fleetd #492 follow-up: carry the unclear detail across detect_supervisor's subshell boundary

b17f37a set SUPERVISOR_UNCLEAR_DETAIL as a global inside detect_supervisor, but the real call site invokes it as $(detect_supervisor) — a subshell — so that global died with the subshell and the die() message's ${VAR:-fallback} silently masked the loss with generic text. - detect_supervisor now packs kind and detail onto its one stdout line (joined by the ASCII unit separator byte, $SUPERVISOR_DETAIL_SEP), the only channel that survives $( ). The real call site unpacks both with in-shell parameter expansion — no extra subshell. - Dropped the ${SUPERVISOR_UNCLEAR_DETAIL:-...} fallback at the die() message: under set -u, a missing detail now fails loudly instead of silently defaulting (same defect class as #497). - Added a constraints comment block above detect_supervisor for future callers: stdout-only, no ${VAR:-default} papering over a lost value, and every case on the return value needs an explicit *) arm. - Added *) arms to the three `case "$SUPERVISOR_KIND"` switches (report/stop/start): report warns and continues (display-only), stop/start die naming the value (they act on it). - Rewrote the "unclear" test to go through the real call-site shape ($(detect_supervisor) then the same split), not a hand-constructed value, and tightened its final assertion to check for the actual detail text rather than $SYSTEMD_UNIT alone (the die() boilerplate names the unit either way, so that check could pass on a lost value).
fleetd #492 follow-up: detect_supervisor must never read "could not tell" as "none"
2026-09-12 09:58:45 +07:00 · 2026-09-12 09:23:21 +07:00 · 2026-09-12 09:00:59 +07:00
2 changed files with 568 additions and 54 deletions
@@ -5,7 +5,7 @@
 # A merge is not a deployment: the running daemon holds the jar it was started with, so code merged
 # to main does nothing until this runs. See CLAUDE.md -> "Redeploying the daemon".
 #
-# This script exists to turn six remembered traps into one auditable command:
+# This script exists to turn eight remembered traps into one auditable command:
 #
 #   1. A piped `mvn` hides BUILD FAILURE behind a zero exit, so the build here is never piped.
 #   2. The daemon must start from a LOGIN shell, or the tokens it hands to members are empty:
@@ -27,6 +27,21 @@
 #      restart of the OLD jar. So this script detects whether the agent is loaded and, only then,
 #      swaps `kill` + manual `nohup` for `launchctl unload`/`load` — the one supervisor in control
 #      at any moment is whichever one you asked to act, never both.
+#   7. fleetd #492 — a systemd --user unit is a THIRD possible supervisor (seen on a second host):
+#      Restart=on-failure treats this JVM's SIGTERM exit code (143, per CB-594 above) as a failure
+#      too, so a bare `kill` there would race systemd's own restart of the OLD jar exactly like
+#      launchd would. This script now tells launchd, systemd, and "genuinely unsupervised" apart as
+#      three different answers, drives whichever one it finds through its own control plane
+#      (`launchctl` / `systemctl --user`), and REFUSES outright — never falls back to `kill` — when
+#      it finds a supervision signal it cannot map to exactly one of the two it knows how to drive.
+#      A wrong guess here is how two daemons end up running against one herdr session. Follow-up:
+#      "not currently loaded" is not the same fact as "unsupervised" — a unit that is installed but
+#      activating/failed/pending-restart, or a `systemctl` call that could not answer at all (e.g.
+#      no user-bus access), both now read as a fifth answer, "unclear", and REFUSE the same way
+#      "ambiguous" does, rather than silently falling through to "none".
+#   8. fleetd #492 — a post-restart check counts running fleetd processes and fails the whole run if
+#      more than one is alive. That is the one thing none of the checks above (healthz 200, jar id,
+#      the fresh "listening" line) can see: every one of them is satisfied by EITHER daemon.
 #
 # Usage:
 #   scripts/redeploy-fleetd.sh              # build, confirm, restart, verify
@@ -55,13 +70,42 @@ HEALTH_WAIT=60    # seconds to wait for /healthz to answer after start
 LAUNCHD_LABEL='dev.ltms.fleetd'
 LAUNCHD_PLIST="$HOME/Library/LaunchAgents/$LAUNCHD_LABEL.plist"

+# fleetd #492: the systemd --user unit this script must not fight with either (see trap 7 above).
+# Measured on the second host: `systemctl --user cat fleetd` names the unit "fleetd" (not
+# "dev.ltms.fleetd" — systemd user units here are not namespaced the way the launchd label is).
+SYSTEMD_UNIT='fleetd'
+
+# fleetd #492 follow-up: detect_supervisor packs TWO values (kind, detail) onto the one stdout
+# line that survives its $(...) call — see the constraints comment above that function. This is
+# the separator between them: the ASCII "unit separator" byte, chosen because it never occurs in
+# any of the prose detail strings and needs no escaping in a `case`/glob pattern.
+SUPERVISOR_DETAIL_SEP=$'\x1f'
+
+# fleetd #492 follow-up: set by systemd_loaded/systemd_installed when the underlying `systemctl`
+# call could not answer cleanly — it exited non-zero AND wrote something to stderr, which is a real
+# tool failure (e.g. it cannot reach the user bus over a non-lingering ssh session), never the same
+# fact as a clean negative answer ("not active", no stderr). Initialized here, not just inside the
+# probes, so detect_supervisor can read them under `set -u` even before either probe has ever run,
+# and so a test that stubs a probe with a plain `return 0`/`return 1` body (leaving these untouched)
+# reads a deterministic 0 rather than whatever a previous probe call left behind.
