlink():
+            os.symlink("real", dest / "link")
+
+    def keepdir_prep(dest: Path):
+        dest.mkdir(parents=True, exist_ok=True)
+        (dest / "realdir").mkdir(exist_ok=True)
+        if not (dest / "dir").is_symlink():
+            os.symlink("realdir", dest / "dir")
+
+    def basis_dirlink(src: Path):
+        build_recvtree(src)
+        build_recvtree(src.parent / "realbasis")
+        os.symlink("realbasis", src.parent / "basislink")
+
+    return [
+        Scenario("intree:src-via-dirlink", src_dirlink, ["-a"], ["src/link/"], "dest/"),
+        Scenario("intree:src-dirlink-noslash", src_dirlink, ["-a"], ["src/link"], "dest/"),
+        Scenario("intree:dest-via-dirlink", build_recvtree, ["-a"], ["src/"], "dest/",
+                 dest_prep=dest_dirlink_prep,
+                 dest_arg=lambda d: str(d / "link") + "/",
+                 snap_dest=lambda d: d / "real"),
+        Scenario("intree:keep-dirlinks", dirtree, ["-aK"], ["src/"], "dest/",
+                 dest_prep=keepdir_prep, snap_dest=lambda d: d / "realdir"),
+        Scenario("intree:link-dest-dirlink", basis_dirlink, ["-a"],
+                 lambda wd: ["--link-dest=" + str(Path(wd) / "basislink"), "src/"],
+                 "dest/"),
+        Scenario("intree:compare-dest-dirlink", basis_dirlink, ["-a"],
+                 lambda wd: ["--compare-dest=" + str(Path(wd) / "basislink"), "src/"],
+                 "dest/"),
+    ]
+
+
+def intree2_sweep():
+    """More in-tree dir-symlink traversal: -k/copy-dirlinks on the source,
+    source files under a symlinked PARENT, and -K update/delete through a
+    symlinked dest dir."""
+    def src_with_dirlink(src: Path):
+        _write(src / "realdir/a", b"aa\n")
+        _write(src / "realdir/b", b"bb\n")
+        _write(src / "top.txt", b"top\n")
+        os.symlink("realdir", src / "dl")
+
+    def src_symlink_parent(src: Path):
+        _write(src / "real/sub/file", b"under symlinked parent\n")
+        os.symlink("real", src / "link")
+
+    def kupd_prep(dest: Path):
+        # symlinked dest dir whose real target holds an OLDER file + an extra
+        dest.mkdir(parents=True, exist_ok=True)
+        (dest / "realdir").mkdir(exist_ok=True)
+        _write(dest / "realdir/f1", b"OLD\n")
+        _ut(dest / "realdir/f1", T_OLD)
+        _write(dest / "realdir/extra", b"extra\n")
+        _ut(dest / "realdir/extra", T_OLD)
+        if not (dest / "dir").is_symlink():
+            os.symlink("realdir", dest / "dir")
+
+    def dirtree2(src: Path):
+        _write(src / "dir/f1", b"new1\n")
+        _write(src / "dir/f2", b"new2\n")
+        _ut(src / "dir/f1"); _ut(src / "dir/f2"); _ut(src / "dir")
+
+    return [
+        Scenario("intree2:copy-dirlinks-k", src_with_dirlink, ["-a", "-k"],
+                 ["src/"], "dest/"),
+        Scenario("intree2:copy-links-L", src_with_dirlink, ["-a", "-L"],
+                 ["src/"], "dest/"),
+        Scenario("intree2:src-symlink-parent", src_symlink_parent, ["-a"],
+                 ["src/link/sub/file"], "dest/"),
+        Scenario("intree2:src-symlink-parent-R", src_symlink_parent, ["-aR"],
+                 ["src/link/sub/file"], "dest/"),
+        Scenario("intree2:src-symlink-parent-dir", src_symlink_parent, ["-a"],
+                 ["src/link/sub/"], "dest/"),
+        Scenario("intree2:K-update", dirtree2, ["-aK"], ["src/"], "dest/",
+                 dest_prep=kupd_prep, snap_dest=lambda d: d / "realdir"),
+        Scenario("intree2:K-delete", dirtree2, ["-aK", "--delete"], ["src/"],
+                 "dest/", dest_prep=kupd_prep, snap_dest=lambda d: d / "realdir"),
+    ]
+
+
+def proto_sweep():
+    """Older protocol versions (negotiation / wire-format regressions)."""
+    s = []
+    for p in (29, 30, 31):
+        for opt in (["-a"], ["-aH"], ["-aHS"], ["-az"],
+                    ["-a", "--no-inc-recursive"]):
+            tag = "+".join(o.lstrip("-") for o in opt if o != "-a") or "a"
+            s.append(Scenario(f"proto{p}:{tag}", build_kitchen,
+                     opt + [f"--protocol={p}"], ["src/"], "dest/"))
+    return s
+
+
+def combo_sweep():
+    """Pairs of options over a stale dest (so update/backup/inplace actually
+    fire) -- non-symlink option-interaction regressions."""
+    import itertools
+    flags = ["-H", "-S", "--inplace", "-z", "-c", "-b", "-O", "-J",
+             "--numeric-ids", "-A", "-X", "-E", "--no-whole-file", "-I",
+             "--size-only", "-u"]
+    s = []
+    for x, y in itertools.combinations(flags, 2):
+        s.append(Scenario(f"combo:{x},{y}", build_recvtree, ["-a", x, y],
+                          ["src/"], "dest/", pre_dest=stale_dest))
+    return s
+
+
+def scale_sweep():
+    """Content scale: many small files, deep nesting, a large file."""
+    def many(src: Path):
+        for i in range(500):
+            _write(src / f"d{i % 12}" / f"f{i:04d}", f"file {i}\n".encode())
+
+    def deep(src: Path):
+        p = src
+        for i in range(40):
+            p = p / f"d{i}"
+        _write(p / "leaf", b"deep\n")
+        _write(src / "shallow", b"s\n")
+
+    def big(src: Path):
+        _write(src / "big.bin", bytes((i * 7) & 0xFF for i in range(3_000_000)))
+        _write(src / "small", b"x\n")
+
+    return [
+        Scenario("scale:many", many, ["-a"], ["src/"], "dest/"),
+        Scenario("scale:many-H", many, ["-aH"], ["src/"], "dest/"),
+        Scenario("scale:deep", deep, ["-a"], ["src/"], "dest/"),
+        Scenario("scale:big", big, ["-a"], ["src/"], "dest/"),
+        Scenario("scale:big-inplace", big, ["-a", "--inplace", "--no-whole-file"],
+                 ["src/"], "dest/"),
+        Scenario("scale:big-z", big, ["-az"], ["src/"], "dest/"),
+    ]
+
+
+def _daemon_scns(chroot):
+    """PUSH scenarios to a [m] daemon module (the daemon receiver path)."""
+    D = {"chroot": chroot}
+    pfx = "daemonchroot" if chroot == "yes" else "daemon"
+
+    def kt(src: Path):
+        _write(src / "dir/f1", b"n1\n")
+        _write(src / "dir/f2", b"n2\n")
+        _write(src / "top", b"t\n")
+
+    def kprep(dest: Path):
+        dest.mkdir(parents=True, exist_ok=True)
+        (dest / "realdir").mkdir(exist_ok=True)
+        if not (dest / "dir").is_symlink():
+            os.symlink("realdir", dest / "dir")
+
+    return [
+        Scenario(f"{pfx}:push", build_recvtree, ["-a"], ["src/"], daemon=D),
+        Scenario(f"{pfx}:push-H", build_recvtree, ["-aH"], ["src/"], daemon=D),
+        Scenario(f"{pfx}:push-X", build_recvtree, ["-aX"], ["src/"], daemon=D),
+        Scenario(f"{pfx}:push-update", build_recvtree, ["-a"], ["src/"],
+                 pre_dest=stale_dest, daemon=D),
+        Scenario(f"{pfx}:push-delete", build_recvtree, ["-a", "--delete"],
+                 ["src/"], pre_dest=stale_dest, daemon=D),
+        Scenario(f"{pfx}:push-K-symlinkdir", kt, ["-aK"], ["src/"],
+                 dest_prep=kprep, snap_dest=lambda d: d / "realdir", daemon=D),
+    ]
+
+
+def daemon_sweep():
+    return _daemon_scns("no")
+
+
+def daemonchroot_sweep():
+    """use chroot = yes (root-only): the daemon chroots into the module path."""
+    return _daemon_scns("yes")
+
+
+def gaps_sweep():
+    """Scenarios observable mainly via the stdout/stderr/itemize signals rather
+    than the dest tree: --list-only listings, --dry-run plans (incl.
+    --mkpath+--dry-run file-to-file), and type-change updates."""
+    def t_tree(src):
+        _mk_reg(src / "f", 100)
+        _mk_reg(src / "dir/g", 50)
+        os.symlink("f", src / "l")
+        _ut(src / "dir")
+
+    def t_one(src):
+        _mk_reg(src / "file", 50)
+
+    def t_typesrc(src):
+        _mk_reg(src / "x", 30)        # src: x is a FILE
+        _mk_reg(src / "keep", 10)
+
+    def pre_typedir(dest):            # dest: x is a DIR (different type)
+        os.makedirs(dest / "x")
+        _mk_reg(dest / "x" / "inner", 5)
+        _ut(dest / "x")
+
+    return [
+        Scenario("gap:list-only", t_tree, ["--list-only", "-a"], ["src/"], "dest/"),
+        Scenario("gap:dry-run", t_tree, ["-ai", "--dry-run"], ["src/"], "dest/"),
+        Scenario("gap:mkpath-dryrun-f2f", t_one, ["-ai", "--mkpath", "--dry-run"],
+                 ["src/file"], "dest/", dest_arg=lambda d: str(d) + "/newdir/file"),
+        Scenario("gap:dry-run-delete", build_recvtree, ["-ai", "--dry-run", "--delete"],
+                 ["src/"], "dest/", pre_dest=stale_dest),
+        Scenario("gap:typechange-force", t_typesrc, ["-a", "--force"], ["src/"], "dest/",
+                 pre_dest=pre_typedir),
+        Scenario("gap:typechange-delete", t_typesrc, ["-a", "--delete"], ["src/"], "dest/",
+                 pre_dest=pre_typedir),
+    ]
+
+
+def misc_sweep():
+    """Genuinely-untested subsystems: filter/include-exclude rules, per-dir merge,
+    CVS-exclude, fuzzy, write-batch, xattr, -R dot-anchoring -- looking for NEW
+    root-cause families beyond the daemon-symlink cluster."""
