* harden background daemon: stdout integrity, lifecycle, single-instance lock
Deep review of internal/daemon (worker pool, launcher, control server,
session fan-out, single-instance lock) found and fixes a stdout-integrity
bug plus several lifecycle/locking hazards. Each fix has a regression test;
the daemon suite passes under -race.
Worker stdout integrity
- Replace the capped bufio.Scanner (64 KiB) on a worker's stdout with an
uncapped bufio.Reader: a legitimately large stream-json line from our own
trusted worker is no longer silently dropped (the 1 MiB cap belongs to
untrusted network frames, not this local pipe).
- Surface a stdout read error as a run failure instead of swallowing it as a
clean success, and kill the worker first so it can't block writing to the
now-unread pipe and hang Wait.
Control-server lifecycle
- Track open connections and close them on Shutdown so a single idle/hostile
client can no longer wedge drain (and SIGTERM cleanup) forever.
- Guard the listener with a mutex and re-check the shutdown signal after bind
so a shutdown requested in the bind window is not lost.
- Honor shutdown during buffered-history replay too.
Session fan-out
- A slow subscriber that drops live lines now receives a namespaced gap
notice when its channel drains, instead of silently losing output.
- Hold the session lock across the (non-blocking) broadcast sends to close a
latent send-on-closed-channel panic when a subscriber cancels mid-send.
Single-instance lock
- Reclaim a stale lock with an atomic rename-with-verify instead of a blind
remove, so two instances starting at once cannot both reclaim and end up
both holding the lock; a holder that reacquires in the gap is restored.
Client + pool
- Bound the control handshake with an I/O deadline so a peer that never
replies can't wedge Dial/NewClientConn.
- Key the active-worker map by a monotonic id rather than PID (the OS reuses
PIDs the instant a worker exits).
- Group-terminate on context cancel so a stuck worker's children are not
orphaned.
* daemon: add -race regression test for the Session.Line lock-hold invariant
Session.Line holds s.mu across its non-blocking subscriber sends so cancel()/
finish() cannot close a subscriber channel mid-send (send-on-closed-channel
panic). That invariant had no concurrency test. Add one that drives Line
concurrently with Subscribe/cancel and finish; it panics with a data race if
the lock-hold is reverted and passes with it (verified both ways under -race).