+SYSTEMD_LOADED_ERRORED=0
+SYSTEMD_INSTALLED_ERRORED=0
+# fleetd #492 follow-up: SUPERVISOR_UNCLEAR_DETAIL is the specific supervisor/reason that
+# require_drivable_supervisor's die() names on an "unclear" answer. Deliberately NOT pre-declared
+# here (unlike the two flags above): it is set only by the real call site, right after it unpacks
+# detect_supervisor's stdout (see the constraints comment above detect_supervisor). If that call
+# site is ever skipped or broken, a bare `set -u` reference to this variable in
+# require_drivable_supervisor must fail loudly with "unbound variable" — a pre-declared empty
+# default would instead silently print an empty reason, hiding exactly the value this ticket
+# exists to surface.
+
 DO_BUILD=1; ASSUME_YES=0; CHECK_ONLY=0
 for arg in "$@"; do
  case "$arg" in
    --yes|-y)   ASSUME_YES=1 ;;
    --no-build) DO_BUILD=0 ;;
    --check)    CHECK_ONLY=1 ;;
-    -h|--help)  sed -n '3,37p' "${BASH_SOURCE[0]}"; exit 0 ;;
+    -h|--help)  sed -n '3,48p' "${BASH_SOURCE[0]}"; exit 0 ;;
    *) echo "unknown option: $arg (try --help)" >&2; exit 2 ;;
  esac
 done
@@ -79,6 +123,193 @@ running_pid() { pgrep -f "$PATTERN" || true; }
 launchd_installed() { [ -f "$LAUNCHD_PLIST" ]; }
 launchd_loaded() { launchctl list "$LAUNCHD_LABEL" >/dev/null 2>&1; }

+# fleetd #492: same two questions for systemd --user. Kept as separate, overridable functions
+# (never an inline `systemctl` call at each use site) so a test on a box with no systemd at all
+# (this repo is developed on macOS) can substitute each one independently — the same seam
+# launchd_installed/launchd_loaded above already use.
+#
+# fleetd #492 follow-up: both functions used to throw `systemctl`'s stderr straight into
+# /dev/null, which meant "systemctl answered no" and "systemctl could not answer at all" (e.g. it
+# cannot reach the user bus over a non-lingering ssh session) looked identical — both a plain
+# nonzero exit. They now capture stderr separately and set their own *_ERRORED flag ONLY when the
+# call exited non-zero AND wrote something to stderr — a real tool failure, never a clean "not
+# installed"/"not active" answer (which exits non-zero with empty stderr). detect_supervisor reads
+# the flag right after calling the probe, so a probe that could not answer routes to "unclear",
+# never silently becomes "none".
+#
+# "installed": a unit FILE by this name exists, regardless of its current state — the systemd
+# analogue of the plist file existing on disk. `list-unit-files` reads unit definitions without
+# depending on runtime state, so this stays read-only and safe under --check.
+systemd_installed() {
+  SYSTEMD_INSTALLED_ERRORED=0
+  command -v systemctl >/dev/null 2>&1 || return 1
+  local err_file out rc=0
+  if ! err_file="$(mktemp -t systemd-installed-err)"; then
+    SYSTEMD_INSTALLED_ERRORED=1
+    return 1
+  fi
+  out="$(systemctl --user list-unit-files "$SYSTEMD_UNIT.service" --no-legend 2>"$err_file")" || rc=$?
+  if [ "$rc" -ne 0 ]; then
+    if [ -s "$err_file" ]; then
+      SYSTEMD_INSTALLED_ERRORED=1
+    fi
+    rm -f "$err_file"
+    return "$rc"
+  fi
+  rm -f "$err_file"
+  printf '%s' "$out" | grep -q .
+}
+# "loaded": systemd currently supervises this unit as an active job — the systemd analogue of
+# `launchctl list <label>` succeeding. Measured on the second host: `systemctl --user is-active
+# fleetd` -> "active". A clean "no" (inactive/failed/activating/deactivating) exits non-zero with
+# nothing on stderr; a probe that could not reach systemd at all exits non-zero WITH a stderr
+# message — see the fleetd #492 follow-up note above.
+systemd_loaded() {
+  SYSTEMD_LOADED_ERRORED=0
+  command -v systemctl >/dev/null 2>&1 || return 1
+  local err_file rc=0
+  if ! err_file="$(mktemp -t systemd-loaded-err)"; then
+    SYSTEMD_LOADED_ERRORED=1
+    return 1
+  fi
+  systemctl --user is-active "$SYSTEMD_UNIT" >/dev/null 2>"$err_file" || rc=$?
+  if [ "$rc" -ne 0 ] && [ -s "$err_file" ]; then
+    SYSTEMD_LOADED_ERRORED=1
+  fi
+  rm -f "$err_file"
+  return "$rc"
+}
+
+# fleetd #492: three real answers, not two — launchd, systemd, or genuinely unsupervised — plus a
+# fourth, "ambiguous", for the one case this script cannot tell apart: both signals firing at once.
+# That is exactly "I cannot tell who supervises this process", and guessing wrong here is how two
+# daemons end up running against one herdr session (see trap 7 in the header).
+#
+# fleetd #492 follow-up: a fifth answer, "unclear", for two more situations that must NEVER be read
+# as "none" (measured — see the report this ticket is a follow-up to):
+#   - installed-but-not-loaded, on EITHER supervisor. `systemctl --user is-active` answers "no" for
+#     `activating`, `deactivating`, `failed`, and while an auto-restart is pending — every one of
+#     those is a host that IS under systemd (or launchd) and whose supervisor is about to act again.
+#     `*_installed` already knows the unit/agent exists; this is the first place that fact is
+#     actually consulted in the decision, not just printed as a warning.
+#   - a probe that could not answer at all. systemd_loaded/systemd_installed set their own
+#     *_ERRORED flag (see the comment above them) when `systemctl` exits non-zero WITH a stderr
+#     message — a real tool failure, e.g. it cannot reach the user bus over a non-lingering ssh
+#     session — never conflated with a clean negative answer.
+# "none" now means only: neither supervisor is installed, neither is loaded, and neither probe
+# errored.