+    def t_filter(src):
+        _mk_reg(src / "keep.txt", 10)
+        _mk_reg(src / "skip.log", 10)
+        _mk_reg(src / "sub/keep2.txt", 10)
+        _mk_reg(src / "sub/skip2.log", 10)
+        _ut(src / "sub")
+
+    def t_dirmerge(src):
+        _mk_reg(src / "a.txt", 10)
+        _mk_reg(src / "b.log", 10)
+        _write(src / ".rsync-filter", b"- *.log\n")
+        _ut(src / ".rsync-filter")
+
+    def t_cvs(src):
+        _mk_reg(src / "keep", 10)
+        _mk_reg(src / "core", 10)
+        _mk_reg(src / "obj.o", 10)
+
+    def t_fuzzy(src):
+        _mk_reg(src / "file.txt", 5000, t=T_NEW)
+
+    def pre_fuzzy(dest):
+        _mk_reg(dest / "file.txt.bak", 5000, t=T_OLD, fill=7)
+
+    def t_xattr(src):
+        _mk_reg(src / "f", 10)
+        try:
+            os.setxattr(src / "f", "user.test", b"val")
+        except OSError:
+            pass
+
+    def t_reldot(src):
+        _mk_reg(src / "sub/deep/f", 10)
+        _ut(src / "sub/deep")
+        _ut(src / "sub")
+
+    return [
+        Scenario("misc:exclude", t_filter, ["-a", "--exclude=*.log"], ["src/"], "dest/"),
+        Scenario("misc:filter-rule", t_filter, ["-a", "-f", "- *.log"], ["src/"], "dest/"),
+        Scenario("misc:filter-incl", t_filter, ["-a", "-f", "+ */", "-f", "+ *.txt", "-f", "- *"], ["src/"], "dest/"),
+        Scenario("misc:dirmerge-F", t_dirmerge, ["-a", "-F"], ["src/"], "dest/"),
+        Scenario("misc:cvs-C", t_cvs, ["-a", "-C"], ["src/"], "dest/"),
+        Scenario("misc:fuzzy", t_fuzzy, ["-a", "--fuzzy"], ["src/"], "dest/", pre_dest=pre_fuzzy),
+        Scenario("misc:write-batch", build_recvtree,
+                 (lambda wd, dest: ["-a", f"--write-batch={wd}/batch"]), ["src/"], "dest/"),
+        Scenario("misc:xattr", t_xattr, ["-aX"], ["src/"], "dest/"),
+        Scenario("misc:relative-dot", t_reldot, ["-aR"], ["src/./sub/deep/f"], "dest/"),
+    ]
+
+
+def daemon_sym_sweep():
+    """PUSH symlink/dirlink/keep-dirlinks scenarios to a daemon module -- the
+    daemon receiver/keep-dirlinks path. Enumerates the family: -K through a
+    symlinked dest dir (plain/nested/update/delete), and -L/-k/-l/--safe-links/
+    --munge-links/--copy-unsafe-links of in-tree and out-of-tree symlinks pushed
+    into a module."""
+    D = {"chroot": "no"}
+
+    def kt(src):
+        _mk_reg(src / "dir/f1", 10)
+        _mk_reg(src / "dir/f2", 10)
+        _mk_reg(src / "top", 5)
+
+    def kt_nested(src):
+        _mk_reg(src / "dir/sub/f", 10)
+        _mk_reg(src / "top", 5)
+
+    def kprep(dest):
+        dest.mkdir(parents=True, exist_ok=True)
+        (dest / "realdir").mkdir(exist_ok=True)
+        if not (dest / "dir").is_symlink():
+            os.symlink("realdir", dest / "dir")
+
+    def kprep_nested(dest):
+        dest.mkdir(parents=True, exist_ok=True)
+        (dest / "realdir" / "sub").mkdir(parents=True, exist_ok=True)
+        if not (dest / "dir").is_symlink():
+            os.symlink("realdir", dest / "dir")
+
+    def kprep_update(dest):
+        kprep(dest)
+        _mk_reg(dest / "realdir" / "f1", 3, t=T_OLD)
+
+    def kprep_delete(dest):
+        kprep(dest)
+        _mk_reg(dest / "realdir" / "extra", 3)
+
+    def src_dirlink(src):
+        _mk_reg(src / "rd/inner", 10)
+        os.symlink("rd", src / "dl")
+        _mk_reg(src / "anchor", 5)
+        _ut(src / "rd")
+
+    def src_abslink(src):
+        os.symlink("/etc/hostname", src / "abs")
+        _mk_reg(src / "anchor", 5)
+
+    rd = lambda d: d / "realdir"
+    return [
+        Scenario("dsym:K-dirlink", kt, ["-aK"], ["src/"], "dest/", dest_prep=kprep, snap_dest=rd, daemon=D),
+        Scenario("dsym:K-nested", kt_nested, ["-aK"], ["src/"], "dest/", dest_prep=kprep_nested, snap_dest=rd, daemon=D),
+        Scenario("dsym:K-update", kt, ["-aK"], ["src/"], "dest/", dest_prep=kprep_update, snap_dest=rd, daemon=D),
+        Scenario("dsym:K-delete", kt, ["-aK", "--delete"], ["src/"], "dest/", dest_prep=kprep_delete, snap_dest=rd, daemon=D),
+        Scenario("dsym:L-src-dirlink", src_dirlink, ["-aL"], ["src/"], "dest/", daemon=D),
+        Scenario("dsym:k-src-dirlink", src_dirlink, ["-ak"], ["src/"], "dest/", daemon=D),
+        Scenario("dsym:l-src", src_dirlink, ["-al"], ["src/"], "dest/", daemon=D),
+        Scenario("dsym:safe-links", src_abslink, ["-a", "--safe-links"], ["src/"], "dest/", daemon=D),
+        Scenario("dsym:munge", src_abslink, ["-a", "--munge-links"], ["src/"], "dest/", daemon=D),
+        Scenario("dsym:copy-unsafe", src_abslink, ["-a", "--copy-unsafe-links"], ["src/"], "dest/", daemon=D),
+    ]
+
+
+def daemon_escape_sweep():
+    """Daemon following symlinks that point OUTSIDE the module (absolute, or ../
+    escape), via -L / --copy-links / --copy-unsafe-links / --safe-links, on both
+    the sender (pull) and receiver (push) side -- the daemon symlink-safety
+    behaviour."""
+    Dpull = {"chroot": "no", "pull": True}
+    Dpush = {"chroot": "no"}
+
+    def s_abs(src):
+        os.symlink("/etc/hostname", src / "abslnk")
+        _mk_reg(src / "anchor", 5)
+
+    def s_escape(src):
+        _mk_reg(src.parent / "secret", 7)          # outside the module (wd/secret)
+        os.symlink("../secret", src / "esc")
+        _mk_reg(src / "anchor", 5)
+
+    def s_filelink(src):
+        _mk_reg(src / "real", 10)
+        os.symlink("real", src / "fl")             # in-tree symlink to a FILE
+        _mk_reg(src / "anchor", 5)
+
+    def s_absdir(src):
+        out = src.parent / "outdir"                # small out-of-module dir (wd/outdir)
+        _mk_reg(out / "x", 8)
+        _mk_reg(out / "y", 8)
+        os.symlink(str(out), src / "extdir")       # absolute symlink to out-of-module DIR
+        _mk_reg(src / "anchor", 5)
+
+    pull = [
+        ("dpull:L-abs", s_abs, ["-aL"]),
+        ("dpull:L-escape", s_escape, ["-aL"]),
+        ("dpull:copyunsafe-escape", s_escape, ["-a", "--copy-unsafe-links"]),
+        ("dpull:L-filelink", s_filelink, ["-aL"]),
+        ("dpull:safe-escape", s_escape, ["-a", "--safe-links"]),
+        ("dpull:copylinks-abs", s_abs, ["-a", "--copy-links"]),
+        ("dpull:L-absdir", s_absdir, ["-aL"]),
+        ("dpull:k-absdir", s_absdir, ["-ak"]),
+        ("dpull:copydirlinks-absdir", s_absdir, ["-a", "--copy-dirlinks"]),
+        ("dpull:copyunsafe-absdir", s_absdir, ["-a", "--copy-unsafe-links"]),
+    ]
+    push = [
+        ("dpush:L-abs", s_abs, ["-aL"]),
+        ("dpush:L-escape", s_escape, ["-aL"]),
+        ("dpush:copyunsafe-abs", s_abs, ["-a", "--copy-unsafe-links"]),
+        ("dpush:copyunsafe-escape", s_escape, ["-a", "--copy-unsafe-links"]),
+        ("dpush:L-filelink", s_filelink, ["-aL"]),
+    ]
+    s = [Scenario(n, fn, o, ["src/"], "dest/", daemon=Dpull) for n, fn, o in pull]
+    s += [Scenario(n, fn, o, ["src/"], "dest/", daemon=Dpush) for n, fn, o in push]
+    return s
+
+
+def daemon_pull_sym_sweep():
+    """PULL symlink/dirlink scenarios FROM a daemon module (the daemon SENDER
+    side -- untested until now). Served source contains the symlinks; the client
+    pulls with -L/-k/-l/-K/--safe-links/--munge-links/--copy-unsafe-links."""
