No test covers the Reaper LiveDaemonPID wiring in startEnginePlane — deleting it silently disables orphan reaping (fail-closed blind spot)

[cajasmota]

Summary

The Reaper's LiveDaemonPID wiring in startEnginePlane
(internal/daemon/engineplane.go) is load-bearing but has no test coverage.
Because the sweep is fail-closed (#5933), deleting that line silently disables
orphan-watcher reaping forever — no error, no log, no failing test.

Mutation evidence

Delete the LiveDaemonPID: line from the ReaperConfig literal in
startEnginePlane, then:

go build ./...                                                              -> OK
go test ./internal/daemon/ -run 'Split|Engine|Serve|Reaper|Watch' -count=1  -> ok

Fully green with the wiring removed. Reproduced independently twice (by the
adversarial reviewer and by the implementer).

Why it could not be covered in #5933

• startEnginePlane / RunEngine have no direct test coverage; the only harness
  that reaches them is the full daemon.Run path in phaseb_test.go.
• Reaper.Start has a hardcoded ~30s startup delay before its first sweep, and a
  5-minute sweep interval.
• ReaperConfig.Interval is never set by engineplane.go, so the interval is not
  test-injectable.

Any real end-to-end assertion would therefore take 30s+ — out of scope for a
bounded fix, and it would land as a slow test in the default suite.

What #5933 does and does not cover

TestReaper_sweepWatchers_LiveDaemonPIDFromPidfile enforces the reaper/watchreg
contract using the exact closure shape engineplane.go wires (real pidfile,
real ReadPIDFile parse path, match-survives / mismatch-reaped). It does not
detect removal of the wiring line itself, because it constructs its own
ReaperConfig rather than going through startEnginePlane.

Suggested fix

1. Make ReaperConfig.Interval (and ideally the startup delay) test-injectable,
   and have engineplane.go pass an explicit value.
2. Add a RunEngine/startEnginePlane-level test asserting the constructed
   Reaper resolves LiveDaemonPID from the daemon pidfile — gated opt-in like the
   existing GRAFEL_FSNOTIFY_TESTS pattern if it remains slow.

Alternatively, a cheap structural guard: have startEnginePlane fail loudly (or
log Warn at startup) if it ever builds a watcher-reaping ReaperConfig with a nil
LiveDaemonPID, converting a silent permanent degradation into an observable one.

Related

• Split-mode reaper kills every watcher forever: LiveDaemonPID defaults to engine os.Getpid but watchers stamp daemon.pid → 5-min reindex treadmill #5933 (the fix that introduced the fail-closed semantics)
• pidAliveProbe uses Signal(0) — reports every live process as dead on Windows, so watchreg sweep drops all live watcher entries #5934 (Windows liveness probe, same call path)