+#
+# fleetd #492 follow-up — constraints every caller of this function depends on (learned the hard
+# way: an earlier version of this fix set a SUPERVISOR_UNCLEAR_DETAIL global from inside here and
+# it was silently lost, because every real call site invokes this as `$(detect_supervisor)`):
+#   1. It is called as `$(detect_supervisor)`, so ONLY STDOUT crosses back to the caller. Anything
+#      this function needs to tell its caller — the "unclear" detail included — must be printed,
+#      never assigned to a global: a global set inside a `$( )` subshell dies with that subshell.
+#      This function packs BOTH values (kind and detail) onto that one stdout line, joined by
+#      $SUPERVISOR_DETAIL_SEP, and the caller unpacks them on its own side of the subshell boundary.
+#   2. This script runs under `set -euo pipefail` (line 50), so an unset variable is a loud
+#      failure. Do not add a `${VAR:-default}` anywhere downstream to paper over a value that
+#      should always be there — that hides a lost value instead of surfacing it (fleetd #497's
+#      defect class).
+#   3. Every `case` on this function's return value needs an explicit final `*)` arm, chosen by
+#      whether that caller ACTS on the value (`die` — an unrecognised value must never be silently
+#      driven) or only DISPLAYS it (`echo`/`warn` and continue — a diagnostic must not go silent on
+#      exactly the value it most needs to report).
+#
+# Pure and side-effect-free besides the two *_ERRORED flags (read back within this same call, never
+# by the caller — see the constraints above): reads the four probes and decides — never mutates
+# anything, so it is safe under --check and testable by overriding
+# launchd_installed/launchd_loaded/systemd_installed/systemd_loaded after sourcing.
+detect_supervisor() {
+  local ld=0 sd=0 li=0 si=0 kind detail=""
+  SYSTEMD_LOADED_ERRORED=0
+  SYSTEMD_INSTALLED_ERRORED=0
+
+  launchd_loaded && ld=1
+  systemd_loaded && sd=1
+  launchd_installed && li=1
+  systemd_installed && si=1
+
+  if [ "$SYSTEMD_LOADED_ERRORED" = 1 ] || [ "$SYSTEMD_INSTALLED_ERRORED" = 1 ]; then
+    detail="the systemd --user probe for '$SYSTEMD_UNIT' could not answer cleanly (systemctl exited non-zero and reported an error on stderr, not a clean negative — e.g. it cannot reach the user bus)"
+    kind="unclear"
+  elif [ "$ld" = 1 ] && [ "$sd" = 1 ]; then
+    kind="ambiguous"
+  elif [ "$li" = 1 ] && [ "$ld" = 0 ]; then
+    detail="the launchd agent ($LAUNCHD_LABEL) is installed ($LAUNCHD_PLIST exists) but is not currently loaded"
+    kind="unclear"
+  elif [ "$si" = 1 ] && [ "$sd" = 0 ]; then
+    detail="the systemd --user unit ($SYSTEMD_UNIT) is installed but not currently active — it may be activating, deactivating, failed, or waiting on an auto-restart"
+    kind="unclear"
+  elif [ "$ld" = 1 ]; then
+    kind="launchd"
+  elif [ "$sd" = 1 ]; then
+    kind="systemd"
+  else
+    kind="none"
+  fi
+
+  printf '%s%s%s' "$kind" "$SUPERVISOR_DETAIL_SEP" "$detail"
+}
+
+# fleetd #492: turns anything detect_supervisor returns that is NOT exactly one of the two
+# supervisors this script knows how to drive into a die() — never a fall-through to the `kill`
+# path. Kept as its own function so a test can call it directly (in a subshell, since it die()s)
+# without running the whole report-state flow or needing a real launchd/systemd.
+require_drivable_supervisor() {
+  local kind="$1"
+  case "$kind" in
+    launchd|systemd|none) ;;
+    ambiguous)
+      die "both launchd ($LAUNCHD_LABEL) and systemd --user ($SYSTEMD_UNIT) report themselves as
+          loaded for this daemon at the same time. This script cannot tell which one actually
+          supervises the running process, and driving either alone risks the OTHER reviving the
+          OLD jar out from under it — the exact failure this ticket (fleetd #492) exists to
+          prevent. Stop one of the two supervisors by hand, confirm only one remains loaded, then
+          rerun." ;;
+    unclear)
+      # fleetd #492 follow-up: SUPERVISOR_UNCLEAR_DETAIL crosses back from detect_supervisor's
+      # subshell via its stdout, unpacked by the caller BEFORE it calls this function (see the
+      # constraints comment above detect_supervisor). No ${VAR:-default} here on purpose: if the
+      # detail is somehow missing, `set -u` makes this reference fail loudly instead of silently
+      # naming nothing — a default that hides a lost value is the same defect class as fleetd
+      # #497.
+      die "a supervisor looks present but this script cannot tell whether it actually drives this
+          daemon: $SUPERVISOR_UNCLEAR_DETAIL. Guessing wrong here is the same failure 'ambiguous'
+          above exists to prevent: driving the daemon while an unseen supervisor revives the OLD
+          jar out from under it (fleetd #492). Check 'launchctl list $LAUNCHD_LABEL' and
+          'systemctl --user status $SYSTEMD_UNIT' by hand, resolve whichever looks unclear, then
+          rerun." ;;
+    *)
+      die "detect_supervisor returned an unrecognized value '$kind' — refusing to guess which
+          supervisor, if any, controls this daemon." ;;
+  esac
+}
+
+# fleetd #492: the exact symptom a racing supervisor produces — count how many fleetd processes are
+# alive right now. Takes the pid list as a parameter (rather than calling running_pid() itself) so a
+# test can pass a canned two-line string without a real second process running. Pure except for the
+# die() in assert_single_daemon below.
+count_daemon_pids() {
+  local pids="$1"
+  if [ -z "$pids" ]; then
+    echo 0
+  else
+    printf '%s\n' "$pids" | grep -c .