+    D = {"chroot": "no", "pull": True}
+
+    def served_dirlink(src):
+        _mk_reg(src / "rd/inner", 10)
+        os.symlink("rd", src / "dl")
+        _mk_reg(src / "anchor", 5)
+        _ut(src / "rd")
+
+    def served_intree(src):
+        _mk_reg(src / "real", 10)
+        os.symlink("real", src / "lnk")
+        _mk_reg(src / "anchor", 5)
+
+    def served_abslink(src):
+        os.symlink("/etc/hostname", src / "abs")
+        _mk_reg(src / "anchor", 5)
+
+    def served_dir(src):
+        _mk_reg(src / "dir/f1", 10)
+        _mk_reg(src / "dir/f2", 10)
+        _mk_reg(src / "top", 5)
+
+    def kprep(dest):
+        dest.mkdir(parents=True, exist_ok=True)
+        (dest / "realdir").mkdir(exist_ok=True)
+        if not (dest / "dir").is_symlink():
+            os.symlink("realdir", dest / "dir")
+
+    return [
+        Scenario("dpull:plain", build_recvtree, ["-a"], ["src/"], "dest/", daemon=D),
+        Scenario("dpull:L-dirlink", served_dirlink, ["-aL"], ["src/"], "dest/", daemon=D),
+        Scenario("dpull:k-dirlink", served_dirlink, ["-ak"], ["src/"], "dest/", daemon=D),
+        Scenario("dpull:l", served_dirlink, ["-al"], ["src/"], "dest/", daemon=D),
+        Scenario("dpull:L-intree", served_intree, ["-aL"], ["src/"], "dest/", daemon=D),
+        Scenario("dpull:safe-links", served_abslink, ["-a", "--safe-links"], ["src/"], "dest/", daemon=D),
+        Scenario("dpull:munge", served_abslink, ["-a", "--munge-links"], ["src/"], "dest/", daemon=D),
+        Scenario("dpull:copy-unsafe", served_abslink, ["-a", "--copy-unsafe-links"], ["src/"], "dest/", daemon=D),
+        Scenario("dpull:K-dest-dirlink", served_dir, ["-aK"], ["src/"], "dest/", dest_prep=kprep, snap_dest=lambda d: d / "realdir", daemon=D),
+        Scenario("dpull:hardlinks", build_recvtree, ["-aH"], ["src/"], "dest/", daemon=D),
+    ]
+
+
+_COMBO_FLAGS = ["-H", "-S", "--inplace", "-z", "-c", "-b", "-O", "-J",
+                "--numeric-ids", "-A", "-X", "-E", "--no-whole-file", "-I",
+                "--size-only", "-u"]
+
+
+def combo3_sweep():
+    """Option TRIPLES over a stale dest -- deeper interaction coverage."""
+    import itertools
+    return [Scenario(f"combo3:{x},{y},{z}", build_recvtree, ["-a", x, y, z],
+                     ["src/"], "dest/", pre_dest=stale_dest)
+            for x, y, z in itertools.combinations(_COMBO_FLAGS, 3)]
+
+
+def combo4_sweep():
+    """Option QUADRUPLES over a stale dest (C(16,4)=1820)."""
+    import itertools
+    return [Scenario(f"combo4:{w},{x},{y},{z}", build_recvtree,
+                     ["-a", w, x, y, z], ["src/"], "dest/", pre_dest=stale_dest)
+            for w, x, y, z in itertools.combinations(_COMBO_FLAGS, 4)]
+
+
+def ssh_sweep():
+    """PUSH over a remote-shell split (support/lsh.sh) -- exercises the real
+    client+server processes / protocol / arg passing, and confirms whether the
+    in-tree-symlink regressions also manifest over the wire."""
+    def kt(src: Path):
+        _write(src / "dir/f1", b"n1\n")
+        _write(src / "dir/f2", b"n2\n")
+        _write(src / "top", b"t\n")
+
+    def kprep(dest: Path):
+        dest.mkdir(parents=True, exist_ok=True)
+        (dest / "realdir").mkdir(exist_ok=True)
+        if not (dest / "dir").is_symlink():
+            os.symlink("realdir", dest / "dir")
+
+    def sym_parent(src: Path):
+        _write(src / "real/sub/file", b"under symlinked parent\n")
+        os.symlink("real", src / "link")
+
+    return [
+        Scenario("ssh:push", build_recvtree, ["-a"], ["src/"], ssh=True),
+        Scenario("ssh:push-H", build_recvtree, ["-aH"], ["src/"], ssh=True),
+        Scenario("ssh:push-X", build_recvtree, ["-aX"], ["src/"], ssh=True),
+        Scenario("ssh:push-z", build_recvtree, ["-az"], ["src/"], ssh=True),
+        Scenario("ssh:push-update", build_recvtree, ["-a"], ["src/"],
+                 pre_dest=stale_dest, ssh=True),
+        Scenario("ssh:push-delete", build_recvtree, ["-a", "--delete"], ["src/"],
+                 pre_dest=stale_dest, ssh=True),
+        Scenario("ssh:push-protect-args", build_recvtree, ["-a", "-s"], ["src/"],
+                 ssh=True),
+        Scenario("ssh:K-symlinkdir", kt, ["-aK"], ["src/"], dest_prep=kprep,
+                 snap_dest=lambda d: d / "realdir", ssh=True),
+        Scenario("ssh:R-symlink-parent", sym_parent, ["-aR"],
+                 ["src/link/sub/file"], ssh=True),
+    ]
+
+
+def redo_sweep():
+    """Resume / redo state machine: a partial or corrupted prior dest forces the
+    delta + verify + resume path (inplace / append-verify / partial-dir, the
+    latter both relative AND absolute). Generalises the single-pass model -- the
+    'failed verification, update discarded' loop and the discard-path NULL-deref
+    both live here. Lifted across transports too."""
+    def big(src: Path):
+        _mk_reg(src / "f", 120000, t=T_NEW, fill=7)
+        _mk_reg(src / "keep", 200, t=T_NEW, fill=3)
+
+    def pre_truncated(dest: Path):        # a shorter prefix of f (older) -> extend
+        _mk_reg(dest / "f", 40000, t=T_OLD, fill=7)
+
+    def pre_corrupt(dest: Path):          # same size, WRONG content -> delta+verify
+        _mk_reg(dest / "f", 120000, t=T_OLD, fill=200)
+
+    def pre_abs_partial(dest: Path):      # corrupt dest + a stale leftover in an
+        pre_corrupt(dest)                 # ABSOLUTE partial-dir (delta-resume shape)
+        pdir = dest.parent / (dest.name + "_part")
+        pdir.mkdir(parents=True, exist_ok=True)
+        _mk_reg(pdir / "f", 60000, t=T_OLD, fill=7)
+
+    abs_part = lambda wd, dest: ["-a", "--no-whole-file", "--partial",
+                                 f"--partial-dir={Path(dest).parent}/{Path(dest).name}_part"]
+    s = [
+        Scenario("redo:inplace-corrupt", big, ["-a", "--inplace", "--no-whole-file"],
+                 ["src/"], "dest/", pre_dest=pre_corrupt),
+        Scenario("redo:append-verify", big, ["-a", "--append-verify"],
+                 ["src/"], "dest/", pre_dest=pre_truncated),
+        Scenario("redo:append", big, ["-a", "--append"],
+                 ["src/"], "dest/", pre_dest=pre_truncated),
+        Scenario("redo:partialdir-rel", big,
+                 ["-a", "--no-whole-file", "--partial", "--partial-dir=.part"],
+                 ["src/"], "dest/", pre_dest=pre_corrupt),
+        Scenario("redo:checksum-corrupt", big, ["-a", "-c", "--no-whole-file"],
+                 ["src/"], "dest/", pre_dest=pre_corrupt),
+    ]
+    s = lift_transports(s)                # resume path over the wire too
+    # absolute partial-dir uses callable opts (not liftable) -> keep local
+    s.append(Scenario("redo:partialdir-abs", big, abs_part, ["src/"], "dest/",
+                      pre_dest=pre_abs_partial))
+    return s
+
+
+def typetrans_sweep():
+    """Type-transition: the existing dest entry has a DIFFERENT type than the
+    source (file/dir/symlink/fifo), crossed with the selection options whose job
+    is to decide whether to replace it. Generalises 'update skips a file of a
+    different type'. The stale-dest sweeps only ever vary content, never type."""
+    def src_file(src: Path):
+        _mk_reg(src / "x", 60, t=T_NEW)
+        _mk_reg(src / "keep", 10, t=T_NEW)
+
+    def src_dir(src: Path):
+        _mk_reg(src / "x/inner", 30, t=T_NEW)
+        _mk_reg(src / "keep", 10, t=T_NEW)
+        _ut(src / "x")
+
+    def src_link(src: Path):
+        _mk_reg(src / "real", 30, t=T_NEW)
+        os.symlink("real", src / "x")
+        _mk_reg(src / "keep", 10, t=T_NEW)
+
+    # dest entries are pinned NEWER than the source (T_NEWER) so the -u
+    # update-skip path is actually exercised across the type change, and so the
+    # special-file mtimes are deterministic (unpinned fifos/symlinks otherwise
+    # carry wall-clock creation time -> spurious A/B mtime diffs).
+    T_NEWER = T_NEW + 10**7
+
+    def d_dir(dest: Path):
+        os.makedirs(dest / "x")
+        _mk_reg(dest / "x/old", 5, t=T_NEWER)
+        _ut(dest / "x", T_NEWER)
+
+    def d_file(dest: Path):
+        _mk_reg(dest / "x", 99, t=T_NEWER, fill=200)
+
+    def d_link(dest: Path):
+        os.symlink("keep", dest / "x")
+        os.utime(dest / "x", (T_NEWER, T_NEWER), follow_symlinks=False)
+
+    def d_fifo(dest: Path):
+        _mk_fifo(dest / "x")
+        os.utime(dest / "x", (T_NEWER, T_NEWER))
+
+    srcs = [("file", src_file), ("dir", src_dir), ("link", src_link)]
+    dests = [("Ddir", d_dir), ("Dfile", d_file), ("Dlink", d_link), ("Dfifo", d_fifo)]
+    opts = [("plain", ["-a"]), ("update", ["-a", "-u"]),
+            ("existing", ["-a", "--existing"]),
+            ("ignore-existing", ["-a", "--ignore-existing"]),
+            ("force", ["-a", "--force"]), ("delete", ["-a", "--delete"])]
+    s = []
+    for sn, sf in srcs:
+        for dn, df in dests:
+            for on, ov in opts:
+                # skip the same-type combo (file src vs file dest etc.)
