Root cause: when the gateway received SIGTERM (from hermes update,
external kill, WSL2 runtime, etc.), it exited with status 0. systemd's
Restart=on-failure only restarts on non-zero exit, so the gateway
stayed dead permanently. Users had to manually restart.
Fix 1: Signal-initiated shutdown exits non-zero
When SIGTERM/SIGINT is received and no restart was requested (via
/restart, /update, or SIGUSR1), start_gateway() returns False which
causes sys.exit(1). systemd sees a failure exit and auto-restarts
after RestartSec=30.
This is safe because systemctl stop tracks its own stop-requested
state independently of exit code — Restart= never fires for a
deliberate stop, regardless of exit code.
Also logs 'Received SIGTERM/SIGINT — initiating shutdown' so the
cause of unexpected shutdowns is visible in agent.log.
Fix 2: PID file ownership guard
remove_pid_file() now checks that the PID file belongs to the current
process before removing it. During --replace handoffs, the old
process's atexit handler could fire AFTER the new process wrote its
PID file, deleting the new record. This left the gateway running but
invisible to get_running_pid(), causing 'Another gateway already
running' errors on next restart.
Test plan:
- All restart drain tests pass (13)
- All gateway service tests pass (84)
- All update gateway restart tests pass (34)
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