+  fi
+}
+assert_single_daemon() {
+  local pids="$1" count
+  count="$(count_daemon_pids "$pids")"
+  if [ "$count" -gt 1 ]; then
+    die "more than one fleetd process is running after this restart (pids: $(printf '%s' "$pids" | tr '\n' ' ')).
+        This is the exact failure a racing supervisor produces: the OLD jar was revived by its
+        supervisor while this script started a NEW copy. Two daemons on one herdr session kill
+        each other's members. Investigate with 'pgrep -f \"$PATTERN\"' and stop the wrong one by
+        hand — do not assume either pid is the one you want."
+  fi
+}
+
 # CB-600: the script computes its own log path from where it sits on disk (REPO, above); the
 # plist hard-codes an absolute StandardOutPath. Nothing forced the two to agree — if this script
 # were ever run from a checkout other than the one the loaded plist names, launchd would start and
@@ -182,25 +413,57 @@ fi
 ok "jar on disk: $(jar_id)  ($([ -f "$JAR" ] && date -r "$JAR" '+%Y-%m-%d %H:%M:%S' || echo 'none'))"
 ok "HEAD: $(git -C "$REPO" log --oneline -1)"

-# CB-594: supervision state. Installed and loaded are different facts — a copied-but-never-loaded
-# plist supervises nothing, and a loaded label with no file backing it (rare, but possible after an
-# edited/moved plist) is still what launchd will act on.
+# CB-594 / fleetd #492: supervision state. Installed and loaded are different facts — a
+# copied-but-never-loaded plist (or an unloaded systemd unit) supervises nothing, and a loaded
+# label/unit with no file backing it is still what its supervisor will act on.
 if launchd_installed; then
  ok "launchd agent installed: $LAUNCHD_PLIST"
 else
-  warn "launchd agent NOT installed (no supervision — a crash will not restart the daemon)."
+  warn "launchd agent NOT installed."
 fi
-SUPERVISED=0
-if launchd_loaded; then
-  SUPERVISED=1
-  ok "launchd agent loaded ($LAUNCHD_LABEL) — launchd supervises this daemon"
-  # CB-600: fail loudly here, before ANY other check runs, if this script and the loaded plist
-  # would read different log files — every check after this point is worthless otherwise.
-  check_log_path_matches_plist "$OUT" "$LAUNCHD_PLIST"
+if systemd_installed; then
+  ok "systemd --user unit installed: $SYSTEMD_UNIT"
 else
-  warn "launchd agent not loaded — this script is the only thing that will restart the daemon."
+  warn "systemd --user unit NOT installed ($SYSTEMD_UNIT)."
 fi

+# fleetd #492: decide which of the two (if either) actually supervises this daemon, and refuse
+# outright — before touching anything — if that cannot be told apart (see require_drivable_
+# supervisor above). --check reaches this same line, so a host with an undrivable supervisor is
+# reported as a failure even in --check, without ever reaching the build/stop/start steps.
+# fleetd #492 follow-up: detect_supervisor runs as $(...), so only the printed line survives —
+# unpack kind and detail from it HERE, in this shell, before calling anything downstream. See the
+# constraints comment above detect_supervisor for why this cannot be done any other way.
+SUPERVISOR_RAW="$(detect_supervisor)"
+SUPERVISOR_KIND="${SUPERVISOR_RAW%%"$SUPERVISOR_DETAIL_SEP"*}"
+SUPERVISOR_UNCLEAR_DETAIL="${SUPERVISOR_RAW#*"$SUPERVISOR_DETAIL_SEP"}"
+require_drivable_supervisor "$SUPERVISOR_KIND"
+ok "supervisor detected: $SUPERVISOR_KIND"
+SUPERVISED=0
+case "$SUPERVISOR_KIND" in
+  launchd)
+    SUPERVISED=1
+    ok "launchd agent loaded ($LAUNCHD_LABEL) — launchd supervises this daemon"
+    # CB-600: fail loudly here, before ANY other check runs, if this script and the loaded plist
+    # would read different log files — every check after this point is worthless otherwise.
+    check_log_path_matches_plist "$OUT" "$LAUNCHD_PLIST"
+    ;;
+  systemd)
+    SUPERVISED=1
+    ok "systemd --user unit active ($SYSTEMD_UNIT) — systemd supervises this daemon"
+    ;;
+  none)
+    warn "no supervisor loaded — this script is the only thing that will restart the daemon."
+    ;;
+  *)
+    # fleetd #492 follow-up: this block only DISPLAYS state, it changes nothing yet — so a value
+    # it doesn't recognise gets reported, not an abort that goes silent on exactly the state most
+    # worth seeing. (Unreachable today: require_drivable_supervisor above already died on
+    # "ambiguous"/"unclear" before this case runs. Guards the value nobody has invented yet.)
+    warn "unrecognised supervisor kind: '$SUPERVISOR_KIND' — detect_supervisor returned a value this block does not know; continuing to report the rest of the state."
+    ;;
+esac
+
 # The trap with no log line. Checked in a LOGIN shell, because that is how the daemon is started
 # below. Never prints the value — only whether it resolved.
 if zsh -lc '[ -n "${WORKER_GITEA_TOKEN:-}" ]' 2>/dev/null; then
@@ -281,25 +544,47 @@ fi

 # ------------------------------------------------------------------ stop
 #
-# CB-594: when SUPERVISED, launchd owns the stop — never a raw `kill` here. A bare SIGTERM makes
-# this JVM exit 143 even with its shutdown hook running to completion (verified separately: a
-# throwaway Java process with an equivalent shutdown hook, sent SIGTERM from a login shell that
-# could `wait` on it directly, reported exit code 143 every time — never 0). launchd's
-# KeepAlive.SuccessfulExit=false treats any nonzero exit as a crash and restarts the OLD jar,
-# which would race this script's own restart of the NEW one. `launchctl unload` avoids that race
-# by deregistering the job first, so no KeepAlive is left armed when the process actually stops.