+                if (sn, dn) in (("file", "Dfile"), ("dir", "Ddir"), ("link", "Dlink")):
+                    continue
+                s.append(Scenario(f"type:{sn}-vs-{dn}-{on}", sf, ov, ["src/"],
+                                  "dest/", pre_dest=df))
+    return lift_transports(s)
+
+
+def tsprec_sweep():
+    """Timestamp precision: sub-second mtimes at nanosecond boundaries, crossed
+    with the time-handling options. Exercises the nsec validate/convert path that
+    integer-second fixtures never reach (caught via rc/error/content/itemize --
+    snapshot records whole-second mtimes, so this is a code-path exerciser)."""
+    NS = [0, 1, 4999, 500000000, 999999999, 999999001]
+
+    def mk(src: Path, nsec):
+        _mk_reg(src / "f", 100)
+        os.utime(src / "f", ns=(T_NEW * 10**9 + nsec, T_NEW * 10**9 + nsec))
+        os.makedirs(src / "d", exist_ok=True)
+        _mk_reg(src / "d/g", 50)
+        os.utime(src / "d/g", ns=(T_NEW * 10**9 + nsec, T_NEW * 10**9 + nsec))
+
+    s = []
+    for nsec in NS:
+        s.append(Scenario(f"ts:nsec{nsec}", lambda src, n=nsec: mk(src, n),
+                          ["-a"], ["src/"], "dest/"))
+    s += [
+        Scenario("ts:modwindow", lambda src: mk(src, 999999999),
+                 ["-a", "--modify-window=1"], ["src/"], "dest/"),
+        Scenario("ts:atimes", lambda src: mk(src, 123456789),
+                 ["-a", "--atimes"], ["src/"], "dest/"),
+        Scenario("ts:crtimes", lambda src: mk(src, 123456789),
+                 ["-a", "--crtimes"], ["src/"], "dest/"),
+    ]
+    return s
+
+
+def bigscale_sweep():
+    """Scale escalation: many empty dirs / many files at SCALE_N (--scale). Pairs
+    with the --cost peak-RSS oracle -- a per-entry allocation/footprint regression
+    only shows up at scale and is invisible to functional outcome alone."""
+    n = max(1, SCALE_N)
+
+    def emptydirs(src: Path):
+        for i in range(n):
+            (src / f"d{i:07d}").mkdir()
+
+    def manyfiles(src: Path):
+        for i in range(n):
+            _write(src / f"b{i % 64:02d}" / f"f{i:07d}", b"x\n")
+
+    def deepdirs(src: Path):
+        for i in range(n):
+            (src / f"a{i % 50:02d}" / f"b{(i // 50) % 50:02d}" / f"c{i:07d}").mkdir(parents=True)
+
+    return [
+        Scenario("scale:emptydirs", emptydirs, ["-a"], ["src/"], "dest/"),
+        Scenario("scale:emptydirs-nir", emptydirs, ["-a", "--no-inc-recursive"],
+                 ["src/"], "dest/"),
+        Scenario("scale:manyfiles", manyfiles, ["-a"], ["src/"], "dest/"),
+        Scenario("scale:deepdirs", deepdirs, ["-a"], ["src/"], "dest/"),
+    ]
+
+
+def rrsync_sweep():
+    """rrsync lane: route the remote side through the restricted rrsync wrapper
+    (subdir-restricted, so its option/path validation is exercised) -- a whole
+    subsystem the other lanes never drive. Covers `-a` and an `-rlpt` control,
+    push and pull. rrsync ships per version, so each build is paired with its own
+    rrsync via --rrsync-a/--rrsync-b."""
+    bt = build_recvtree
+
+    def tree(src: Path):
+        _mk_reg(src / "dir/f1", 50)
+        _mk_reg(src / "dir/f2", 50)
+        _mk_reg(src / "top", 20)
+        os.symlink("top", src / "lnk")
+        _ut(src / "dir")
+
+    def abslink(src: Path):
+        _mk_reg(src / "anchor", 10)
+        os.symlink("/etc/hostname", src / "abs")
+
+    def mk(name, setup, opts, pull=False, pre_dest=None):
+        sc = Scenario(name, setup, opts, ["src/"], "dest/", pre_dest=pre_dest)
+        sc.rrsync = {"pull": pull}
+        return sc
+
+    return [
+        mk("rr:push-a", bt, ["-a"]),                       # D-bundled -> subdir deny decides
+        mk("rr:push-rlpt", tree, ["-rlpt"]),               # no-D control: should transfer
+        mk("rr:push-rlptD", tree, ["-rlptD"]),             # explicit D into restricted subdir
+        mk("rr:push-update", bt, ["-a"], pre_dest=stale_dest),
+        mk("rr:push-copy-unsafe", abslink, ["-rlpt", "--copy-unsafe-links"]),
+        mk("rr:pull-a", bt, ["-a"], pull=True),
+        mk("rr:pull-rlpt", tree, ["-rlpt"], pull=True),
+        mk("rr:pull-symlink", tree, ["-rlpt"], pull=True),
+    ]
+
+
+def tcpdaemon_sweep():
+    """Real-TCP-daemon lane: a genuine `rsync --daemon` on a bound port (greeting/
+    handshake/socket path), and an auth variant (challenge-response) -- the daemon
+    code the stdio-pipe lane bypasses. Push + pull, with/without auth."""
+    bt = build_recvtree
+
+    def mk(name, opts, pull=False, auth=False, pre_dest=None):
+        sc = Scenario(name, bt, opts, ["src/"], "dest/", pre_dest=pre_dest)
+        sc.daemon = {"tcp": True, "pull": pull, "auth": auth}
+        return sc
+
+    return [
+        mk("tcp:push", ["-a"]),
+        mk("tcp:push-H", ["-aH"]),
+        mk("tcp:push-X", ["-aX"]),
+        mk("tcp:push-update", ["-a"], pre_dest=stale_dest),
+        mk("tcp:push-delete", ["-a", "--delete"], pre_dest=stale_dest),
+        mk("tcp:pull", ["-a"], pull=True),
+        mk("tcp:pull-H", ["-aH"], pull=True),
+        mk("tcp:push-auth", ["-a"], auth=True),
+        mk("tcp:pull-auth", ["-a"], pull=True, auth=True),
+    ]
+
+
+SWEEPS = {"options": options_sweep, "pathshape": pathshape_sweep,
+          "recv": recv_sweep, "destshape": destshape_sweep,
+          "name": name_sweep, "filesfrom": filesfrom_sweep,
+          "intree": intree_sweep, "intree2": intree2_sweep,
+          "proto": proto_sweep, "combo": combo_sweep, "combo3": combo3_sweep,
+          "combo4": combo4_sweep, "scale": scale_sweep, "ssh": ssh_sweep,
+          "daemon": daemon_sweep, "daemonchroot": daemonchroot_sweep,
+          "mode": mode_sweep, "size": size_sweep, "filetype": filetype_sweep,
+          "selection": selection_sweep, "behavior": behavior_sweep,
+          "placement": placement_sweep, "wire": wire_sweep,
+          "pairwise": pairwise_sweep, "daemonsym": daemon_sym_sweep, "daemonpull": daemon_pull_sym_sweep, "daemonesc": daemon_escape_sweep, "misc": misc_sweep, "gaps": gaps_sweep,
+          "redo": redo_sweep, "typetrans": typetrans_sweep, "tsprec": tsprec_sweep,
+          "bigscale": bigscale_sweep, "rrsync": rrsync_sweep,
+          "tcpdaemon": tcpdaemon_sweep,
+          "priv": priv_sweep}
+# "all" excludes the root-only sweeps (priv, daemonchroot) and the very large
+# combo4; run those explicitly. Parallelism (-j) makes the broad benign set
+# (incl. the daemon symlink/escape + misc/gaps sweeps) affordable by default;
+# combo4 stays out of a single pass (the --loop ladder reaches order 4 anyway).
+# bigscale stays out of the default pass (heavy + meant for --cost runs); reach
+# it via --sweep bigscale --cost --scale N.
+ALL_SWEEPS = ["options", "pathshape", "recv", "destshape", "name", "filesfrom",
+              "intree", "intree2", "proto", "combo", "combo3", "scale", "ssh",
+              "daemon", "mode", "size", "filetype", "selection", "behavior",
+              "placement", "wire", "pairwise", "daemonsym", "daemonpull",
+              "daemonesc", "misc", "gaps", "redo", "typetrans", "tsprec",
+              "rrsync", "tcpdaemon"]
+
+
+def _compare(a, b, has_times=True, ign_types=(), incl_item=True, incl_lit=True,
+             incl_out=False, incl_err=False):
+    """Issue list for one result vs another (A-vs-B, and the per-binary stability
+    check). incl_item/incl_lit/incl_out/incl_err drop the itemize / Literal-data /
+    normalised-stdout / normalised-stderr-text signals (used when those are
+    themselves nondeterministic, or for content-only checks). Empty list ==
+    indistinguishable on the requested signals."""
+    issues = []
+    if a["rc"] != b["rc"]:
+        issues.append(f"  exit: A={a['rc']} B={b['rc']}")
+    a_err = any(m in a["err"] for m in ERR_MARKERS)
+    b_err = any(m in b["err"] for m in ERR_MARKERS)
+    if a_err != b_err:
+        issues.append(f"  stderr-error: A={a_err!r} B={b_err!r}\n     A:{a['err'][:300]}")
+    if incl_lit and a["lit"] != b["lit"]:
+        issues.append(f"  Literal-data: A={a['lit']} B={b['lit']}")
+    issues += diff_snapshots(a["snap"], b["snap"], ignore_mtime=not has_times,
+                             ignore_mtime_types=ign_types)
+    if incl_item and a["item"] != b["item"]:
+        issues.append("  itemize differs:\n     A=" + repr(a["item"])
+                      + "\n     B=" + repr(b["item"]))
+    if incl_err and a.get("errn", "") != b.get("errn", ""):
+        issues.append("  stderr-text differs:\n     A:" + a.get("errn", "")[:300]
+                      + "\n     B:" + b.get("errn", "")[:300])
+    if incl_out and a.get("out", "") != b.get("out", ""):
+        issues.append("  stdout differs:\n     A:" + a.get("out", "")[:300]
+                      + "\n     B:" + b.get("out", "")[:300])
+    return issues
+
+
+def run_scenario(scn: Scenario, workroot: Path):
+    wd = workroot / scn.name.replace("/", "_").replace(":", "_").replace("+", "_")
+    if wd.exists():
+        shutil.rmtree(wd, ignore_errors=True)
+    # opts may be a list, or a callable(wd, dest)->list for placement options
+    # whose aux dir is workdir/dest-relative (dest differs per A/B tag).  Resolve
+    # a representative copy (dest_A) for flag inspection; resolve per-tag below.
