The shared daemon's inactivity backstop (#692) reaped the daemon after
maxIdleMs (default 30 min) of no inbound query bytes whenever a client was
still connected — without ever checking whether that client was actually
alive. lastActivityAt is fed only by inbound socket data and MCP has no
keepalive, so a genuinely-live session that just hadn't queried CodeGraph in
30 min tripped it. The daemon then exited, and the proxy's onDaemonLost
degrades that session (and every other session sharing the daemon) to an
in-process engine for the rest of its life. On one dev machine over a day the
backstop fired 20 times on live sessions (clients=1) and the liveness sweep
caught 0 real dead peers — net harm.
The backstop exists only to catch a phantom client (one counted but gone,
whose socket-close was never delivered). It now consults the peer pids the
daemon already tracks: after the inactivity window it sweeps provably-dead
peers, then reaps the daemon only if NO remaining client can be proven alive
(every one is an unknown-pid connection the sweep can't verify — the sole
phantom class it can't catch). One provably-alive client keeps the daemon up.
Extracted the decision into Daemon.backstopShouldExit(isAlive) so it's unit-
testable with an injected liveness probe, mirroring reapDeadClients. All #692
guarantees preserved; the only behavior change is that a provably-alive quiet
session is no longer reaped.
- daemon-client-liveness.test.ts: 7 new deterministic cases for
backstopShouldExit (live kept, phantom reaped, mixed protects the live one,
dead-peer swept-then-held, within-window, zero-client).
- mcp-daemon.test.ts: the integration test that asserted the backstop reaps a
live connected client (it encoded the bug) now asserts the opposite — a
live-but-quiet session survives several backstop windows with its lockfile
intact and no backstop shutdown logged.
Validated end-to-end on the built bundle: a quiet session's daemon stayed up
across 4 backstop windows (maxIdle=3s), same pid throughout, zero backstop
fires. Found while fixing #1185.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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