+# CB-594 / fleetd #492: when SUPERVISED, the supervisor owns the stop — never a raw `kill` here. A
+# bare SIGTERM makes this JVM exit 143 even with its shutdown hook running to completion (verified
+# separately: a throwaway Java process with an equivalent shutdown hook, sent SIGTERM from a login
+# shell that could `wait` on it directly, reported exit code 143 every time — never 0). launchd's
+# KeepAlive.SuccessfulExit=false and systemd's Restart=on-failure both treat any nonzero exit as a
+# crash and restart the OLD jar, which would race this script's own restart of the NEW one.
+# `launchctl unload` avoids that race by deregistering the job first, so no KeepAlive is left
+# armed when the process actually stops. `systemctl --user stop` needs no such dance: unlike
+# KeepAlive, systemd's Restart= does not fire on a deliberate stop, only on an unexpected exit of
+# an active unit.

 if [ -n "$OLD_PID" ]; then
  say "stop"
  RESTART_MARK="$(wc -l < "$OUT" 2>/dev/null || echo 0)"   # verify a FRESH line appears later
-  if [ "$SUPERVISED" = 1 ]; then
-    echo "   supervision is ON: using 'launchctl unload' (not kill) so launchd's own KeepAlive"
-    echo "   cannot restart the OLD jar out from under this script — see the CB-594 comment above."
-    launchctl unload -w "$LAUNCHD_PLIST" \
-      || die "launchctl unload failed — the daemon may still be under supervision; investigate before retrying"
-  else
-    kill "$OLD_PID"
-  fi
+  case "$SUPERVISOR_KIND" in
+    launchd)
+      echo "   supervision is ON (launchd): using 'launchctl unload' (not kill) so launchd's own"
+      echo "   KeepAlive cannot restart the OLD jar out from under this script — see the CB-594"
+      echo "   comment above."
+      launchctl unload -w "$LAUNCHD_PLIST" \
+        || die "launchctl unload failed — the daemon may still be under supervision; investigate before retrying"
+      ;;
+    systemd)
+      echo "   supervision is ON (systemd --user): using 'systemctl --user stop' (not kill) so"
+      echo "   systemd's own Restart=on-failure cannot restart the OLD jar out from under this"
+      echo "   script — see the fleetd #492 comment above."
+      systemctl --user stop "$SYSTEMD_UNIT" \
+        || die "'systemctl --user stop $SYSTEMD_UNIT' failed — the daemon may still be under supervision; investigate before retrying"
+      ;;
+    none)
+      kill "$OLD_PID"
+      ;;
+    *)
+      # fleetd #492 follow-up: this block ACTS (stops the daemon one specific way per kind) — an
+      # unrecognised value must never fall through to a default action, silently picking the wrong
+      # one (or none at all) while reporting success. (Unreachable today: require_drivable_
+      # supervisor already died before this runs. Guards the value nobody has invented yet.)
+      die "detect_supervisor returned an unrecognized value '$SUPERVISOR_KIND' at the stop step —
+          refusing to guess how to stop a daemon under an unknown supervisor. The daemon was NOT
+          stopped." ;;
+  esac
  for _ in $(seq "$STOP_WAIT"); do
    [ -z "$(running_pid)" ] && break
    sleep 1
@@ -310,13 +595,20 @@ if [ -n "$OLD_PID" ]; then
        leave worktrees and panes behind. Investigate, then kill -9 by hand if you accept that."
  fi
  ok "pid $OLD_PID exited"
-elif [ "$SUPERVISED" = 1 ]; then
+elif [ "$SUPERVISOR_KIND" = "launchd" ]; then
  # Loaded but not currently running (e.g. throttled after a crash loop). Unload it anyway so the
  # start step below does a clean load, never a load stacked on an already-loaded label.
  say "stop"
  RESTART_MARK="$(wc -l < "$OUT" 2>/dev/null || echo 0)"
  launchctl unload -w "$LAUNCHD_PLIST" 2>/dev/null || true
  ok "launchd agent unloaded (was already not running)"
+elif [ "$SUPERVISOR_KIND" = "systemd" ]; then
+  # Same case for systemd: the unit is known/active-capable but not currently running. `stop` on an
+  # already-stopped unit is a harmless no-op — kept for symmetry with the launchd branch above.
+  say "stop"
+  RESTART_MARK="$(wc -l < "$OUT" 2>/dev/null || echo 0)"
+  systemctl --user stop "$SYSTEMD_UNIT" 2>/dev/null || true
+  ok "systemd --user unit stopped (was already not running)"
 else
  RESTART_MARK="$(wc -l < "$OUT" 2>/dev/null || echo 0)"
 fi
@@ -324,34 +616,56 @@ fi
 # ------------------------------------------------------------------ start
 # Unsupervised: login shell (zsh -l) is what puts the secrets on the daemon's environment, and cwd
 # must be fleetd/ because the daemon resolves fleetd.yaml, logs/ and target/ relative to it.
-# Supervised: launchd does both — deploy/dev.ltms.fleetd.plist points ProgramArguments at
+# Supervised (launchd): launchd does both — deploy/dev.ltms.fleetd.plist points ProgramArguments at
 # scripts/fleetd-launchd-wrapper.sh (CB-594), which is what execs the login shell in launchd's
 # place, and WorkingDirectory in the plist already pins fleetd/.
+# Supervised (systemd --user): the unit does both too — measured on the second host, ExecStart is
+# `/bin/zsh -lc "exec java -jar target/fleetd.jar fleetd.yaml"` (a login shell, same reason as
+# above) and WorkingDirectory is already pinned to fleetd/.

 say "start"
-if [ "$SUPERVISED" = 1 ]; then
-  echo "   supervision is ON: using 'launchctl load' so launchd starts and keeps supervising this"
-  echo "   process, instead of a manual nohup that launchd would know nothing about."