+    def resolve_opts(dest):
+        return scn.opts(str(wd), str(dest)) if callable(scn.opts) else scn.opts
+    opts0 = resolve_opts(wd / "dest_A")
+    # skip if either binary lacks an option (can't A/B compare)
+    for o in opts0:
+        flag = o.split("=", 1)[0]
+        if flag.startswith("--"):
+            if not (supports(RSYNC_A, flag) and supports(RSYNC_B, flag)):
+                return ("SKIP", f"a binary lacks {flag}")
+    # build the source ONCE so both binaries see identical input (incl. mtimes)
+    src = wd / "src"
+    src.mkdir(parents=True, exist_ok=True)
+    scn.setup(src)
+    src_args = scn.src_args(str(wd)) if callable(scn.src_args) else scn.src_args
+    # ignore mtime only when neither -t nor -a is in effect; some dest times are
+    # left unmanaged by rsync (-O dirs, -J links, -b backup) -> ignore those.
+    has_times = any(o in ("-a", "-t", "-rlptD", "--times") or
+                    (o.startswith("-") and not o.startswith("--") and "t" in o)
+                    for o in opts0)
+    ign_types = _ign(opts0)
+    # a "/"-rooted or absolute source (e.g. --files-from with absolute names)
+    # makes rsync create IMPLIED parent dirs it has no source time for -> their
+    # mtime is wall-clock and differs between the A and B runs. Ignore dir mtimes
+    # for those, like -O/backup dirs.
+    if isinstance(src_args, list) and any(a == "/" or a.startswith("/")
+                                          for a in src_args):
+        ign_types = set(ign_types) | {"d"}
+
+    def one_run(tag, binary, rep):
+        dest = wd / f"dest_{tag}{rep}"
+        optr = resolve_opts(dest)     # per-tag (aux dir is under this tag's dest)
+        if scn.dest_prep:
+            scn.dest_prep(dest)        # e.g. create dest as a symlink to a real dir
+        elif scn.pre_dest:
+            dest.mkdir(parents=True, exist_ok=True)
+            scn.pre_dest(dest)
+        _tls.measure = COST            # have sh() sample peak process-group RSS
+        _tls.rss = None
+        if scn.daemon is not None:
+            port = (20000 + (abs(hash(scn.name)) % 2000) * 16
+                    + (0 if tag == "A" else 8) + rep)
+            D = scn.daemon
+            if D.get("tcp"):           # real bound TCP port (not the stdio pipe)
+                rc, err, lit, item, out = _tcp_daemon(
+                    binary, str(wd), str(wd / "src") if D.get("pull") else str(dest),
+                    optr, src_args, localdest=str(dest), pull=D.get("pull", False),
+                    chroot=D.get("chroot", "no"), auth=D.get("auth", False))
+            elif D.get("pull"):
+                rc, err, lit, item, out = run_daemon_pull(
+                    binary, str(wd), str(wd / "src"), optr, str(dest), port,
+                    chroot=D.get("chroot", "no"))
+            else:
+                rc, err, lit, item, out = run_daemon_xfer(
+                    binary, str(wd), str(dest), optr, src_args, port,
+                    chroot=D.get("chroot", "no"))
+        elif scn.rrsync is not None:
+            rrs = (RRSYNC_A if tag == "A" else RRSYNC_B) or str(_RRSYNC_SRC)
+            if scn.rrsync.get("pull"):
+                rc, err, lit, item, out = run_rrsync_pull(
+                    binary, rrs, str(wd), str(wd / "src"), optr, str(dest))
+            else:
+                rc, err, lit, item, out = run_rrsync_push(
+                    binary, rrs, str(wd), optr, src_args, str(dest))
+        elif scn.ssh:
+            rc, err, lit, item, out = run_ssh_xfer(binary, str(wd), optr,
+                                                   src_args, str(dest))
+        else:
+            dest_arg = scn.dest_arg(dest) if scn.dest_arg else str(dest) + "/"
+            rc, err, lit, item, out = run_xfer(binary, str(wd), optr, src_args,
+                                               dest_arg)
+        _tls.measure = False
+        snap_target = scn.snap_dest(dest) if scn.snap_dest else dest
+        return dict(rc=rc, err=err, lit=lit, item=item, snap=snapshot(snap_target),
+                    out=_norm_out(out, wd, dest), errn=_norm_err(err, wd, dest),
+                    rss=getattr(_tls, "rss", None))
+
+    # STABILITY GATE: run each binary REPEAT times (cheap), and if a candidate
+    # A/B diff appears, ESCALATE to more samples and require the diff to be stable
+    # across ALL of them. A binary whose own runs disagree (or a diff that doesn't
+    # reproduce) is nondeterministic -> quarantine FLAKY, never a false regression.
+    # (A ~50% flake fools 2 repeats too often; escalation makes false DIFFs rare.)
+    # CONTENT-level instability within one binary's repeats = real nondeterminism
+    # (rc / error / dest content+existence; mtime, itemize, Literal-data excluded
+    # -- those carry incidental wall-clock/dir-time variance, not a correctness
+    # flake).  This is what makes an A/B diff untrustworthy -> quarantine FLAKY.
+    def content_unstable(rs):
+        for other in rs[1:]:
+            d = _compare(rs[0], other, has_times=False, ign_types=set(),
+                         incl_item=False, incl_lit=False)
+            if d:
+                return d
+        return None
+
+    def attr_stable(rs, k):
+        return all(rs[0][k] == r[k] for r in rs[1:])
+
+    base = max(1, REPEAT)
+    ra = [one_run("A", RSYNC_A, r) for r in range(base)]
+    rb = [one_run("B", RSYNC_B, r) for r in range(base)]
+    item_ok = lit_ok = out_ok = err_ok = True
+    if base >= 2:
+        # escalate sampling when a candidate A/B diff appears (on ANY signal),
+        # to confirm stability before trusting it
+        if _compare(ra[0], rb[0], has_times, ign_types, incl_out=True, incl_err=True):
+            confirm = max(base, 5)
+            ra += [one_run("A", RSYNC_A, r) for r in range(base, confirm)]
+            rb += [one_run("B", RSYNC_B, r) for r in range(base, confirm)]
+        cu = content_unstable(ra) or content_unstable(rb)
+        if cu:
+            if not KEEP:
+                shutil.rmtree(wd, ignore_errors=True)
+            return ("FLAKY", ["  scenario content is nondeterministic across "
+                              "repeats (quarantined, not a regression):"] + cu[:6])
+        # itemize / Literal-data / stdout / stderr-text are kept as A/B signals
+        # only if each is stable per binary (else incidental run-variance noise)
+        item_ok = attr_stable(ra, "item") and attr_stable(rb, "item")
+        lit_ok = attr_stable(ra, "lit") and attr_stable(rb, "lit")
+        out_ok = attr_stable(ra, "out") and attr_stable(rb, "out")
+        err_ok = attr_stable(ra, "errn") and attr_stable(rb, "errn")
+    a, b = ra[0], rb[0]
+    # When BOTH builds error on this (often edge) input, neither produced a clean
+    # benign transfer; the exact stderr/stdout/itemize/literal wording is low
+    # signal (different errno/message for the same failure). A real regression --
+    # A worse than B -- still shows in the exit code and the dest tree, which stay
+    # compared. Drop the free-text/itemize signals in that case.
+    both_failed = (any(m in a["err"] for m in ERR_MARKERS)
+                   and any(m in b["err"] for m in ERR_MARKERS))
+    issues = _compare(a, b, has_times, ign_types,
+                      incl_item=item_ok and not both_failed,
+                      incl_lit=lit_ok and not both_failed,
+                      incl_out=out_ok and not both_failed,
+                      incl_err=err_ok and not both_failed)
+    # COST oracle: directional peak-RSS blow-up. Require the gap to hold across
+    # ALL samples (min A vs max B) so run-to-run RSS noise can't trip it; only a
+    # gross ratio + absolute floor counts -- a real resource regression, not jitter.
+    if COST:
+        arss = [r["rss"] for r in ra if r.get("rss")]
+        brss = [r["rss"] for r in rb if r.get("rss")]
+        if arss and brss:
+            amin, bmax = min(arss), max(brss)
+            if amin > 3 * bmax and (amin - bmax) > 64 * 1024 * 1024:
+                issues.append(f"  peak-RSS blow-up: A>={amin // (1<<20)}MB "
+                              f"B<={bmax // (1<<20)}MB ({amin / bmax:.1f}x)")
+    if not (KEEP or issues):
+        shutil.rmtree(wd, ignore_errors=True)
+    if not issues:
+        return ("OK", issues)
+    # Classify: if A's only divergence is an INTENTIONAL refusal that B did not
+    # do (an entry in ALLOWLIST), mark ALLOW -- a documented behaviour change,
+    # recorded separately, not a silent regression.
+    for sub, note in ALLOWLIST:
+        if sub in a["err"] and sub not in b["err"]:
+            return ("ALLOW", [f"  intentional behaviour change: {note}",
+                              f"     A:{a['err'][:200]}"] )
+    # Direction matters for "regression": only A being WORSE than B counts.