-  # CB-600: 'launchctl unload -w' above already persisted Disabled=true for this label. A load -w
-  # that succeeds clears it; a load -w that FAILS leaves the agent both stopped and disabled — worse
-  # than before this script ran, because a later reboot or login will not bring it back either. One
-  # retry covers a transient race (e.g. launchd not yet fully done deregistering); if it still fails,
-  # die with the exact recovery command rather than a bare "failed".
-  if ! launchctl load -w "$LAUNCHD_PLIST" 2>/dev/null; then
-    warn "launchctl load failed on the first attempt — retrying once after a short pause"
-    sleep 2
-    launchctl load -w "$LAUNCHD_PLIST" || die "launchctl load failed twice.
-        The agent is now STOPPED and DISABLED — it will NOT come back on its own, not even after a
-        reboot or login, because 'launchctl unload -w' above persisted Disabled=true and load -w
-        never got the chance to clear it. Recover with:
-            launchctl load -w \"$LAUNCHD_PLIST\"
-        If that still fails, check 'launchctl list $LAUNCHD_LABEL', validate the plist with
-        'plutil -lint \"$LAUNCHD_PLIST\"', and check $OUT before assuming a retry will succeed."
-  fi
-else
-  # Absolute jar path so `ps` names which checkout is running.
-  ( cd "$MODULE" && zsh -lc "nohup java -jar '$JAR' >> fleetd.out 2>&1 &" )
-fi
+case "$SUPERVISOR_KIND" in
+  launchd)
+    echo "   supervision is ON (launchd): using 'launchctl load' so launchd starts and keeps"
+    echo "   supervising this process, instead of a manual nohup that launchd would know nothing"
+    echo "   about."
+    # CB-600: 'launchctl unload -w' above already persisted Disabled=true for this label. A load -w
+    # that succeeds clears it; a load -w that FAILS leaves the agent both stopped and disabled — worse
+    # than before this script ran, because a later reboot or login will not bring it back either. One
+    # retry covers a transient race (e.g. launchd not yet fully done deregistering); if it still fails,
+    # die with the exact recovery command rather than a bare "failed".
+    if ! launchctl load -w "$LAUNCHD_PLIST" 2>/dev/null; then
+      warn "launchctl load failed on the first attempt — retrying once after a short pause"
+      sleep 2
+      launchctl load -w "$LAUNCHD_PLIST" || die "launchctl load failed twice.
+          The agent is now STOPPED and DISABLED — it will NOT come back on its own, not even after a
+          reboot or login, because 'launchctl unload -w' above persisted Disabled=true and load -w
+          never got the chance to clear it. Recover with:
+              launchctl load -w \"$LAUNCHD_PLIST\"
+          If that still fails, check 'launchctl list $LAUNCHD_LABEL', validate the plist with
+          'plutil -lint \"$LAUNCHD_PLIST\"', and check $OUT before assuming a retry will succeed."
+    fi
+    ;;
+  systemd)
+    echo "   supervision is ON (systemd --user): using 'systemctl --user start' so systemd starts"
+    echo "   and keeps supervising this process, instead of a manual nohup it would know nothing"
+    echo "   about."
+    systemctl --user start "$SYSTEMD_UNIT" || die "'systemctl --user start $SYSTEMD_UNIT' failed.
+        Check 'systemctl --user status $SYSTEMD_UNIT' and $OUT before assuming a retry will succeed."
+    ;;
+  none)
+    # Absolute jar path so `ps` names which checkout is running.
+    ( cd "$MODULE" && zsh -lc "nohup java -jar '$JAR' >> fleetd.out 2>&1 &" )
+    ;;
+  *)
+    # fleetd #492 follow-up: this block ACTS (starts the daemon one specific way per kind) — an
+    # unrecognised value must never fall through to a default action, silently picking the wrong
+    # one (or none at all) while reporting success. (Unreachable today: require_drivable_
+    # supervisor already died before this runs. Guards the value nobody has invented yet.)
+    die "detect_supervisor returned an unrecognized value '$SUPERVISOR_KIND' at the start step —
+        refusing to guess how to start a daemon under an unknown supervisor. The daemon was NOT
+        started." ;;
+esac

 for _ in $(seq 10); do
  NEW_PID="$(running_pid)"
@@ -411,6 +725,13 @@ FRESH_LOG="$(mktemp -t fleetd-fresh-log)"
 trap 'rm -f "$FRESH_LOG"' EXIT
 tail -n "+$((RESTART_MARK + 1))" "$OUT" > "$FRESH_LOG" 2>/dev/null || true
 classify_amqp_connection_errors "$FRESH_LOG"
+
+# fleetd #492: checked here, after healthz and the fresh-log check have both had time to run, so a
+# supervisor that revives the OLD jar a few seconds late is caught too. Every check above (healthz
+# 200, jar id, the fresh 'listening' line) is satisfied by EITHER daemon if two are alive — this is
+# the only one that can tell.
+assert_single_daemon "$(running_pid)"
+
 say "result"
 ok "pid $NEW_PID, jar $(jar_id)"
 if [ "$REDEPLOY_ERROR_COUNT" -eq 0 ]; then
@@ -25,6 +25,187 @@ classify_fixture() {
  classify_amqp_connection_errors "$TMP/$name"
 }

+# fleetd #492 follow-up: detect_supervisor's stdout is now "kind<SEP>detail" (see the constraints
+# comment above detect_supervisor in redeploy-fleetd.sh) — every test below that only cares about
+# the kind must split it out with the SAME in-shell parameter expansion the real call site (:438)
+# uses, never a bare string comparison against the raw output.
+supervisor_kind_of() {
+  printf '%s' "${1%%"$SUPERVISOR_DETAIL_SEP"*}"
+}
+supervisor_detail_of() {
+  printf '%s' "${1#*"$SUPERVISOR_DETAIL_SEP"}"
+}
+
+
+# fleetd #492 — supervisor detection. Detect_supervisor() reads launchd_loaded/systemd_loaded, so
+# each test overrides BOTH pairs (installed + loaded) explicitly, rather than relying on either
+# being naturally absent: this machine may itself be running a real fleetd under launchd right now
+# (see CLAUDE.md/MEMORY.md — launchd supervision has been live here since 2026-08-26), so leaving
+# launchd_loaded unmocked in a "systemd only" test would silently read this host's own live state
+# instead of the fixture.
+test_detect_supervisor_launchd_only() {
+  launchd_installed() { return 0; }
+  launchd_loaded()    { return 0; }
+  systemd_installed() { return 1; }
+  systemd_loaded()    { return 1; }
+  assert_equals "launchd" "$(supervisor_kind_of "$(detect_supervisor)")" "launchd-only detection"
+}
+
+test_detect_supervisor_systemd_only() {
+  launchd_installed() { return 1; }
+  launchd_loaded()    { return 1; }
+  systemd_installed() { return 0; }
+  systemd_loaded()    { return 0; }
+  assert_equals "systemd" "$(supervisor_kind_of "$(detect_supervisor)")" "systemd-only detection"
+}
+
+test_detect_supervisor_none() {
+  launchd_installed() { return 1; }
+  launchd_loaded()    { return 1; }
+  systemd_installed() { return 1; }
+  systemd_loaded()    { return 1; }
+  assert_equals "none" "$(supervisor_kind_of "$(detect_supervisor)")" "unsupervised detection"
+}
+
+# fleetd #492 follow-up — detect_supervisor must never answer "none" when the truth is "could not
+# tell". `systemd_installed`/`systemd_loaded` already know a unit file exists; this proves that
+# fact is now actually consulted, not just printed as a warning: an installed-but-not-loaded unit
+# reads as unclear, because is-active answers "no" for activating/deactivating/failed/pending
+# auto-restart too, and every one of those is a host that IS under systemd.
+test_detect_supervisor_systemd_installed_not_loaded_is_unclear() {
+  launchd_installed() { return 1; }
+  launchd_loaded()    { return 1; }
+  systemd_installed() { return 0; }   # the unit file IS there
+  systemd_loaded()    { return 1; }   # is-active says no — could be activating/failed/pending restart
+  local raw
+  raw="$(detect_supervisor)"
+  assert_equals "unclear" "$(supervisor_kind_of "$raw")" "systemd installed-but-not-loaded must read as unclear, not none"
+  printf '%s' "$(supervisor_detail_of "$raw")" | grep -qF "$SYSTEMD_UNIT" \
+    || fail "detail does not name the systemd unit it found installed-but-not-loaded"
+}
+
+# Same fact, the launchd side: a plist on disk that is not currently loaded (unloaded without being
+# removed, or about to be reloaded) must not read as "no supervisor" either.
+test_detect_supervisor_launchd_installed_not_loaded_is_unclear() {
+  launchd_installed() { return 0; }   # the plist IS there
+  launchd_loaded()    { return 1; }   # launchctl list says not loaded
+  systemd_installed() { return 1; }
+  systemd_loaded()    { return 1; }
+  local raw
+  raw="$(detect_supervisor)"
+  assert_equals "unclear" "$(supervisor_kind_of "$raw")" "launchd installed-but-not-loaded must read as unclear, not none"
+  printf '%s' "$(supervisor_detail_of "$raw")" | grep -qF "$LAUNCHD_LABEL" \
+    || fail "detail does not name the launchd label it found installed-but-not-loaded"
+}
+
+# Drives the REAL systemd_loaded/systemd_installed bodies (never stubbed) through a `systemctl`
+# stub placed first on PATH that exits non-zero AND writes to stderr — the shape of a systemctl
+# that runs but cannot reach the user bus (measured elsewhere as a headless ssh session with no
+# lingering). This must read as unclear, never none: a probe that could not answer at all is not
+# the same fact as "no supervisor is loaded".
+test_detect_supervisor_systemd_probe_error_is_unclear() {
+  # Re-source first to restore the REAL launchd_*/systemd_* probe bodies. Earlier tests in this
+  # file permanently override them with stub `return 0`/`return 1` bodies (that is the whole point
+  # of those tests), and a bash function definition is global for the rest of the process — without
+  # this, systemd_loaded here would still be whatever the previous test left it as, never touching
+  # a real `systemctl` call at all.
+  source "$ROOT/scripts/redeploy-fleetd.sh"
+  local bin_dir result rc=0
+  bin_dir="$TMP/stub-bin-systemctl-errors"
+  mkdir -p "$bin_dir"
+  cat > "$bin_dir/systemctl" <<'STUB'
+#!/usr/bin/env bash
+echo "Failed to connect to bus: No such file or directory" >&2
+exit 1
+STUB
+  chmod +x "$bin_dir/systemctl"
+
+  PATH="$bin_dir:$PATH" systemd_loaded && rc=0 || rc=$?
+  [ "$rc" -ne 0 ] || fail "systemd_loaded must not report loaded=true when systemctl only errored"
+  assert_equals "1" "$SYSTEMD_LOADED_ERRORED" "systemd_loaded must flag a probe error, not a clean negative"
+
+  launchd_installed() { return 1; }
+  launchd_loaded()    { return 1; }
+  result="$(PATH="$bin_dir:$PATH" detect_supervisor)"
+  assert_equals "unclear" "$(supervisor_kind_of "$result")" "a systemd probe error must read as unclear, not none"
+  printf '%s' "$(supervisor_detail_of "$result")" | grep -qF "$SYSTEMD_UNIT" \
+    || fail "detail does not name the systemd unit whose probe errored"
+}
+
+# fleetd #492 follow-up (Item 1): this must go through the REAL call-site shape at :437-440, not a
+# hand-constructed "unclear" value — a test that builds "unclear" directly proves the switch, not
+# the handoff, and that is exactly the gap that let SUPERVISOR_UNCLEAR_DETAIL never reach the real
+# caller in b17f37a. detect_supervisor runs as $(detect_supervisor): a subshell. Only stdout
+# survives that boundary, so kind AND detail must both cross on it — this test proves they do.