+    a_ok = a["rc"] == 0 and not any(m in a["err"] for m in ERR_MARKERS)
+    b_ok = b["rc"] == 0 and not any(m in b["err"] for m in ERR_MARKERS)
+    if a_ok and not b_ok:
+        return ("ABETTER", ["  A succeeds where B FAILS -- an "
+                            "improvement/behaviour change, not a regression:",
+                            f"     B:{b['err'][:200]}"])
+    return ("DIFF", issues)
+
+
+_mport = [25500]
+
+
+def _ssh1(cbin, sbin, wd, opts, src, dest, pull):
+    e = ["-e", f"sh {_LSH}", f"--rsync-path={sbin}"]
+    if pull:   # remote side (sbin) is the SENDER
+        argv = [cbin, "--stats", "-i", *e, *opts, f"lh:{src}/", f"{dest}/"]
+    else:      # client (cbin) is the SENDER
+        argv = [cbin, "--stats", "-i", *e, *opts, f"{src}/", f"lh:{dest}/"]
+    return _parse_out(sh(argv, cwd=str(wd)))
+
+
+def _daemon1(cbin, sbin, wd, module, opts, localdir, pull):
+    _mport[0] += 1
+    port = _mport[0]
+    conf = Path(wd) / f"d{port}.conf"
+    Path(module).mkdir(parents=True, exist_ok=True)
+    conf.write_text(f"use chroot = no\nport = {port}\nlog file = {wd}/d{port}.log\n"
+                    f"pid file = {wd}/d{port}.pid\n[m]\n  path = {module}\n"
+                    f"  read only = no\n  hosts allow = 127.0.0.1\n")
+    proc = subprocess.Popen([sbin, "--daemon", "--no-detach", f"--config={conf}",
+                             f"--port={port}", "--address=127.0.0.1"],
+                            stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+    try:
+        if not _wait_port(port):
+            return (99, "daemon failed to start", None, "")
+        url = f"rsync://127.0.0.1:{port}/m/"
+        if pull:   # daemon (sbin) is the SENDER
+            argv = [cbin, "--stats", "-i", *opts, url, f"{localdir}/"]
+        else:      # client (cbin) is the SENDER
+            argv = [cbin, "--stats", "-i", *opts, f"{localdir}/", url]
+        return _parse_out(sh(argv, cwd=str(wd)))
+    finally:
+        proc.terminate()
+        try:
+            proc.wait(timeout=5)
+        except subprocess.TimeoutExpired:
+            proc.kill()
+
+
+def run_matrix(workroot, logf):
+    """Cross-build role matrix: mixed A<->B over the wire, both directions
+    (push/pull => which build is sender vs receiver), both transports
+    (ssh/daemon).  Each config's dest is compared to the PURE-B baseline; an
+    A-involved config that is WORSE than pure-B is a regression (protocol/interop
+    or role-specific)."""
+    A, B = RSYNC_A, RSYNC_B
+    scns = [("basic", build_recvtree, ["-a"]),
+            ("H", build_recvtree, ["-aH"]),
+            ("X", build_recvtree, ["-aX"]),
+            ("z", build_recvtree, ["-az"]),
+            ("c", build_recvtree, ["-ac"])]
+    combos = [("Ac_As", A, A), ("Ac_Bs", A, B), ("Bc_As", B, A)]
+    nreg = 0
+
+    def one(cbin, sbin, wd, tag, transport, direction, opts):
+        dest = Path(wd) / f"d_{tag}"
+        if transport == "ssh":
+            rc, err, lit, item, _out = _ssh1(cbin, sbin, wd, opts, Path(wd) / "src",
+                                             dest, pull=(direction == "pull"))
+        else:
+            if direction == "pull":
+                rc, err, lit, item, _out = _daemon1(cbin, sbin, wd, Path(wd) / "src",
+                                                    opts, dest, pull=True)
+            else:
+                rc, err, lit, item, _out = _daemon1(cbin, sbin, wd, dest, opts,
+                                                    Path(wd) / "src", pull=False)
+        return rc, err, snapshot(dest)
+
+    for name, setup, opts in scns:
+        for transport in ("ssh", "daemon"):
+            for direction in ("push", "pull"):
+                wd = workroot / f"mx_{name}_{transport}_{direction}"
+                shutil.rmtree(wd, ignore_errors=True)
+                (wd / "src").mkdir(parents=True)
+                setup(wd / "src")
+                brc, berr, bsnap = one(B, B, wd, "base", transport, direction, opts)
+                bok = brc == 0 and not any(m in berr for m in ERR_MARKERS)
+                for tag, cv, sv in combos:
+                    rc, err, snap = one(cv, sv, wd, tag, transport, direction, opts)
+                    ok = rc == 0 and not any(m in err for m in ERR_MARKERS)
+                    diffs = diff_snapshots(snap, bsnap)
+                    label = f"matrix:{name}/{transport}/{direction}/{tag}"
+                    if not diffs and ok == bok:
+                        print(f"OK    {label}")
+                        continue
+                    if ok and not bok:
+                        print(f"BETTER {label} (A cfg ok; pure-B failed)")
+                        continue
+                    nreg += 1
+                    kind = "exit/err" if ok != bok else "tree"
+                    print(f"DIFF  {label}  [{kind}; client={cv.split('/')[-1]} "
+                          f"server={sv.split('/')[-1]}]")
+                    detail = ([f"  exit/err: cfg ok={ok} (rc={rc}) base ok={bok}",
+                               f"     err:{err[:200]}"] if ok != bok else []) + diffs[:8]
+                    for ln in detail:
+                        print(ln)
+                    logf.write(f"\nMATRIX-DIFF {label}  opts:{' '.join(opts)} "
+                               f"vs pure-B  *** REGRESSION CANDIDATE ***\n")
+                    for ln in detail:
+                        logf.write(ln + "\n")
+                    logf.flush()
+    print(f"\n=== matrix: {nreg} cross-version regression candidates ===")
+    return nreg
+
+
+def _fx_hardlinks(src):
+    for i in range(6):
+        _write(src / f"f{i}", f"data{i % 3}\n".encode())
+    for i in range(6):
+        try:
+            os.link(src / f"f{i % 3}", src / f"hl{i}")
+        except OSError:
+            pass
+    os.symlink("f0", src / "sl")
+
+
+def _fx_weird(src):
+    for n in ["a space", "café", "semi;colon", "dollar$x", "paren(s)", "-dash",
+              "tab\tt"]:
+        _write(src / n, b"x\n")
+
+
+def _fx_deep(src):
+    p = src
+    for i in range(25):
+        p = p / f"d{i}"
+    _write(p / "leaf", b"deep\n")
+    _write(src / "top", b"t\n")
+
+
+def _fx_sparse(src):
+    with open(src / "sp.bin", "wb") as f:
+        f.seek(2 << 20)
+        f.write(b"end")
+    _write(src / "reg", b"r\n")
+
+
+def _fx_many(src):
+    for i in range(200):
+        _write(src / f"d{i % 8}" / f"f{i:03d}", f"{i}\n".encode())
+
+
+FUZZ_FIXTURES = [("recvtree", build_recvtree), ("kitchen", build_kitchen),
+                 ("hardlinks", _fx_hardlinks), ("weird", _fx_weird),
+                 ("deep", _fx_deep), ("sparse", _fx_sparse), ("many", _fx_many)]
+FUZZ_FLAGS = ["-H", "-S", "--inplace", "-z", "-c", "-b", "-O", "-J",
+              "--numeric-ids", "-A", "-X", "-E", "--no-whole-file", "-I",
+              "--size-only", "-u", "-k", "-K", "-L", "--copy-unsafe-links",
+              "--safe-links", "--munge-links", "--delete", "--existing",
+              "--ignore-existing", "--max-size=100000", "--compress-choice=zstd",
+              "--no-inc-recursive", "--checksum-choice=md5"]
+
+
+def _perturb(src, dest):
+    """dest = an older/partial copy of src (so update/backup/delete/-u/-I bite)."""
+    shutil.copytree(src, dest, symlinks=True)
+    for p in sorted(dest.rglob("*")):
+        if p.is_file() and not p.is_symlink():
+            p.write_bytes(b"OLDER CONTENT\n")
+            os.utime(p, (T_OLD, T_OLD))
+            break
+    (dest / "_obsolete").write_text("x\n")
+    os.utime(dest / "_obsolete", (T_OLD, T_OLD))
+
+
+def _fuzz_run(sndr, rcvr, transport, direction, wd, tag, opts, src):
+    dest = wd / f"d_{tag}"
+    _perturb(src, dest)
+    if transport == "ssh":
+        if direction == "push":
+            rc, err, _, _, _ = _ssh1(sndr, rcvr, wd, opts, src, dest, pull=False)
+        else:
+            rc, err, _, _, _ = _ssh1(rcvr, sndr, wd, opts, src, dest, pull=True)
+    else:  # daemon
+        if direction == "push":
+            rc, err, _, _, _ = _daemon1(sndr, rcvr, wd, dest, opts, src, pull=False)
+        else:
+            rc, err, _, _, _ = _daemon1(rcvr, sndr, wd, src, opts, dest, pull=True)
+    return rc, err, snapshot(dest)
+
+
+def run_fuzz(workroot, logf, n, seed):
+    """Stochastic differential fuzzer: random fixture x option-subset x transport
+    x direction x build-pair, each compared to the pure-B baseline for the same
+    config. Finds cross-build / option-interaction regressions broadly."""
+    import random
+    rnd = random.Random(seed)
+    A, B = RSYNC_A, RSYNC_B
+    seen = set()
+    nreg = nrun = 0
+    print(f"fuzz: {n} iterations, seed={seed}")
+    for i in range(n):
+        fxname, fx = rnd.choice(FUZZ_FIXTURES)
+        flags = sorted(rnd.sample(FUZZ_FLAGS, rnd.randint(0, 4)))
+        opts = ["-a"] + flags
+        transport = rnd.choice(["ssh", "daemon"])
+        direction = rnd.choice(["push", "pull"])
+        sndr, rcvr = rnd.choice([(A, A), (A, B), (B, A)])
+        # skip option unsupported by either binary
+        if any(o.startswith("--") and not (supports(A, o.split("=")[0])
+               and supports(B, o.split("=")[0])) for o in opts):
+            continue
+        wd = workroot / f"fz{i}"
+        shutil.rmtree(wd, ignore_errors=True)
+        (wd / "src").mkdir(parents=True)
+        fx(wd / "src")
+        try:
+            brc, berr, bsnap = _fuzz_run(B, B, transport, direction, wd, "base",
+                                         opts, wd / "src")
+            crc, cerr, csnap = _fuzz_run(sndr, rcvr, transport, direction, wd,
+                                         "cfg", opts, wd / "src")
+        except Exception as ex:
+            continue
+        nrun += 1
+        bok = brc == 0 and not any(m in berr for m in ERR_MARKERS)
+        cok = crc == 0 and not any(m in cerr for m in ERR_MARKERS)
+        diffs = diff_snapshots(csnap, bsnap, ignore_mtime_types=_ign(opts))
+        if not diffs and cok == bok:
+            if not KEEP:
+                shutil.rmtree(wd, ignore_errors=True)
+            continue
+        if cok and not bok:        # config better than pure-B
+            if not KEEP:
+                shutil.rmtree(wd, ignore_errors=True)
+            continue
+        which = f"sndr={'A' if sndr==A else 'B'} rcvr={'A' if rcvr==A else 'B'}"
+        sig = (fxname, tuple(flags), transport, direction,
+               "exit" if cok != bok else "tree")
+        if sig in seen:
+            if not KEEP:
+                shutil.rmtree(wd, ignore_errors=True)
+            continue
+        seen.add(sig)
+        nreg += 1
+        label = f"fuzz:{fxname}/{transport}/{direction}/[{' '.join(opts)}]/{which}"
+        print(f"DIFF  {label}")
+        detail = ([f"  exit: cfg rc={crc}(ok={cok}) base rc={brc}(ok={bok})",
+                   f"     err:{cerr[:200]}"] if cok != bok else []) + diffs[:8]
+        for ln in detail:
+            print(ln)
+        logf.write(f"\nFUZZ-DIFF {label}  *** REGRESSION CANDIDATE ***\n")
+        for ln in detail:
+            logf.write(ln + "\n")
+        logf.flush()
+    print(f"\n=== fuzz: {nrun} configs run, {nreg} distinct regression candidates ===")
+    return nreg
+
+
+def _ign(opts):
+    s = set()
+    if "--omit-dir-times" in opts or any(o[:1] == "-" and o[1:2] != "-" and "O" in o for o in opts):
+        s.add("d")
+    if "--omit-link-times" in opts or any(o[:1] == "-" and o[1:2] != "-" and "J" in o for o in opts):
+        s.add("l")
+    if "--backup" in opts or any(o[:1] == "-" and o[1:2] != "-" and "b" in o for o in opts):
+        s.add("d")
+    # aux/implied dirs (temp/partial/backup, and --mkpath's created parents) sit
+    # in the dest tree but get no source time -> their mtime is wall-clock and
+    # differs between the A and B runs.
+    if any(o.startswith(("--temp-dir", "--partial-dir", "--backup-dir",
+                         "--partial", "--mkpath"))
+           for o in opts):
+        s.add("d")
+    return s
+
+
+# ---------------------------------------------------------------------------
+# --loop: infinite scenario generators (random novel combos + systematic ladder)
+
+# extra flags worth randomizing beyond the combo set (symlink / selection / wire)
+_RAND_FLAGS = _COMBO_FLAGS + ["-k", "-K", "-L", "-l", "--copy-unsafe-links",
+                              "--safe-links", "--munge-links", "--delete",
+                              "--existing", "--ignore-existing",
+                              "--no-inc-recursive", "--compress-choice=zstd",
+                              "--checksum-choice=md5", "--sparse", "--fuzzy"]
+
+
+def _random_scenarios(rnd, seen):
+    """Infinite stream of randomized benign A/B scenarios: random fixture x random
+    option subset (size 2-6), optional stale dest. Deduped by signature, fed
+    through the same A/B oracle as the fixed sweeps."""
+    idx = 0
+    while True:
+        fxname, fx = rnd.choice(FUZZ_FIXTURES)
+        k = rnd.randint(2, 6)
+        flags = tuple(sorted(rnd.sample(_RAND_FLAGS, k)))
+        stale = fxname == "recvtree" and rnd.random() < 0.5
+        sig = ("rand", fxname, flags, stale)
+        if sig in seen:
+            continue
+        seen.add(sig)
+        idx += 1
+        nm = (f"rand{idx}:{fxname}:" + "_".join(f.lstrip("-") for f in flags)
+              + ("+stale" if stale else ""))
+        yield Scenario(nm, fx, ["-a", *flags], ["src/"], "dest/",
+                       pre_dest=(stale_dest if stale else None))
+
+
+def _systematic_combos(seen):
+    """Infinite stream walking the option-combination ladder combo2->3->4->...
+    over a stale dest; when the top order is exhausted the ladder restarts."""
+    k = 2
+    while True:
+        for combo in itertools.combinations(_COMBO_FLAGS, k):
+            sig = ("sys", combo)
+            if sig in seen:
+                continue
+            seen.add(sig)
+            yield Scenario(f"sys{k}:" + ",".join(combo), build_recvtree,
+                           ["-a", *combo], ["src/"], "dest/", pre_dest=stale_dest)
+        k += 1
+        if k > len(_COMBO_FLAGS):       # exhausted every order -> restart ladder
+            for s in [s for s in seen if s and s[0] == "sys"]:
+                seen.discard(s)
+            k = 2
+
+
+def _mixed_scenarios(rnd, seen):
+    """Alternate random / systematic so a parallel pool runs ~half of each."""
+    rg = _random_scenarios(rnd, seen)
+    sg = _systematic_combos(seen)
+    while True:
+        yield next(rg)
+        yield next(sg)
+
+
+class _Tee:
+    """Minimal write/flush fan-out so matrix/fuzz diffs land in both the curated
+    findings log and the full per-run log."""
+    def __init__(self, *files):
+        self._f = files
+
+    def write(self, s):
+        for f in self._f:
+            f.write(s)
+
+    def flush(self):
+        for f in self._f:
+            f.flush()
+
+
+def main():
+    global RSYNC_A, RSYNC_B, RRSYNC_A, RRSYNC_B, KEEP, REPEAT, CMD_TIMEOUT, COST, SCALE_N
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--rsync-a", default="./rsync")
+    ap.add_argument("--rsync-b", default="old_versions/rsync_3.4.1")
+    ap.add_argument("--rrsync-a", default=None,
+                    help="rrsync wrapper script paired with A for the rrsync lane "
+                         "(default: in-tree support/rrsync)")
+    ap.add_argument("--rrsync-b", default=None,
+                    help="rrsync wrapper paired with B (e.g. a baseline version's "
+                         "rrsync); rrsync regressions live in the script, so this "
+                         "must match B's version to A/B the rrsync lane")
+    ap.add_argument("--sweep", default="all",
+                    choices=["options", "pathshape", "recv", "destshape",
+                             "name", "filesfrom", "intree", "intree2", "proto",
+                             "combo", "combo3", "combo4", "scale", "ssh",
+                             "daemon", "daemonchroot", "mode", "size",
+                             "filetype", "selection", "behavior", "placement",
+                             "wire", "pairwise", "daemonsym", "daemonpull",
+                             "daemonesc", "misc", "gaps", "redo", "typetrans",
+                             "tsprec", "bigscale", "rrsync", "tcpdaemon",
+                             "priv", "all"])
+    ap.add_argument("--workdir", default="/tmp/abdiff")
+    ap.add_argument("--findings", default="abdiff-findings.txt")
+    ap.add_argument("--only", default=None, help="run only scenarios containing this substring")
+    ap.add_argument("--keep", action="store_true")
+    ap.add_argument("--list", action="store_true")
+    ap.add_argument("--matrix", action="store_true",
+                    help="run the cross-build role matrix (mixed A<->B, "
+                         "push+pull, ssh+daemon) instead of the sweeps")
+    ap.add_argument("--fuzz", type=int, default=0, metavar="N",
+                    help="stochastic differential fuzzer: N random "
+                         "fixture/option/transport/direction/version-pair configs")
+    ap.add_argument("--seed", type=int, default=1, help="fuzz RNG seed")
+    ap.add_argument("--repeat", type=int, default=REPEAT, metavar="N",
+                    help="stability gate: run each binary N times per scenario; "
+                         "scenarios whose own runs disagree are quarantined FLAKY "
+                         "(default 2; use 1 to disable, 3+ to catch rarer flakes)")
+    ap.add_argument("-j", "--jobs", type=int, default=20, metavar="N",
+                    help="run N scenarios in parallel (default 20)")
+    ap.add_argument("--loop", action="store_true",
+                    help="after the fixed sweeps, keep generating new randomized + "
+                         "higher-order-combo scenarios indefinitely (Ctrl-C to stop)")
+    ap.add_argument("--cmd-timeout", type=int, default=CMD_TIMEOUT, metavar="SECS",
+                    help="per-rsync wall-clock timeout; 0 disables (default 120)")
+    ap.add_argument("--log-dir", default=".",
+                    help="directory for the full per-run abdiff-log_<TIME>.txt "
+                         "(default: current directory)")
+    ap.add_argument("--cost", action="store_true",
+                    help="also compare peak process-group RSS (resource oracle); "
+                         "flags only gross, stable A-worse blow-ups. Pair with "
+                         "--sweep bigscale --scale N.")
+    ap.add_argument("--scale", type=int, default=SCALE_N, metavar="N",
+                    help=f"element count for the bigscale fixtures (default {SCALE_N}; "
+                         "use e.g. 100000 with --cost to surface footprint regressions)")
+    ap.add_argument("--timelimit", type=float, default=0, metavar="SECS",
+                    help="stop after SECS seconds (0 = no limit); in --loop it ends "
+                         "the loop, in a finite sweep it stops queuing new scenarios. "
+                         "In-flight scenarios finish, then a summary is written.")