+test_require_drivable_supervisor_refuses_unclear() {
+  launchd_installed() { return 1; }
+  launchd_loaded()    { return 1; }
+  systemd_installed() { return 0; }
+  systemd_loaded()    { return 1; }
+
+  local SUPERVISOR_RAW SUPERVISOR_KIND SUPERVISOR_UNCLEAR_DETAIL output rc=0
+  # Exactly what :437-439 does — do not shortcut this by constructing "unclear" by hand.
+  SUPERVISOR_RAW="$(detect_supervisor)"
+  SUPERVISOR_KIND="${SUPERVISOR_RAW%%"$SUPERVISOR_DETAIL_SEP"*}"
+  SUPERVISOR_UNCLEAR_DETAIL="${SUPERVISOR_RAW#*"$SUPERVISOR_DETAIL_SEP"}"
+
+  assert_equals "unclear" "$SUPERVISOR_KIND" "setup: expected unclear before testing the refusal"
+  [ -n "$SUPERVISOR_UNCLEAR_DETAIL" ] \
+    || fail "detail did not survive the \$(...) call-site boundary — SUPERVISOR_UNCLEAR_DETAIL is empty in the parent shell"
+  printf '%s' "$SUPERVISOR_UNCLEAR_DETAIL" | grep -qF "$SYSTEMD_UNIT" \
+    || fail "detail that crossed the subshell boundary does not name the systemd unit it found installed-but-not-loaded"
+
+  output="$(require_drivable_supervisor "$SUPERVISOR_KIND" 2>&1)" || rc=$?
+  [ "$rc" -ne 0 ] || fail "require_drivable_supervisor accepted an unclear (undrivable) supervisor"
+  # Check for the ACTUAL DETAIL TEXT, not just "$SYSTEMD_UNIT" — the die() message's boilerplate
+  # recovery instructions name the unit unconditionally either way ("systemctl --user status
+  # $SYSTEMD_UNIT"), so a bare unit-name grep here would pass even on a lost/fallback detail. Only
+  # the specific detail string proves the crossed value, not the boilerplate, reached the message.
+  printf '%s' "$output" | grep -qF "$SUPERVISOR_UNCLEAR_DETAIL" \
+    || fail "refusal message does not contain the specific detail that crossed the subshell boundary"
+}
+
+# The heart of the ticket: a supervisor this script cannot drive must refuse, never fall through to
+# `kill`. require_drivable_supervisor die()s, so it is invoked inside a command substitution — that
+# forks a subshell, so its exit() only ends the subshell and this test script keeps running under
+# `set -e`.
+test_require_drivable_supervisor_refuses_ambiguous() {
+  local output rc=0
+  output="$(require_drivable_supervisor "ambiguous" 2>&1)" || rc=$?
+  [ "$rc" -ne 0 ] || fail "require_drivable_supervisor accepted an ambiguous (undrivable) supervisor"
+  printf '%s' "$output" | grep -qF "$LAUNCHD_LABEL" \
+    || fail "refusal message does not name the launchd label it found"
+  printf '%s' "$output" | grep -qF "$SYSTEMD_UNIT" \
+    || fail "refusal message does not name the systemd unit it found"
+}
+
+test_require_drivable_supervisor_accepts_known_kinds() {
+  require_drivable_supervisor "launchd" || fail "refused a drivable launchd supervisor"
+  require_drivable_supervisor "systemd" || fail "refused a drivable systemd supervisor"
+  require_drivable_supervisor "none"    || fail "refused the unsupervised case"
+}
+
+# fleetd #492 — the one-daemon check. Two live pids is the exact symptom a racing supervisor
+# produces, and none of the other post-restart checks (healthz, jar id, the fresh log line) can see
+# it because either daemon alone satisfies them.
+test_count_daemon_pids() {
+  assert_equals 0 "$(count_daemon_pids "")" "count of an empty pid list"
+  assert_equals 1 "$(count_daemon_pids "4242")" "count of a single pid"
+  assert_equals 2 "$(count_daemon_pids "$(printf '4242\n4343\n')")" "count of two pids"
+}
+
+test_assert_single_daemon_accepts_one_pid() {
+  assert_single_daemon "4242" || fail "assert_single_daemon rejected a single running pid"
+}
+
+test_assert_single_daemon_rejects_two_pids() {
+  local output rc=0
+  output="$(assert_single_daemon "$(printf '4242\n4343\n')" 2>&1)" || rc=$?
+  [ "$rc" -ne 0 ] || fail "assert_single_daemon accepted two simultaneously running pids"
+  printf '%s' "$output" | grep -qF '4242' || fail "refusal message does not list the pids it found"
+  printf '%s' "$output" | grep -qF '4343' || fail "refusal message does not list the pids it found"
+}
+
 test_no_errors() {
  cat > "$TMP/no-errors.log" <<'LOG'
 2026-09-05 12:00:00 INFO fleetd listening
@@ -224,6 +405,18 @@ test_unattributable_quiet_mutation_is_caught() {
  printf 'Unattributable mutation: FAIL: cross-unattributable recovered: expected 0, got 2\n'
 }

+test_detect_supervisor_launchd_only
+test_detect_supervisor_systemd_only
+test_detect_supervisor_none
+test_detect_supervisor_systemd_installed_not_loaded_is_unclear
+test_detect_supervisor_launchd_installed_not_loaded_is_unclear
+test_detect_supervisor_systemd_probe_error_is_unclear
+test_require_drivable_supervisor_refuses_ambiguous
+test_require_drivable_supervisor_refuses_unclear
+test_require_drivable_supervisor_accepts_known_kinds
+test_count_daemon_pids
+test_assert_single_daemon_accepts_one_pid
+test_assert_single_daemon_rejects_two_pids
 test_no_errors
 test_recovery_patterns_match_source
 test_attributed_recovered_connection_error