+    ap.add_argument("--root-extra", dest="root_extra", action="store_true",
+                    default=None,
+                    help="with --sweep all, also include the root-only sweeps "
+                         "(priv, daemonchroot); auto-on when running as root")
+    args = ap.parse_args()
+    RSYNC_A = os.path.abspath(args.rsync_a)
+    RSYNC_B = os.path.abspath(args.rsync_b)
+    RRSYNC_A = os.path.abspath(args.rrsync_a) if args.rrsync_a else None
+    RRSYNC_B = os.path.abspath(args.rrsync_b) if args.rrsync_b else None
+    KEEP = args.keep
+    REPEAT = args.repeat
+    CMD_TIMEOUT = args.cmd_timeout
+    COST = args.cost
+    SCALE_N = args.scale
+
+    if args.matrix or args.fuzz:
+        workroot = Path(args.workdir)
+        workroot.mkdir(parents=True, exist_ok=True)
+        fp = Path(args.findings)
+        fp.parent.mkdir(parents=True, exist_ok=True)
+        logf = open(fp, "a", buffering=1)
+        ld = Path(args.log_dir)
+        ld.mkdir(parents=True, exist_ok=True)
+        runlog_path = ld / f"abdiff-log_{time.strftime('%Y%m%d_%H%M%S')}.txt"
+        runlog = open(runlog_path, "a", buffering=1)
+        mode = "FUZZ" if args.fuzz else "MATRIX"
+        header = (f"\n===== abdiff {mode} {time.strftime('%Y-%m-%d %H:%M:%S')} "
+                  f"A={RSYNC_A} B={RSYNC_B} =====\n")
+        logf.write(header)
+        runlog.write(header)
+        print(f"A (under test): {RSYNC_A}\nB (baseline): {RSYNC_B}\n"
+              f"full log: {runlog_path}\n")
+        tee = _Tee(logf, runlog)
+        n = (run_fuzz(workroot, tee, args.fuzz, args.seed) if args.fuzz
+             else run_matrix(workroot, tee))
+        logf.close()
+        runlog.close()
+        return 1 if n else 0
+
+    sweeps = ALL_SWEEPS if args.sweep == "all" else [args.sweep]
+    # as root (or --root-extra), fold the root-only sweeps into an "all" run so a
+    # privileged session exercises owner/device/specials/fake-super + chroot daemon.
+    root_extra = args.root_extra if args.root_extra is not None else (os.geteuid() == 0)
+    if args.sweep == "all" and root_extra:
+        sweeps = sweeps + ["priv", "daemonchroot"]
+        if COST:
+            sweeps = sweeps + ["bigscale"]
+    scns = []
+    for s in sweeps:
+        scns += SWEEPS[s]()
+    if args.only:
+        scns = [s for s in scns if args.only in s.name]
+    if args.list:
+        for s in scns:
+            print(s.name, s.opts)
+        return 0
+
+    print(f"A (under test): {RSYNC_A}  ({sh([RSYNC_A,'--version']).stdout.splitlines()[0] if sh([RSYNC_A,'--version']).returncode==0 else '?'})")
+    print(f"B (baseline):   {RSYNC_B}  ({sh([RSYNC_B,'--version']).stdout.splitlines()[0]})")
+
+    # absolute: scenarios run rsync with cwd=workdir and also pass dest/aux paths
+    # under it, so a relative --workdir would double-resolve (rsync writes to
+    # wd/wd/...) and every scenario would go silently vacuous.
+    workroot = Path(os.path.abspath(args.workdir))
+    workroot.mkdir(parents=True, exist_ok=True)
+    # Curated, cross-run findings log: open once, write a run header, and flush
+    # each anomaly the moment it is found so the log is a live record mid-run.
+    fp = Path(args.findings)
+    fp.parent.mkdir(parents=True, exist_ok=True)
+    logf = open(fp, "a", buffering=1)  # line-buffered
+    logf.write(f"\n===== abdiff run {time.strftime('%Y-%m-%d %H:%M:%S')} "
+               f"sweep={args.sweep} A={RSYNC_A} B={RSYNC_B} jobs={args.jobs}"
+               f"{' loop' if args.loop else ''} =====\n")
+    logf.flush()
+    # Per-run findings log: fresh timestamped file in the current dir (or
+    # --log-dir). Holds ONLY findings (DIFF/TIMEOUT/ERROR/FLAKY/ALLOW/BETTER) --
+    # no OK/SKIP noise; stdout shows a live "test N" counter instead.
+    ld = Path(args.log_dir)
+    ld.mkdir(parents=True, exist_ok=True)
+    runlog_path = ld / f"abdiff-log_{time.strftime('%Y%m%d_%H%M%S')}.txt"
+    runlog = open(runlog_path, "a", buffering=1)
+    runlog.write(f"# abdiff findings  {time.strftime('%Y-%m-%d %H:%M:%S')}\n"
+                 f"# A={RSYNC_A}\n# B={RSYNC_B}\n"
+                 f"# sweep={args.sweep} jobs={args.jobs} repeat={REPEAT} "
+                 f"loop={args.loop} cmd_timeout={CMD_TIMEOUT}\n")
+    runlog.flush()
+    print(f"findings log: {runlog_path}\njobs: {args.jobs}"
+          f"{'  (loop: Ctrl-C to stop)' if args.loop else ''}")
+
+    def ostr(s):  # opts may be callable (placement scenarios)
+        return ' '.join(s.opts) if not callable(s.opts) else '(dynamic)'
+
+    LABEL = {"OK": "OK   ", "SKIP": "SKIP ", "FLAKY": "FLAKY", "ABETTER": "BETTER",
+             "ALLOW": "ALLOW", "DIFF": "DIFF ", "TIMEOUT": "TIME ", "ERROR": "ERROR"}
+    counts = {k: 0 for k in LABEL}
+    done = [0]
+    rec_lock = threading.Lock()
+
+    def progress(total=None):
+        sys.stdout.write(f"test {done[0]}{('/' + str(total)) if total else ''}"
+                         f"{('  ' + str(counts['DIFF']) + ' DIFF') if counts['DIFF'] else ''}\r")
+        sys.stdout.flush()
+
+    def record(s, status, info, total=None):
+        if isinstance(info, str):
+            info = [info]
+        # a subprocess timeout surfaces as a DIFF whose err carries the marker;
+        # promote it to its own TIMEOUT class so it's easy to triage.
+        if status == "DIFF" and any("[abdiff: TIMEOUT]" in ln for ln in info):
+            status = "TIMEOUT"
+        with rec_lock:
+            counts[status] = counts.get(status, 0) + 1
+            done[0] += 1
+            if status not in ("OK", "SKIP"):   # a finding: print it + log it
+                sys.stdout.write("\r")          # clear the progress line
+                print(f"{LABEL.get(status, status)} {s.name}  [#{done[0]}]")
+                for line in info:
+                    print(line)
+                suffix = (" *** REGRESSION CANDIDATE ***"
+                          if status in ("DIFF", "TIMEOUT", "ERROR") else "")
+                for f in (runlog, logf):
+                    f.write(f"\n{status} {s.name}  opts: {ostr(s)}{suffix}\n")
+                    for line in info:
+                        f.write(line + "\n")
+                    f.flush()
+            progress(total)
+
+    def worker(s):
+        try:
+            return run_scenario(s, workroot)
+        except Exception as ex:        # never let one scenario kill the pool
+            return ("ERROR", [f"  exception: {ex!r}"])
+
+    def summary_line(prefix):
+        return (f"{prefix} {counts['OK']} OK, {counts['SKIP']} skipped, "
+                f"{counts['FLAKY']} FLAKY, {counts['ALLOW']} ALLOW (intentional), "
+                f"{counts['ABETTER']} BETTER (A>B), {counts['TIMEOUT']} TIMEOUT, "
+                f"{counts['ERROR']} ERROR, {counts['DIFF']} DIFF "
+                f"(regression candidates) ===")
+
+    deadline = (time.time() + args.timelimit) if args.timelimit else None
+    if deadline:
+        print(f"time limit: {args.timelimit:.0f}s\n")
+
+    if not args.loop:
+        total = len(scns)
+        print(f"scenarios: {total}\n")
+        with ThreadPoolExecutor(max_workers=args.jobs) as ex:
+            futs = {ex.submit(worker, s): s for s in scns}
+            for fut in as_completed(futs):
+                status, info = fut.result()
+                record(futs[fut], status, info, total=total)
+                if deadline and time.time() > deadline:
+                    print("\n[time limit reached -- cancelling pending scenarios]")
+                    for p in futs:
+                        p.cancel()
+                    break
+        summary = summary_line("===")
+        print("\n" + summary)
+        runlog.write("\n" + summary + "\n")
+        logf.write(summary + "\n")
+        runlog.close()
+        logf.close()
+        return 1 if (counts['DIFF'] or counts['TIMEOUT'] or counts['ERROR']) else 0
+
+    # --loop: run the fixed sweeps first, then an endless mixed stream of new
+    # randomized + systematic-combo scenarios, keeping the pool full.
+    rnd = random.Random(args.seed)
+    seen = set()
+    mixed = _mixed_scenarios(rnd, seen)
+    fixed = iter(scns)
+
+    def next_scn():
+        try:
+            return next(fixed)
+        except StopIteration:
+            return next(mixed)
+
+    ex = ThreadPoolExecutor(max_workers=args.jobs)
+    inflight = {}
+
+    def submit_one():
+        s = next_scn()
+        inflight[ex.submit(worker, s)] = s
+
+    try:
+        for _ in range(args.jobs * 2):
+            submit_one()
+        while True:
+            fdone, _pending = wait(list(inflight), return_when=FIRST_COMPLETED)
+            for fut in fdone:
+                s = inflight.pop(fut)
+                status, info = fut.result()
+                record(s, status, info)
+                if not (deadline and time.time() > deadline):
+                    submit_one()
+            if deadline and time.time() > deadline and not inflight:
+                print("\n[time limit reached -- writing summary]")
+                break
+    except KeyboardInterrupt:
+        print("\n[interrupted -- cancelling pending, writing summary]")
+        ex.shutdown(wait=False, cancel_futures=True)
+    finally:
+        summary = summary_line("=== loop stopped:")
+        print("\n" + summary)
+        runlog.write("\n" + summary + "\n")
+        runlog.flush()
+        logf.write(summary + "\n")
+        logf.flush()
+        runlog.close()
+        logf.close()
+    return 1 if counts['DIFF'] else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())